The

NW _.
e“ \.

,
to acquire 1-
in this my
GeMann:
identify and
transit ions .
PreviouSly p:

EXCit
located at 1;
1657-2 keV '

11‘;

inprm at l

 

159-

3'59 1449.:

Vtile the deca

 

 

ABSTRACT

GAMMA RAY SPECTROSCOPY STUDIES OF THE EXCITED
STATES OF ODD PROTON (ODD MASS) NUCLEI
IN THE Z=50-62, N=64-82 REGION
By

Dwight Beecher Beery

The beta, gamma decay schemes of Nd141m+g’ Nd139m+g,
Ce137m+g, Ba133m, and Ba131m have been studied in an effort
to acquire information about the energy level systematics
in this region of the table of nuclides. Ge(Li) singles and
Ge(Li)-Nal(T1) coincidence spectrometers were employed to
identify and establish the sequence of many new gamma ray
transitions. Some significant additions to and changes from
previously proposed schemes are suggested by the data.

Excited levels accommodating 10 gamma rays have been
located at 145.4, 1126.8, 1292.5, 1298.4, 1580.0, 1607.9, and
1657.2 keV in Prlhl following the decay of 2.6-h Ndlulg. States
in Pr139 at 113.8, 405.0, 589.2, 916.8, 1074.4, 1311.8, 1328.2,
1405.5, 1449.5, and 1501.2 keV are populated by 30 min Ndi399,
while the decay of 5.5-h Nd139m pOpulates levels at 113.8, 821.9,
828.1, 851.9, 1024.0, 1369.6, 1523.2, 1624.5, 1834.1, 1927.1,
2048.8, 2174.3, and 2196.7 keV. These two sets of states in-
corporate 17 and 38 transitions, respectively. The 19 gamma rays
seen in the decay of 34.4—h Ce137m in equilibrium with 9.0-h

Cel379 depopulate excited states of La137 at 10.5, 447.1, 493.1,

J

 

 

 

739.1, 7!
these lex.
populated
ulated by
he directl
0.12 of th
direct fem
‘50 keV.
Li:
investigatec
from the pre
mltiplet of
is interpret
multl'plet d9,
tion Can be e
me only for
A sur
iatiCs of the
in the 2:504;
predictions ar
The detailed n

58 datemined

 

 

Dwight Beecher Beery

709.1, 762.2, 781.5, 835.4, 926.6, 1004.8, and 1171.9 keV. Of
these levels, only those at 762.2, 835.4, and 1004.8 keV are

137m

populated directly by Ce and none of these states is pOp-

133 was seen to

ulated by both isomers. Only one state of Cs
be directly populated by 38.9-h Ba133m and an upper limit of

0.12 of the l4.6~min Ba131m disintegrations is placed on the
direct feeding to high spin states of C3131 with energies

>60 keV.

Limits on the possible spin assignments of the states
investigated have been placed from calculated log ft values and
from the presence of transitions to states with known spins. A
multiplet of six high-spin, odd parity states near 2 MeV in Pr139
is interpreted as a set of three—quasiparticle states. The
multiplet decay pattern suggests the possibility that informa—
tion can be extracted from these states that is normally avail-
able only for lower—lying levels.

A survey has been made of all of the energy state syste-
matics of the low energy levels of odd proton (odd mass) nuclides
in the Z350-62, N=64-82 region. From these observations, some
predictions and suggestions for future experiments have been made.

The detailed nature of these and the higher-lying states can only

be determined after further experimental data become available.

 

 

 

 

 

in

GAMMA RAY SPECTROSCOPY STUDIES OF THE EXCITED
STATES OF ODD PROTON (ODD MASS) NUCLEI

IN THE Z=50—62, N=64-82 REGION

By

Dwight Beecher Beery

’ A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Department of Physics

1969

 

 

 

 

 

ACKNOWLEDGEMENTS

I wish to thank Dr. W. H. Kelly for the suggestion of
this region of study. His readily available aid and skillful
guidance during the experimental work and the preparation of this
thesis are sincerely appreciated. I also thank Dr. Wm. C. McHarris
for his very significant help with many aspects of these investi-
gations.

Dr. H. G. Blosser, Mr. H. Hilbert, and Dr. W. P. Johnson
assisted with the operation of the Michigan State University sector—
focused cyclotron. The variable energy and high proton beam flux
has facilitated the irradiation of the isotOpes used in this thesis
project.

Dr. C. R. Gruhn directed the detector development program
at Michigan State University. The fabrication of high quality Ge(Li)
detectors has been a prerequisite for the sucess of the experiments
performed in this study.

Dr. R. L. Auble, Dr. G. Berzins, Dr. L. M. Beyer, Mr. J.
Black, Mr. W. B. Chaffee, Mr. R. E. Doebler, Mr. R. E. Eppley,

Dr. R. C. Etherton, Mr. G. C. Geisler, Mr. R. Goles, Dr. J. J.
Kolata, Mr. K. Kosanke, and Mr. R. Todd have aided greatly in use-
ful discussions and in the acquisition of data.

Mr. N. R. Mercer and his machine shOp staff were of assis-

tance in the fabrication of some of the apparatus used in this

ii

 

 

‘quLw

 

 

State 07::

Unique CI”.

and staff
fiélds of
V.

and the Va:

 

 

investigation.

Mr. R. Belgard helped with the drafting of figures.

Miss T. Arnette, Mr. and Mrs. W. Merritt, and the Michigan
State cyclotron computer staff have been helpful in clarifying the
unique characteristics of the M. S. U. Computer.

The willingness of each of the M.S.U. cyclotron faculty
and staff members to answer questions relating to their individual
fields of interest has been of considerable value.

Mrs. Ina Samra cheerfully and accurately typed this thesis
and the various publications which emerged from the investigations
described here.

I acknowledge the financial assistance of the National
Science Foundation, U. S. Atomic Energy Commission, and Michigan
State University.

Finally, I thank my wife Helen for her understanding,

inspiration, and support throughout the course of this study.

iii

 

 

ACKNOKLEI

 

LIST OF I.
LIST OF E?
Cuapter

LI

II. E:

 

2.

TABLE OF CONTENTS

Page

ACKNOWLEDGEMENTS ......... . .................. . ................ ii

LIST OP TABLES ............................ . ..... ............. ix

LIST OF FIGURES ........................... . .................. x
Chapter

I. INTRODUCTION.... ..... ............... ......... . ...... 1

II. EXPERIMENTAL APPARATUS AND METHODS .................. 6

2.1. The Gamma Ray Spectrometer .................... 6

2.1.1. Singles Experiments........ ........... 7

2.1.2. Coincidence EXperiments............... 8

2.1.2.A. Split-ring NaI(Tl) Annulus-
Ge(Li) Spectrometer......... 10

2.1.2.B. 3-in. x 3-in. NaI(Tl)—
Ge(Li) Spectrometer.... ..... l4

2.1.2.C. Multiparameter Ge(Li)-
NaI(Tl) Spectrometer ........ 14

2.2. Data AnaJ-YSiS...0.00.00.00.000...0.00.00.00.00 17

2.2.1. Gamma Energy and Intensity Measure-
ments...0.000000000000000IOOO...0.0... 17

2.2.2. Double and Single Escape Peaks........ 19

2.2.3. X-ray and Annihilation Photon

Measurements......................... 20

2.2.4. Gamma-Gamma Coincidence Spectra ...... 20

2.3. Decay Scheme Construction ...... ..... .......... 21

III. EXPERIMENTAL RESULTS............... ......... . ...... . 26
3.1. Decay Schemes of Ndlulg and Ndlulm..... ...... 26

iv

Chapter

flflJ

 

{if H

I ... H3... H111...

Chapter

3.2.

3.1.1. Introduction... .......... .............
3.1.2. Source Preparation........ ............
3.1.3. Ndlklg Gamma Ray Spectrum .............
3.1.3.A. Singles Spectra....... ......
3.1.3.B. Coincidence Spectra .........
3.1.4. Ndlulm Gamma Ray Spectra. .......... ...
3.1.5. Decay Scheme and Discussion ...........
3.1.6. Comparison with Recent Investigations.
Decay Schemes of Nd139m and Nd1399 ............
3.2.1. Introduction ............. . ............
3.2.2. Source Preparation ....................
3.2.3. Experimental Results for Nd139m .......
3.2.3.A. Gamma Ray Singles Spectra...
3.2.3.B. Gamma-Gamma Coincidence
Studies.....................
3.2.3.C. Delayed Coincidence EXperi-
ments .......................
3.2.4. Nd139m Decay Scheme ...................
3.2.4.A. The 113.8-keV Level and
Those that are Depopulated
through It... ............ ...
3.2.4.B. The 828.1-, 851.9-, 1024.0-,
and 2174.3-keV States.......
3.2.4.C. Remaining Gamma Rays ..... ...
3.2.4.D. Comparison with Another
(B,Y) Study.... .............
3.2.5. Spin and Parity Assignments from

Nd139mDeCaYOOOOOOOOOOOOO0.0.0.0....O.

Page

26

28
28
33
37
39
47
49
49
52
53

53

57

68

78

81

83

83

85

85

Chapter

 

 

T 1-5!! I l!

 

Chapter

3.3.

3.2.5.A. Electron Data and Multi-
pOIaritieSOOCOOOOOOOO ..... O.

3.2.5.B. Ground and Metastable
States of Nd139 ..... . .......

3.2.5.C. The Ground, 113.8-, and
821.9-keV States in Pr139

3.2.5.B. The 828.1-, 851.9-, and

1024.0—keV States ...........
3.2.5.B. The "High Odd-Parity
States"........... ..........
3.2.5.F. The Remaining States.. ......
3.2.6. Experimental Results for Nd1399 .......

3.2.6.A. Gamma Ray Singles Spectra...

3.2.6.B. Gamma-Gamma Coincidence
StudieSOCCCQOCOCC ..... O .....

3.2.7. Nd1399 Decay Scheme.. .................

3.2.8. Spin and Parity Assignments from
NdlsggDecay.......COOOIOOOOOOOOOOO...

3.2.9. Discussion ..... ..... ...... . ...........
3.2.9.A. Single-Particle States......
3.2.9.B. Three-Quasiparticle States..

3.2.9.C. Vibrational States -— the
Remaining States............

3.2.9.D. Shell Model Ca1cu1ations....

Decay Schemes of Ba133m, Ba131m, Ce137m, and
Ce13g ............. ......C...‘......OCCICCOOCO

3.3.1. Instrumentation.......... .......... ..

3.3.2. :perimental Results for 38. 9-h

vi

Page

85

88

91

93

95
101
101

101

106

114

116
119
120

124

129

129

130

130

IV.

 

4.1.

Chapter

3.3.2.A. Introduction ................
3.3.2.B. Source Preparation ..........
3.3.2.C. Gamma Ray Spectrum..........

3.3.2.B. Ba133m+g Decay Scheme and

Discussion ..................
3.3.3. Experimental Results for Ba131m .......
3.3.3.A. Introduction.. ..... .........

3.3.3.B. Source Preparation..........
3.3.3.0. Gamma Ray Spectra ...........

3.3.3.B. Bal31m Decay Scheme and
DiSCUSSion.......OOOOOOOOOOO

3.3.4. Experimental Results for Ce137m+9 .....
3.3.4.A. Introduction................
3.3.4.B. Source Preparation ..........
3.3.4.C. Gamma Ray Spectra ..........

3.3.4.B. Ce137m+9 Decay Scheme and
Discussion..................

IV. DISCUSSION OF RESULTS AND SYSTEMATICS ...............

4.1.

4.2.

Three-Quasiparticle Multiplets in Other
NUClideS ..... .........OOOOOOOIOOOOOO0.0.0.0...

Experimental Energy Level Systematics in the
Odd Proton (Z = 50-62) Odd Mass Region.. ......

4.2.1. Log f% values for 3/2+ Ground State to
Lowest 5/2+ State Transition..........

4.2.2. Energy Systematics of the Low-Lying
7/2+, 5/2+, 3/2+, and 1/2+ States in

theRegion....-......OOIOOOOO......OOO

4.2.3. Beta Decay of 11/2—Levels to 7/2+
Low-Lying Daughter States...... .......

vii

Page
132
132

133

136
139
139
140

140

143
147
147
148

148

151

157

157

159

159

164

167

 

 

“09‘ '

‘hfi' 3’

 

 

 

Czapter

I
l
1
1

l
.4

'1'!v,.

'n‘r ‘. “A I

.‘JL‘VC-fli'ua:

Chapter Page
4.2.4. Characteristics of Similar 11/2—

States in Odd Proton Odd Mass
NUCleiooooo ..... .....OOOOOOOOOOOOOOOOO 171

4.3. General Summary ............................... 173

BIBLIOGRAPHY.OO0.0.000.........OOOOOOOOOOO0.00......000...... 175

viii

87’

‘\

u.
12,
13,

14,

 

10.

ll.
12.
13.

14.

LIST OF TABLES

Page
Gamma—rays used as energy standardsfor Ndlhlm+g decay... 30

Energies and relative intensities of gamma rays from
the decay of Nd1H1 ...................... .......... ...... 32

Experimentally determined levels and Spins of Prll"l ..... 48

Energies and relative intensities of gamma rays present
in the decay of Nd139m

............. ...... ............... 58

Relative intensities of photons in the decay of Nd139m
Observed in coincidence experiments..................... 61

Summary of gamma—gamma anti-coincidence and coincidence
experiment results...................................... 64

Cascade energy relations for Nd139m gamma rays.......... 82
Multipolarity of gamma transitions...................... 86

WeisskOpf single-particle estimates for gamma rays de-
pOpulating the "high odd-parity states" in Pr139........ 97

Energies and relative intensities of gamma rays observed
inNdlagg SpeCtraOOOOOOOOO0.0.0.000.........OOOOOOOCOOOO 105

Relative intensities of photons in the decay of Nd1399
observed in several gamma-gamma coincidence experiments. 108

Gamma rays used as energy standards..................... 131
Ba131m gamma ray data ........... .. ......... ............. 144

0637"“? photon data 152

Characteristics of similar 11/2- states in odd photon
Oddmass nuc1e1000000000O0.0............OOOOOOOOOOOOOOOO 172

ix

 

Figure

"J
o

.l:~

98.

9b,

10,

Figure

9a.

9b.

10.

LIST OF FIGURES

Singles y-ray Spectrum from the decay of Nd”1
taken with a 7-cm3 Ge(Li) detector...................

Ndlul anti-coincidence spectrum. ..... . ...............

Singles y-ray spectrum from the decay of Nd“+1
taken with a 7-cm3 Ge(Li) detector...................

Spectrum of y-rays in coincidence with the 145.4-

kev YO............OOIOOOO0.00.00.00.00. ....... 0......

Integral y—ray coincidence Spectrum ............ ......

Anti-coincidence spectrum recorded by the 7-cm3

Ge(Li) detector when placed inside the tunnel of an

Page

12

13

31

35

36

8-in.x 8-in. NaI(T1) split annulus with a 3-in. X 3—in.

NaI(T1) detector at the other end of the tunnel......
Singles y-ray spectra of Nd1“1m + Ndl719.... ..... ....
Decay scheme of Nd1“19+m..... ........ ................

Nd139m singles Y-ray spectrum taken with a 7-cm3

Ge (Li) detector -— low-energy Portion. o o o o o o o o o o o o o o o

Nd139m singles y—ray Spectrum taken with a 7-cm3
Ge(Li) detector -— high-energy portion.......... .....

Nd139m anti-coincidence spectrum recorded by the 7—cm3

Ge(Li) detector when placed inside the tunnel of an
8-in. x 8-in. NaI(T1) split annulus, with a 3-in. x
3-in. NaI(T1) detector at the other end of the

tunnel-......OIOOOOOOOOOO......IOOOOOOOOOO ......... .0.

38
40

41

55

56

60

I
fir]

k

 

i .n- 3“

 

Figu:

12

14

13

16

17

18

19

20

21

23.

[‘0
{x

Figure Page

11. Spectrum of Nd139m y—rays in prompt coincidence with
the11308-keVYOOOO ..... ......OOOOOOOOOOOOOOOOO ..... 63

12. Spectrum of Nd139m y-rays in coincidence with the
680-720-kev energy intervaIOOOOOOO......OOOOOOOOOO... 66

13. Same as Figure 12, except that the NaI(Tl) gate was
set on the adjoining 720-760-keV interval........... 67

14. a) Nd1399+m integral coincidence spectrum.
b) The annulus gate was set on the 405—keV energy

regionOOOOOOO00.0.0000.........OOOOOOOOIOOOOO.... 69

15. Spectrum of Nd1399+m y-rays in coincidence with the
450—550—kev energy interval.......................... 7O

16. Spectrum of Nd139m y—rays in coincidence with the
500-600-keV energy region...................... ...... 71

17. Spectrum of Nd13971 y-rays in coincidence with the
790—840-kev energy interval.sococoa-00000000000000... 72

18. Same as Figure 17, except that the NaI(T1) gate was
set on the adjoining 840-900—keV energy interval..... 73

19. Same as Figure 17, except that the NaI(Tl) gate was
set on the 950-1150-keV energy interval............. 74

20. Same as Figure 17, except that the NaI(Tl) gate was
set on the 1180-1300-keV energy interval............ 75

21. Spectrum of Nd13%n-y-rays in coincidence with the
1900-2200-kev energy interval. . O O C C O C 0 O . 0 . C . . C C C O . O C 76

22. Spectrum of Ndl39nly-rays in delayed coincidence with
the 113.8-keVoosoc-00.000000000000000.0000000000.... 77

23. Time—to—amplitude converter decay curve for the 821.9-

kev State inPr139O......OOOOOIOOCOOOOOCO0.0.0.000... 79

24. Decay schemes of Nd139m and Nd1399 ..... ....... . 80

xi

 

I 1‘ J.
‘ "I " "Fro-IL
'I F , I- V ‘ ..|V_

 

Figure

27a,

27b. .: :

29.

30.

31.

32.

33.

34.

35_

L O'A'

for

Nd‘
Ge(L

Ge(;
Edi-3

a)
b)

 

SPEC
113.
811C
Spec
EXpe
Stra

the

CEO

Figure

25.

26.

27a.

27b.

28.

29.

30.

31.

32.

33.

34.

35.

Page
A comparison of experimental and theoretical K-
conversion coefficients for some of the y-tran-
sitions following Nd139m decay......... ...... ... ..... 87
Upper: Energies of the metastable states in the
N=79 and N=81 isotones.
Lower: Values of the squared radial matrix elements
for the isomeric transitions in the same nuclei ...... 89
Nd1399+m singles y-ray spectrum taken with a 7-cm3
Ge(Li) detector -- low-energy portion..... ..... ...... 102
Nd1399+m singles y-ray Spectrum taken with a 7-cm3
Ge(Li) detector -- high-energy portion. ....... ....... 103
Nd1399+m anti-coincidence spectrum.......... ......... 107
a) Ndl399+m integral coincidence spectrum...... ..... 110

b) The annulus gate was set on the 405—keV energy

region.0.00.0000...000......000.000.000.00.0..0000 110

Spectrum of Nd1399+m y-rays in coincidence with the
11308—kev Y00000000000000000000000000.00.00.000000000 112

Slices from two—dimensioal (megachannel) y-ray
Spectrlm for Ndl3gghn...0 ..... 0......00. ....... 0.0... 113

Experimental levels in odd-mass Pr isotOpes, demon—
strating the effects of changing neutron number on

the positions of the states....... ...... ..... ....... . 121

Experimental levels in odd—mass N=80 isotones, demon—
strating the effects of changing proton number on the

positions of the states ..................... ......... 122

Symbolic shell-model representations of some impor—
tant transitions between Nd139 and Pr139 states...... 126

Ba133m+9 singles y-ray spectrum taken with a 7-cm3
Ge(Li) deteCtorooo00000000000000...0000000000.o. 00000 134

xii

Figure
36.

37.

38.

39.

40.

41.

42.
43.
44.
45.

46.

47.

48.

49.

Page
Decay schemes of Ba133m and Ba1339 ......... .......... 137
331319 singles spectrum taken with a 7—cm3 Ge(Li)
detector 000000000000000 0.00 0000000 0.000000000..0000.0 141

Ba131m+9 singles Spectrum taken ‘30-min after a 5-

min bombardment with the proton beam................. 142

131m

Decay scheme of Ba suggested by Horen et al. (100)

and confirmed by our measurements.................... 145

Systematics of the energy level separations be—
tween low—lying 1/2+ and 3/2+ states in odd mass
Sn, Te, Xe, Ba, and Ce isotOpes (6)............. ..... 146

Ce137m+9 singles y-ray spectrum recorded with a 7-cm3

Ge (Li) detector -- low-energy portion. 0 o o o o o o 00000000 149

Ce137m+9 singles spectrum -- high—energy portion..... 150

Decay schemes of Ce137m and Ce1379 ................... 153
LOg fT values. ................ ..... ............. ..... 161
Log f% values. ..... ............................. ..... 162

Systematics of the energy level separations between
low-lying 5/2+ and 7/2+ states in odd proton (odd

mass) nuclei......................................... 165

Systematics of the energy level separations be-
tween low—lying 3/2+ and 7/2+ states in odd proton

(odd mass) nuclei.................................... 168

Systematics of the energy level separations between
lowblying l/2+ and 7/2+ states in odd proton (odd

maSS)nuc1810.00.0000.0.0.0.00000000000000000... ..... 169

Energy gaps between the lowest lying 2+ and 0+

states in even—even nuclei in the Z = 50-62 region... 170

xiii

CHAPTER I

INTRODUCTION

A significant measure of our understanding of nuclear
structure is given by the capability of current nuclear models
to predict previously unobserved properties of nuclei. These
models, in part, are based on experimental observations of
the same nuclear state properties. Therefore, an increase
in the quality and quantity of experimental results can aid
both in the testing of available models and in the develOpment
of improved models.

Comparisons of the experimental results reported here
have been.made with the currently available models. The
latter include the Kisslinger - Sorensen model with a short-
ranged pairing force and a long-ranged quadrupole interaction
(1), Wildenthal's calculations using the shell model with
selected configuration mixing (2), and qualitative concepts
from the weak-coupling particle—core model (3).

Past studies of radioactive decay, in conjunction with
nuclear reaction work, have played basic and complementary
roles in advancing our current understanding of nuclear struc—
ture. The investigations included in this thesis are intended
to extend and improve some of the information acquired from

radioactive decay by means of beta and gamma ray spectroscoPy.

 

 

 

 

have bet

A

L023 1:11;]
aCIlCn s

Praiictic

During the past several years, many eXperimental studies
have been undertaken of the beta and gamma ray decay schemes of
odd proton odd mass neutron deficient nuclei lying in the 505Zs62,
645N582 region of the table of nuclides (4—6). In general, these
studies characterize the daughter states as fully as possible by
combining their decay schemes with the results of available re—
action studies and other experimental results and theoretical
predictions from the literature.

The "magic numbers" of 50 protons and 82 neutrons corre-
spond to tightly bound or spherical nuclei. In this region the
available shell model states in the approximate order of fill-
ing are 97/2, d5/2’ 81/2, h11/2’ and d3/2. Far from these in-
ert nucleon cores a permanently deformed region is eXpected.

The transitional region between the spherical and deformed re-
gions appears to be characterized by more and lower energy states

which suggest a softer nucleus. Within this region the line of

21 1127 C8133 L8139

1
B-stability runs roughly through SISb 70, 53 7”, 55 78’ 57 82

and 59Prlgé. These stable nuclei have a nearly ideal location for
probing this transitional region with radioactive nuclei produced
by the (p,xn) and (He3,xn) reactions employed in this study.
Evidence of considerable experimental and theoretical in-
terest in the region selected for this investigation is seen in
current reference compilations (4—6). Recent beta and gamma ray

spectroscopy studies performed at Michigan State University (7-15)

have played a central role in the acquisition of the currently

accepted experimental data on odd mass antimony and iodine energy
level structure. One curious result of these investigations has
been the observation of a very smooth systematic trend (i.e. quad-
ratic) in energy of four low-lying states of spins 7/2+, 5/2+,
3/2+, and 1/2+ in both the odd mass antimony and iodine isotopes
(7,15). It is of interest to see if this smooth trend exists

in the odd mass neutron deficient cesium, lathanum, and
praseodymium nuclides.

Just below the 82 neutron shell are seen many systematic
examples of h11/2-+d3/2 neutron isomerism. Some of these isomers
and ground state decays might be expected to populate levels with
spins ranging from 1/2 to 15/2. These many levels near a closed
shell might then be interpreted in relatively tractable shell
model terms.

The selection of specific nuclides within the region of
interest was initiated by sufficiency requirements for available
energy for electron-capture, half-life, and feasibility of source
production and chemical separation. A determination of the current
state of experimental and theoretical studies from available pub-
lications was also an important early stage of the selection pro-
cess.

In this study, the investigations of the beta and gamma
ray decays of Ba131m, Bal33m, Cel37m+9, Nd1”1m+9, and Nd139m+9
by means of gamma ray spectroscopy are reported. A brief study

Of systematic trends in this region of the periodic table suggests

several patterns which will be interesting to compare with pre-
dictions of future nuclear models.

During the course of this study, other decay schemes (e.g.
Pr139 and Pr1”°) were investigated in detail in order to verify
that the low intensity gamma rays were assigned to the correct
nuclide. The results of these studies are published elsewhere
(16,17) and are not included in this thesis.

Chapter II describes in a general way some of the ex—
perimental apparatus and several of the methods of gamma ray
spectroscopy employed in the present investigation. Emphasis
is placed on methods used most recently to significantly increase
the amounts and quality of data which can be obtained and analyzed
in a given time interval. Also, a sequence of 14 steps used to
establish important features of positron and electron capture de-
cay schemes is outlined.

Chapter III describes the experimental results obtained.
Following Ndlulg decay, six new gamma rays were discovered and
placed in a decay scheme containing three new Prlnl excited
states. From the Nd139m+9 decay scheme study, 72 gamma rays were
observed and 22 excited states (14 new) were placed in Pr139. A
set of six states between 1.6 and 2.2 MeV are interpreted as mem-
bers of a high spin odd—parity three-quasiparticle multiplet hav—
ing the (wds/Z)(vd3/2)'1(vh11/2)'1 configuration. Gamma ray
studies of the decays of Ba133m, Bal31m, and Ce137m+9, are also

described. A new high—spin state of C3133, the first measurement

of two intense gamma rays following Ba131m with a Ge(Li) de-
tector, and an energy level scheme for La137 which includes
ten excited states are among the results obtained.

In Chapter IV the systematic behavior of some of the
nuclear prOperties in the region are discussed. Here also,
some characteristic prOperties of the interesting three-

quasiparticle multiplets in Pr139 are summarized.

 

 

 

x
,—
‘ .

Eation use

tIOSCOpv.

CHAPTER II

EXPERIMENTAL APPARATUS AND METHODS

The 8+le decay schemes constructed during this investi—
gation used both standard and new techniques of gamma ray spec-
troscopy. Section 2.1 describes in a general way the apparatus
in current use at Michigan State University for gamma ray meas-
urements, including a somewhat more detailed description of a
multiparameter coincidence system developed during the course
of the present study. In section 2.2 some general methods used
to analyze data are outlined. Section 2.3 outlines the pattern
of Operations used in the present study to construct parts of
each nuclear decay scheme with the aid of the analyzed data.
The descriptions of the production and chemical separations
that were used for the several different nuclides are deferred
to Chapter III where they are included with a discussion of

experimental results.

2.1. The Gamma Ray Spectrometer

 

The process of data accumulation by gamma ray spectrom-
eters is becoming increasingly efficient,and at the same time
the quality of the data is improving. This progress is aided
by the rapidly improving technology involved in the manufacture
of larger Ge(Li) detectors, amplifier-preamplifier systems with

improved performance characteristics (e.g., higher resolution),

 

 

faster as

 

lyzers vi
ADC-Comm
section (12‘
gamma ray
Coinciden;

apparatus ‘

 

 

faster and more stable analog-to—digital converters (ADC), ana-
lyzers with larger memories, more flexible and more SOphisticated
ADC—computer interfacing, and increasing computer memory. This
section describes some significant general characteristics of the
gamma ray spectrometers employed in this study for singles and
coincidence experiments. Special purpose applications of the

apparatus are discussed in Chapter III.

2.1.1. Singles Experiments

 

The basic components of the singles gamma ray spectrom—
eter used in the present study were: a) A 7—cm3 Ge(Li) detector
(18) cooled to liquid nitrogen temperature (77°K), b) a room tem—
perature FET preamplifier and high-voltage supply, c) a pulse
shaping amplifier with pole-zero compensation, d) an analog-to—
digital converter (ADC) and multi-channel analyzer (MCA), and
e) a data readout system. The performance of a given spectrom-
eter was limited by its weakest link, which could not be isolated
to a single component in all cases but depended upon each exper-
iment and its objectives. For example, where gamma ray energies
324MeV were measured, the number of channels in the MCA was some—
times the weakest link. 0r, low detector efficiency would obscure
the full-energy peaks corresponding to gamma rays with very low
intensity. A common limitation on gamma ray energy resolution was
introduced by the performance of the preamplifier—amplifier system.

For a given set of available components, a balance normally

was required between competing parameters to optimize spectrometer

 

5Ut too I

broadenir

tillation

9‘ v
-~.e‘ g}? and

“Ted bv
Nita lfet .

performance. A few of the many typical examples of competing
parameters are listed below. If the potential difference applied
to the Ge(Li) detector was too low, incomplete charge collection
would occur, but too high a potential led to breakdown and in-
creased noise. Too low a counting rate could sometimes produce
widened peaks because of long term instability of the electronics,
but too high a counting rate could cause pulse pileup, again
broadening the peak. This balance was especially important for
the detection of weak photons in the presence of a continuous
background. Other Optimizations employed included those between
small and large numbers of MCA channels, short and long shaping
amplifier time constants, high and low amplifier gains, and be-
tween a large and small number of measurements of gamma ray ener-
gies and intensities.

The gamma ray singles Spectrometer resolution achieved
with Ge(Li) detectors exceeded that obtained with NaI(Tl) scin-
tillation counters by factors of 210. During the present study,
the Ge(Li) detectors were used exclusively for singles gamma ray

energy and intensity measurements.

2.1.2. Coincidence Experiments

 

All but one of 41 excited nuclear states seen to be pOp-
ulated by 8+,e decay in the course of this study are suggested to
have lifetimes <<lOO nanoseconds (ns). It was thus found useful
to employ two or three singles spectrometers in different coin-

cidence configurations to perform anti-coincidence, prompt-coin-

 

 

cidence,

tern of e:

 

‘

stJdies cg
coincidenc
three very

merinem

 

decay Scher
scribed in
Periments, .

Study with :

 

“Patients.

Ant
vere pen-Om
annulus as d
delayed COin
orientEd at
dioactin So
“*3

\‘ erments

cidence, and delayed-coincidence experiments. The standard pat-
tern of eXperiments employed in the course of the decay scheme
studies described in Chapter III was: a) singles Spectrum, b) anti-
coincidence spectrum, and c) coincidence spectra. Within these
three very general categories of experiments, many special purpose
experiments were suggested by unique features of each particular
decay scheme under investigation. The study of Nd1u1M+g decay de-
scribed in section 3.1 illustrates the "classical" pattern of ex-

d139m study in section 3.2 represents a

periments, whereas the N
study with many unexpected features and a wider range of different
experiments.

Anti-coincidence and many prompt—coincidence experiments
were performed with Ge(Li) detectors inside a split-ring NaI(T1)
annulus as described below in section 2.1.2.A. Other prompt and
delayed coincidence experiments were performed with Ge(Li) detectors
oriented at 90° from a 3-in. X 3-in. NaI(Tl) detector with the ra—
dioactive source under investigation at the apex of the angle. These
experiments are outlined in section 2.1.2.B.

During the study of Nd139m decay, the cyclotron and coinci—
dence photon spectrometer were required on ten separate occasions
between March, l968,and September, l968,for intervals of :24 hours.
This large expenditure of time for coincidence measurements, which
essentially differed only in the energy regions gated by the NaI(Tl)

detector spectrometers, suggested the develOpment of a multiparam-

eter coincidence system. This system stores on magnetic tape the

 

pan of c':
in each de
cozputer p
Eultipara:

course of

10

pair of channel numbers corresponding to each coincidence event

in each detector. The tapes are later scanned with the aid of a
computer program in order to extract the coincidence data. A
multiparameter coincidence apparatus that was develOped during the

course of these investigations is described in section 2.1.2.C.

2.1.2.A. -—Split—Ring NaI(Tl) Annulus -- Ge(Li) Spectrometer—-

 

Several specific applications of the split-ring 8-in. x
8-in. NaI(Tl) annulus vs. Ge(Li) gamma ray Spectrometer are described
and suggested in reference 19. In the present study, the anti-coin-
cidence application with the radioactive sources placed inside the
annulus tunnel and on tOp of the Ge(Li) detector was found to be
the most useful of the configurations suggested in reference 19. In
this configuration it was possible to obtain convenient and signif—
icant reductions of the Compton edges caused by backscattering in
the active region of the Ge(Li) detector. This was done with the
aid of an additional 3-in. x 3-in. NaI(T1) anti-coincidence de-
tector placed in the annulus tunnel Opposite the Ge(Li) detector.

An essential feature of spectra taken with the anti-coinci-
dence spectrometer was the enhancement of the primarily electron—
capture-fed gamma ray transitions to the ground states of nuclei.
That is, gamma rays which are not detected by one detector of the
coincidence spectrometer within the coincidence resolving time
(usually 5100 us) of the detection of a photon in the other de—
tector were enhanced typically by factors of 2 to 10 over ratios

seen in singles experiments with respect to other photons.

11

One might suspect that the anti-coincidence spectrometer
would necessarily make the gamma rays involved in cascades diffi-
cult to detect. However, the effect of Compton background reduc-
tion can sometimes outweigh the cascading photon full-energy peak
intensity reduction. For example, compare the 145.4— and 981.3—
keV full-energy peaks in Figure 1 (singles) with the correspond—
ing peaks in Figure 2 (anti-coincidence with the configuration
described above). A definite "peak/background" enhancement is
seen in Figure 2 even though the 145.4- and 981.3~keV gamma rays
of Nd“+1 are in cascade. A quantitative explanation of this
enhancement would require consideration of the numbers and kinds
of contributions to the Compton background. However, the pres—
ence of a scattered photon associated with each count in the
Compton distribution of the Spectrum taken with the Ge(Li) de—
tector suggests a partial explanation. The Compton scattered
photon of even a primarily electron-capture-fed ground state
transition gamma ray can be picked up by a NaI(T1) detector, and,
undistinguished from a coincident photon, trigger the anti-coin-
cidence circuit. Thus a background count can be removed by a
process which is not associated with the production of a full—
energy peak.

Both integral- and gated-coincidence experiments were
also performed with this spectrometer. The effects of summing of
coincident photon pulses in the annulus were removed to first

approximation by comparing spectra taken with adjacent gates.

 

 

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O 255 BOO 755 IOOO
CHANNEL NUMBER
Fig. 2. NdIQI anti—coincidence spectrum. Note the larger

peak/background ratios for the 145.4— and 981.3-keV
full-energy peaks here as compared with the corres-
ponding ratios in Figure 1. The text explains quali—
tatively how this can happen even though the 145.4—
and 981.3—keV gammas are in cascade with each other.
It is noted that "anti-Compton" usually implies place-
ment of the radioactive source outside the annulus

and "anti-coincidence" most commonly suggests an in-
ternal location of the source. This distinction is
not employed here.

eter. TE
resoluti;

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14

2.1.2.B.--3-in. X 3-in. NaI(Tl)-Ge(Li) Spectrometer--
Somewhat cleaner coincidence spectra were obtained by the use
of a 3-in. X 3-in. NaI(Tl) detector gated gamma ray spectrom-
eter. The smaller NaI(Tl) detector has the advantages of higher
resolution (8% rather than 10% for the 661—keV gamma ray of 03137)
and reduced summing.

This photon coincidence Spectrometer was employed for a
very useful delayed-coincidence run described in section 3.2.3.C.
A ZOO-ns delay in one branch of the coincidence circuit enhanced
several full energy peaks by two orders of magnitude with respect
to peaks involved in prompt coincidences. The enhancement is due\
to the fact the gamma rays pOpulated a state that was found to de-
cay with a 40-ns half-life.

In addition to its use in prompt coincidences where
source intensity was not a significant limitation, the 3-in. X 3-in.

NaI(Tl)-7-cm3 Ge(Li) spectrometer was used for multiparameter coin-

cidence experiments as described in the following section.

2.1.2.C.-~Mu1tiparameter Ge(Li)-NaI(Tl) Spectrometer-~In
the conventional coincidence experiments described in the previous
section, signals from the Ge(Li) detector (only) are recorded if
certain coincidence requirements are satisfied. Separate coin—
cidence experiments are required for each set of pulse height re-
quirements placed on output signals from the NaI(T1) electronics
which trigger the coincidence circuit. Each conventional coin-

cidence experiment employs two detectors, a considerable collec-

 

 

expert
more ,

0f dat;
winch

Prepan

15

tion of electronics, and many man—hours in order to assemble

and Operate the apparatus and analyze the data. The only basic
parameter change from one prompt coincidence experiment to the
next in the study of a particular decay scheme is the single
channel pulse height analyzer window chosen. However, since the
experiments were usually conducted at intervals of one week or
more, the spectrum energy calibrations changed and each analysis
of data had to be conducted independently. In addition, each
coincidence experiment required additional accelerator time to
prepare the radioactivity.

One way to remove the massive duplication involved in
these experiments is to record the signals from both detectors
following each fast-coincidence event. It would not be prac—
tical to store each ordered-pair of channel numbers in sep—
arate computer locations since >106 locations would be re—
quired for even an array of 1024X1024 channels. However, if
each ordered pair of channel numbers would be written on magnetic
tape with the aid of dedicated buffer storage of :240 events,
$2000 events/sec could be recorded with no deadtime beyond the
ADC (20),

Near the end of the present study, such a system was de-
veloped with the aid of a dual 4096-channe1 ADC interfaced to an
SDS SIGMA-7 computer and magnetic tape readout. Typically, 20
coincidence events/sec are recorded for 15 hours whereupon one

2400 foot tape is filled with 1.1><106 events. A computer pro-

gran, E1
dence da
to scan

fiD96-cha1

eter coir
gral Coir
gates frc
SUbtracti
and d) an
the Ge(Li
this Powe
the 30‘01

33.6.3.

16

gram, EVENT RECORDER, was written in order to extract coinci—
dence data from the magnetic tape. Only ten minutes is required
to scan the entire tape, punch-out and print-out one to five
4096-channel spectra of data, and rewind the tape.

The sequence of operations at the end of a multiparam-
eter coincidence experiment was: a) Scan the tape for the inte-
gral coincidence spectrum obtained from each detector, b) choose
gates from each Spectrum judiciously with or without background
subtraction, c) scan the tape for the gated coincidence spectra,
and d) analyze the data, noting that each Spectrum displaying
the Ge(Li) detector data has the same gain. An application of
this powerful tool was employed, in the present study, only to
the 30-min Nd1399 decay. The results are described in section
3.2.6.B.

Future applications and development of the multiparameter
coincidence spectrometer may be expected to increase the efficiency
of decay scheme studiesat Michigan State significantly. A Ge(Li)-
Ge(Li) multiparameter spectrometer is presently in use, and future
developments may include triple-coincidence and anti-coincidence-
coincidence experiments, delayed-coincidence experiments with time
as one of three parameters, and experiments with other combina-
tions of three or more parameters which would be recorded on mag-
netic tape and removed later. A primary weakness of the current
system is that it is not easily monitored. Monitoring has been
accomplished to date with a separate MCA and scaler to count all

of the coincident events.

 

 

spectros

of data

17

2.2. Data Analysis

 

Rapid advances in many aspects of experimental nuclear
spectrOSCOpy have resulted in a significant increase in the rate
of data accumulation during the course of this study. For ex—
ample, 1ong-lived sources from Oak Ridge have been replaced by
short-lived sources from the Michigan State University sector-
focused cyclotron, 4-keV FWHM resolution of Ge(Li) detector
systems have been replaced by spectrometers with 2-keV FWHM res-
olution, and 1024 channel analyzers have been replaced in our
gamma ray spectrometers with 4096 channel analyzers and 8092
channel ADC'S coupled to the Sigma-7 and PDP-9 computers.

Most of the data analysis in the present study was
carried out on the Michigan State University Cyclotron Laboratory
SDS SIGMA—7 computer with FORTRAN programs written or adapted
for this computer. Brief and general descriptions of the gamma
ray energy and intensity measurements, x-ray and annihilation
photon measurements, and the analysis of the gamma-gamma coin-
cidence spectra are given below. Discussions related to the
special characteristics of individual experiments are given in

Chapter III.

2.2.1. Gamma Energy and Intensity Measurements

 

The centroids and areas of photon spectra peaks were
determined following the subtraction of linear or cubic inter-
polated backgrounds. The computations were performed with the

aid of the MOIRAE program, written in machine language for the

l8

SIGMA-7 computer by R. Au. The major functions of this program
are found in the MIKIMAUS program originally written by G. Berzins
and outlined in Appendix D of reference 21.

A significant feature of the MOIRAE program is that it is
Operated on—line with the SIGMA-7 computer and allows a more rapid
analysis than did earlier methods.

Gamma ray energy measurements were made by first computing
least squares quadratic calibration equations from centroid channel
numbers of well-known standard energies and then computing the en-
ergies of "unknown" gammas from their measured centroids. Rough en-
ergy approximations (i1 keV) were made from "external" calibrations
taken in successive measurements of calibration, unknown, and cal-
ibration sources. Serious energy measurements ($20.2 keV) were
made from "internal" calibrations taken by simultaneously counting
the unknown and standard calibration sources.

The choice of standard calibration sources and how they
are employed are significant factors in gamma energy measurements.
Ideally one would like standards that emit only the gamma rays
actually used as standards and a small number of them because the
Spectral distribution of the calibration photons often obscures
other photon peaks of interest. On the other hand, many good cal-
ibration points are useful in order to establish a reliable cali-
bration curve.

Gamma ray relative intensities were established with the

aid of a detector efficiency versus photon energy curve for energies

 

 

......

l9

ranging from 30-keV to 3—MeV. A linear relationship between the
log of the i-Ell full energy peak efficiency (effi) and the log of
the iEE-photon energy (Ei) between zlSO-keV and zB-MeV was ob-

served for each detector to within eXperimental uncertainties.

1.64

This dependence (effi/eff for the 7—cm3 detector)

j = (Ej/Ei)

was employed in the "MOIRAE E(I)" computer program.

2.2.2. Double and Single Escape Peaks

 

Double and single escape peaks were used to check the
energies of full—energy peaks and their intensities. Their en—
ergies are 1022.0- and 511.0-keV reSpectively below the corre-
sponding full—energy peak, and the intensities were derived from
empirical efficiency curves. Recent evidence (22) suggests that
the energy differences may need to be corrected by a "field in-
crement effect" factor depending on the detector-source geometry
and the electric field in‘the detector created by the diode bias
voltage. This correction was negligible (<0.1-keV) for the ex-
perimental conditions of the present study. In particular, the
electric fields produced by the detector biases were relatively low.
Both double and single escape peaks are depressed by anti-coin-
cidence runs, enhanced in Sll—keV annihilation photon gated coin-
cidence experiments, and characterized in singles Spectra by the
lack of Compton edges. A careful consideration of these charac-

teristics was necessary and useful in the process of data

analysis.

20

2.2.3. X—Ray_and Annihilation Photon Measurements

 

K x-rays accompanied 84 to 90% of the decay processes of
electron capture and internal conversion in nuclei with 50§Z560 (23).
With measured K x—ray intensities and measured K—fluorescent yields
(the latter are tabulated on page 570 of reference 6 as a function
of daughter Z), a rough check can be obtained on prOposed decay
schemes and elements present in a given Spectrum.

Since almost every positron emission event is followed by
an annihilation process involving two 511.0-keV annihilation photons,
it is commonly feasible to determine ground state beta branching
ratios with the aid of the intensity of the annihilation photons,as
described in section 2.3. To eliminate the possibility of the pos-
itrons penetrating into the detectors and to allow studies to be
made on the total positron annihilation, COpper absorbers were

placed around the samples during some of the counting intervals.

2.2.4. Gamma—Gamma Coincidence Spectra

 

Chance coincidence rates were maintained below 1/10 of
the rate Of true coincidences at all times. The Compton distribu-
tion underlying the full energy peaks of interest in a coinci-
dence gate can contribute significantly to coincidence spectra. To
remove this effect to first order, spectra in coincidence with the
Compton background in adjacent gates were compared. In the multi-
parameter coincidence experiments referred to in section 2.1.2.C
the subtraction of weighted spectra taken with adjacent gates can

be performed as the magnetic tape is scanned.

 

 

spectro:
I‘ne use
merely re
detector
this lab

quate fu

21

A major weakness of Ge(Li)—NaI(T1) photon coincidence
spectrometers is the poor resolution of the NaI(T1) detector.
The use of Ge(Li)-Ge(Li) coincidence spectrometers has been se—
verely restricted because of the limitations introduced by low
detector efficiency. However, this restriction is decreasing in
this laboratory as the large Ge(Li) detectors required for ade-

quate full-energy peak efficiency become more readily available.

2.3. Decay Scheme Construction

 

A nuclear decay scheme presents significant results of
experimental studies. In the present investigation these results
include the energies, spins, parities, and half—lives of the
states, the disintegration energy for electron-capture, the gamma
and beta transition intensities, the gamma decay multipolarities,
and the log ft values for beta decay to each state. The methods
used to construct decay schemes are so varied that a new pattern
must be devised in the process Of studying each individual decay.
Abundant evidence of this is seen in Chapter III as eight decay
schemes are described in eight quite different sequences.

The location of states may be suggested initially by
gamma energy sums, anti-coincidence data, prompt coincidence data,
delayed coincidence data, gamma ray relative intensities, or other
parameters. These parameters become more difficult to translate
into a consistent system of energy levels as the number of gamma
rays increases.

After the energy levels are established, the determination

22

of the gamma and beta transition intensities and the log ft values
is a fairly routine Operation involving a rather long sequence of
Operations. Thus, it lends itself readily to computer analysis.
Tentative values of these latter parameters were commonly useful

in the process of updating partial decay schemes. For these rea-
sons and in order to reduce decay scheme construction time and
numerical errors, a computer program called DECAY SCHEME has been
written.

The sequence of Operations used to perform these "routine"
aspects of the decay scheme construction was suggested by data
derived from many different experiments. It was independent of
the order in which the experiments were performed. Therefore, an
understanding of this sequence is a prerequisite to an understand-
ing of the calculation of gamma and beta transition intensities and
lot ft values. This sequence, described below, is not entirely in-
corporated into the current version of the DECAY SCHEME computer
program, but it was employed in full as part of the construction
of each of the decay schemes discussed in Chapter III. For these
reasons, this chapter concludes with a list Of steps which out-
lines briefly the pattern of operations used in the present study
to derive the gamma and beta transition intensities and the log ft
values from the remainder of each decay scheme and the full-energy
peak areas after the energy levels had been established.

1) Calculate the relative photon intensities I from I E A/(eff)
where A 5 net full-energy peak area in 3 singles run and

eff E relative efficiency of the Ge(Li) detector employed at

 

 

l-J
V

.15-
v

ml

2)

3)

4)

5)

6)

23

the energy of the photon of interest. If absorbers are
employed for special purpose runs, eff is reduced by a
factor given by reference 23 as a function of the energy of
the photon, and of the thickness, densitygand atomic num—
ber (Z) of the absorber.

If reliable direct measurements are not available, calcu—
late the internal conversion coefficients by least-squares
fits of calculated conversion coefficients tabulated in the
current literature (24) as a function of gamma ray energy,
Z of the nucleus in question, and a measured or assumed
multipolarity.

Calculate the total transition relative intensities It 3
I(l+atot),where atot 5 total internal conversion coefficient.
Calculate the total beta-feeding relative intensity I8 5
2(It)out —E(It)in for decay to each excited daughter state,

where £(It) ) E sum of all total transition intensities

out(in
depopulating (populating) the excited state.

Tabulate EK/B+ ratios for decay to each daughter state (from
Figure 3 on page 575 of reference 6) as a function of the
parent nucleus Z and the positron endpoint energy. 8K 2 K
electron-capture transition probability and 8+ E positron
emission transition probability for the competing modes of
beta decay to each daughter state.

Calculate eK/Stot for the decay to each daughter state as

a function of Z of the daughter nucleus, the total energy of

a
C

 

re

Ca

re

51.

an:

Ca.

81.

in-

Ve-

l’i

7)

8)

9)

10)

ll)

24

the capture transition, and the binding energies of the

K and LI.shells of the daughter nucleus,where Etot E total

electron—capture transition probability. The theoretical
values for the ratios of the relative intensities of K,

LI’ LII’ LIII’ and MWN+--- capture are given in the formulas

and graph of page 576 of reference 6.

Calculate Etot/B+ for decay to each daughter state from the

results of steps 5 and 6 above.

Calculate ZIB+ = %I where ZIB+ E sum of all positron

SlP

relative intensities and I511 E relative intensity of the

511.0—keV annihilation photons measured with total
annihilation.

Calculate BIKEC =(IKX/wK)-ZIKIC.where ZIKEC E sum of all K

electron-capture relative intensities, IKX E K x-ray relative

intensity (see step 1), 2 E sum of all K internal-con—

IKIC'
version relative intensities (see steps 1 and 2), and wK E
K-fluorescent yield, or fraction of K-vacancies which give
rise to K x—rays, as listed on page 570 of reference 6 as
a function of Z of the parent isotope.

Calculate the total beta—feeding relative intensity to the

ground state of the daughter from I = IB+ (1+etot/B+), where

8
each quantity refers here to the ground state beta—decay
alone.

Set N E 100/(2IB+IM),where ZIB E sum of all beta-decay re-
lative intensities of the parent decay and 1% E total

12)

13)

14)

 

 

25

transition relative intensity of an isomeric parent to

(a) lower state(s) of the parent. N E normalization con-
stant which converts each of the above relative intensities
into percent of the total parent disintegrations.

Apply steps 7 and 11 to steps 4 and 10 in order to deter—
mine the percent of parent beta decay populating each
daughter state by 8+ and Etot decay individually.

Calculate QKi = QKE --Ei for each daughter state,where QKi E
available energy for K electron capture to the iEE.daughter
state, QK€ E available energy for K electron capture to the
daughter ground state (tabulated in reference 6), and E1 is
the energy of the iEE.state.

Calculate log (ft) = log (fat) + log C + A log (ft) for

beta decay to each daughter state,where log fat is a
function of Qi and the partial half-life of the decay, log

0 is a function of Z of the parent, and A log ft is a func—
tion of the fraction of parent disintegrations which proceed
by K electron capture decay to the daughter state of interest.

This calculation is clearly described on page 574 of ref—

erence 6 which includes graphs of each of these functions.

 

. w J

 

 

 

 

CHAPTER III

EXPERIMENTAL RESULTS

3.1. Decay Schemes of Nd1”19 and Nd141m

 

3.1.1. Introduction

 

Since the first production of Nd1H1 in 1937 by Pool and
Quill (25), who used fast neutrons to induce the Nd1”2(n,2n)
Ndll+l reaction, this nuclide has been produced by a number of
reactions involving the use of protons, deuterons, alpha parti-
cles, and photons as projectiles (4-6). Similarly, there have
been various more or less successful studies of its decay
scheme (4-6). Among the more complete gamma ray Spectroscopic
studies that utilized only NaI(Tl) detectors (26-29), however,
there exist some serious discrepancies. Also, in none of these

studies are more than three Pr”1

excited states reported, whereas,
as a result Of the study of inelastically scattered deuterons on
Prlkl, Cohen and Price (30) have reported levels at 140, 1140, 1300,
1500, and 1630 keV, with additional levels at 1800 keV and higher
energies. Some studies on inelastically scattered neutrons (31)
and NIH-induced Coulomb excitation (32), although with poorer

resolution, have also indicated the existence of a number of levels

starting in the vicinity of 1 MeV. In the only previous published

26

 

tector
using a
azplifi
A 3-cm?
used to
game Is

in this

the .‘Jd 1~

ibration

 

aCtivitie
5P€Ctra w
grOUHd CO
tion to SE
t1'aCting_
mined and
tune. In
and the Co
Came. The
obscured bF
then°V~we1
”an. ta}.
Figure 3.

Al;

git
en in Ta‘;

: I

I

 

29

tector was ”4.3 keV FWHM for the 661.6—keV gamma ray of C3137,
using a room temperature FET preamplifier, a low—noise RC linear
amplifier with pole—zero compensation, and a 1024—channel analyzer.
A 3-cm3 planar Ge(Li) detector mounted in a similar fashion was
used to confirm the energy values and intensity ratios of the
gamma rays Observed. Both of these detectors were manufactured

in this laboratory (18,38).

The energies of the gamma rays were measured by counting
the Ndlnl sources simultaneously with a number of well-known cal—
ibration sources, which are listed in Table 1. In order that
activities decaying with different half-lives could be identified,
spectra were recorded periodically as the sources aged. A back-
ground correction was made for each peak by fitting a cubic equa—
tion to several channels above and below the peak and then sub—
tracting. The centroid of each calibration peak was then deter-
mined and a least-squares fit made to a quadratic calibration
curve. The centroids of unknown peaks were similarly determined
and the corresponding energies calculated from the calibration
curve. The energies of weak Nd”1 gamma rays, which would be
obscured by the calibration standards, were determined by using
the nowawell-determined stronger gamma rays as internal standards.
A gamma ray spectrum taken with the 7-cm3 detector is shown in
Figure 3.

A list of gamma ray energies and relative intensities is
given in Table 2. The energies assigned are mean values taken

from a number of different measurements recorded at different

 

 

 

 

 

30

Table l. Gamma rays used as energy standards

for Nd141m+g decay.

 

Nuclide y—ray energy (keV) Reference
0057 121.97:0.05 a
C057 136.33i0.04 a
Ce139 165.84i0.03 b
0513“ 644.744i0.027 c
03137 661.632i0.069 d
c3131+ 795.806i0.050 c
Mn5“ 834.84i0.07 e
Y88 898.01i0.07 e
Sc“6 1120.50:o.07 e
C060 1173.226i0.040 f
C060 1332.483i0.046 f
Na21+ 1368.526i0.044 f
T1208(D.E.) 1592.46i0.10 f
Na2“(D.E.) 1731.91:o.012 f
Y38 1836.08i0.07 e

 

8Reference 39.
bReference 40.
cReference 41,
dReference 42,
eReference 43.

fReference 44.

 

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Table 2. Energies and relative intensities of
gamma rays from the decay of Ndlul.
Measured
y—ray energy Relative intensity
(keV)
Singles 145.4—keV y—Y , Integral y—y Antico-
spectraa coincidence coincidence incidence
Spectrum Spectrum spectrum
K x-rays (8.0i2.0)x10 ——— ——- ---
145.4i0.3 30.3:3.0 46:5 81:10 ll.l:1.1
(16:6) (51:11)
511.006 (annih.) 832:83C 955:300 1340:150 60:6
981.3i0.6 3.0:0.3 147:30 90:25 1.1:O.3
(144:30) (87:25)
1126.8:0.4 5100 E100 E100 E100
(0) (0)
ll47.1:0.4 38.2:3.8 1810:200 720:70 14.3:l.4
(1770:200) (680:70)
1292.8:0.6 61.2:6.l 67:12 87:12 58.7:5.9
1298.7:0.7 l6.3:2.0 (O) (O) 15.5:2.0
1434.6:0.5 3.0:0.3 107:40 27:9 1.6:O.2
(104:40) (24:9)
1579.9:1.o 0.74:0.12 -—— ——— 0.83:0.15d
l607.9:0.6 2.3:0.2 ——- --— 2.2:O.2
1657.2:1.0 0.12:0.04 -—- —-- 0.15:0.05

 

8A total electron conversion coefficient of a=0.
(40) for the 145.4-keV transition.
intensity of 44.2:4.4 on the above scale.

46 has been reported
This indicates a total transition
Although all the intensities

in the table are photon intensities, the conversion coefficients for
the higher—energy transitions should be small enough such that the

photon and transition intensities should be nearly the same.

bThe intensities given in parentheses are those corrected for chance
coincidences.

cTwo 0.32-cm Cu absorbers forming a sandwich around the Nd1H1 source
were used for the determination of the intensity of 511—keV photons in
total annihilation.

dAfter subtraction of the 1575—keV Pr”2 contaminant peak produced
by a Prll“1(n,y)Pr1“2 reaction, where the neutrons were produced
predominantly in the degrading foils.

 

IESU
l

3) Calc

 

rela

511.

inter
Versj
X‘flu
riSe

a fun
Calcu.
groUn<
Each (
a10ne.
set N

lative

 

7)

8)

9)

10)

11)

24

the capture transition, and the binding energies of the

K and LI.shells of the daughter nucleus,where Etot E total

electron—capture transition probability. The theoretical
values for the ratios of the relative intensities of K,

LI, LII’ LIII’ and MHWHu~~ capture are given in the formulas

and graph of page 576 of reference 6.
Calculate etot/B+ for decay to each daughter state from the
results of steps 5 and 6 above.

Calculate 218+ = %I where ZIB+ E sum of all positron

51F

relative intensities and I511 E relative intensity of the

511.0-keV annihilation photons measured with total

annihilation.
Calculate ZIKEC =(IkX/mK)—ZIKIC.where XIKEC E sum of all K
electron-capture relative intensities, IKX E K x—ray relative

intensity (see step 1), 2 E sum of all K internal-con—

IKIC
version relative intensities (see steps 1 and 2), and wk E
K—fluorescent yield, or fraction of K-vacancies which give
rise to K x—rays, as listed on page 570 of reference 6 as
a function of Z of the parent isotOpe.

Calculate the total beta-feeding relative intensity to the

ground state of the daughter from I = IB+ (1+etot/B+), where

8

each quantity refers here to the ground state beta—decay
alone.

Set N E lOO/(ZIB+Im),where ZIB E sum of all beta—decay re—

lative intensities of the parent decay and Im E total

 

 

 

14

 

:iné
dang
Calc
avai
stat
daug
the
C310
beta
fUnct
C is
tiOn
by X
This .

erEHCe

12)

13)

14)

25

transition relative intensity Of an isomeric parent to

(a) lower state(s) of the parent. N E normalization con-
stant which converts each of the above relative intensities
into percent of the total parent disintegrations.

Apply steps 7 and 11 to steps 4 and 10 in order to deter-
mine the percent of parent beta decay pOpulating each
daughter state by 8+ and Etot decay individually.

Calculate QKi = QKE —E for each daughter state,where QKi E

i
available energy for K electron capture to the iEh.daughter
state, QKE E available energy for K electron capture to the
daughter ground state (tabulated in reference 6), and E1 is
the energy of the iEE.State.

Calculate log (ft) = log (fbt) + log C + A log (ft) for

beta decay to each daughter state,where log fee is a
function of Qi and the partial half-life of the decay, log

C is a function of Z of the parent, and A log ft is a func-
tion of the fraction of parent disintegrations which proceed
by K electron capture decay to the daughter state of interest.

This calculation is clearly described on page 574 of ref—

erence 6 which includes graphs of each of these functions.

 

Quill (
xdifi r
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C163, 3:
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Starting

CHAPTER III

EXPERIMENTAL RESULTS

3.1. Decay Schemes of Ndlulg and Ndlhlm

 

3.1.1. Introduction

 

Since the first production of Nd”1 in 1937 by Pool and
Quill (25), who used fast neutrons to induce the Nd1”2(n,2n)
Nd”1 reaction, this nuclide has been produced by a number of
reactions involving the use of protons, deuterons, alpha parti-
cles, and photons as projectiles (4-6). Similarly, there have
been various more or less successful studies of its decay
scheme (4—6). Among the more complete gamma ray spectroscopic
studies that utilized only NaI(T1) detectors (26-29), however,
there exist some serious discrepancies. Also, in none of these

lkl excited states reported. whereas,

studies are more than three Pr
as a result of the study of inelastically scattered deuterons on
Prlul, Cohen and Price (30) have reported levels at 140, 1140, 1300,
1500, and 1630 keV, with additional levels at 1800 keV and higher
energies. Some studies on inelastically scattered neutrons (31)

and Nlu-induced Coulomb excitation (32), although with poorer

resolution, have also indicated the existence of a number of levels

starting in the vicinity of 1 MeV. In the only previous published

26

27

work on Nd“+1 decay utilizing Ge(Li) detectors, Koehler and Grissom
(33), using a very small detector, report only four gamma rays, de-
populating the first three Of the excited states reported by Cohen
and Price.

Since the energy for Ndlkl electron—capture decay is 1800
keV (28,34), one might expect some of the other higher—lying Pr“+1
levels also to be populated from its decay. For this reason and
because Of the discrepancies among the earlier studies, it was felt
that a re—investigation of Ndlul decay was in order. Ndll+1 has
been produced by the relatively clean Prl"'1(p,rz)Nd1l*1 reaction.
Ge(Li) and NaI(Tl) detectors were used in singles, coincidence, and
anti-coincidence configurations to study its gamma rays. Addition-
ally, as no Specific rare-earth chemical separations had been per-
formed on the targets in any of the previous work, Nd was separated
chemically from the other rare earths in order to insure that the
activities which were observed came from a Nd isotope. Six new
gamma rays were found as a result of these studies. These can be
fitted into a decay scheme that includes the pOpulation of three
additional excited states in Prlul. The decay of the short-lived
Ndlhlm isomer (35,36) was also investigated to see if it might de-
cay directly to high—spin states in Prlul; the results allow an
upper limit of 0.1% of the total Ndlklm disintegrations to be

placed on any direct population of such levels.

3.1.2. Source Preparation

 

The Nd”l sources were prepared by bombarding 99.97% pure

 

 

P1313
focusej
a :l-;,
for at
away, a:
source E
relative
Ema ra
four ha:

vhiCh far

miltlple

Singles 2
50Urce .
any Other
Mtiomuc

Th
targets f0

theSe Sourtl

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adw

m.
is wall th:

is t
C was r/

 

28

Pr203 with 9 MeV protons from the Michigan State University sector-
focused cyclotron. Typically, z100—mg targets were bombarded with
a zl-uamp beam for 1 hour. Sources were normally allowed to decay
for at least one hour to let any short—lived contaminants decay
away, and then they were counted for two to four half-lives, more
source being added with the passage of time in order to retain a
relatively constant counting rate. It was found that no competing
gamma rays with different half-lives were observed for approximately
four half-lives of the Ndlul. Most of the coincidence experiments,
which required several days' counting time, were performed with
multiple bombardments.

To insure that only radiations from Nd were Observed, the
singles gamma ray Spectrum.was confirmed with a chemically—separated
source. In this source the Nd was separated from the Pr target and
any other contaminating rare earths by eluting it from a Dowex-SO
cation-exchange column with a—hydroxy—isobutyrate (37).

The Ndlulm sources were produced by bombarding similar Pr203
targets for 210 sec. No chemical separations were performed on

these sources.

3.1.3. Ndlulg Gamma Ray Spectrum

 

3.1.3.A. -~Sing1es Spectra-- A 7-cm3 five sided coaxial

 

Ge(Li) detector was used to determine the energies and intensities
of the Nd“+1 gamma rays. It was mounted in a dip-stick cryostat.
The wall thickness of the evacuated aluminum can covering the de-

tector was 0.16 cm. Typical resolution Obtained with this de-

 

 

the Ndl“
ibration
activiti

SPGCtra

 

29

tector was 2'4.3 keV FWHM for the 661.6-keV gamma ray of C3137,
using a room temperature FET preamplifier, a low-noise RC linear
amplifier with pole—zero compensation, and a 1024—channel analyzer.
A 3-cm3 planar Ge(Li) detector mounted in a similar fashion was
used to confirm the energy values and intensity ratios of the
gamma rays Observed. Both of these detectors were manufactured

in this laboratory (18,38).

The energies of the gamma rays were measured by counting
the Nd1H1 sources simultaneously with a number of well-known cal—
ibration sources, which are listed in Table 1. In order that
activities decaying with different half-lives could be identified,
spectra were recorded periodically as the sources aged. A back—
ground correction was made for each peak by fitting a cubic equa—
tion to several channels above and below the peak and then sub-
tracting. The centroid of each calibration peak was then deter-
mined and a least-squares fit made to a quadratic calibration
curve. The centroids of unknown peaks were similarly determined
and the corresponding energies calculated from the calibration
curve. The energies of weak Nd1H1 gamma rays, which would be
obscured by the calibration standards, were determined by using
the nowdwell-determined stronger gamma rays as internal standards.
A gamma ray spectrum taken with the 7—cm3 detector is shown in
Figure 3.

A list of gamma ray energies and relative intensities is
given in Table 2. The energies assigned are mean values taken

from a number of different measurements recorded at different

30

 

 

 

Table l. Gamma rays used as energy standards
for Ndlq1m+9 decay.

Nuclide y—ray energy (keV) Reference
0057 121.97:o.os a
C057 136.33:0.04 a
oe139 165.84:0.03 b
0313“ 644.744:0.027 c
c3137 661.632:0.069 d
Cs13“ 795.806:0.050 c
MnS“ 834.84:0.07 e
Y88 898.01:0.07 e
Sc”6 1120.50:o.07 e
C060 1173.226:0.040 f
€060 1332.483:0.046 f
Naz“ 1368.526:0.044 f
T1208(D.E.) 1592.46:0.10 f
Na2“(D.E.) 1731.9110.012 f
188 1836.08:0.07 e

aReference 39.
bReference 40.
cReference 41,
dReference 42.
eReference 43.
fReference 44.

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32

 

 

 

Table 2. Energies and relative intensities of
gamma rays from the decay of Ndlul.
Measured
y-ray energy Relative intensity
(keV)
Singles 145.4—keV y-y Integral y-y Antico—
spectraa coincidence coincidence incidence
Spectrum spectrum Spectrum
K x-rays (8.0:2.0)x10 --— --- -——
145.4i0.3 30.3:3.0 46:5 81:10 11.1:1.l
(16:6) (51:11)
511.006 (annih.) 832:83c 955:300 1340:150 60:6
981.3:0.6 3.0:0.3 147:30 90:25 l.l:0.3
(144:30) (87:25)
1126.8:0.4 3100 5100 E100 E100
(0) (0)
1147.1:0.4 38.2:3.8 1810:200 720:70 l4.3:l.4
(1770:200) (680:70)
1292.8:0.6 61.2:6.1 67:12 87:12 58.7:5.9
1298.7:0.7 16.3:2.0 (0) (O) 15.5:2.0
1434.6:0.5 3.0:0.3 107:40 27:9 1.6:O.2
(104140) (2419)
1579.911. 0.74:0.12 ——— --- 0.83:0.15d
1607.9:0. 2.3:0.2 -—- -—— 2.2:0.2
1657.2:1. 0.12:0.04 -—- -—- 0.15:0.05

 

8A total electron conversion coefficient of a=0.46 has been reported

(40) for the 145.4-keV transition.

intensity of 44.2:4.4 on the above scale.
in the table are photon intensities, the conversion coefficients for
the higher-energy transitions should be small enough such that the

photon and transition intensities should be nearly the same.

This indicates a total transition
Although all the intensities

bThe intensities given in parentheses are those corrected for chance

coincidences.

cTwo 0.32-cm Cu absorbers forming a sandwich around the Ndlul source
were used for the determination of the intensity of 511-keV photons in
total annihilation.

dAfter subtraction of the 1575—keV Prll+2 contaminant peak produced
by a Prll“1(rz,y)Pr1“2 reaction, where the neutrons were produced
predominantly in the degrading foils.

33

times, different system gains, and with each of the two Ge(Li)
detectors. The corresponding uncertainties in energies are based
on the reproducibilities both of the standard energies and the
Ndll“1 energies from the calibration curves, the sizes of the Nd”1
photopeaks above the background, and the quoted errors of the
standard energies listed in Table 1.

The relative peak areas obtained are also averages from
a number of runs, and the associated statistical uncertainties in—
clude estimated uncertainties in the backgrounds. Relative photo-
peak efficiency curves for the Ge(Li) detectors were obtained in
two ways: First, a set of standard gamma ray sources whose re—
lative intensities had been measured with NaI(Tl) detectors was
used. Second, a set of points was Obtained from sources emit—
ting several gamma rays whose relative intensities were known
from well—established decay schemes. The efficiency curves re-
sulting from the separate methods were in very good agreement.

The K x-ray intensity was obtained by comparing the low~
energy portion of the spectrum directly with the gamma ray spectrum
of Celhl; the comparison was made using the 3-cm3 detector. Celul
also decays to Prlul, with 70% of its decay populating the 145.4—
keV state. Its ratio of K x-rays to 145.4-keV gamma rays has been
measured (45) to be 0.34l:0.010, and, using this value, the corre-

sponding ratio for Nd”1 decay was found to be 264:71.

3.1.3.B. ——Coincidence Spectra-— From the Nd11+1 disinte-

 

gration energy (28,34) of 1800 keV and the measured gamma ray

34

energies listed in Table 2, it is evident that only coincidences
involving the 145.4—keV gamma ray are energetically allowed. Thus,
a 3-in X 3-in. NaI(Tl) detector was gated on the 145.4—keV photo—
peak and the resultant coincidence spectrum seen by the 7-cm3 Ge(Li)
detector was displayed; the resolving time of the system was z50'ns.
Figure 4 shows this coincidence spectrum. In Table. 2 the relative
intensities from this experiment are also included; these have been
corrected for chance coincidences. Comparison of the relative in-
tensities of the 145.4-keV gamma-gamma coincidence spectrum with
those of the singles spectra clearly indicate that the 981.3—,
1147.1-, and 1434.6-keV gamma rays are in coincidence with the
145.4 keV gamma, whereas the 1126.8—, 1292.8-, and 1298.7—keV gamma
rays are not.

In order to search for additional weak gamma rays that
might have passed unobserved in the other measurements, an experi-
ment was also performed in which the NaI(Tl) gate was set to accept
all transitions greater than 130 keV in energy. This "integral"
gamma-gamma coincidence spectrum is shown in Figure 5, and the rel-
ative intensities from it are listed in Table 2. They verify the
results of the 145.4-keV gamma-gamma coincidence experiment, but no
new, weak gamma rays are indicated.

To complement these experiments and confirm which gamma rays
appeared in cascades and which came from primarily electron-capture-
fed ground—state transitions, an 8—in. X 8-in. NaI(T1) split annulus

detector was employed (19) in an anti-coincidence experiment with

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37

the 7-cm3 Ge(Li) detector. The single—channel analyzer on the
annulus gate was set so that the gate would be active for all gamma
rays above 80 keV. The Ndlulsources were placed inside the annulus
tunnel and on top Of the Ge(Li) detector. An additional 3-in. X 3-in.
NaI(T1) anti—coincidence detector was placed in the tunnel above the
sources and the Ge(Li) detector to reduce further the sharp Compton
edges formed by backscattering in the Ge(Li) detector. The result-
ing anti—coincidence spectrum is shown in Figure 6. The intensities
of all ten of the Nd”1 gamma rays, which were seen in this spectrum,
are included in Table 2. Only four of these gamma rays, the same
four indicated by the other coincidence experiments, appear to be in
coincidence with another gamma ray because of the large reductions
in their intensities as compared with the intensities from the sin-

gles spectra.

3.1.4. Ndlulm Gamma RayiSpectra

The energy of the 60-sec Ndlulm isomeric transition to the
ground state was measured to be 756.5:0.3 keV, in excellent agree-
ment with the recent work of Geiger and Graham (46), who obtained
756.8:1.3 keV. A search was also conducted for gamma rays result-
ing from direct electron-capture transitions from Nd1“1m to states
Of Pr141 and/or from alternate transitions depOpulating Ndlulm to
Ndlnlg. Approximately 1 min afterzalo-sec bombardment Of Pr203
with the 94MeV protons, a 59-sec count of the Ndlulm (+Nd1u19)
spectrum was stored in the first quadrant of an analyzer having 4096
channels of memory. The Ndlulm (+Nd1h19) source was gradually moved

toward the Ge(Li) detector during this time in order to maintain the

 

 

 

 

COUNTS I CHANNEL

 

 

 

38

 

‘2
IO
1'

 

l I I

Nd'“ ANTI-COMPTON SPECTRUM

 

 

 

 

 

 

 

 

 

O
Id’r- :' ‘9 ~
0 n w
r 93 °°
- N
m
L s
'3 h-
| - .
°‘ : s
P n ‘3 q
a ‘ I
0, JR 3 a. a) 23
P I: . o -1
'0: 1‘: '3 é ‘3
. 1 ... 4
s It 3%
I‘D-1' E N‘
o V 0
fire 0 -
'oz_ '-ao co N ~
I‘D : N
- '1', I' I0 4
I: B ID
a .3 -
"‘6: ..
. . ”If: '51
IO'» :‘S‘FTI‘ _
.L i :1
O 250’ 4500 750 I000
CHANNEL NUMBER
Fig. 6. Anti-coincidence spectrum recorded by the 7-cm3

Ge(Li) detector when placed inside the tunnel of
an 8-in. X 8—in. NaI(Tl) split annulus with a
3-in,X 3-in. NaI(T1) detector at the other end
of the tunnel. For details, see the text or
reference 19.

39

analyzer dead time at approximately a constant 12%; this procedure
allowed the data to be collected more rapidly than with a fixed source
position. Following intervals of :1 sec for switching analyzer quad-
rants, 59—sec counts were stored successively in the three remaining
quadrants. The entire process was performed 60 times to reduce sta-
tistical errors and to search carefully for weak gamma rays.

The resulting four spectra, each representing 59 min of
counting time, are shown in Figure 7. The 756.5-keV gamma is clearly
the only observable gamma ray that decays with a 60-sec half-life.
The other gamma rays in Figure 7 are the three most intense Ndlqlg
decay transitions. From these Spectra, an upper limit of 0.1% of
the 756.5-keV gamma intensity was placed on any gamma ray with an
energy between 130 and 2600 keV following direct electron-capture
transitions from 11/2- Ndlhlm to high-spin states in Prlul; the same

limit applies to alternate transitions to lower-lying states in

Ndlb'l.

341.5. Decay Scheme and Discussion

The decay scheme that was deduced from the foregoing meas-
urements is shown in Figure 8. Transition energies and excited state
energies are given in keV, the 8+ energy coming from the work of
Biryukov and Shimanskaya (28). The B+/e ratio for decay to the Pr“+1
ground state (also the limits placed for decay to the 145.4-keV state)
is a calculated value, using the method of Zweifel (47). The other
transition intensities, both for electron-capture and for the (total)
Electromagnetic transitions, are adjusted to this value and read in

Percent of the total Nd11+1 disintegrations. Using the measured value

‘ 1. I 4 In In I
e o o... ale mm 3 5 fl.

JUZZ<IO \ WPZDOO-

 

 

 

COUNTS / CHANNEL

 

7

IT

10‘

 

l l

t. H—.

 

756.5

 

 

6.5

75

 

 

 

I I I

SINGLES

“—1
“I
N —
a . m
‘0 ~00 (3
‘4531,
£1"e- -
o .Q“. O o
o o no 0.
EzE:.- - ”1
00-00 a... .0 o.- ooooo 0.0 ooq
m v?
o 9
N“: N ._.
.7." 0'1
S N
.3.0 'fi- —
\‘10 .
\- o .
' ..'<‘..4 . . —1
.....uoiocoo 1 3“..- 0.... 0nd

o .00 -0... ounce-...“

d r“
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'- ’;‘-o I —
. 1.
‘ “’1'" A
3.1 1.31,
-.’.‘o. 4.. fl
-.. O ‘
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w (I)
§T & II
—N a,
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:""'".f'f.: I. —*
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...”-C ...“..O- ...“. .0...

 

L

l“““ l“‘—_‘-

 

Fig. 7.

|25

250

375 625

CHANNEL NUMBER

Singles y-ray spectra of Ndlulm + Ndlulg.

A) The spectrum

from the sum of 60 runs, each started ‘1 min after a 10 sec

activation and lasting 59 sec.
C) 120 sec later. D) 180 sec later.

sec later.

B) Same, except started 60

41

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

ll/ 2-
'——l— "'605
'0.
w M4
10
N
$0553 _4 312+ 2.6h
(112+ 312+ 512+) -- g. 0.2 l657.2 ..(s-Io °1.)8.2 N d1.“
0° - s - /"‘—‘
(112+ 312+ 512+) " I607,9 ((0. 02 °1.) 7.2 60 8|
(512+) I580.0 ((0.03 °1.17.0
(112+,312+,512+1 if 3, P3, I298.4/(0. 1 °1.)7.1
(512+) 9— 9 9 ._ / 1292.5 {1031.163
’1: C:
(312+) S2 9. ”26.8 /(0.7 °1.16.5
NQQQEQTQQ
r~ r~ a) v o N r~ w _
n Oh-n m m «cum
2 ‘2 Q ‘1 ‘1' ‘3 : : m
*3 .
712+ 2.6 ns 9 145.4 (<3-10“°1.B.<0.03°1.e)>8.s
".
5/2+ 31W 0 [(2.4%Bfi96J °/.€)5.3
141
59 Pr 82

Fig. 8.

Decay scheme of Ndxulg+m. Excited—state and y—ray
energies are given in keV. The intensities of all
transitions are total transition intensities and are
given in per cent of the total NdIulg disintegrations.
Log fl values are based on a 2.6—hr half-life. The
spin and parity assignments to the upper six states
in Prlh] are tentative; see text.

42

of 9.6 for the intensity ratio of K x-rays to 511-keV gamma rays,
which is in good agreement with that measured by Biryukov and
Shimanskaya (28), and making reasonable assumptions about the K
fluoresence yield and the ratio of K capture to capture from higher
shells (see, e.g., reference 23), values of 4.3% 8+ and 94.3% S were
obtained. However, since this is quite clearly an allowed transi-
tion, there appears to be at least as much uncertainty in the ex-
perimental value as in the theoretical value of B+/e. The theoret-
ical value was adOpted since any needed future adjustments could be
made more easily with respect to it. The log ft values were calcu—
lated on the basis of a 2.6-hr half-life (28) for Ndlul.

The 981.3-, 1298.7-, 1434.6-, 1579.9-, 1607.9-, and 1657.2-
keV transitions have not been previously reported in decay schemes.
The energy of the 145.4-keV state in Pr1l+1 has been well calibrated
from Ce1I+1 decay (40), the photon energy being given as 145.43-keV.
The evidence for the 1126.8- and 1292.5—keV states, as well as the
new state at 1580.0 keV, is based both on the coincidence results
with the 145.4-keV gamma and the enhancement of the 1126.8-, 1292.8-,
and 1579.9-keV gamma rays in the anti-coincidence experiment, in-
dicating that they are ground—state transitions. The energies Of
these states were chosen on the basis of the best-defined gamma rays
depopulating them, although it can be seen that the cascade energy
sum gives excellent agreement with the crossover energy in each case.

The placement of states at 1298.4, 1607.9, and 1657.2 keV is based

on the enhancement of the respective gamma rays in the anti-coin-

43

cidence experiment and the fact that these gamma rays were sup—
pressed in both coincidence experiments. No evidence was seen for
the state at 880 keV reported by Cybulska and Marquez (27).

dlul and Pr1l+1 have been

The ground-state spins of both N
measured by atomic beam methods, that of the former (48) being 3/2
and of the latter (49) being 5/2. In shell—model terms, Nd”1 is
predicted to be a (613/2)“l neutron state, while the ground state of
Prlgl should be a d5/2 proton state outside a closed 97/2 proton
subshell. Thus, 98.5% of the Nd141 disintegrations consist of its
3/2+ ground state pOpulating the Prll+l 5/2+ ground state directly,
and the log ft value of 5.3 is about what one would expect for an
allowed transition between such similar states.

Now, the 145.4-keV transition in Pr“1 has been well charac-
terized (40) from Celul decay as an R-forbidden M1 with an E2 ad—
mixture of 0.4:O.3% having a mean life of 2.63:0.10 us. The state
itself is presumed to have a (97/2)'3(d5/2)6 configuration. This
configuration forms the ground state of Pr“3 and the 5/2+ state
in this nucleus (50,51) is placed at 57 keV, so the 5/2+ and 7/2+

ll+3

states cross over between Pr and Prlul. The 7/2+ 145.4-keV

state in Pr”1

would not be expected to receive observable direct
pOpulation from 3/2+'Nd1"19, again in accord with the measurements
reported here.

Considering that Pr1l+1 is a single closed shell nucleus one
encounters unexpected difficulties in characterizing its higher—
lying states. Basically, the problem is as follows: Pr”1 can be

1110

considered to be a single proton outside a Ce even-even core, so

44

one is tempted to use the core—coupling model in describing the
Prll’1 higher—lying states. Celko, with a closed neutron shell and
a closed 97/2 proton subshell, is expected to be rather rigid and
not subject to low—lying vibrations. This appears to be true, for
its first excited state is a 2+ state at 1.596 MeV that decays via
a non-enhanced E2 transition (52). Currie (53), in trying to
account for the retardation of the E2 transition from a 4+ level
at 2.083 MeV to this level, applied a quasi-particle representa-
tion for both states, but his best numerical results implied a
[(g7/Z)(d5/2)]2+ configuration instead of the anticipated (and pro-
bably more likely) [(g7/2)2]2+ or perhaps [(d5/2)2]2+. This means
that, although the Ce1H0 2+ state does appear to be a two quasi-
particle state, its exact structure is not clear.

On the other hand, the first excited state of Celuz, hav—
ing only two additional neutrons, lies at 0.65 MeV and appears to
be a 2+ quadrupole vibrational state (54). The first few PrM3

excited states, which lie much lower than those in Pr“+1

, can pro—
bably be explained by a coupling of the 7/2+ ground state and the
5/2+ 57-keV state to this Celqz 2+ collective state (50,55).

The known Pr”1 states lie at an intermediate energy, so
one cannot decide without further evidence whether they are three
quasi-particle states, one quasi-particle states coupled to a
vibrational core, or perhaps a mixture of the two. Although the
two neutrons of Ce1H2 are probably more effective in softening the

Cell‘0 core than is the single proton (outside only a subshell) of

Pr1”1, the E2 transition probabilities may or may not be enhanced

45

over the single—particle estimates. In the following, keeping in
mind the different kinds of states possible, tentative predictions
are made for the Spins and parities of the six upper states on the
basis of beta and gamma decay systematics. It must be kept in
mind, however, that these are only tentative, and for quite def-
inite assignments one needs more information about the levels.
High-resolution scattering reactions of various kinds that pOp—
ulate these states are particularly valuable. A summary of re—
sults from some recently published (B,y), (n,n'y), (n,n'),
(He3,d), and (d,He3) studies are included in the following sub-
section.

The log ft values are all more or less in the range ex—
pected for allowed transitions. First—forbidden decay cannot be
excluded, especially to the highest—lying states, on the basis of
the log f% values, but then the only negative parity states would

be those resulting from the h shell—model state or from octu—

11/2

pole vibrations. The d3/ ground state of Nd”1 should not pOp-

2

ulate the former, although the h Ndlulm might. The latter

11/2
have not been reported near this excitation in any of the neigh-
boring even-even nuclei. Thus, all six states are probably l/2+,
3/2+, or 5/2+. This set is consistent with either interpretation
of the states -- by coupling the 5/2+ or 7/2+ Single quasi-particle
states to a 2+ vibrational core, one can get l/2+ through 11/2+,
with two sets of 3/2+ through 9/2+, and on the basis of three

quasi-particles, the range is even broader.

Assignments for the three states that exhibit gamma ray

 

 

46

branching can be narrowed down from the above limits. The inten-
sity ratio of the 1126.8—keV gamma ray to the 981.3—keV gamma from
the 1126.8-keV state is 35. The mere existence of the 981.3—keV
gamma rules out a 1/2+ assignment, for this would require the
981.3-keV transition to be M3. For a 5/2+ assignment, the single-
particle estimate (23) yields a ratio (both Ml's) of less than 2,
while for a 3/2+ it predicts a ratio (Ml/E2) of about 200. Even

a slight E2 enhancement or M1 retardation would therefore favor

a 3/2+ assignment.

For the 1292.5- and 1580.0—keV states, which have ground-
state to cascade transition ratios of 1.33 and 0.35, respectively,
the 1/2+ assignment can similarily be eliminated. Unless there is
some quite unusual M1 retardation or E2 enhancement, the 3/2+ spin
can also be eliminated, so a 5/2+ assignment is preferred.

The 756.5-keV excited state of Nd”1 has been shown to
have a half—life of 60.3:l.0 sec (46). It is one of the series of

h isomers found just below the N=82 shell. Since the 11/2—

11/2
(presumably single quasi-particle hll/z) state lies (56) at 822
keV in Pr139, it is possible that the same state lies in the 1-MeV
vicinity in Pr“+1 and that there could be come direct pOpulation
of it from Nd151m. From Figure 5, however, it can be seen that
such pOpulation must be less than 0.1% of the intensity of the
756.5-keV isomeric gamma ray. Depending on the exact location of
the h11/2 state in Prlul, this upper limit means simply that the

log ft for electron capture has to be greater than approximately

6.0. The same upper limit can be placed on any branching gamma

47

decay to lower states in Nd11+l itself, if at least one gamma ray

having an energy greater than 130—keV is involved.

3.1.6. Comparison with Recent Investigations

After the completion of the present work, some additional
publications became available (88,89,90,ll8). The energy levels
and spins prOposed in these studies are summarized in Table 2a.

A (y,n) reaction on Nd203 (enriched to 95% Ndlqz) was employed to
obtain the Nd1H1 parent for the (8,7) study described in reference
118. The essential features of the Nd”l decay scheme prOposed
there (118) confirm the corresponding features of the decay scheme
proposed here. No interpretations were made of the prOperties of
the states.

Although the experimental uncertainties of the reaction
energy measurements were relatively large, some common levels are
suggested in Table 2a. Probably the 1657.2-keV state corresponds
to the 1160 (1645) keV state prOposed in reference 90 (88). By
incorporating the results of the (He3,d) Study (88), the Spins
prOposed in the present study can be narrowed down for the 1298.4,
1607.9, and 1657.2 keV states to l/2+, (3/2,5/2)+, and 1/2+;re-

Spectively.

48

.>Ox a“ psomfia mum moamuoao HH<O

.mw oodouomomw
.ww monouomomo
.om monouomomn
.wHH monouomomm

 

 

-11 1-1 oooa «sod 111 1-1
111 111 oNoH mood 11- 111
111 1-- 111 oooa 111 111
1-1 111 ooAH Nona 111 111
111 111 111 Amoaav 11- 11-
1-1 11- 111 Amoofio .1. 111
111 H+N\Hlmsoa oooa 111 111 HA+~\m.+N\m.+N\HoLN.amoH
-11 -11 111 omoH 111 111
oNoH E+Am\o.o\moloooa 1.. Boos a.oooa HA+N\m.+~\m.+~\HVLo.AooH
111 111 ooms ohms o.oama HA+~\mo_o.oomH
111 111 mNmH Emma .11 111
111 111 woos moss 111 -11
111 -11 omoa omsa 111 111
111 1-1 11- qua 1-1 -11
111 111 11- oooa 111. 111
onH E+N\Hlooma moms oomH HA+N\m.+~\HoLm.ooNH HA+~\m.+N\m.+N\HVLo.ooNH
111 111 111 HA+N\ooLmoNH EA+~\ooao.No~H HA+N\mo_m.~o~H
omHH 111 HoHH ENHH HA+N\mo_o.oNHH _A+N\mo_o.o~HH
111 H1N\HHLSHHH 111 HA+N\o.+N\EoLoHHH 111 111
mes H+N\almsa mod was _+N\~os.mofl _+N\a_o.noa
o _+N\m.o o o H+~\mo o _+~\ml so
Amom.oo Ao.mamo A.:.:o A>.:.eo A>.oo A>.oo
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.1;

um mo mafiam was mHo>OH voawEHOuov maaouaoafiuomxm

.mm magma

49

3.2. Decay Schemes of Nd139m and Nd1399

3.2.1. Introduction

 

One of the most interesting regions of the nuclidic chart
for current study is the region just below N=82, for here many
systematic examples of rather extreme isomerism can be observed.
The neutron—deficient side of the A=139 decay chain extends into
this region, and its members are well suited for probing the re-
gion because nuclei rather far removed from B-stability are
reached not too far below the closed shell. Thus, many interest-
ing states should be pOpulated by their decay, and these states
should still be amenable to eXplanation in relatively straight—
forward shell-model terms -- the number of nucleons making
substantial contributions to a given configuration should not be
so large as to be completely unmanageable.

58Celg? is the first radioactive member on this side of
the chain, and it decays directly to stable La139 with a Q8 of only
270 keV and a half—life of 140 d; it has a very simple decay scheme
that has been known for a long time (57). It does, however, have
an interesting h11/2 isomeric state (746 keV; t1 = 55 sec), a
ummflber of the extensive N=81 series. The decay of the second
radioactive member, 4.5-h 59Pr123, to Ce139 is considerably more

complex; the results on this decay scheme are published elsewhere

(16).

139 9

79 and has a

Nd is three beta decays from stable L313

60

rather large amount of energy available for B—decay (QE = 2.8 MeV;

cf. below). As in other N=79 odd—mass isobars, the h d3

11/2- /2

 

 

 

50

(metastable—ground state) separation is fairly small, making the

M4 isomeric transition quite slow. This means that here one is
presented with two dissimilar isomers decaying almost independently,
and because each can pOpulate reasonably high-lying states in Pr139,
a wealth of information about many quite different states in this
daughter nucleus is available from the study of these decays.

Nd139m was first observed by Stover (58) in 1950 as part
of an investigation of the products of bombardment of Pr“+1 with 40-
and 50-MeV protons. Chemical identification was performed by ion
exchange, and the mass number was established with reference to
the granddaughter, Ce139. The half—life was measured to be 5.5:0.2 h.

Later studies (59,60) of conversion electron intensities and
energy differences for a 231—keV transition accompanying this decay
indicated it to be an M4 and to originate in Nd not Pr. Four neigh—
boring odd—mass isobars with 79 neutrons were known (6) to have
isomeric states involving an: 11/2- + 3/2+ transition. From the
trends in the isomeric level energies and in the reduced transition
probabilities, Gromov and his co-workers concluded that here we have
a like pair of states (59) and that the 5.5-h activity was the 11/2—
metastable state.

The 3/2+ ground state was not seen so easily, and its half-
1ife was only recently measured (61) to be 29.7:0.5 min. For that
experiment it was produced by bombarding Pr“+1 with 30- and 33-MeV
deuterons.

The only previous studies of Nd139m decay (56,60) resulted

in rather sketchy decay schemes containing serious disagreements.

51

Because of this and the absence of any decay scheme for Nd1399,
it was felt that this would make a good system for investigation.
This study has indicated the presence of 51 gamma rays accompany—
ing Nd139m decay and 21 that follow Nd1399 decay. Of these gamma-
rays, 56 have been placed in decay schemes containing a total of
22 excited states. Fourteen of these states have not been seen
before.

The decay scheme of Nd1399 turns out to be unexceptional,
having much in parallel with the decay scheme of Nd“+1 (seen in
Figure 8) and some other nuclei in this region below N=82. The

139 can be characterized

low-spin states that it pOpulates in Pr
reasonably well and follow eXpected systematics. On the other
hand, the decay scheme of Nd139m is anything but standard. This
high—Spin isomer decays only 12.7% by the 231.2-keV isomeric tran-
sition, the rest being by 8+/e to mostly high-spin, high-lying
states in Pr139. Six of these, between 1624.5 and 2196.7 keV,

are pOpulated by decay that is less hindered (log ft's between

5.5 and 6.3) than the decay to an h isomeric state at 821.9

11/2
keV in Pr139 (log ft=7.0), which is almost certainly an allowed
transition. This would seem to indicate that the transitions to
these six states are also allowed, which would imply odd-parity
states .

This is interpreted as the configuration of Nd139m being
peculiarly suited for pOpulating a multiplet of three-quasiparticle

states. During the eXplanation, the problem associated with mul-

thile particle rearrangements in beta and gamma decay is discussed.

 

 

52

3.2.2. Source Preparation

 

The 5.5-h Nd139m activity was produced for most of the ex-
periments by the relatively clean (p,3n) reaction on 100% abundant
Prlul. Targets of 99.999% pure (62) Pr203 were bombarded typically
for :1 h with 22—11A of 29—MeV protons from the Michigan State
University sector-focused cyclotron. Sources were allowed to decay
for about five hours to let the 30-min Nd1399 produced by the bom-
bardments reach transient equilibrium with Nd139m. Experiments
were then performed with the sources for approximately twenty hours,
until the Pr”0 produced by the 3.3-d decay of Nd1H0 became a sig—
nificant contaminant.

From crude excitation function studies of reactions fol-
lowing the bombardment of Pr“*1 with protons of various energies,
it was possible to distinguish the Nd139m+9 activities from.weak
contaminant activities. Following each bombardment with 29-MeV pro—
tons, it was possible to identify every contaminant peak observed
in Spectra recorded between 20 min and 40 h after the end of the
bombardment. These weak contaminants, roughly in decreasing order
of importance, were Prlko, Pr139, Ce139, Ndlul, and Prluz. It is
significant that no 22-min Nd138 was produced, for its daughter,
2.1-h Pr138, could prove a troublesome contaminant.

Nd139m sources were also produced following the bombard-
ments of Nd1H2 with 36-MeV I's (He3 ions) and of Pr“+1 by 48- and
60- MeV T's, all from the MSU cyclotron. These reactions were not
so clean as the (p,3n) reaction on Prlhl, but they confirmed the

relative intensities of the Nd139m gamma rays.

 

 

53

Most of the Nd1399 sources were produced by bombarding similar
Pr203 targets with 29—MeV protons for 245 sec. Experiments were
carried out immediately upon concluding each of the bombardments,
and the gamma rays resulting specifically from Nd1399 decay were
followed as their intensities drOpped from their initial values to
those when Nd1399 was in transient equilibrium with Nd139m.

The relative intensities of all the Nd1399 gamma rays
which were observed were confirmed by measurements of the activity

141. These reactions would

produced by 48— and 60- MeV T's on Pr
produce Pm139, which, were it a low—spin nucleus as anticipated,
would populate Nd1399 by B-decay much more strongly than Nd139m.
They did in fact yield Nd1399/Nd139m isomer ratios some 30 times

1111

as large as the (p,3n) reaction Pr , but they yielded many more

interfering short-lived activities as well.

3.2.3. Experimental Results for Nd139m

 

3.2.3.A. --Gamma RaygSinglesgSpectra—— A 7—cm3 five—sided

 

coaxial Ge(Li) detector manufactured (18) in this laboratory was em—
ployed to determine the energies and intensities of the Nd139m gamma
rays. The wall thickness of the evacuated aluminum can enclosing
the detector was 0.16 cm. Under typical Operating conditions, a
resolution of 12.5 keV FWHM for the 661.6-keV gamma of C8137 was
obtained, using a room temperature FET preamplifier, a low-noise

RC linear amplifier with pole-zero compensation, and a 4096-channel
analyzer or ADC coupled to a computer.

Energies of the prominent Nd139m gamma rays were measured by

54

d139m sources simultaneously with several well-known

counting the N
calibration sources. To determine the energy calibration curve, a
least-squares fit of the photopeak centroids of the calibration
transitions to a quadratic equation was used after the background
had been subtracted from under the peaks. The background correc—
tion for each peak was made by fitting a linear equation to sev-
eral channels adjacent to both sides of the peak and then sub—

tracting. The energies of the lower-intensity Nd139m

gamma rays,
which were obscured by the calibration standards, were then de-
termined similarly by using the stronger Nd139m gamma rays as the
standards. Some gamma ray singles spectra are shown in Figures 9a
and 9b.

The Spectrum shown in Figure 9b was used to place an Upper
limit of 0.1% of the disintegrations of Nd139m on any gamma tran-
sition with an energy above 2300 keV. This would appear to rule
out the 2350- and 2500-keV gamma rays proposed earlier (6,56) to
have intensities ”50 times as large as the present upper limit.

The events observed above 2300 keV in Figure 9b come from long-
1ived room backgrounds that were not subtracted out.

The contaminant peaks seen in Figure 9a accompany the
reaction, Pr1“1(p,2n)Nd1”OE+Pr1“0, and Pr139 and Ge139 disinte-
grations following Nd139m+9 decay. Their energies, relative in—
tensities, and intensity changes as functions of time were seen
to be consistent with the prOperties of the associated decay schemes

established in this study and elsewhere (6,63-67).

A summary of the Nd139m gamma ray energies and relative in—

 

 

55

.ESCuomam osu mo MO©CHmE

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57

tensities is given in Table 3. The energies assigned are mean
values taken from a number of different measurements recorded at
different times, different locations, with different system com-
ponents, and with different parameters. Corresponding energy un-
certainties are based on the reproducibilities of the Nd139m en-
ergies from the calibration curves, the sizes of the Nd139m photo—
peaks both before and after background subtraction, and the quoted
errors on the standard energies (68). The relative gamma ray in-
tensities listed in Table 3 are also averages from a number of runs
and were obtained using experimentally determined efficiency curves
(cf. section 3.1.3.A). Associated with these intensities are sta-
tistical uncertainties that include estimated uncertainties in the
underlying backgrounds.

3.2.3.B. -—Gamma Gamma Coincidence Studies-- Coincidence

 

and anti-coincidence eXperiments were performed using Ge(Li)-NaI(T1)
spectrometers. For the first experiment, in order to determine
which gamma rays appear in cascades and which come primarily from
e-fed ground-state transitions, and 8—in. X 8—in. Nai(Tl) Split
annulus detector was employed in an anti-coincidence experiment

with the 7-cm3 Ge(lI) detector (19). The Nd139m source was in-
serted into the center of the annulus tunnel, which was then blocked
by a 3—in X 3—in. NaI(T1) detector at one end and by the Ge(Li) de-.
tector at the other end. By including the 3-in. X 3-in. NaI(Tl)
detector in anti-coincidence with the Ge(Li) detector, the Compton
edges from backscattering in the Ge(Li) detector were reduced over

what they would have been with only the annulus in anti-coincidence.

58

Table 3. Energies and relative intensities of gamma rays

present in the decay of Nd139m.

 

 

Measured y-ray Relative Measured y-ray Relative
energy (keV) intensity energy (keV) intensity
92.9:0.2 3.2: 0.6 828.1i0.2 29 i 2
101.3:O.8 0.7: 0.2 851.9:o.5b 1.4: 0.4b
113.8:0.l 133 :25 895.1:0.6 0.8: 0.2
147.9i0.1 2.5i 0.5 900.3i0.6 1.1: 0.3

209.710.1 6.2i 0.6 910.2i0.2 21.6i 2
214.6i0.2 1.4i 0.4 982.2i0.2 79 i 2
231.2i0.2 2.4i 0.2 1006.1i0.2 9.5i 0.7
254.9i0.3 3.7i 0.6 1011.9i0.2 8.0i 0.6
302.7i0.3 1.4i 0.2 1024.6i0.3 3.6i 0.4
340.4i0.5 2.7i 0.5 1075.1i0.2 9.6i 1
362.610.2 6.2i 0.5 1105.2i0.2 7.4i 0.4
403.9:0.3 8.0i 1.0 1165.8i0.5 1.0i 0.5
424.3i0.3 2.0i 0.4 1220.9i0.3 5.0i 0.5
511.0(y:) 3.2: 2.88 1226.9i0.3 4.0i 0.4
547.7i0.3 7.5i 0.7 1233.7i0.5 0.8i 0.4
572.1i0.5 1.7i 0.4 1246.511.0 0.9i 0.4
601.2i0.8 1.3i 0.4 1322.4i0.3 7.0i 0.9
673.5i0.5 2.5i 0.7 1344.8i0.6 1.3i 0.4
701.3i0.3 13 i 2 1364.8i0.6 1.7i 0.6
708.1:0.l 72 i 2 1374.710.5 1.8i 0.5
733 :1 b 1.0: 0.6b 1463.6i0.5 1.0: 0.3
738.1i0.2 E100 1470.2i0.3 2.0i 0.5
796.6i0.3 13 i 2 1680.7i0.8 0.8i 0.2
802.4i0.3 21 i 2 2060.4i0.2 15.5i 1.0
810.1i0.3 18 i 2 2085.0i0.5 0.1i 0.05
821.9i0.3 3.7i 0.4 2201.2i0.8 0.3i 0.1

 

 

8Calculated from the decay scheme prOposed later in the present
study. Components of the Observed annihilation photon intensity
from Nd1399 and/or Pr139 decay always exceed the Nd139m component.

bSeen in coincidence spectra only. The intensities given here are
inferred from the completed decay scheme and the behavior of these
photons in the coincidence spectra.

59

The single-channel analyzers associated with the NaI(T1) detectors
were set to accept all gamma rays above 80 keV. A resolving time
(21) of 3100 ns was used, and the true—to—chance ratio was usually
z100/1. The resulting Spectrum is shown in Figure 10.

The 231.2— and 821.9-keV gamma ray peaks seen here were
enhanced in the anti-coincidence experiments (relative to their
singles intensities) far more than were any of the other Nd139m
transitions, as indicated in Table 4. Thus, each of the other
Nd139m gamma rays appeared to be involved in one or more coinci-
dences with Z80—keV photons. The following coincidence eXperiments
elucidated most of these cascades.

A coincidence spectrum gated by the split annulus detector
on the 113.8-keV gamma is shown in Figure 11. The gamma ray in-
tensities seen in this experiment, normalized to 100 for the 738.1-
keV gamma intensity, are listed in Table 4. Four gamma intensities
are reduced by factors of about 10, viz., the 796.6-, 828.1-,
1006.1-, and 1220.9-keV gamma rays, whereas several other prominent
peaks appear to be in coincidence with the intense 113.8-keV gamma.
These:resu1ts are indicated in Table 5.

Figures 12 and 13 show the spectra resulting from gating
this same spectrometer on two adjacent energy intervals, 680-720
and 720-760 keV. Because the resolution of the annulus in these
experiments was only 313%, there was considerable overlap between
these gated regions; however, as can be seen in Figure 9a a single

gamma ray dominates each region, so a comparison of the intensities

observed in coincidence with these adjacent gated regions was quite

i.‘IaI-l.1!'l|IIIII

 

COUNTS PER CHANNEL

IO

 

60

 

 

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Fig. 10. NdI3qm anti-coincidence spectrum recorded by the 7-

cm3 Ge(Li) detector when placed inside the tunnel

of an 8—in. X 8-in. NaI(T1) split annulus, with a
3-in. X 3-in. NaI(T1) detector at the other end of
the tunnel. For details, see the text or reference
19. Characteristic of this type of spectrum is the
noticeable absence of Compton edges. The 231.2—keV
Y is the only y—ray enhanced over its singles in—
tensity.

 

 

 

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Spectrum of Nd

Fig. 12.

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NaI(Tl) split annulus.

67

 

 

 

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Fig. 13.

68

useful in constructing the decay scheme.
A comparison of the spectra recorded with adjacent coinci-
dence gates also aided in determining the effects of the under—
lying Compton backgrounds inevitably in the gates. In all,12
different gated regions were used to obtain coincidence spectra
similar to those of Figures 12 and 13. The results are seen in Fr

Figures 14-21 and summarized in Tables 4 and 5. fl

3.2.3.C. -—Delayed Coincidence Experiments—- Possibly the
single most useful coincidence experiment was a delayed coinci—
dence experiment using a 3—in. x 3-in. scintillator and the Ge(Li)
detector. The 3—in. x 3-in. NaI(Tl) scintillator was gated on the
113.8-keV gamma, the coincidence timing resolution (21) was 2100 ns,
and a delay of 2200 us was added to the Ge(Li) side of the coinci-
dence circuit. The resulting spectrum is shown in
Figure 22. Several peaks are enhanced up to two orders of magni-
tude relative to the 708.1-, 738.1-, and 910.2-keV peaks, which were
seen earlier to be in prompt coincidence with the 113.8-keV gamma.
The intensities from this spectrum are listed in Table 4. Later,
in section 3.2.4.A, it will be described how this delayed coinci-
dence Spectrum confirms the placement of nine states in Pr139.

The state responsible for the delays lies at 821.9 keV, and
in order to measure its half-life, a fast—slow coincidence system
with two 2-in. x 2-in. NaI(Tl) detectors and a time-to—amplitude

converter were used. Now, from the prompt and delayed coincidence

data there was no evidence for delays connected with states other

(KNIVES PEI! CiuumNEL.

 

 

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This spectrum was recorded by the 7—cm‘
Ge(Li) detector with the 8—in. x 8—in.
NaI(Tl) split annulus set to accept all y~
rays above 350 keV.

B) The annulus gate was set on the ADS-keV
energy region.

 

 

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Nd'39'“ COINCIDENCE WITH 840-900 Rev
A

mfi“
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l m...—.—_—,—_—,—_;::"

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Same as Figure 17 except that the NaI(T1) gate was set on the

adjoining 840—900-keV energy interval.

Fig. 18.

 

 

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N

ES

75

 

 

 

 

 

 

 

 

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q Nd'39'“ COINCIDENCE WITH IIao-Isoo keV
'? N. 33‘ m
i II a 3‘3
m,-°“'.fl l 4... °..
. '32:" i5;v§'=‘¥'—‘¢"~*7"
5*" _
I
l .5
if?
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l l
0 250 500 750
CHANNEL NUMBER
Fig. 20. Same as Figure 17 except that the NaI(Tl) gate

was set on the 1180—1300-keV energy interval.

COUNTS PER CHANNEL

76

 

I I l l I
Nam" COINCIDENCE
IooL WITH moo-2200 kev -
ll§.8 kev
80* A "
.. - ,I‘. ..
. I
40 *- 1 A

20

 

 

\
r I \ ‘4

 

 

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Fig. 21.

......lmu .A-"" --.”°~'+""-! I M

l0 |20 I40 l60 IBO ZIO 220

CHANNEL NUMBER

Spectrum of Nd13qm y—rays in coincidence
with the l900—2200-keV energy interval.
The gate signals came from the 3—in. x
3—in. NaI(Tl) detector.

 

77

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78

than the 821.9-keV state. For this reason and because of leading-
eflge walk problems with lower-energy gamma rays, the system was
triggered with (prompt and delayed) pulses above 600 keV. The timing

was chosen so that the prompt coincidence peak would be centered in

the SlZ-Channel analyzer used.

The time spectrum that remained following subtraction of F
the 17 counts/channel background is shown in Figure 23. The resol— [—
ution of the system was 3.3 ns FWHM, and the timing calibration was :L

made by inserting precisely measured pieces of delay cable into the
circuit. No difficulties connected with channel widths or non-
linearities in the TAC were noted, so no corrections were made for
these. The half-life calculated following a least-squares fit of a
straight line to the logarithms of the data points in Figure 23 was
40:2 us. No evidence of decays with different half-lives was ob—
served in this experiment. The ratio of the areas under the prompt
peak and the delayed curves are consistent with the decay scheme

and the interpretation that the 821.9-keV state decays with a 40—ns

half-life.

3.2.4. Nd139m Decay Scheme

A decay scheme has been constructed for Nd139m from the
results of the coincidence studies and the energy sums and rel—
ative intensities of the transitions. This decay scheme is shown
in Figure 24, together with the decay scheme for Nd1399, which will
be discussed later. The striking difference between the two decay

schemes is worthy of note, with the 113.8—keV gamma being the only

 

 

 

 

 

 

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81

common transition. All energies are given in keV, and the QE is

a calculated value (69). Total transition intensities are given in
units of percent per disintegration of the parent Nd139m. The B+/e
ratios are also calculated values, using the method of Zweifel (47).
In general, the energy sums of competing crossover and cascade tran—
sitions agree to within i0.2 keV. Because there are so many coin—
cident, cascading transitions in this nucleus, there are many checks
as to the energies of most of the levels. The energy assigned for
each level is therefore a weighted value based both on the tran-
sitions that feed into and out of that level. Both because there
are an abnormally large number of gamma ray branchings in this
nucleus and because the interpretation of the higher-lying states
makes it essential that they be convincingly placed, the relevant
sets of sums have been presented in Table 6, where it can be seen

that the self-consistency is excellent.

3.2.4.A. -—The 113.8-keV Level and Those that Are Depopulated

through It-- The large relative intensity of the 113.8-keV gamma com-

 

bined with its coincidence behavior leads one to place a first-ex-
cited state at 113.8 keV, in agreement with earlier studies (56,60).
The isomeric state at 821.9 keV was first placed on the basis
of sevemal prompt coincidence experiments having timing resolutions
(21) of z100 ns (see, e.g., Table 5). It was then confirmed by the
delayed coincidence experiment of Figure 22, which suggested that
seven levels above 821.9 keV are depOpulated through the 821.9-keV

state. The gamma transitions presumed to originate from these levels

 

82

 

 

Table 6. Cascade energy relations for Nd139m y-rays.

y—Rays in Suma Suma State Energy Adopteda
821.9 821.9 821.9
113.8 + 708.1 821.9

851.9 851.9 851.9
113.8 + 738.1 851.9

802.4 + 821.9 1624.3 1624.5
796.6 + 828.1 1624.7

601 + 910.2 + 113.8 1625

254.9 + 547.7 + 821.9 1624.5

101.3 + 701.3 + 821.9 1624.5
1011.9 + 821.9 1833.8 1834.1
1006.1 + 828.1 1834.2

982.2 + 851.9 1834.1

810.1 + 910.2 + 113.8 1834.1

209.7 + 802.4 + 821.9 1834.0
1105.2 + 821.9 1927.1 1927.1
1075.1 + 851.9 1927.0

403.9 + 701.3 + 821.9 1927.1

302.7 + (1624 5 state) 1927.2

92.9 + (1834 1 state) 1927.0
1226.9 + 821.9 2048.8 2048.8
1220.9 + 828.1 2049.0
1024.6 + 910.2 + 113.8 2048.6

424.3 + (1624.5 state) 2048.8

214.6 + (1834.1 state) 2048.7
2060.4 + 113.8 2174.2 2174.3
1322.4 + 851.9 2174.3

340.4 + (1834.1 state) 2174.5
1374.7 + 821.9 2196.6 2196.7
1344.8 + 851.9 2196.7

673.5 + 701.3 + 821.9 2196.7

572.1 + (1624.5 state) 2196.6

362.6 + (1834.1 state) 2196.7

147.9 + (2048.8 state) 2196.7

 

aAll energies in keV.

83

were enhanced by roughly two orders of magnitude over their inten-
sities in prompt coincidence experiments (cf. Table 4), and the

ratio of each intensity to that of, say, the 547.7—keV gamma is
within 320% of what it was in the singles Spectra. Not only were

the direct transitions from these levels to the 821.9—keV state
enhanced, but so were many interconnecting transitions. [At gamma

ray energies below 300 keV, quantitative comparisons of the de-

layed gamma ray intensities with the decay scheme were significantly
less precise because the earlier crossovers of the lower-energy pulses
artificially introduced enhancement factors of >2 into the delayed

139

coincidence spectrum.] fflua energies of these seven Pr states, at

1369.6, 1523.2, 1624.5, 1834.1, 1927.1, 2048.8, 2196.7 keV, were

assigned from the weighted energy sums listed in Table 6.

3.2.4.B. --The 828.l—, 851.9e, 1024.0-, and 2174.3-keV States—-

 

These four states are suggested by energy sums and relative gamma ray
intensities (cf.Table 3), as well as by the prompt coincidence data
(Tables 4 and 5). The absence from Figure 22 of all ten of the gamma
rays indicated in the decay scheme to feed the 828.1-, 851.9—, and
1024.0—keV states is consistent with the interpretation of these states'
positions.

The 828.1-keV state is also confirmed by the suppression of
796.6-, 828.1-, 1006.1-, and 1220.9-keV gamma rays in the prompt coin-
cidence eXperiments gated on the 113.8-keV gamma; see Figure 11 and

Table 4.

3.2.4.C. -—Remaining_0amma Rays-- The twelve very weak gamma

 

 

84

rays observed at 733, 895.1, 900.3, 1165.8, 1233.7, 1249.9, 1364.8,
1463.6, 1470.2, 1681, 2085.0, and 2201.2 keV have not been def-
initely placed in the level scheme. These gamma rays do not fit
between any existing states and do not significantly change the in—
terpretation of the level scheme or its comparison with other nuclei
or with theoretical calculations. The sum of these gamma ray inten-
sities amounts to only 1.5% of the observed Nd139m gamma ray intensity.
Some tentatively suggested placements follow.

The 733-keV gamma ray was seen only in coincidence with the
738.1—keV gamma. The energies of these two sum to 1471.2 keV, within
the measured uncertainty of the 1470.2-keV gamma, thus tentatively
suggesting a level at 1584.0 keV.

Evidence involving poor statistics indicates that the 2085.0-
keV gamma ray is in coincidence with the 113.8-keV gamma, whereas the
2201.2-keV gamma is not. On these grounds alone tentative states at
2198.8 and 2201.2 keV may be inferred.

In order to obtain a lower limit for the log ft values of
transitions to these "unplaced" states, it was assumed that each un—
placed state was fed directly by e—decay and de-excites entirely by
the unplaced gamma rays. It then followed that the corresponding
log f% values would all be larger than about 7.8.

The prOperties of a few of the remaining five very weak
gamma rays (1165.8, 1233.7, 1249.9, 1463.6, and 1681 keV)can be
seen in Table 4. It was not possible to find a unique location for
them in the decay scheme. The sum of their intensity is less than

0.7% of the total observed Nd139m gamma rays.

85

3.2.4.D. --Comparison with Another;(8,y) Study—- After the

 

completion of the present study, a publication became available which
describes an investigation of Nd139m decay following the Nd1”2(y,3n)
Nd139m reaction produced by bremsstrahlung on Nd203 enriched to 95%
Ndll+2 (119). The decay of Nd1399 was not reported and a competing
Nd1”2(y,4n) reaction produced 5.2—h Nd138m which was distinguished
from 5.5—h Nd139m Only by coincidence measurements. A number of
confirmations are obtained from reference 119 but no additions or
corrections to the present study are required. No interpretation

was made of the prOperties of the states.

3.2.5. Spin and Parity Assignments from Ndl39m Decay

 

3.2.5.A. --Electron Data and Multipolarities-- The gamma

 

intensities from the present measurements were compared with the
conversion-electron intensity data of Gromov, et al. (56,70) in
order to gain multipolarity information about some of the more in-
tense, lower-energy transitions following Nd139m decay. These com-
parisons and predicted multipolarities are listed in Table 7 and
plotted in Figure 25. It was not possible to use the conclusions
of Gromov, et a1. directly because their gamma intensities were
obtained with NaI(T1) scintillators and differ markedly from our
data. However, from K/L conversion intensity ratios, the 231.2—
and 708.1—keV transitions have been established to be M4 and.M2,I
respectively, by both sets of previous workers (56,60). The
theoretical conversion coefficients (71) for these transitions
were then used as a basis for determining the remaining coeffi-

cients. At lower energies ($300 keV) it appeared that the coeffi-

86

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K CONVERSION COEFFICIENT

 

87

 

 

 

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Fig. 25.

200 500 IOOO

TRANSITION ENERGY (keV)

A comparison of experimental and theoretical K—conversion
coefficients for some of the y-transitions following Nd139m
decay. The lines are the theoretical values of Sliv and
Band (71). The data points were obtained by comparing the
electron intensities of Gromov, et al. (56,70) with the
y—ray intensities measured in the present study, assuming
the theoretical values of the 231.2«keV (M4) and 708.1—

keV (M2) transitions to be correct for purposes of normal—

ization. Each circle (triangle) refers to reference 56 (70).

88

cients were affected by absorption in the electron counter window.

3.2.5.B. -—Ground and Metastable States of Nd139—- Here at
N=79 one ought to consider three-quasiparticle (hole) states, but
to a reasonable first approximation the low-lying ones can be thought
of as single hole states, so there are some similarities with the N=8l
nuclides. Among the latter there are now seven known that have d3/2
ground states and h11/2 isomeric states connected by M4 isomeric
transitions (72). Also, the N=79 nuclei Te131, Xe133, Ba135, and
Ce137 (references 14 and 73,74,75, and 76, respectively) have 3/2+
ground states and 11/2- metastable states. Thus, when K-L conver—
sion electron energy differences suggested that the 231.2-keV tran—
sition occurs in Nd rather than in Pr and the conversion line in-
tensity confirmed that the transition was an M4, this indicated a

similar d3/2-h isomer pair. The energies of the N=79 and N=81

11/2
isomers have been plotted in Figure 26, including Nd139m and a
projection for Smlhlm (77).

It is instructive to compare the reduced transition probabil-
ity of the 231.2—keV gamma with those of the other Mm gamma rays,
for these isomeric transitions should be among the best examples of
true single-particle transitions. In Figure 26 the squares of the
radial matrix elements, LMIZ, of these transitions are also plotted.

These were calculated using Moszkowski's approximations for single-

neutron transitions (78,79):

(a in 10‘13cm)2[’—1

T(ML) _ 0.19(L+1) IMIZ Mm 2L+1
197MeV

sp ‘ [(2L+1)!!]2 "7"—

x 5(j1’L’jf) x 1021 sec-1.

Y ENERGY (keV)

IMI2

8
o

§§§

L”

J)

Fig.

89

 

 

 

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MI 143 I45
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A= I3I I33 I35 I37 I39 |4I I43 I45

MASS NUMBER

26. Upper: Energies of the metastable states in the N=79
and N=81 isotones. (The SmMl point is a predicted
one.)

Lower: Values of the squared radial matrix elements
for the isomeric transitions in the same nuclei.

 

90

(ML)
T SP

for M4's) is the multipolarity, a (=1.2x10-13 cm) is the effec—

Here is the single—particle transition probability, L (=4
tive nuclear radius, and S(ji,L,jf) is a statistical factor
(i.e., angular momentum portion of the matrix element), which for
ll/2+3/2 transitions has the value 15/11.

The resulting values obtained are consistently smaller

than the approximation of a constant wave function,

|M|2 = [#1201 L)?- = 14.6
n+2 n ’

where “n is the magnetic moment of the neutron, but this fact
should not be of concern, for M4 transitions are normally retarded
over such estimates and one needs much more detailed information
about the nuclear wave functions in order to make detailed com—
parisons meaningful. What is of more importance is the fact that
the values of IMIZ are not constant but show a definite trend in
both the N=79 and N=81 nuclei. [It is unusual for IMI2 not to
be constant over such a series. For example, in the odd—mass
neutron-deficient lead isotopes, IMI2 was constant to the point
that an apparent 15% discrepancy at Pb203 suggested that an unob-
served transition was competing with the mm isomeric transition,
and this competing transition was later discovered (80).]

Both because collective modes of the core would not be ex-
pected to contribute appreciably to an M4 multipole field and be—
cause the hll/z states cannot be mixed readily with other states in

these nuclei, these M4 transitions should prove a more sensitive

admixtures in the d states

test of, say, d 1/2 3/2

5/2 and perhaps 3

than would normally be possible from electromagnetic transition

91

rates. The fact that the IM[2 values for the N=79 nuclei are
consistently larger than those for the N=8l nuclei goes along with
this, for the N=79 three—quasiparticle states would be expected to
be much less pure, and only to a (good) first approximation can

)2

the transitions be characterized as proceeding from a pure [(d3/2
h11/2]11/2_to a pure [dB/2)3]3/2+ configuration. A more complete

analysis of these.M4 transitions, using the occupation number

formalism, is presently underway (72).

3.2.5.0. —-The Ground, 113.85,_and 821.9—keV States in Pr139--

 

The ground state of Pr139 is fed by the d ground state of Nd139.

3/2
In the present study of this decay (see below, sections 3.2.6 and
3.2.7) a log ft of 5.1 was obtained for this transition, which sug—
gests l/2+, 3/2+ or 5/2+ for the ground state of Pr139. Any of these
assignments could be consistent with the observed (16) 99% of Pr
B-decay (log ft = 5.3) to the 3/2 ground state of Ce139. The simple
shell model, predictions by Kisslinger and Sorensen (81), and sys-
tematics of odd-mass nuclei with odd proton numbers between 51 and
63indicate5/2+ and 7/2+ configurations for the two lowest levels of
Pr139. Sixteen nuclei in this region have ground state and first-
excited states well characterized (6), and in every case the
assignments are 5/2+ and 7/2+ or 7/2+ and 5/2+.

The measured K and L conversion electron intensities for the
113.8-keV transition and its 2.5-ns half-life (82) are characteristic

of the i-forbiddenIMl transitions between 97/2 and dS/z states. Six-

teen of these also have been measured (6) in odd-proton nuclei between

92

Z=51 and Z=63.

No direct B-pOpulation of the 113.8-keV state was observed
from either the 3/2+ or ll/2- states of Nd139. This is consistent
with a 7/2+ assignment for this state. The upper limit of 3% e—decay
to it from Nd139m places a lower limit for the log ff at 7.6, although
the log ft is expected to be appreciably higher: The e—decay from

some of the h Te and Sn isomers to g7/2 states in their daughter

11/2
nuclei has been observed (6,9—12,14), and the log ft's cluster around
9. For an estimated log ft = 9 for decay to the 113.8-keV state in
Pr139, the corresponding e-decay is only 0.1%.

The above cumulative evidence rather strongly suggests 5/2+
and 7/2+ assignments for the ground and first-excited states in Pr139,
which would imply d5/2 and (g7/2)‘1(d5/2)2 configurations.

The measured GK (Table 7) of the 708.1-keV transition in-
dicated it to be an M2, and this, combined with the evidence of direct
feeding of the 821.9—keV state by Nd139m suggests this state to be
11/2-. The measured 40—ns t; is consistent with this assignment. The
amounts of admixing in the gamma transitions were not determined, but
WeisskOpf single-particle estimates (23) for the t%'s of a 708.1—keV
pure.M2 and an 821.9-keV pure E3 are 1.2><10-9 and 1.3><10—6 sec, re-

Spectively. Thus, the M2 (partial t = 42 ns) appears to be retarded

%
over the single-particle estimate; this is not particularly sur-
prising, however, as M2's are customarily retarded. More interesting,
the E3 (Partial t1 = 600 ns) appears to be enhanced over the single-
particle estimate, and E3's also are most often retarded (83).

However, there are three other known enhanced E3's, in La137,

 

 

94..

93

Eulk7, andAEu149 (references 84,85, and 85 respectively), all just

below or above the N=82 shell. More will be said about this in
section 3.2.9 below in terms of possible octupole admixtures in
the 821.9—keV state, but the dominant characteristics of this

state warrant the assignment h The log ft of 7.0 for the

11/2'
e-population of this state is high but certainly within the realm
of possibilities for an 11/2— -+'1l/2- allowed transition. It will
be seen later (section 3.2.9) that the reason for this is that a

multiparticle rearrangement is necessary in order for Nd139m to

pOpulate this state.

3.2.5.D. —-The 828.1e,_851.9fi,_and 1024.0—keV States-- The

 

828.1-keV gamma appears to be of E2 and/or ME multipolarity, which
sets limits of 1/2+ through 9/2+ on the 828.1-keV state. This

state is fed strongly by the 1624.5-, 1834.1—, and 2048.8—keV states,
each of which is populated directly by 11/2— Nd139m, so 1/2+, 3/2+,
and possibly 5/2+ can probably be eliminated. If the state were
9/2+, one might expect some direct e—feeding (first forbidden);

none was seen, but the limits are not too precise on this. This is
mentioned in anticipation of the problems that will arise concern-
ing some of the higher-lying states. A 9/2+ assignment would also
suggest that the 828.1—keV transition be pure E2, but again the

precision in a does not allow one to say concretely whether this

K
transition does or does not contain some Ml character. An upper
limit of 0.27% (of Nd139m disintegrations) can be placed on the in-

tensity of the missing 714.3-keV gamma ray to the 7/2+ 113.8-keV

94

gamma ray to the 7/2+ 113.8-keV state. The absence of this trans-
ition is slightly surprising, considering either a 7/2+ or 9/2+
assignment, but, for example, a core-coupled configuration involv—

ing the d ground state could result in either but would explain

5/2
the absence of such a transition. Both 7/2+ and 9/2+ are retained
as possible assignments.

Using the same approach with the 851.9-keV state, 9/2+ was
obtained as the probable assignment, with 7/2+ as a somewhat less
likely alternate. Again, the 738.1—keV gamma appears to be M1
and/or E2, which sets limits of 3/2+ through ll/2+ for the state.
This state is fed strongly by the 1834.1—, 1927.1—, 2174.3-, and
2196.7—keV states, each of which is pOpulated directly by what
looks like an allowed transition. In particular, the intense 982.2-
keV gamma from the 1834.1—keV state —- the state with the strong—
est claim to being a high-spin (9/2, 11/2) odd-parity state —- is
characterized as an El. This permits one to narrow the assignments
down to 7/2+, 9/2+, and 11/2+. ll/2+ can be ruled out on the basis
of the branching ratio of the 738.1— and 851.9-keV gamma rays, for
it would force the 851.9—keV gamma to be an M3, which has a pre-
dicted (single-particle estimate) t% of 1.0><10"6 sec, as compared
with only 2.7><10"11 sec for a 738.1—keV E2. The branching ratio
would also favor 9/2+ (M1,E2 vs pure E2) over 7/2+ (both M1,E2),
but, as pointed out in connection with the 828.1-keV state, one
has to know more about the internal structures of such states be-
fore other than gross decisions based on branching ratios can be

made.

95

On the basis of the 910.2—keV gamma to the 7/2+ 113.8-keV
state, a gamma ray that is at least five times as intense as the
unobserved 1024.0-keV ground—state gamma ray, one can probably
limit the spins of the 1024.0-keV state to a range of 2 units on
either side of 7/2. Because the state competes favorably for feed-
ing from the 1624.5-, 1834.1-, and 2048.8-keV states, which again
are fed directly by 11/2- Nd139m, this range is biased toward the
high-spin side of 7/2. Finally, the lack of direct e-pOpulation
(upper limit :0.4%) suggests even parity. Conclusion: (5/2+),

7/2+, 9/2+, or ll/2+ for this state.

3.2.5.B. --The "High Odd—Parity States"-- The most intrigu-

 

ing aspect of this study is the pOpulation of (at least) six high-
lying states in Pr139 by what appear to be allowed transitions from
11/2- Nd139m. These six states, at 1624.5, 1834.1, 1927.1, 2048.8,
2174.3, and 2196.7 keV, are pOpulated by e-decay with log ft's that
range from 5.6 to 6.3. This would seem to imply that these states
have Spins of 9/2, 11/2, or 13/2, all with odd parity. Granted
that log ft values by themselves are not always reliable indicators
of the degree of forbiddeness in B-decay, still it is much more
common for allowed transitions to be abnormally slow than for
first-forbidden transitions to be abnormally rapid (86). Also,

the decay to the presumed h 821.9—keV state should be, super-

11/2
ficially at least, the most straightforward of the B—transitions
from Ndl39m, and it has a log ft of 7.0. Thus, these six high-

lying states are favored for receiving pOpulation over the hll/Z

state, and from this point of view the e—decay to them is undoubt—

edly allowed. There are other indications, as well (to be described

96

later), that they have odd parity. These six states also have
other peculiarities, among which are the large number of low-energy
inter-connecting gamma transitions and the lack of transitions to
the low-lying states. In this section these states will be dis-
cussed somewhat phenomenologically, arriving only at estimates of
the simplest external structures (i.e., spins and parities) con—
sistent with our data, and the problems of detailed internal
structure will be postponed to section 3.2.9 where it will be shown
that they are three-quasiparticle states.

Now, although the foregoing conclusions based on gamma ray
branching ratios from the lower-lying states may have been overly
conservative, the sheer number of competing gamma rays from these
"high odd-parity" states makes it worthwhile to determine at least
whether or not useful information can be obtained by analyzing
their various branchings. Therefore, the single-particle estimates
for the half-lives of all the gamma rays originating from these
states have been assembled in Table 8, assuming possible E1, M0, or
E2 multipolarities. .M2 and higher multipolarities were excluded on
the basis of there being no likely mechanisms for enhancing them
to the point that they could complete with the many possibilities
for de-excitation by lower multipolarities. This tabulated infor—
mation must be used with caution, however, for the El's and Ml's
could easily be retarded, as noted before, whereas the E2's could
be either enhanced or not enhanced, depending on the collective
or non-collective nature of the states involved. Also, because

some common internal structure is expected among these states,

97

 

 

 

 

Table 8. Weisskopf single—particle estimates for gamma rays
depOpulating the "high odd-parity state" in Pr139.
State y-ray Relative Single—particle estimateb
energy energy intensity8 for t% (sec)
(keV) (keV) (1%)
(corrected for conversion)
E1 M1 E2
1624.5 101.3 6.5 1.1(—13) l.6(-ll) 3.8(-07)
254.9 20 9.5(-15) l.4(—12) 5.1(-09)
601 6.5 7.8(-l6) l.1(-13 7.6(—ll)
796.6 61 3.4(-l6) 4.9(—l4) 1.9(-ll)
802.4 3100 3.3(-l6) 4.8(—l4) l.8(-ll)
1834.1 209.7 9.0 l.6(—l4) 2.3(-12) l.3(—08)
810.1 23 3.2(-16) 4.7(-14) 1.7(—11)
982.2 2100 1.8(—l6) 2.6(-14) 6.6(—12)
1006.1 12 l.7(-16) 2.4(-14) 5.8(—12)
1011.9 10 l.7(-16) 2.4(-14) 5.7(—12)
1927.1 92.9 89 8.2(—14) l.2(—ll) 3.3(—07)
302.7 14 5.8(-15) 8 4(—l3) 2.2(—O9)
403.9 86 2.5(—15) 3 7(-13) 5.4(—10)
1075.1 5100 l.4(-l6) 2 0(-14) 4.2(—12)
1105.2 77 l.3(—l6) l 8(—l4) 3.6(-12)
2048.8 214.6 33 l.5(-l4) 2.2(-12) 1.l(-08)
424.3 39 2.2(-15) 3.2(-13) 4.3(-10)
1024.6 72 l.6(—16) 2.3(-13) 5.3(—12)
1220.9 5100 9.5(—17) l.4(-l4) 2.2(-12)
1226.9 83 9.3(—l7) l.3(-l4) 2.2(-12)
2174.3 340.4 18 4.2(-15) 6 0(-l3) l.3(—O9)
1322.4 46 7.4(—l7) l l(-l4) l.5(-12)
2060.4 3100 2.0(-l7) 2 8(-15) l.6(-l3)
2196.7 147.9 57 3.7(-14) 5 4(-12) 5.9(-08)
362.6 3100 3.5(-15) 5 0(-l3) 9.2(—10)
572.1 26 9.l(-l6) 1.3(—13) 9.7(-ll)
673.5 39 5.6(-16) 8 0(-l4) 4.3(-11)
1344.8 22 7.l(-l7) l 0(-l4) l.4(-12)
1374.7 30 6.6(-l7) 9 5(—15) l.2(-12)

 

aThe strongest y-ray from each level is arbitrarily given a relative
intensity of 100% and the others are compared with this.

bReferences 78 and 79 as treated in reference 23.

98

differences between M1 and E2 transition rates may not be pre-
dictable; therefore, the most useful information will be expected
to come from comparing transitions that lead to states not in the
group of six.

The "prototype” state at 1834.1 keV receives 37.4% of the
e—decay, with log ft = 5.6, and the argument for its being 9/2-,
11/2-, or 13/2— is clearly stronger than for any of the other
states. 0f the five gamma rays that de—excite it, the intense
982.2—keV gamma to the 851.9-keV state seems rather unambiguously
to be an El (Table 7, Figure 25). This is additional evidence for
odd parity, as 9/2+ or possibly 7/2+ was previously assigned to the
851.9-keV state. The 9/2+ assignment would imply either 9/2— or
11/2- for the 1834.1-keV state, while the 7/2+ assignment would
limit it to 9/2-.

At this point only the consistency of the other gamma rays
can be checked with these assignments. The 1006.1-keV gamma to
the (7/2+, 9/2+) 828.1—keV state presumably is a parity-changing
transition like the 982.2-keV gamma whereas the 1011.9-keV gamma
to the 11/2- 821.9-keV state is not. The simplest explanation is
for the 1011.9-keV gamma to be M1 and the 1006.1-keV gamma to be E1.
The pronounced difference in the rates of the 982.2- and 1006.1—
keV "El" gamma rays must be attributed to internal structures of
the states. It will be seen later that there are strong impli-
cations that the transitions out of the "high odd-parity" mul-
tiplet are rather highly hindered, so small admixtures in the states

involved could have strong effects on the transition rates. The

99

9/2- and 11/2— assignments remain for the 1834.1—keV state, where
the latter spin is recalled to be incompatible with 7/2+ for the
828.1-keV state.

The relatively intense 810.1-keV gamma ray would also appear
to be an El transition, allowing the 5/2+ possibility to be removed
for the 1024.0-keV state. The M1 and/or E2 assignment (Table 7,
Figure 25) for the 209.7-keV gamma adds nothing new, but it is noted
that the 1624.5-keV state must be quite similar to the 1834.1-keV
state for this transition to be so enhanced.

Arguments for the 1927.1-keV state, which receives 12.8% of
the e-population, follow along similar lines. In particular, the
92.9-keV transition must be a collectively-enhanced Ml and/or E2,
making the 1927.1- and 1834.1-keV states quite similar in origin.
The 1105.2-keV gamma to the 821.9-keV state may be M1, and the
1075.1-keV gamma to the 851.9-keV state may be El, all of which is
consistent with 9/2— or 11/2- (equally probable) for the 1927.1-keV
state. An educated guess for the 403.9-keV gamma is El, which would
imply positive parity for the 1523.2—keV state.

The 2196.7-keV state also appears to be very closely related
to the 1834.1-keV state, viz., by the strong 362.6-keV gamma. Argu—
ments parallel those above, resulting in 9/2- or ll/2— as possible
choices.

The 1624.5— and 2048.8-keV states are similar in that both
favor depOpulating to the 828.1- rather than the 851.9—keV state.

In each case what would appear to be an M1 transition to the 11/2-

821.9-keV state competes most favorably with an apparent E1 to the

100

828.1—keV state. Arguments for odd parity are also weakest for
these two states (log ft = 6.3 for s-decay to each), but the de-
excitation pattern would be no easier to interpret if high-Spin,
even—parity states were assumed. Thus, 9/2(-) or 11/2(-) were
tentatively chosen as possible assignments. It is perhaps worth
noting that, if these assignments are correct and the six "high
odd-parity" states are indeed closely related, there seems to be

an interesting gradation in prOperties, with the 1834.1—keV state
standing toward the middle, being the only state directly connected
to all the others by gamma transitions. One example of this grada-
tion is the strong transition between the 1927.1- and 1834.1-keV
states, between the 1834.1- and 1624.5-keV states, and (less
strong) between the 1624.5- and 1369.6—keV states -- this contrasts
with the weak transition between the 1927.1- and 1624.5-keV states
and the absence of a transition (upper limit :0.2% of the parent
disintegrations) between the 1927.1- and 1369.6—keV states. [This
sort of behavior ought to aid in sorting the states when shell-model
calculations are done on the three-quasiparticle configuration pro-
posed here for these states.]

The 2174.3-keV state stands somewhat apart from the other
five in that it is the only one to de—excite directly to the lowest
states in Pr139 and to miss pOpulating several of the other five
with quite intense gamma rays. Its large e-pOpulation (log ft = 5.9)
does, however, indicate 9/2-, 11/2—, or 13/2-. And its 2060.4-keV
gamma to the 7/2+ 113.8-keV state, 1322.4-keV gamma to the (9/2+,

7/2+) 851.9-keV state, and lack of a transition (1352.4-keV gamma

 

101

intensity 50.3% of all parent decays) to the 11/2- 821.9—keV state
favor the 9/2- assignment. The presence of the 2060.4-keV gamma ray
also implies, if it is a three—quasiparticle state, that this state

includes some fig character in its composition, being thus less

7/2

"pure" than the other five.

3.2.5.F. -—The Remaininngtates-- The only remaining states

 

in Pr139 excited by Nd139m e-decay that were known with enough assur-
ance to be placed in the decay scheme are the 1369.6- and 1523.2—keV
states. The 1369.6-keV state receives 1.3% of the e-decay, with log
ft = 7.3. Thus, one cannot decide between allowed and first-forbid—
den non—unique decay, and the assignment can be 9/2i, ll/Zi, or 13/2i.
Even less can be said about the 1523.2—keV state, which receives no
direct population from Nd139m. On the basis of the strength of the
101.3—keV gamma from the 1624.5-keV state, a weak argument can be

made for spins between 7/2 and 13/2 with perhaps even parity.

3.2.6. Experimental Results for Nd1399

 

3.2.6.A. --Gamma Ray Singles Spectra—- A gamma ray singles

 

Spectrum of Nd1399+m taken with the 7-cm3 Ge(Li) detector described
in section 3.2.3.A is Shown in Figures 27a and 27b. This spectrum
represents the sum of six runs taken z30 min after the end of ”45—
sec proton bombardments. The duration of each of these runs was :20
min. Spectra were recorded periodically as the sources aged in order
to identify activities with different half-lives and to follow the
Nd1399 as it reached equilibrium with Nd139m. Most of the gamma ray

intensity, even this soon after the bombardments, originates from

102

 

 

 

 

 

 

 

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detector - high energy portion.

Fig. 27b.

104

Nd139m decay, for some 88% of Nd1399 beta decay proceeds directly
to the ground state of Pr139.

A list of the energies and relative intensities of the
gamma rays identified with the decay of Nd1399 is given in Table 9.
These were measured as described in section 3.2.3.A except that the
now well-determined Nd139m gamma ray energies were used as internal
calibration standards. Of the 21 gamma rays listed in Table 9, only
the 405.0—keV gamma has been reported previously (61).

A basic cause of experimental difficulties encountered in
the study of Nd1399 decay is that the annihilation photons are an
order of magnitude more intense than any of the gamma rays follow—
ing its decay. This means that even the low activity of 5.5—h
Nd139m produced by the 45—sec bombardments Significantly masks the
30-min Nd1399 gamma rays shown in Figures 27a and 27b.

As mentioned briefly in section 3.2.2, an attempt was made
to populate Nd1399 selectively apart from Nd139m by using the Pr“+1
(T,5n)Pm139E:;gNd1399 reaction. It was expected that the ground
state of Pm139 would be a 5/2+ state and would pOpulate 3/2+ Nd1399
in preference to 11/2- Nd139m, thus producing a cleaner Spectrum.
The attempt was a partial success because the Nd1399/Nd139m isomer
ratio was indeed increased by an order of magnitude. However, the
presence of many other short- and long-lived contaminants from
competing reactions nullified any net advantage of this method for
producing clean Nd1399 sources. One would need to use this re—

action in conjunction with a rapid ion-exchange separation (not yet

feasible but perhaps available within a few years) for it to be

105

Table 9. Energies and relative intensities of gamma rays

observed in Nd1399 spectra.

 

 

Measured y-ray Relative y-ray Measured y-ray Relative y—ray

energy (keV) intensitya energy (keV) intensitya
113,3:o,2 10,1:10b 1074.5:O.5 11.9: 1
134,4io,4 4,2: 0.4 1096.7:1.0 0.9: 0.4
405.0i0.4 36.4i 3.0c 1214.5:o.4 2.2: 0.3
475.5i0.4 7.9i 0.6 1247.0i 1.0 0.6i 0.3
485,9io,8 2.8i 0,7 1311.8i0.6 2.0: 0.7
511.0 (7:) 360 isod.e 1328.4:0.6 1.1: 0.3
589.0i0.5 5.3i 0.6 1405.5i0.7 3.3: 0.5
622,3iO,3 6.4i 1.0 1449.5i0.7 0.8: 0.3
669,3i0,5 8.3i 2 1464.1:0.5 2.3: 0.4
916.8i0.4 8.5i 0.6 1500.8i0.8 2.0i 0.5
923.0i0.4 6.9i 0.8 1531.2il.0 1.li 0.4

 

 

8Relative to 100 for the intensity of the 738.1-keV y—ray in Nd139m :30
min after the end of #45 sec proton bombardments.

bBased on the sum of y-intensity feeding the 113.8-keV level as in-
dicated in the decay scheme (Figure 24) because most of the 113.8—keV
y-intensity originates from population by the 5.5-h Nd139m, even
30 min after Nd139m+9 is produced.

cResult after the 403.9-keV component of the 403.9-, 405.0—keV doublet
is subtracted out on the basis of the Nd139m relative intensities
(see Table 3).

dApproximately 98% of the annihilation hotons come from Nd1399 decay
=30 min after the production of Nd139m39.

eFrom the decay scheme an upper limit of 16.1 can be placed on the
intensity of a hypothetical 511.8—keV y-ray depopulating the 916.8-
keV state.

106

really clean. It did, however, verify the relative intensities

of most of the Nd1399 gamma rays.

3.2.6.B. --Gamma Gamma Coincidence Studies-— by analogy

 

with the decay scheme of d3/2 Nd“I1 (seen in Figure 8), it was ex—
pected that a number of states would be present, which, upon re-
ceiving direct B-population, would de-excite directly to the Pr139
ground state. For this reason the 8-in. X 8—in. NaI(Tl) Split
annulus (19) and a 3-in. X 3-in. NaI(Tl) detector was used in an
anti-coincidence experiment with the 7-cm3 Ge(Li) detector; the
geometry was as described in section 3.2.3.B. Again, the single-
channel analyzer for the NaI(Tl) detectors was set so that the gate
would be active for all gamma rays above 100 keV. The resulting
anti-coincidence spectrum is shown in Figure 28, and the resulting
intensities of the Nd1399 gamma rays (relative to 100 for the 738.1—
keV Nd139m gamma ray) are listed in Table 10. Seven states in Pr139
were indicated by these results.

In order to complement the anti—coincidence data, a coin-
cidence spectrum was obtained using the same apparatus. The gate
from the NaI(Tl) detectors was Open for gamma rays above 350 keV.
This "integral" coincidence Spectrum is shown in Figure 29A, and
the relative intensities derived from it are also included in
Table 10. As expected, they verify the results of the anti—coin—
cidence data.

The high intensity of the 405.0—keV gamma suggests the

139

presence of a state in Pr at this energy. Four energy sums also

107

 

Nd'39°‘"‘ ANTI-COMPTON SPECTRUM

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108

 

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III III III m.eV e.oV e.oV e.o o.NeNN
III III N N NV m.e N.N m.eNNN
III III III N N.o a.oV m.o N.G¢¢N
III mNV NNV NNV NV om a.NN m.eNON
SNG em m eNN sm.o o.N a.e o.NNa
III NV eV a.N N.o sN.m m.w N.eNa
sNe em eV NN e.N N.N N.N N.aee
III NV NV NV N.o N.N e.e N.NNe
III III mV N.aV N.e e.N N.m o.mwm
III III III III III III ANN.SNV NN.NNmV
NNN NN eon NNN eom oNN com so.NNm
seN sw.N NV so.e SN.N m.o N.N a.mme
III sm.e NN N.m sN.N N.o m.N m.mNe
es smm NNV NN saeV N.mV e.em e.moe
sNN sw.e III III sw.m N.oV N.e e.eNN
NNNV oeNV III III III eNV N.oN N.NNN
mosmwaocwoo moaowwoawoo
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SIN GIN >msIN.NNN SssImoe NssmmssN IsNostss ssstst »
NN .NNs NN .NNN om .NNN SN .NNN SN .NNN NN .NNN NN .NNe N

 

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mmmNuz mo mwumw mnu :N muouona mo mwaufimcoucw m>wumamm .oa Nanny

 

 

 

 

sN seemssst mN SNNNNstmoe mNsN
zufimamucfi cm nuNB m.» >oxlm.HHm mo manuxfiabm aw usn .maouonm cowumaanfianm mo vmmoaaoo haomumqn

109

mzu

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110

 

 

 

I I v T r

Nd'39°*"‘ OOINCIDENCE SPECTRA
g AIINTEGRAI. GATE >350kev ‘
a ssooINCIoENcE WITH 405de

 

 

 

 

 

02" 8 £8
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. .I a

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lo I.
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250 500 750 I000
CHANNEL NUMBER

Fig. 29. égec§§13qg+m integral coincidence spectrum. This

um was recorded by a 7—cm‘ Ge(Li) detector with
the 8—in. x 8—in. NaI(T1) split annulus set to accept

all y-rays above 350 keV.

B) The annulus gate was set on the 405—keV energy
region.

lll

indicate possible gamma ray cascades involving this transition.

To obtain evidence supporting these cascades, the NaI(Tl) annulus
detector was gated on the 380—430-keV region and the coincident
Spectrum seen by the 7—cm3 Ge(Li) detector was displayed. The re-
solving time (21) of the coincidence circuit was 2100 ns. This
spectrum is shown in Figure 29B, and the relative intensities of
the Nd1399 gamma rays are included in Table 10, where the ones that
are thought to be in coincidence with the 405.0—keV gamma are so
indicated.

The same coincidence spectrometer was then gated on the
113.8-keV gamma. The measured relative intensities from the spec-
trum seen in Figure 30 are also listed in Table 10. This experi-
ment verified the energy-sum indication that the 113.8-keV gamma
is in cascade with the 475.5- and 1214.5—keV gamma rays.

Confirmations of several of the coincidences described
above and new evidence for a 405.0-184.4-keV cascade were obtained
with a 3-in. x 3-in. NaI(Tl), 7-cm3 Ge(Li) two-parameter (mega-
channel) Spectrometer employing dual 4096—channel ADC's. These
data are summarized in Table 10. Following each coincident event,
the channel numbers representing the photon energies were stored
in a dedicated buffer in the SDS Sigma 7 computer. When the buffer
filled, its contents were written on magnetic tape. It was then
possible to recover the coincidence information in slices in order
to construct useful spectra. In Figure 31A, the integral Nd1399+m
coincidence spectrum obtained on the Ge(Li) side is shown, and in

Figure 31B, the results of gating on the 405-keV region of the

112

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.mSHBGGO madam AHHvaz .cHIm x .cwlw onu mm3 Houomumv mumw OSH
.> >waw.MHH mnu nuwa mocmvaoaNou SN mhmul> E+mmvaz wo asuuommm .Om .

Egg.

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113

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114

NaI(Tl) side and displaying the resulting Ge(Li) spectrum are

shown.

3.2.7. Nd1399 Decay_Scheme

 

The decay scheme for Nd1399 that was deduced from the
measurements was presented in Figure 24 for comparison with the
Nd139m decay scheme. Again, all transition energies and excited
state energies are given in keV and the B+/s ratios are calculated
values (47). All of the (total) transition intensities are given
in percent of the Nd1399 disintegrations.

None of the ten excited states prOposed here has been re-
ported previously in published Nd1399 decay-scheme studies. The
only one of these states for which there is evidence of pOpulation
from Nd139m decay (i.e., B-decay) is the 113.8-keV state. The 113.8-
keV gamma was seen to have a 30-min decay component in addition to
its dominant 5.5-h component. It was also observed to be in cascade
with the 475.5- and 1214.5—keV gamma rays accompanying Nd1399 decay.

It was mentioned earlier that the high intensity of the
405.0—keV gamma indicates the probability of a state in Pr139 at
405.0 keV. This placement was confirmed by coincidences of four
gamma rays (five, if a tentative 511.8-keV gamma is included) with
the 405.0-keV gamma. In the process of constructing the decay
scheme, it is assumed that the imbalance of gamma ray intensities
leaving and entering the 405.0-keV state is removed entirely by B-

feeding of this state. However, the possibility that a 511.8-keV

transition from a level at 916.8 keV to this state is present

115

(52.5%) but obscured by the intense annihilation photons cannot
be ruled out;

Higher-lying states at 589.2, 1074.4, 1328.2, and 1501.2
keV are suggested by energy sums and relative photon intensities
and confirmed by coincidence and anti-coincidence information. The
states at 916.8, 1311.8, 1405.5, and 1449.5 keV were placed on the
basis of the enhancement (reduction) of the 916.8-, 1311.8-, 1405.5-,
and 1449.5—keV gammas in anti-coincidence (coincidence) experiments
as seen in Figures 28-31 and Table 10.

The Qs = 2800 keV is a calculated value (69), which ought
to be good to within several hundred keV. There have been several
attempts to measure the 8+ end points, but at this time their pre—
cision is not particularly good. Several measurements of the (total)
annihilation photon relative intensity component due to Nd1399 were
used in order to calculate the 88% B-branching to the ground state.
In the Nd139m+9-->Pr139--+Ce139 decay chain, Nd1399 accounts for 298%
of the annihilation photon intensity at Z30—min after the 45-sec
bombardments.

Four unplaced gamma rays identified with Nd1399 decay were
observed with energies (relative intensities) of 622.3 (6.4), 1247.0
(0.6), 1464.1 (2.3), and 1531.2 keV (1.1). The sum of these inten-
sities yields 8.3% of the observed Nd1399 gamma ray intensity and
1.5% of the observed Nd1399 total disintegrations. Some properties
of these rays can be deduced from Tables 9 and 10. _The relatively
strong 622.3-keV gamma perhaps suggests placing a state at 622.3 keV,

but in view of the lack of any supporting evidence, it is omitted

116

from the decay scheme. The log ft for pOpulation of such a state

would be 26.8.

3.2.8. Spin and Parity Assignments from Nd1399 Decay

 

Spin and parity assignments to the lowest two levels have
been discussed in section 3.2.5.0 in connection with the decay of
Nd139m. The 63% e, 27% 8+ decay to the ground state is quite con-
sistent with a nd5/2—+vd3/2 transition, and the log ft = 5.1 is
remarkably close to that found for the analogous transition in Nd1H1
decay shown in Figure 8 (log ft = 5.3).

It is difficult to set a precise upper limit on direct B-
decay to the 7/2+ 113.8-keV state because of the intense 113.8-keV
gamma ray component from Nd139m decay. An upper limit of 0.03% of
the parent disintegrations, with log ft >8.8, was placed on the
analogous and much cleaner Ndlu19—+Pr1“19 B-transition. For Nd1399
decay, of course, such precision is out of the question, but the
fact that no indication of direct B-population is seen is clearly
consistent with a (d5/2)2(g7/2)—1 configuration, as discussed be-
fore.

The remaining nine levels all are populated by B+/e-decay
from 3/2+ Nd1399 with log ft's ranging from 5.6 to 7.2. These all
fit quite nicely in the range expected for allowed decay, and, al-
though one cannot rule out first—forbidden decay on the basis of
these alone, no indication is seen that any states other than the
l/2+, 3/2+ or 5/2+ states are pOpulated directly by Nd1399. In

fact, all the log ft values are slightly smaller than those listed

117

in Figure 8 for the analogous transitions in Nd“+1 decay. For some
of the states, especially those exhibiting gamma ray branching, the
assignments may be narrowed further:

The states at 405.0 and 916.8 keV are tentatively assigned
l/2+ or 3/2+ because they both decay to the 5/2+ ground state and
miss the 7/2+ 113.8-keV state. The 916.8-keV state may or may not
decay also to the 405.0-keV state via the unobserved 511.8-keV tran-
sition, which just might have appreciable intensity, but this fact
is more concerned with the internal structure of (both) states than
with their spin and parity -- although the presence of the transi-
tion would lend further support to the assignments prOposed here.
Solely on the prediction of the shell model that the 31/2 state

ought to lie between the h state and the d5/2 and g7/2 states,

11/2
one is tempted to identify the 405.0-keV state with it. There is
no supporting evidence, however, and one must ask why the 81/2
state should be pOpulated so easily here when it has not been seen
in either Nd”1 decay (see section 3.1) or Ce“+3 decay (87) to the
next heavier Pr isotopes, which otherwise show much the same single-
particle state positions (within a few hundred keV). It will be
seen that gamma ray branchings to this state tend to support the
3/2+ rather than the l/2+ assignment.

Next the states that decay through the 7/2+ 113.8-keV state
are considered, namely those at 589.2- and 1328.2—keV. The mere pre—

sence of the 475.5- and 1328.4-keV gammas rules out the l/2+ possi—

bility for these states. Both assignments can be tentatively nar-

118

rowed down to 5/2+ with the aid of the gamma ray branchings.

For the 589.2—keV state, a 3/2+ assignment would lead to
single-particle estimates (23) of the relative intensities of the
589.0/475.5/184.4-keV gammas (Ml/EZ/Ml, with possible E2 admixing
in the Ml's) of 1/0.005/0.04. A 5/2+ assignment (all Ml's would
lead to roughly l/0.5/0.06. Although considerable E2 enhancement
is to be expected (because of the softness of this nucleus to
vibrations) and Ml's might be somewhat retarded, the latter ratio
is clearly preferable when compared with the eXperimental ratio,
l/l.5/0.75. The 405.0- and 589.2-keV states may well be core-

coupled states involving the d ground state. That they lie

5/2
so low is not too surprising, for Pr139 (two neutrons fewer than

82) (87), which, being again somewhat soft to vibrational excita-
tions, appears to have core-coupled states at this same energy.

A 5/2+ assignment for the 589.2-keV state would exclude a l/2+
assignment for the 405.0—keV state.

Quite similar reasoning holds for the 1328.2-keV state, ex—
cept that it lies high enough that one can deduce little about its
internal makeup. The corresponding single-particle predictions
for the relative intensities of the 1328.4/1214.5/923.0-keV gammas
are 1/0.005/0.3 and l/0.8/0.3 for 3/2+ and 5/2+ assignments, re-
spectively. Although neither can be called a satisfactory fit (ex-
perimental ratios are l/l.5/5.5), the latter is in the ball park.

Considering the obvious enhancement of the gamma rays to the 405.0-

keV state from both the 1328.2— and 589.2-keV states, one is tempted

119

to look for the 739.0—keV gamma between the latter two. Unfortu—
nately, it could be as intense as 0.7% and have escaped detection
because of the presence of the intense 738.1-keV gamma from Nd139m
decay.

The states at 1074.4 and 1501.2 keV are tentatively assigned
l/2+ or 3/2+ because they hit the ground state but miss the 7/2+
113.8-keV state in their depOpulation. This is indeed tentative,
however, and one must know more about the internal structure of these
states before definite assignments can be made. It would be quite
possible, for example, to postulate a hypothetical 5/2+ state con-
sisting of a dS/Z quasiparticle coupled to a 2+ phonon excitation
that would clearly pOpulate the ground state to the exclusion of the
113.8—keV state.

The remaining states, at 1311.8, 1405.5, and 1449.5 keV,
which were placed on the basis of their ground-state transitions
alone, might have their assignments narrowed down to 3/2+ or 5/2+;
however, the pOpulation is quite weak for all three, with even par-

ity even being somewhat in doubt, so they are left as 1/2, 3/2, 5/2(+).

3.2.9. Discussion

 

A total of at least twenty-three states in Pr139, practically
none of which had been reported before, were observed from the com-
bined decays of Nd139m and Nd1399. These states apparently can be
classified in three quite distinct categories: 1) single-quasipar—
ticle states, 2) single-quasiparticle states coupled to various vi—
brational configurations, and 3) three-quasiparticle states. The

conclusions drawn can be most definite about the states in the first

120

category and, because of an unusual feature in the B—decay prOper-
ties of Nd139m, the third category. As this is an experimental re-
port, the discussions that follow will remain empirical, but some
directions are proposed, both experimental and theoretical, that
might be taken for further clarification of the prOperties of this

most interesting nucleus.

3.2.9.A. -—Single—Particle States—— Again, the term "single-

 

particle" states is used here to label those states with primarily
single-quasiparticle amplitudes in their wave functions. These range
from the more or less pure states near the ground to highly fraction—
ated and complicated states at higher energies, and when these states
are spoken of in simple shell—model terms, this is not to imply that
they are really pure shell-model states.

On the neutron—deficient side of N=82 in the lanthanide re-
gion, practically nothing has been done in the way of even quali—
tative calculations of the positions of nuclear states -- even the
pairing-plus—quadrupole force calculations of Kisslinger and Sorensen
(81) give out at Ndlul. This means that empirical data must be used
for the most part, although the large number of states excited in Pr139
in this study makes this more practicable than usual. Thus, in Fig-
ures 32 and 33 respectively, the known states in the light odd-mass Pr
isotOpes and in the odd-mass N=79 isotones are plotted. Here the
nuclides are beginning to get far enough from B-stability that no

scattering reactions have been performed to excite states, so the

number of states recorded is very much a function of 08.

121

 

 

 

 

 

 

 

 

 

LOW LEVELS IngPr: ISOTOPES
2200 ._ (WELL/2') _
(eff—JV?)
(9151/21
2000 *— -—
(Will/2"
(9/2’ um
l800 -— ‘— —
(9/2',l|/2') "’ .
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311 W
(ll2’,3/2°)-—
A '400 swans/23:: «mags/2') _
% 9’2""2("3’3,’Z; L2? (vats/2251?)
é (I/2.,3/Z',5/2‘) (5’20)
>_ l200 — 3,2. (veg/235M) ‘
8:) k? “(l/2412‘) /I l/2' (vans/2712')
,9/2‘.u/2 - ,I
% '000 ("33./2V “Q's—’2” 7
Lu (sag/2%
3/2*
500 -— (1.?) _
5_/2_‘
(3/2‘ 5/2’)
400 L -—'— . —
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m1-———— 12”", m\\\ 5’2+
o __ <2a‘»_____-§12:__-__s_/2:/><fz _.
N 7 BLQ £2 84
A I31 I39 |4l I43
Fig. 32. Experimental levels in odd—mass Pr isotopes,

demonstrating the effects of changing neutron
number on the positions of the states. Unam—
biguously related states are connected by the
dashed lines. References: Pr137, ref.70;
Pr139, this section of this study; Prlkl, sec—
tion 3.1 and Chapter 4 of this study and refs.
88, 89, 90; and Pr193, ref. 87.

 

LOW LEVELS IN N=80 ISOTONES

 

 

 

 

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0 \\ //’
O P- 1Z2...___\ ‘/ iz___(§£2:) —
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A I35 I37 |39 |4|

 

Fig. 33. Experimental levels in odd—mass N=80 isotones,
demonstrating the effects of changing neutron num—
ber on the positions of the states. Unambiguously
related states are connected by the dashed lines.
References: C5135, refs. 91 and 92; La137, sec—
tion 3.3 of this study; Pr139, this section of this
study; and Pm1”1, refs. 61 and 77.

123

Evidences of all the available single-proton states between
Z=50 and Z=82 are probably seen in Pr139. The most clearcut single-
quasiparticle states are those at 0, 113.8, and 821.9 keV. The

ground state undoubtedly consists primarily of a single d proton

5/2
outside a closed 97/2 subshell, and the 113.8—keV state simply pro-
motes a 97/2 proton, resulting in a (d5/2)2(g7/2)‘1 configuration.
As mentioned previously, the retarded Ml transition between them is
characteristic of the £—forbidden.M1's between 97/2 and d5,2 states
in a wide variety of nuclei in this region. The relatively small
Spacing between the first two states is consistent with trends in

both proton and neutron numbers in neighboring nuclei, for the

7/2+ and 5/2+ states cross over between Pr11+1 and Prll+3 and also

137 139.

between La and Pr

The 821.9—keV state shows evidence of being a single h11/2
proton outside the closed 97/2 subshell. As mentioned in section
3.2.5.C, the.M2 transition from this state to the 113.8-keV state
is retarded, while the E3 to the ground state is enhanced over
single—particle estimates. Van Hise, Chilosi, and Stone (84)
suggest that a similarly enhanced E3 transition in La137 could be
eXplained in terms of a coupling of the dS/2 proton to a 3— oc-
tupole vibration, resulting in a nearby 11/2- state that could be
mixed into this state. Superficially, one might ask why it is
not also possible to admix a similar 11/2- state, this time based
on the g7/2 proton, into this state, thereby enhancing the M2

as well as the E3 transition. As it turns out, one cannot really

test either hypothesis, for the positions of possible octupole

124

states are unknown. With the above shell—model assignments, how-
ever, the E3 is the better single-particle transition, involving
principally the de-excitation only of a proton from the h11/2 to
the d5/2 orbit. The M2, conversely, involves breaking the 97/2
subshell in addition, so its retardation is suggested by these
simple arguments.

The positions of the d and 8 states are not so

3/2 1/2

clear, but they are probably fragmented and contribute to several
states above 1 MeV. The state at 405.0 keV (and at 589.2 keV, for
that matter, if the spin assignments prOposed here are incorrect)
is not likely to be either of these single-particle states. In the

141

more rigid Pr , other than the g7/2 and d5/ states, there are

2

no single—particle states below 1114 keV that were pOpulated either
by Ndlul decay (described in section 3.1) or by (T,d) (88) or (d,T)
(89) scattering. 0f the number of levels just above 1 MeV that

were possible contenders, it was not possible to identify specific

levels with either the d3,2 or 81/2 states because of uncertainties

related to the vibrational character of that nucleus. Pr139 is

much easier to deform than Prlul, and thus many more low-lying

states are expected, but there is no reason to expect either the

d or 8 states to drOp drastically in energy, so they may

3/2 1/2

be partly associated with a number of the higher levels.

3.2.9.B. --Three-Quasiparticle States-- In section 3.2.5.E

 

arguments were presented to the effect that the six states at 1624.5

1834.1, 1927.1, 2048.8, 2174.3, and 2196.7 keV appear to be high-

125

spin, odd—parity (9/2— or ll/2-) states. The only straightfor—
ward explanation that has been found to eXplain their enhanced
e-p0pulation relative to the 821.9-keV state plus the many low-
energy gamma transitions among them and the lack of direct tran-
sitions down to the ground or 113.8-keV states is that these six
states are part of a three-quasiparticle multiplet having the con—

figuration (nd5/2)(vd )'1(vh )‘1. The particle transitions

3/2 11/2

postulated here are outlined in Figure 34.

In the extreme single—particle approximation, eoNdlggg
can be represented as three d3/2 neutron holes in the N=82 shell
(i.e., a single neutron in the d3/2 orbit) and eight 97/2 (closed

subshell) and two d protons above Z=50. Due to the isomeric

5/2
prOperties discussed in section 3.2.5.E, Nd139m ought to differ

only in the promotion of an h neutron into the d3/ level,

11/2 2

resulting in eleven h11/2 and two d3/2 neutrons. The only change
involved in the decay of Nd1399 to the ground state of Pr139 is

the conversion of a d5/2 proton into a d3/2 neutron. This accounts

for the low log ft value of 5.1 for this transition.
The analogous transition from Nd139m, i.e., nd5/2—»vd3/2,

however, results in the three-particle configuration (“dB/2)

(vd )’1(vh

)‘1. Hence, the apparent abnormally large pOpu-

3/2 11/2

lation to these states is in fact the expected mode of decay. The
821.9-keV 11/2-state, on the other hand, should have the config-
uration (nh11/2)(vd3/2)2, so decay to it would require converting

one d proton into an h neutron, either in one step or perhaps

5/2 11/2

126

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127

through an intermediate d3/2 neutron state, and a simultaneous

promotion of the remaining d5/2 proton to the h state. The

11/2
resulting relatively large log ft of 7.0 is thus not unexpected.
Although the above interpretation qualitatively explains
most of the gamma ray branchings between members of the negative-
parity multiplet, there are several places involving very highly
hindered transitions where it runs into difficulties. This is
taken to mean that small admixtures in the states are very im—
portant in determining these transition rates. However, it is
instructive to consider specifically one of the more extreme ex—
amples -- the 1011.9-keV gamma (2.9%) from the 1834.1—keV state
to the ll/2- 821.9-keV state as compared with the unobserved (<0.5%)
1834.1-keV gamma to the 5/2+ ground state. With an 11/2- assign—
ment for the 1834.1-keV state one would not expect to see the 1834.1-
keV gamma, but with a 9/2- assignment the arguments are not so clear.
Single-particle estimates (23) for the t%'s of the 1011.9- (M1) and
1834.1-keV (M2 or E3) gammas are 2.4XIO’1” and 8><10"12 or 4><lO"9 sec,
respectively. According to the above description, the missing M2
or E3 would involve an apparently very simple vd3/2—+vh11/2 tran—
sition (the M2 would be i-forbidden); there may also be some hin—
drance from uncoupling and recoupling the states. 0n the other
hand, the observed 1011.9-keV Ml gamma requires the simultaneous

changes vd3/2—+vh and “dS/z—fimhlllz’ each of which is doubly

11/2

fi—forbidden. However, l-forbiddeness loses much of its meaning

in multiparticle transitions and would depend on the relative

128

phases of the transforming states; also core polarization in
multiparticle states tends to obscure the 2 selection rules (93).
However, multiparticle gamma decay is formally absolutely forbidden,
and, although there are known cases where such transitions take
place at enhanced rates (e.g., the 63-keV El gamma in Bk250 fol-

25“ o-decay (94)),these are not common. When such invol-

lowing Es
ved rearrangements are compared, the single-particle estimates lose
all meaning, and minute admixtures could easily be the deciding
factors.

In this multiplet of three-quasiparticle states there are
two different and potentially very rewarding sources of information:
1) The enhanced transitions between the various members of the mul-
tiplet. These should give information about the gross features of
the states and should allow one to perform calculations on states
at several MeV that normally can be done only near the ground
state. 2) The very retarded transitions to states not in the
multiplet. These should allow one to determine some of the ad-
mixtures in the states and also something more about the structures
of many of the lower-lying states.

Relatively few three-quasiparticle states are known. The
mechanism prOposed here for pOpulating three-quasiparticle multiplets
has rather stringent requirements which are listed in section 4.1

together with a discussion of what nuclides may satisfy these re-

quirements.

129

3.2.9.C. —-Vibrational States——the Remaining States—- The

 

"quasiparticle" was used advisedly in the previous section,

term
for the simple shell model becomes less and less of a good approxi—
mation for states at these energies. Thus, the states and the
transition rates will need to be calculated from the occupation-
number approach. When this is done, it is expected that much more
information will also be forthcoming about the remaining states in
Pr139, most of which are probably core-coupled vibrational states.
At this time it would be especially interesting to know more about
the nature of the 828.1- and 851.9-keV states, which receive con-
siderable pOpulation from the three-quasiparticle states. Assuming
that our interpretation of the latter is correct, the 828.1- and

851.9-keV states would seem to be constructed from the d ground

5/2
state coupled to a 2+ quadrupole vibration. That they receive so
much population could be explained partly by their receiving it in
default of other states being available (cf. the gamma ray branch-
ing discussion in the previous section) and perhaps partly by the
fact that the three-quasiparticle states undoubtedly contain vi-
brational admixtures. After all, from one vieWpoint three-quasi-

particle states and single-quasiparticle—plus-core states are only

extreme examples of the same thing.

3.2.9.D. --Shell Model Calculations-- Calculations are

 

presently being performed by Dr. R. Muthukrishnan using the shell
model to predict the state energies in Pr139. The preliminary cal-
culations look very promising, especially for the negative parity

States .

130

3.3. Decay Schemes of Bal33m,

Ba131m, Ce137m, and Ce1379

 

The study reported here consists of three parts which are
introduced and described separately. Each of the activities was
produced by bombardments of the respective targets with protons
from the Michigan State University variable-energy sector-focused

cyclotron.

3.3.1. Instrumentation

 

The apparatus used for counting included a 7-cm3 Ge(Li)
detector, room-temperature FET preamplifiers, low-noise RC linear
amplifiers with pole-zero compensation, and 1024 or 4096 channel
pulse-height analyzers.

Gamma ray energy measurements were accomplished by count-
ing "unknown" radioactive sources together with judiciously chosen
calibration sources entered in Table 11. A background correction
was made for each peak by fitting a linear equation to several
channels above and below the peak and then subtracting. A least-
squares fit of the peak energies and centroids was made to a
quadratic curve from which energies corresponding to the centroids
of the unknown peaks were determined. The energies of weak gamma
rays, which would be obscured by the calibration standards, were
determined by using the previously determined stronger gamma rays
as internal standards.

The energies assigned are mean values taken from a number

of different measurements recorded at different times and with

 

 

 

 

 

 

131

 

 

Table 11. Gamma rays used as energy standards.
Nuclide Gamma Ray Energy (keV) Reference
Amzul 59.543:0.015 a
0657 121.97 10.05 a
0657 136.33 10.04 3
0e139 165.84 10.03 a
T1208 238.61 :0.01 a
Cs137 661.595:0.976 b
11208 583.139i0.023 C
CO60 ll73.226i0.040 c
C060 1332.483:0.046 c
Naz“ 1368.526i0.044 c
11208 (0.3.) 1592.46 10.10 c

 

3Reference 39.
bReference 40.

cReference 44.

 

 

 

 

 

 

132

different system gains. The uncertainties in energies are based
on the reproducibilities both of the standard energies and the
"unknown" energies from the calibration curves, the sizes of the
full-energy peaks above the background, and the quoted errors
of the standard energies listed in Table 11.

The relative gamma ray intensities are also averages from
a number of runs and were obtained using experimentally determined
efficiency curves (cf. section 3.1). Associated with these in-
tensities are statistical uncertainties that include estimated un-

certainties in the underlying backgrounds.

3.3.2. Experimental Results for 38.9-h Ba133m

 

3.3.2.A. --Introduction—- The decay of the ground state of

 

Ba133 has been well characterized by many authors (see reference
95 and op. cit. therein). No evidence for direct feeding of high
spin states of C5133 by electron capture of Ba133m was reported in
a recent study (96) of this metastable state. Ba133m has 775-keV
available for electron capture (96-98) and has a 38.9 hour half—
1ife (99). It would seem to be a rather sensitive probe of any
low-lying high-spin states which might be present in C3133. In

this study evidence has been found for such a transition.

3.3.2.B. --Source Preparation-- The Ba133m activity was

 

produced by bombardment of natural CsN03 (CF-grade) with protons.
Bombardments were carried out for approximately 30-min with 0.4-
uA of l4—MeV protons. Chemical separations were performed.

SrClz, RbCl, and Ba012 carriers were added to an aqueous solution

133

of CsNO3 and then chilled. Chilled red fuming HNO3 was then added
to co-precipitate Sr and Ba. The precipitate was then redissolved
in dilute HCl and, following this, HCl gas was bubbled through the
chilled solution to precipitate Ba alone. Amongthe contaminant

activities isolated from the Ba133m acticity by the chemical sep-
aration were Sr85m (70—min, 233-keV) (6), Sr87m (2.8-h, 387—keV)

(6), and C5132 (6.5—d, 667.65- and 629.8-keV) (101). The remain-

ing transitions of these contaminants were too weak to be seen.

3.3.2.C. ——Gamma-Ray Spectrum—- A Ba133m+9 singles gamma

 

ray Spectrum is shown in Figure 35. This Spectrum was taken after
chemical separations were performed as described earlier. The com-
peting gamma rays were easily identified by their well—known ener-
gies, relative intensities, and half-lived (41,95,102). The de-
cay of the 276 keV peak was carefully followed for 8 half-lives of
Ba133m. This information together with a comparison of the 355.99-
keV gamma ray intensity with the branching ratio of the 276.45- and
355.99—keV gamma rays from the 437.0-keV level of C5133 (from ref-
95, I(276.45—keV Y)/I(355.99-keV y) = 0.116) suggest that only

0.1% of the area of the peak at 276-keV seen in Figure 35 belongs

to Ba1339. Hence this peak is actually a doublet. By careful com—
parison of the 276.45:0.08-keV Ba1339 gamma ray (95) and =276-kev
Ba133m gamma ray in alternate calibrations of photopeak centroids it
was possible to determine that the Ba133m gamma ray has 0.36:0.12-keV
less energy than the 276.45:0.08—keV Ba1339 gamma ray (95). There—

fore an energy of 276.09iO.lS—keV is assigned to the isomeric tran—

COUNTS PER CHANNEL

134

 

 

3053”” SINGLES

 

 

 

 

“2
Q
._ {”1 E? 3; as“ _
319’!“ .~ ' ; h 9':
‘5...” .A Ell g
”a? 'th ”-
. _.;;{;‘)¢:ll {1,11% . .§.~ _ \ .
- -.':- v.-'-.-::.:-.'-:-:w-:-
|o|_ ." .1! .'..
I ' ‘ 4 I
250 500 750 IOOO
CHANNEL NUMBER
Fig. 35. B3133m+g singles y—ray Spectrum taken with a 7—cm3

Ge(Li) detector. This run was started 46-h after
the end of a 2—h activation and lasted 4~h. Only
0.1% of the area of the peak at 276—keV belongs to
BalRRQ. The 632.510.5 keV y-ray intensity was mea—
sured to be 0.05500.010% of the 276.09—keV isomeric
transition Y—ray.

135

sition in Ba133m.

The 632.5-keV gamma ray was observed to decay with the
same 38.9-h half-life as did the 276.09-keV isomeric Ba133m transi-
tion. Both decays were carefully followed for four half-lives of
the Ba133m parent. Repeated chemical separations did not change
the relative intensities of these two gamma rays. Therefore, the
632.5-keV gamma ray is indicated to belong to the decay of Ba133m.
The intensity of the new 632.5:0.5-keV gamma ray was measured to be
0.055i0.010% of the 276.09-keV isomeric transition gamma ray inten-
sity. The relatively large uncertainty is due to the long counting
time required to obtain adequate statistics for the weak phot0peak
and the small gain shifts that inevitably occurred during these
times. This long counting time also made coincidence measurements
impractical.

Because of the very high intensity of the 276.09-keV

isomeric transition in Ba133m

and the necessity of a reasonable
counting rate, some chance pileup of these gamma rays occured as

seen in Figure 35. The characteristic shape of the "peak" at :552_
keV and the dependence of its relative intensity and apparent half-
life on the counting rate and absorber employed identified it to be

a chance pileup of pairs of intense 276.09-keV gamma rays. The
possibility that the peak at 632.5-keV is due to a chance sum of the
276.45— and 355.99-keV gamma rays was ruled out because such a pileup

peak is calculated to be more than two orders of magnitude smaller

than the 276 + 276-keV chance pileup peak seen in Figure 35.

136

3.3.2.B. -—Ba133m+9 Decay Scheme and Discussion-- The Ba133g

 

decay scheme has been studied extensively (6,95) and the currently
accepted one is shown in Figure 36. The disintegration energy is
taken from reference 97 and the other energies and gamma ray inten-
sities accompanying Ba1339 decay are taken from reference 95. The
gamma ray energy and intensity values were consistent with these
measurements and verify this decay scheme. Energy level systematics
in the region suggest that the 8l—keV level is the 5/2+ single par-
ticle level (see Chapter IV) but any difference in the character of
the two 5/2+ levels is not strongly suggested by the gamma ray tran-
sition rates (95).

The Ba133m decay scheme seen in Figure 36 includes the two—

transition cascade to the Ba133

ground state proposed by Thun et al.
(96), except that the 276.09i0.15-keV transition energy measured in
the present study replaces the 275.7-keV energy in the earlier decay
scheme (96). Thus, the energy available for electron-capture from
Ba133m is 288.410.2—keV larger than the correSponding energy from
Ba133g. From this information and the 487:2-keV disintegration en—
ergy (97) of Ba133g, it follows that the energy available for Ba133m
electron-capture would support a cascade of the 632.5—keV gamma ray
with any other transition below approximately 142-keV.

Ba133m gamma rays within 1150-

No evidence was observed for
keV of 632.5-keV. From the data, an upper limit of 5% (10%) of the
632.5-keV gamma ray intensity was placed on the intensity of any
higher (lower) energy Ba133m gamma ray within the 632.5:150 keV

interval.

137

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[was soum coxMu mum >momc mmmfimm wcamsmasooom mmfiuuoco was mofiufimaou
Isa mmul> cam no consummmu Eouw ameu ma awumsm sowumuwoucfimav use

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2. 8
8
mm.
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m 9
( 9

138

The two energetically allowed alternative placements of
the 632.5—keV gamma ray appear to be from a possible 713.5-keV level
to the 5/2+ first excited state and from a possible 632.5-keV
level to the 7/2+ ground state. These alternatives suggest very
different gamma ray branching ratios to the ground and first—ex-
cited states. Comparisons with branching ratios in Similar nuclei
strongly suggest that the new gamma ray depOpulates a 632.5-keV
state in C8133. In particular, 19 high spin (9/2 or 11/2) ex—
cited states are found in odd proton (odd A) nuclei reported in
this region (6,10,13-15,103) which gamma decay to both of the 7/2+
and 5/2+ ground and first excited states, respectively. If, for
each of these high spin states, R is defined to be the ratio (in-
tensity of the gamma ray feeding the 7/2... state)/(intensity of
the gamma ray feeding the 5/2+ state), it is found that R>4.9 for
17 of these 19 states. For the other two excited states (both 9/2+),
R = 0.23 and 0.39. R<0.05 (>10) is suggested by the data of the
present study for branching from a 713.5-(632.5-) keV state of C8133
so the latter alternative is prOposed as being more likely.

The 276.09-keV Ba133m isomeric transition K+L+M conversion
coefficient of 4.80:0.30 obtained from measurements by Thun et al.
(96) was used in determining a 0.009:0.003% branching ratio to the
632.5-keV state in C3133. The log ft = 8.0 is comparable with the
other high Spin states in this region populated by ll/2— isomers
(6,10,13-15,103). This log ft suggests that allowed, first-forbid—

den, or unique first-forbidden beta transitions are possible but

 

 

 

CO
81,

In

 

139

systematics and gamma feeding ratios suggested that the latter al-
ternative is quite unlikely.

A study of the gamma rays following inelastic neutron
scattering in C3133 has recently been reported (104) which includes
(n,n'y) cross-section and threshhold evidence for excited states of
03133 at 632.811, 706.211, and 768.411 keV. The 632.8-keV state
was seen there to populate only the 7/2+ ground state. This evi-
dence confirms the prOposed placement of the gamma ray with an
energy which was measured in the present study to be 632.5:0.5 keV.
The upper limit of 5% of the 632-keV gamma ray intensity which has

133m gamma ray places a lower

been placed on any higher energy Ba
limit of 29.0 on the log ft for Ba133m decay to the 706- or 768-

keV state.

3.3.3. Experimental Results for Ba131m

 

3.3.3.A. -—Introduction—- An earlier study of the 14.6 min

 

131 suggests three possible alternatives for the Ba131m—+

isomer of Ba
Ba1319 cascade spin sequence (100). In the same study, a search was
conducted with scintillation detectors for possible missing transi-
tions and directfeeding of high spin states in CS131. Although
none were found, low-lying high—Spin states in Cs133 have been dis—
covered recently with the aid of Ge(Li) detectors as described in
the previous section. In the present investigation, a search was
conducted for possible missing transitions in Ba131m and for direct

electron capture feeding of low-lying high spin states in 03131.

In the process, the Ba131m decay scheme is confirmed which includes

140

more precise energies and shows essential agreement with the ear-

lier' scintillation study (100).

3.3.3.B. --Source Preparation-- The 14.6—min Ba131m activ-

 

ity was produced by proton bombardments of Natural CsN03 (GP-grade).
A 34—MeV proton beam irradiated the targets for S—min durations

with Z-uA of beam current. No chemical separations were performed.

3.3.3.0. --Gamma Ray Spectra-- The decay of the ground

 

state of Ba131 has been well characterized recently (105-108). In
Figure 37 is shown a singles gamma ray spectrum of Ba1319 decay taken
in the course of the present study. The energies are taken from ref-
erences 101 and 108. Data for the singles Spectra were taken with
the gamma ray spectrometer described earlier. The gamma ray energies
and relative intensities were determined as outlined there. These
data confirmed the Ge(Li) data of Karlsson (107) and verified the
identification of the Ba1319 component of the Bal31m+9 spectra.

A Ba131m+g singles gamma spectrum taken =30-min after a
5-min bombardment with the proton beam is shown in Figure 38. From
this, and other similar spectra, the energies of the 78.5:0.2 and
108.010.3-keV gamma rays were determined following Ba131m decay. The
Ba1319 and Cs132 decay energies are taken from references 108 and 101
respectively.l As seen in Table 12, these values are in good agree-
ment with the scintillation results of Horen, Kelly, and Yaffe (100).
The 108.0-keV state in Ba131 has recently been observed following
La131 decay (109) and its energy was measured to be 108.110.5-keV in

agreement with the measurement reported here. An upper limit of

141

 

86'3'9 SINGLES SPECTRUM

276.09(Ba'33"'L

 

 

 

 

 

 

 

 

 

 

 

 

 

g ..
.18 E g 13 31
a LLJVE .978“ § —
§ “8% 11“" ~‘
I 149%
KI! _. 58.3
g Io’- Ig<fifl$ ¢ g ‘*
ff.) 4 g Q g, _
g . "8&1 ? .
'07 “next I -
“~2l ,
IO - ' _. 13’“ n
' 116 586 760 Ioloo

CHANNEL NUMBER

Fig. 37. Ba1319 singles Spectrum taken with a 7-cm3 Ge(Li)
detector. The energies shown here are taken from
references 107 and 108. This Spectrum was one
of several taken to aid in the identification of
the Ba1319 component of the Ba131m+9 spectra.

142

 

 

 

 

 

 

 

1090‘" f} l l I l
5 I0
>2 8
x at!" m SINGLES SPECTRUM
IOSb'r!
I" -
In
:1 2
3 Q
RI 3 a?
d Io‘é i g I _
g ' I 5’ 1|
Ls .
3 ‘VO lo.
31 I0 1- 11:.
VI i!
g 311955;: .
I02- 3
. 8
o. n
a... .I_
I0 — '
l 1 l l J
o 250 500 .750 I000
CHANNEL NUMBER
Fig. 38. Ba131m*9 singles Spectrum taken 530~min after a

5~min bombardment with the proton beam. The en-
ergies of 78.510.2 and 108.010.3 keV are in good
agreement with the scintillation results of Horen,
Kelly, and Yaffe (100).

143

0.1% of the Ba131m disintegrations was placed on the feeding to high
Spin states of C8131 with energies >60-keV, in agreement with an
earlier study (110) which used a differnet method. The same upper
limit applies to other possible transtions in B3131 following the
131m.

decay of Ba

3.3.3.B. --Ba131m Decay Scheme and Discussion-— In Figure

 

39 is the Ba131m decay scheme suggested in reference 100 and con-
firmed by the present measurements. All odd-mass odd—N nuclei with
mass numbers between 113 and 143, having fewer than 82 neutrons, and
directly measured ground state Spins, have l/2+ or 3/2+ ground states.
The shell model also suggests that Ba1319 has a Spin of l/2+ or 3/2+.
From the systematics of differences between low-lying l/2+ and 3/2+
states in odd-mass Ba, Xe, and Te isotopes seen in Figure 40, the
331319 spin is suggested to be l/2+. This trend is in agreement

with the absence of observed beta-decay to the measured 5/2+ ground
state of C3131 from Ba1319.

The multipolarities of the 108.0- and 78.5—keV transitions
respectively were measured to be M1+E2 and E3 respectively by Horen,
Kelly, and Yaffe (100). From these multipolarities, and since no
other transitions have been identified, the 9/2- —+ 3/2+-—+~l/2+ de-
cay sequence shown in Figure 39 is tentatively prOposed. This 9/2—
level can be explained in shell model terms as a projection of three
h11/2 holes as proposed by Horen et al. (100).

Recently Kisslinger (111) has suggested that, in this region,
one of the 9/2— states derived from coupling three quasiparticles in

the ll/2- level may be expected at an energy significantly lower than

144

Table 12. Ba131m gamma ray data.

 

 

 

 

Energy Intensity
(keV) (relative)
Present Study Horen, Present Study Horen,
et al.8 et al.a
78.5i0.2 7815 2.1:0.5 2.4il.2
lO8.0:O.3 10713 2100 5100

 

 

 

8Reference 100.

Fig. 39.

145

 

 

 

l3l
568075

Q€=l I64

Decay scheme of Ba131m suggested by Horen
et al. (100) and confirmed by our measure—
ments. The energies are in keV and the ten—

tative spins proposed are discussed in the
text.

 

 

 

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146

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.on meOUOmfi mu mam .mm .mx .0H .:m mama mmo :H mmumum +N\m mam

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147

the other (single quasiparticle plus phonon) odd-parity levels, so
it "intrudes" among the low-lying states. The 9/2- state at 321—
keV in Te125 has been prOposed (112) as an example. Possibly the
Ba131 isomer may be another example.

A Nilsson-type 9/2- level arising from the oblate equilib-
rium deformation has been predicted by Kumar and Baranger (113) and
suggested by W. G. Winn and D. D. Clark (114) for the Ba131m case.
Similar explanations have been prOposed (114) for the Xe125 and Xe127

isomers. The available data on the 9/2- and 11/2— isomeric states

in other Ba, Xe, and Te isotOpes are not yet sufficient to permit

meaningful extrapolations to Ba131m.

3.3.4. Experimental Results for Ce137m+9

3.3.4.A. --Introduction-- The decays of 9.0—h Ce137g and
34.4—h Ce137m have been examined in some detail in one earlier un—
published study with Ge(Li) detectors (76). The present work was
doen independently and complements the earlier study which added
eight states to the decay schemes. The present work differs from
the earlier study in regard to a few key photon intensities and
log ft values. It is noted that in recent compilation of decay

schemes (6) a different parent for one of these eight new states

is listed.

3.3.4.B. --Source Preparation-- The 9.0—h Ce1379 and 34.4—h
Ck2137m activities were produced by the relatively clean (p,3n) re-

action on 99.99% pure natural La302 with 25-MeV protons. A 0.5—11A

 

 

148

beam was employed for a duration of :90-min. The only competing

reaction products with comparable half-lives were Ba135m (28.7-h,

did not hamper the investigation significantly. No chemical

separations were performed.

3.3.4.C. ——Gamma Ray Spectra-- In Figure 41 is shown the

low energy region of the Ce137m+9 spectrum recorded with the 7—cm3

.-.-Awh‘i
.. - ..- .

Ge(Li) detector. The duration of this counting interval was 15.5

..F—_
ii.
1‘

hours and it was initiated 12 hours after the end of a l-hour bom-
bardment. The 254.3-keV gamma ray seen here is 3300 times as in—
tense as 12 of the 19 other gamma rays measured following the on-
set of transient equilibrium. Thus the weak gamma ray full-energy
peaks can be concealed easily by competing reactions.

In Figure 42 is shown the high energy region of the
Ce137m+9 Spectrum. The run used for this Spectrum also had a 15.5
hour duration but it was initiated 45 hours after the end of a one
hour bombardment, near the onset of transient equilibrium. The en-
ergies and relative intensities of the gamma rays observed in Fig-
ures 41 and 42 are listed in Table 13 along with results of an
earlier study (76). All of the relative gamma intensities reported
here are based on measurements taken while the Ce137m and Ce1379
parents were in transient equilibrium (380 hours after their pro-
ductixui). From the variation of the gamma ray intensities with
time soon after bombardments, it was possible to distinguish the

states pOpulated by the 34.4-h Ce137m and 9.0—h Ce1379 parents.

 

149

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Although energy differences and other considerations clearly

indicate a state in La137 at 10.5-keV fed by :eight gamma rays,

the total conversion coefficient (115) of 130 for the 10.5—keV

transition, the difficulty in measuring such low energy photons

 

or electrons, the low intensity of the other gamma rays, and the

long half-life of the parent decays makes coincidence experiments

with this transition impractical at present.

3.3.4.B. --Ce137m9 Decay Scheme and Discussion-- The de— L”

cay scheme that was deduced from the foregoing measurements is

shown in Figure 43. Transition energies and excited—state energies

are given in keV, the disintegration energy for electron—capture

is a calculated value (69). The B+le ratio for decay to the La137

10.5-keV state is also a calculated value, using the method of

Zweifel (47). Spins and parities of the states are taken from ref-
erence 76 and op. cit. therin. Each assignment is consistent with

the log ft values calculated from the data taken in the present

study.
The decay scheme and conclusions regarding Cel37m+9 decay

and La137 states prOposed here essentially confirm the unpublished

report of Frankel (76). The measurements reported here were ob—

tained independently and the 25% reduction factor due to transi-

ent equilibrium was included in the Cel379 gamma intensities in-

volved in the calculations of the 8+ and EK-feeding and log ft

values. It was also noted that the 1160.4-keV gamma ray was seen

to follow Ce1379 decay rather than (as listed in a recent decay

152

Table 13. Ce137m+9 photon data.

 

_— _..

 

 

Energy Intensity Parenta
(keV) (relative)

Present Study Frankelb Present Studyc Frankeld
10.5i0.4e 10.0 —-— —..._ g
169.3i0.5 168 15 i5 8.3 m
254.3i0.3 255.8 600 i40 202 m
433.3110 433.0 2 1 1 g
436.5i0.5 436.1 16.5 ”10.8 16.2 g
447.1i0.3 446.5 E100 E100 g
479.3i1.0 479.0 0.5 10.2 g
482.7i0.5 481.5 2.0 “$0.4 3.2 g
493.1i0.5 492.5 0.4 10.2 0.6 9

511.0 (annihil) 511.0 (annihil) 51.8f 1.2

698.6i0.3 698.0 1.6 ”$0.4 2.4 g
762.2i0.4 762.1 8.1 i0.8 13.5 m
771.1i0.5 771.1 0.3 i0.1 0.8 g
781.5i0.6 781.5 0.10i0.03 0.32 g
824.9i0.3 825.0 19 i2 33.5 m
835.4i0.4 835.8 4.2 i0.6 7.3 m
916.3i0.3 915.8 3.0 10.3 5.5 g
926.6i0.3 925.8 1.7 i0.4 3.2 g
994.1i0.4 994.1 $0.14 0.12 m
1004.810.3 1004.0 1.1 i0.3 1.70 m
1160.410.6 1160.3 0.08i0.03 0.16 g

 

 

a959.0—11 Ce1379 parent and mE34.4—h Ce137m parent. The assignments
given in reference 76 and these made in the present study agreed

in every case.
bReference 76. Uncertainties listed as within 1 keV.

CAll relative y intensities are measured after transient equilibrium
has been reached (:80 hours after bombardment).

dReference 76.

8From six energy differences. The range of the differences was 0.4
keV.
fDue to the presence of contaminants, we were not able to definitely

identify this minimum value of annihilation photon intensity as en-
tirely belonging to Ce1379 decay.

 

1,.

 

 

153

 

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be e:

 

 

154

scheme compilation (6)), Ce137m decay. Comparisons of the exper-
imental levels with the calculations of Kisslinger and Sorenson (l)
are recorded in reference 76. Fair agreement is seen.

The decay schemes of Ce137m and Ce137g are distinctly dif-
ferent. No evidence of common transitions was observed. The 7/2+
and 5/2+ assignments for the ground and first-excited states of
57La133 would appear to suggest (1197/2)‘1 and (TrdS/Z)(Irg7/2)"2
shell model configurations respectively. The close proximity of
these two levels, separated by only 10.5-keV, is consistent with
the energy level systematics of the Z = 57, N = 80 region for odd-
proton even—neutron nuclei (see Chapter IV). The ground state Spin
assignments of odd—proton even—neutron nuclei for 51fo59 nuclides
are in general 5/2+ for low and 7/2+ for high mass number nuclides
of a given Z. The crossover point is between A = 127 and 129 for
Z = 53, between A = 131 and 133 for Z==55, and between A = 141 and
143 for Z = 59. This trend also suggesmsthat these two states might
be expected nearly to coincide in 57La137.

A recent study (see section 3.2 and reference 115) of
59Pr139 levels pOpulated by 60Nd133m+9 B+/€ decay suggest a resem-

80

blance between the 5 Celggnlg and 60Nd133m'9 decay patterns. The

8
log ft of the dominant 3/2+ Nd1399—+5/2+ Pr1399 transition is 5.1

and for the 3/2+ Ce1379—+5/2+ 10.5-keV La137 state transition, log
f1 = 5.3. Both the (5/2+) 589.2-keV state of Pr139 and the 5/2+,3/2+
447.1-keV state of La139 predominately pOpulate the lowest 7/2+

state in their respective nuclides and are pOpulated by their 3/2+

wh

smoI
Xe If

SEE

sta

stat

 

 

 

155

parents with log ft values of 6.4 These may be corresponding 5/2+
states but the situation is somewhat less clear for the other low
spin states in Pr139 and La137. Eight other low Spin states in Pr139
are populated by Nd1399 B+/€ decay with log ft values ranging 5.9 to
7.2. Five other low spin states in La137 have log ft values rang-
ing from 6.7 to 8.3. The larger log ft values for the Ce1379—+La137
B+/€ transitions may be due to the initial Shell configurations in
the decays. In particular, Nd1399 has (097/2)8(fld5/2)2(vd3/2)
which is more favorable for a (nd5/2)-+(vd3/2) transition than the
probable (ng7/2)8(vd3/2) configuration of Ce1379.
Ce137m decay also resembles Nd139m decay in several ways.
These h11/2_7d3/2 isomeric transitions have "eXperimental" matrix
elements (see section 3.2) differing by <10% and they fit into the
smooth trend of both energies and matrix elements seen in Telgé,
Xelgg, Balgg, Celgg, and Ndlgg. The 99.3% isomeric transitions
seen in Ce137m decay differs considerably with the corresponding
12.7% in Nd139m due to the accessibility of six three-quasiparticle
states (see section 3.2 and reference 115) in Pr139. These latter
states range from 1624.5- to 2196.7-keV in 91139 while only =1450-
keV is available for elctron-capture in the decay of Ce137m.
The ll/2- state at 1004.8—keV in La137 has been reported
to have an enhanced E3 (:8) to the 5/2+ 10.5-keV state (84) and an
0W2,E3) transition to 7/2+ ground state (76). In Pr139 a 4012 nS

11/2— state at 821.9-keV has an E3 enhancement of :2.2 to the 5/2+

ground state (see section 3.2). In Eu1“7 and Eulug, E3 enhance-

 

 

 

ments

state

Staté

ha nce

 

__ ..__._
x

___-.__

______

 

156

ments have also been seen for transitions depOpulating low-lying ll/2—
states (85). It may be suggested that the 713, 962, 1114, and 1439 keV
states of Eulus, Pm1”3, Prlul, and Ce139 (all ll/2—) may also have en—
hanced E3 transitions to their lowest 5/2+ states. In the N = 50-126
region, the four known cases of E3 enhancement (in P1138, Celgg, Eulgz,
and Eulgg) listed in section 3.2 and references 84 and 85 (25 measured

retarded E3 transitions were also listed in this region) all are quite

near the N = 82 shell.

 

 

 

 

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CHAPTER IV

DISCUSSION OF RESULTS AND SYSTEMATICS

Descriptions of characteristics of the states investi-
gated are included in the previous chapter along with some compar-
isons with neighboring nuclei and current nuclear models. One
of the most interesting and significant results of this study was
the observation of the pOpulation by Ndl39m of a multiplet of six
high-lying, high-Spin, odd-parity states in Pr139. These states
are interpreted to be three-quasiparticle states and are discussed
in Chapter III in some detail. Here these studies are related to
other nuclides in the region. This thesis then concludes with a

brief survey of experimental energy level systematics in the neutron

deficient odd mass, odd proton (50fZE62) region.

4.1. Threefiguasiparticle Multiplets in Other Nuclides
Well-characterized three-particle states in nuclei are
comparatively rare, and recognizing them most often has depended
on the isomeric prOperties of a few high-spin states. Consequently,
the excitation of a multiplet of such states in one nucleus, each
state decaying to a number of lower—lying states, has many inter-
esting theoretical implications.

It is worth noting that there are stringent requirements

157

Sta!

figu

even

 

 

 

158

for the mechanism suggested in section 3.2.9.B for pOpulating the
particular three-quasiparticle multiplets that were seen to be pop-
ulated by the electron capture decay of Nd139m. These include a

high-spin parent nucleus, such as the h isomer, and a sufficient

11/2
decay energy to pOpulate states well above the pairing energy gap in
its daughter nucleus. Additionally, the parent nucleus must be un-
able to decay readily by other modes, e.g., an isomric transition,
if present, must be of low—enough energy to allow the B+/e-decay to
compete. Finally, the nucleus must have an intrinsic configuration
that forces the preferred decay path to be into the three-quasipar—
ticle states. Such arrangements would appear to be present only

for B+/E-decay of nuclides with N<82. (Below N=50 the correct con-
figuration occurs at Kr83 and Sr85, but these are too close to B-
stability for populating high-lying states. Below N=126 the con-

211, a region that is not

figuration is projected to occur around Pu
even particle stable.)

Below N=82 the appr0priate configurations can be found only
at N=79 and N=77, with the possibility of N=75, depending on the
states. 0n the neutron-rich

relative spacing of the h and 8

11/2 1/2

Side of N=79 there are some peculiar and complex decays of 11/2—
isomers, e.g., Te131m decays primarily to high-spin states at 1899
and 1980 keV (14). However, these cannot be definitely described
as decay to three-particle states. 0n the neutron—deficient side,
Ce137m has a possible configuration, although it lacks the dS/2
protons, so decay would be forbidden (ng7/2—+vd3/2). However its

QE is small enough to preclude such decay anyway (76). This leaves

 

 

 

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159

Nd139m as the nucleus closest to B-stability with the requisite
prOpertieS. Other possible candidates in this region among cur-

rently known nuclei are Sm1”1(m) and Nd137(m?). Smlul is now

being investigated (77).

4.2. Experimental Energy Level Systematics

in the Odd Proton (Z=50-62) Odd Mass Region

Because of the small amount of experimental information
available concerning the levels of the highly neutron deficient

nuclei in this region, this discussion will be restricted to

3 121—135 129—137 135-139 137-141
2- 53I 68- 82' 55C3 74- 82' 57La 78— 82’ 59Pr 78- 82‘
1
o

:lgg. Of these 31 nuclei, six are stable. These

nuclides and their abundances ( where i 100%) are Sb121 (57.25%),

36123 (42.751), 1127, 05133, La139 (99.9112), and Prlul. All

Pm isotopes are unstable.

4.2.1. Longt Values for 3/2+ Ground State to Lowest 5/2+ State

Transitions.

 

In searching for the systematic variations of the prOper-
ties of the nuclear states of this region, a start could be made
with the beta decay which pOpulates the states of interest. In
section 2.3 of Chapter II a sequence of operations used for cal—
culating log ft values was outlined. The difficulties encountered
in determining relative intensities for B and Y transitions and the
length and complexity of this process could suggest that errors

might easily slip into the literature where calculation is made of

 

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160

the fraction of beta decay which proceeds by the decay mode of
interest. The theory from which log ft values are derived does
not include the detailed nature of the states involved in the
transitions. For these reasons, as the tools of gamma ray Spec-
trosc0py and the understanding of state configurations improve,
the departures from the approximation that log f1 values are
independent of the available energy for electron capture and the
number of protons of the parent may lead to new insights concern-
ing the nature of the states involved.

As pointed out in section 3.3.5.B, there are a large num-

ber of d ground states in the even proton, N=79,81 nuclides.

3/2
The low log ft values for transitions between these states and the
lowest-lying ds/2 states in the adjacent odd proton isobars suggest
that these states may be quite similar in nature.

Figures 44 and 45 have been constructed from references
6—15, and from Chapter III of the present study. Parent and daughter
nuclei are listed along the abscissas in order of increasing Z and
A. The effect of increasing numbers of nucleon pairs on the (vd3/2)—+
(figs/2) and (figs/2)—+(vd3/2) transitions are shown separately. Since
large fractions of the beta decay proceed in the decay modes of interest,
these log ft values might be expected to be somewhat more accurate than
usual. An error of 223% (350%) in fractional beta feeding intensity
contributes an error of z0.1(0.3) in the log f1.

One of the experimental difficulties encountered during this

investigation was that such large fractions of the decays of 3/2+

161

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163

Ndlulg, Nd1399, and Ce1379 proceeded directly to the ground or first
excited state of the daughter of each. These fractions were 98.5%,
88%, and 96.7% respectively of the parent decays. In consequence,
the higher~energy gamma rays were so weak that they could be obscured
or concealed by the more intense gamma rays from contaminant ac-
tivities even though the contaminants themselves were quite weak.o
In Figure 44 the largest log ft values are seen relatively
far from closed shells. The general pattern is relatively smooth
and suggests that lower log ft values may be associated with beta
transitions near closed shells. In Figure 45 a similar pattern
is seen in the 5/2-+3/2+ even proton daughter transitions, possibly
at somewhat lower values of log ft. The relatively smooth pro-
gression of log ft values was interrupted by a value of 4.9 for
1125~+Te125 decay from the compilation of data found in reference 6.
A look at the original references disclosed that this value was
out of date and that the more recently determined value (116) of
5.25 fits nicely into the relatively smooth pattern. An ex-
trapolation of this curve to Pmlul-eNd1H1 decay suggesnsa log ft
of 4.8 for the eK/B+ decay.
Kisslinger and Sorensen (1) suggest that the effect of
pairing correlations on the B—decay matrix elements may account
for these trends. Their calculations for "odd-jumping" beta de—
cays show an increase in the log ft values for A increasing from
117 to 133. ("Odd-jumping" beta transitions are accompanied by
the transformation of an odd p(n) into an odd n(p).) For the

'Navenrjumping" beta decays, a decrease is obtained for A increasing

 

 

par

 

I... IIIIIIIIIJ

164

from 137—141. ("Even—jumping" 8 transitions are accompanied by
an even p(n)-+even n(p) transformation.) These latter log ft
values are predicted to lie lower than the ones for the odd-jumpw
ing cases. In the more complete experimental data Shown in Fig-
ures 44 and 45, the log ft values of both the odd and even-jump—
ing transitions are seen to dr0p as the N=82 shell fills. These

data then seem to suggest that other effects may be important.

4.2.2. Energy Systematics of the Low-Lying 7/2+, 5/2+, 3/2+,

 

and l/2+ States in the Region

 

Figure 46 shows the relative energy spacings of the low—
lying 5/2+ and 7/2+ states of odd proton odd mass nuclei in the
region of interest. The data are taken from references 6,7-15,
117, and from Chapter 3 of the present study.

The parabolic appearancesof the Sb and I curves have been
previously noted by G. Berzins (7) and L. M. Beyer (15). The pat-
tern may be followed in Cs but the number of data points (four)
is too small to yield significant evidence. The first sharp de-
parture from the smooth trend of increased spacing as pairs of
neutrons are added is seen in Prlgg and Prlgg which were discussed
in Chapter III, and in Lalgg and Lalgg. This pattern change may
be related to the coupling of the 59th proton in Pr”1 to the two

82

d3/2 neutron holes present in Prlgg. The effect of the ("dB/2)

(Vd3/2)-2 coupling in Prlgg may be to depress the 7/2+ state and/or

raise the 5/2+ state energy.

165

 

 

 

 

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66 70 74 78 82 86 90
NEUTRON NUMBER

Fig. 46. Systematics of the energy level separations
between low-lying 5/2+ and 7/2+ states in
odd proton (odd mass) nuclei. The Spin
assignment of at least one of the states
connected by each dashed line is tentative.

 

 

pa
0f
(193

m0d(

the

 

 

166

The effect of the closure of the neutron shell appears
to be negligible on these states. This observation suggests that
the lowest lying 7/2+ and 5/2+ states in the odd proton odd mass
nuclei of the region are fairly pure quasiparticle states. Spec-
troscopic factors from a recent study of (He3,d) reactions on the
even Sn isotopes (117), in good agreement with the predictions of
Kisslinger and Sorensen (120), also suggest relatively pure one-
quasiparticle states for the lowest lying 7/2+ and 5/2+ states.

A recent study and compilation (40) of M1 and E2 transi-
tion probabilities in the region of interest includes a comparison
of experimental and theoretical hindrance factors for g7/22d5/2 £-
forbidden M1 and E2 transitions. These are obtained from a combin-
ation of Ml/EZ mixing ratios and mean life measurements. The exper-
imental Ml hindrance factors were noted in reference 40 to range
from :10_100 and the E2 enhancements range from =1-lOO.' Sorensen's
pairing—plus-quadrupole force calculation gives a fair description
of the E2 transition rates but fails to provide a satisfactory
description of the M1 transition rates. Calculations with the shell
model and configuration. mixing have had more sucess in accounting for
the measured M1 transition rates (40,121).

It is also noteworthy that Wildenthal (2) has performed a
shell model calculation with six adjustable parametensfor ten N=82
nuclei. The 7/2+, 5/2+ energy difference trends in the odd proton
nuclides and the energy gaps (energy of the first 2+ states) in the
even—even N=82 nuclides are well described by the model.

In Figure 47 the current eXperimental relative energy Spac-

167

ings of 3/2+ and 7/2+ states of odd proton odd mass nuclei (6-15,
117) are diSplayed. A parabolic pattern is also observed for
iodine as noted by L. M. Beyer (15). The remainder of the data
are too sketchy to be convincing.

The current experimental relative energy Spacing of the
l/2+ and 7/2+ states is shown in Figure 48 (6-15,ll7). Again, def-
inite conclusions cannot be drawn from the available data although
the parabolic pattern persists. Also, shell effects are apparent
in Figures 47 and 48 at N=82 where large energy differences are
seen. These behaviors are very similar to that seen in Figure 49
which givesthe energy gaps of the adjacent even-even nuclei. The
similarity of the behaviors seen in these three figures may suggest
the presence of significant components of core coupling in the wave

functions for the l/2+ and 3/2+ states in these nuclei.

4.2.3. Beta Decay of ll/2- Levels to 7/2+ Low-Lying Daughter States

 

.Seven first forbidden unique beta transitions from the 11/2-
isomeric state of even proton nuclei to the lowest l/2+ states con-
tained in Figures 46-48 have been reported (7-15) in the region of
interest. The log ft values for these seven beta transitions are
7.9, 8.7, 8.9, 9.2, 9.3, 9.6, and 9.9. During the present investi-
gation, an upper limit of 0.1% of the intensity of the 756.5-keV
isomeric gamma ray in Nd11+1 was placed on the pOpulation of the 7/2+
state at 145.4 keV in Prlul. This limit, however, leads to a lower

limit of only =6.5 on the log f% for this transition. Similarily,

 

STATE ENERGY DIFFERENCE

168

 

 

 

l

l

l

 

 

 

_ 11
200 66707478 82 86

NEUTRON NUMBER

Fig. 47.

Systematics of the energy level separations
between low-lying 3/2+ and 7/2+ states in

odd proton (odd mass) nuclei.

The spin

assignment of at least one of the states
connected by each dashed line is tentative.

7/2"

Lc- It‘ll-I-‘ . I

 

 

ll"
I’IIIIII’

 

 

 

STATE ENERGY DIFFERENCE

Ema+ - E7/2‘ (keV)

Fig. 48.

IOOO

169

 

IZOO

BOO

7/2+

 

 

 

 

 

 

66 7o 74 7s 82
NEUTRON NUMBER

Systematics 01 the energy level separations
between low—lying 1/2+ and 7/2+ states in odd
proton (odd mass) nuclei. The spin assignment
of at least one of the states connected by
each dashed line is tentative.

 

ENERGY GAP = E2.-E0. (keV)

§

§

§

Fig. 49.

170

 

 

 

 

 

l I l I T l l
* ENERGY GAP ‘
'— IN EVEN-EVEN _1
*— NUCLEI "‘1
Z ,..§_n\ _
- \-x.-i_ :
L—. —.
" Te / T
.. )F‘jfi-¥f" -+
.... ...4I
_ KM/ 4
h— Eag’lp’Jr/’ -d
T *CLOSED -
- SHELL 4
l l l l l
62 66 70 74 78 82 86
NEUTRON NUMBER
Energy gaps between the lowest lying 2+ and

0+ states in evenreven nuclei in the Z=50-62
region.

 

 

th
St
fo:
val

thI

qui
dec
ene
the
ing

cle

here

in P

SYStI

 

the ‘

 

171

the attempts to detect direct feeding of the lowest lying 7/2+
states in the daughters were also futile in the corresponding cases
for Nd139m, Ce137m, and Ba133m decay. The range for the log ft
values stated above (all in the 8.9:1.0 range) is consistent with
the fact that the corresponding transitions were not detected in

these decay schemes.

4.2.4. Characteristics of Similar 11/2- States in Odd Proton

Odd Mass Nuclei

 

In Pr139, the 40 ns delayed 11/2- state at 821.9-keV was
quite important in the determination of the prOperties of the Nd139m
decay scheme as discussed in section 3.2.3.C. Table 14 lists the
energies, half-lives, and E3 enhancement values (where available) of
the nearby 11/2— states which are suggested here to be correspond-
ing states. The effect of the N=82 closed shell on the energies is
clearly seen but little can be said about the unusual enhancement

of the E3 transitions (ll/2- —+ 5/2+). The pattern of energies seen
here seems to suggest that a corresponding ll/2- state may be found
in Pm“+1 at circa 700 keV. As mentioned in section 4.1, the decay

of $1111!”1 to Pmll+1 is presently being studied as another likely

system in which three-quasiparticle states might be pOpulated.

4.3. General Summary

 

Gamma ray spectrosc0py has been employed to investigate
the behavior of the Ndlu1m+9, Nd139m+9, Ba133m, Bal31m, and Ce137m+9

decay schemes.

 

1.
ESE“ i

 

 

 

172

 

 

Table 14. —-Characteristics of similar 11/2- states
in odd proton odd mass nuclei.
State State E3

Nuclide Energy (keV) tl/z (ns) Enhancement Reference
63Eu132 497 2400 1.4 a
63EuIZZ 625 710 2.1 a
63Eu133 713 b

u
61Pm183 962 b

141
sgPr 82 1114 b

1
59?: 33 822 '40 2.2 c

139
57La 82 1420 b

137
57La 80 1005 $0.41 37.8 C,d

 

aReference 85.

bReference122.

cChapter III of this thesis.

dReference 84.

 

 

WE
ar

in

abc
EXp
vat:
betw
fore
due I
that
Michi

3 mas

 

173

A general overview of the apparatus and methods employed
is given. A multiparameter coincidence system, used for the first
time during the course of this study has been described. An outline
of those aspects of nuclear decay scheme construction which could be
treated in a routine way is presented. This 14-step sequence forms
the basis of a useful computer program called DECAY SCHEME.

The utility of the beta, gamma method of studying prOper—
ties of nuclear states has been illustrated in the present investi—
gation which placed 56 gamma rays in decay schemes containing 22
excited states below 2200-keV in Pr139 alone. Six of these states
were identified as three-quasiparticle states. Reaction studies
are seen to be useful sources of complementary information regard-
ing nuclear state characteristics.

Each of the odd-proton odd—mass nuclides included in the
above discussion of systematics by virture of the availability of
experimental data are very close to stable nuclei. As the obser-
vations described here are extended into the transition region
between the spherical and deformed nuclei, two main problems are
foreseen. The short half-lives of the nuclides and complexity
due to numerous contaminating activities encountered will require
that additional techniques be employed. Plans are underway at
Michigan State to employ in-beam gamma studies in conjunction with

a mass separator in order to achieve this goal.

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I74

 

 

10.
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