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LIBRARY

Michigan State
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This is to certify that the

dissertation entitled

Decay Studies of the
Neutron-Rich Nuclides

1
9N and 20N

presented by

Don William Anthony

has been accepted towards fulﬁllment

of the requirements for

 

July 133 2000

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Ph.D. degreein Chemical Rhysics

 

 

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Decay Studies of the N eutron-Rich Nuclides
19N and 20N

By

Don William Anthony

A DISSERTATION

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

DOCTOR OF PHILOSOPHY

Department of Chemistry

2000

ABSTRACT
DECAY STUDIES OF THE NEUTRON-RICH NUCLIDES '9N AND 20N
By

Don William Anthony

Secondary, radioactive ion beams of 19N and 20N were produced at the National
Superconducting Cyclotron Laboratory at Michigan State University by fragmenting a
primary beam of 22Ne (80 MeV/A) in a beryllium target. The isotopes of interest were
implanted within a plastic scintillator and their decay was monitored. Beta events were
recorded by the implantation scintillator, delayed-neutron emission was observed by the
neutron bar array as well as three 6Li inorganic scintillators, and gamma-ray events
were recorded by two germanium detectors. Quantitative measurements of the
branching ratios for delayed-neutron and gamma ray emission were made for both '9N
and 20N, and the half-lives of the species were deduced.

The 19N data suggests that there are twelve delayed-neutrons emitted during the
decay comprising 50.9(62)% of the total decay strength. The gamma-ray spectroscopy
yields several branches to bound, excited levels of 19O contributing 39.4(38)% of the
total decay strength. There is also evidence of Signiﬁcant delayed-neutron emission to
bound excited states of 18O. The half-life is determined to be 299(19) milliseconds and
the ground state spin and parity is restricted to (1/2- or 3/2-). From the delayed—neutron
data, the resonant capture strength for the l8O(n,’y)'90 reaction has been estimated and
found not to compete well with the direct capture component except at the highest

temperatures possible during the Inhomogeneous Big Bang scenario. The measured

ranching ratios and inferred properties of the populated intermediate States do not
compare well with shell model predictions.

The 20N data suggests eight delayed—neutron branches with energies in excess of
1.1 MeV. Above this threshold, delayed neutron emission accounts for 34.0(40)% of
the total decay strength. The gamma-ray analysis leads to 27.0(46)% of the decay
proceeding through bound excited states of the 200 daughter. There is also evidence
that delayed-neutron. emission to I80 populates excited bound states rather than the
ground state. The half-life has been re-measured to be 121(15) milliseconds and the

ground state spin and parity of 20N is restricted to be low-spin, most likely (1-).

Dedicated to the memory of my grandparents
Bill and Mary Anthony

George and Edna Friess

ACKNOWLEDGEMENTS

I would ﬁrst like to thank my supervisor and friend, Dave Morrissey, for his
guidance and support through this process. Without his good counsel and eleventh-hour
heroics, this experiment could not have been successful. Second, I want to thank Paul
Mantica for permitting me to take part in his experimental program, and for his
continual willingness to help me learn to become an experimentalist.

I also wanted to express my gratitude to the graduate students, past and present,
at the NSCL. I think that I learned more with and from them than any other source. I
look forward to continuing the lifelong friendships that have been forged here over the
last four years.

To my parents, Dave and Marcelle Anthony and my brother and Sister, David
and Dawna, you all must know that your support and love have made it possible for me
to meet with success in my life. I owe you all more than I could express with mere
words.

To my wife, Paola, thank you for everything. There is absolutely no doubt that
these achievements are as much yours as they are mine. I can’t imagine life without
you and I love you more than anything in the world. You have been the most blessed
gift in my life and everything I have done, or will do, is simply because of you.

And ﬁnally, Abigail, you have changed my life. Now I have a sense of scale, of

what is important, and of what “unconditional love” means. I am forever yours.

TABLE OF CONTENTS

LIST OF FIGURES ................................................................................ ix
LIST OF TABLES ............................................................................... xiiv
CHAPTER 1 INTRODUCTION ................................................................. l
1.1 General Introduction .................................................................. 1

1.1.1 Structure ................................................................... 2

1.1.2 Nucleosynthesis .......................................................... 3

1.2 Literature Search - ”N, 20N ......................................................... 5

1.2.1 Prior Measurements of '9N Decay Properties ........................ 6

1.2.2 Prior Measurements of 20N Decay Properties ........................ 9

1.2.3 Literature Search Summary ........................................... 10

1.3 Shell Model Predictions for the Decay of 19N ................................... 12

1.4 Experimental Goals ................................................................. 13
CHAPTER 2 EXPERIMENTAL SETUP ..................................................... 14
2.1 Production of Secondary Radioactive Beams ................................... 14

2.2 Experimental End-Station ......................................................... 15

2.2.1 Implantation System .................................................... 16

2.2.2 Neutron Bar Array ...................................................... 17

2.2.3 6Li Glass Scintillators .................................................. 19

2.2.4 High Purity Germanium Detectors ................................... 19

2.3 Electronics and Data Acquisition ................................................. 20

2.4 Secondary Beam Purity ............................................................. 21

vi

CHAPTER 3 EXPERIMENTAL RESULTS ................................................. 27

3.1 Overview of Analysis Procedure .................................................. 27
3.2 Half-Life Measurement of 19N .................................................... 28
3.3 Half-Life Measurement of 20N .................................................... 29
3.4 Total Number of Events ............................................................ 30
3.4.1 Dead Time Considerations ............................................ 31
3.4.2 Summary ................................................................. 35
3.5 Neutron Bar Array ................................................................... 36
3.5.1 Time Calibration of the Neutron Bar Array ......................... 37
3.5.2 Neutron Bar Array Efﬁciency Calibration .......................... 45
3.5.3 Neutron Bar Results - 19N and 20N ................................... 49
3.5.4 '9N Delayed Neutron Spectrum ....................................... 49
3.5.5 20N Delayed Neutron Spectrum ....................................... 52
3.6 6Li Inorganic Scintillators ........................................................... 54
3.6.1 Efﬁciency Calibration ................................................. 57
3.6.2 19N Results — 6Li Detectors ........................................... 60
3.7 Gamma-Ray Data ................................................................... 63
3.7.1 Gamma-ray Spectra of the Isotopes of Interest ..................... 66
3.7.2 Background Gamma Rays ............................................. 71
3.7.3 Ratio Of Efﬁciencies ................................................... 71
3.7.4 Gamma Ray Segregation .............................................. 72
3.7.5 Gamma-Ray Half-Life Measurements .............................. 73
3.7.6 Gamma Ray Assignments 19N , 20N Secondary Beams ........... 75

vii

3.7.7 Efﬁciency Calibration — Standard Sources .......................... 77

3.7.8 Efﬁciency Calibration - On-Line Sources .......................... 80

3.7.9 Efﬁciency Calibration — Germanium Coincidences ................ 82

3.7.10 Efﬁciency Calibration — Summary ................................... 83

3.7.11 Gamma Ray Intensities -— '9N ......................................... 87

3.7.12 Intensities of Higher Energy l9N Gamma Rays .................... 88

3.7.13 Gamma Ray Intensities — 20N ......................................... 90
CHAPTER 4 INTERPRETATION ............................................................ 93
4.1 Interpretation of 19N Gamma Rays ................................................ 93
4.2 Interpretation of 19N Delayed Neutrons .......................................... 96
4.3 Delayed Neutron Assignments 19N ............................................... 99
4.4 Interpretation of 20N Gamma Rays ............................................... 101

. 4.5 Interpretation of 20N Neutron Spectrum ........................................ 104
4.6 19N Shell Model Considerations ................................................. 106
4.7 Beta Decay of 20N .................................................................. 108
4.8 Astrophysics Results l9N ......................................................... 110

4.8.1 Direct Capture Part of the 18O(n,y)'9O Reaction Rate. . ....1 11

4.8.2 Resonant Capture Part of the 18O(n,y)'90 Reaction Rate........114

4.8.3 Comparison of Direct and Resonant Capture Strengths ......... 116

CHAPTER 5 SUMMARY AND CONCLUSIONS ........................................ 118
5.1 Summary and Conclusions — l9N ................................................ 118

5.2 Summary and Conclusions — 20N ................................................ 121

LIST OF REFERENCES ....................................................................... 122

viii

LIST OF FIGURES

FIGURES IN THIS DISSERTATION ARE PRESENTED IN COLOR
Figure 1 Energy level diagram of the mass 19 system pertinent to the decay of 19N. . ..7

Figure 2 Energy level diagram of the mass 20 system pertinent to the decay of 20N...11

Figure 3 Schematic diagram of the implantation detection system ........................ 17
Figure 4 Schematic diagram of the Neutron Bar Array ..................................... 18
Figure 5 Schematic diagram of the entire detector end-station ............................. 20

Figure 6 Electronics diagrams of the PPAC, Real-Time clock and RF pulsing control
elements of the end-station .............................................................. 22

Figure 7 Electronics diagrams for the neutron bars, 6Li detectors and germanium

detectors ................................................................................... 23
Figure 8 Electronics diagrams for the components of the Implantation system ......... 24
Figure 9 Fit to the half-life spectrum of 19N .................................................. 29
Figure 10 Fit to the half-life spectrum of 20N ................................................ 30
Figure 11 Delayed neutron spectrum of ”’C Showing the gates used to reﬂect back

onto the half-life Spectrum for the dead-time analysis .............................. 33
Figure 12 Best ﬁt curves to the neutron-gated half—life spectra of 16C .................... 34

Figure 13 Fit to the full half-life Spectrum recorded by the implantation scintillator
during the decay of 16C .................................................................. 35

Figure 14 Spectrum collected by one of the array bars from the precision time
calibrator ................................................................................... 38

Figure 15 Delayed-neutron spectrum collected by one of the array bars during the
decay of 16C ............................................................................... 40

Figure 16 Plot of inverse velocity versus channel centroid for the calibration of
the neutron bar array ..................................................................... 44

Figure 17 Aggregate TOF delayed-neutron spectrum recorded by the neutron bar array

during the decay of 16C ................................................................... 47
Figure 18 Aggregate TOF delayed-neutron spectrum recorded by the neutron bar array

during the decay of 17N ................................................................... 47
Figure 19 Calibration curve for the total efﬁciency of the neutron bar array ............ 49

Figure 20 Delayed-neutron TOF Spectrum collected by the neutron bar array
during the decay of 19N ................................................................... 50

Figure 21 Delayed-neutron TOF spectrum collected by the neutron bar array
during the decay of 20N .................................................................. 53

Figure 22 Delayed-neutron TOF spectrum collected by 6Li detector #3 during
the decay of 16C ........................................................................... 55

Figure 23 Delayed-neutron TOF spectrum collected by 6Li detector #3 during
the decay of 17N ........................................................................... 55

Figure 24 Time calibration curve for 6Li detector #3 from the 16C and 17N decays. . ...56
Figure 25 Calibration curve for the total efﬁciency of 6Li detector #3 .................... 58

Figure 26 Parameters necessary to determine the geometric efﬁciency of an
aperature ................................................................................... 59

Figure 27 Energy spectrum of delayed neutrons recorded by 6Li detector #1
during the decay of '9N .................................................................. 61

Figure 28 Energy spectrum of delayed neutrons recorded by 6Li detector #2
during the decay of 19N .................................................................. 61

Figure 29 Energy spectrum of delayed neutrons recorded by (’Li detector #3
during the decay of '9N .................................................................. 62

Figure 30 Gamma-ray spectrum collected in Ge#2 from the 60Co calibration sourc ..... 65

Figure 31 Energy calibration curves for Ge#l and Ge#2 .................................... 65
Figure 32 Gamma-ray spectrum collected by Ge#l during the decay of 16C ............ 67
Figure 33 Gamma-ray spectrum collected by Ge#2 during the decay of 16C ............ 67
Figure 34 Gamma-ray Spectrum collected by Ge#l during the decay of 17N ............ 68

Figure 35 Gamma-ray Spectrum collected by Ge#2 during the decay of '7N ............ 68

Figure 36 Gamma-ray Spectrum collected by Ge#l during the decay of '9N ............ 69
Figure 37 Gamma-ray spectrum collected by Ge#2 during the decay of '9N ............ 69
Figure 38 Gamma-ray spectrum collected by Ge#l during the decay of 20N ............ 70
Figure 39 Gamma-ray spectrum collected by Ge#2 during the decay of 20N ............ 70

Figure 40 Ratio of counts detected in Ge#l to Ge#2 during the beam-off
period of the I9N experimental runs .................................................... 72

Figure 41 Ratio of counts detected by Ge#2 over all times to beam-off during
the decay of 19N ........................................................................... 74

Figure 42 Gamma-ray gated half-life spectra for two transitions observed in Ge#l . . ..75
Figure 43 Gamma-ray spectrum of 60Co collected by the NaI start detector ............. 79

Figure 44 Gamma-ray spectrum collected in Ge#2 gated on the 60Co 1332 keV

line in the NaI start detector ............................................................ 80
Figure 45 Data points taken for the efﬁciency determination of Ge#2 .................... 84
Figure 46 Best ﬁt to the efﬁciency calibration data for Ge#2 .............................. 85

Figure 47 Interpolation and extrapolation error bars for the efﬁciency calibration
of Ge#2 .................................................................................... 86

Figure 48 Partial energy level diagram of '90 showing the bound state transitions
observed in this experiment during the decay of '9N ................................ 94

Figure 49 The beta decay of '9N Showing the transitions that can be ascribed
to 19N on the basis of the experimental data .......................................... 101

Figure 50 Partial energy level diagram of 200 showing the bound state transitions
observed in this experiment during the decay of 20N .............................. 104

Figure 51 The beta decay of 20N Showing the transitions that can be ascribed
to 20N on the basis of the experimental data .......................................... 108

Figure 52 Comparison of the direct capture and resonant capture cross sections
for the 18O(n,y)'9O reaction as derived from this work ............................ 119

xi

Figure 53 The beta decay of 19N Showing the transitions that may be conclusively
identiﬁed from the experimental data ................................................ 122

Figure 54 The beta decay of 20N showing the transitions that may be conclusively
identiﬁed from the experimental data ................................................ 125

xii

LIST OF TABLES

Table 1 Literature search for experimental measurement of 19N and 20N decays. . . . . ...10

Table 2 Beam-on and Beam—off pulsing times for each nuclide investigated ............ 15

Table 3 Beam purity at the A1200 focal plane and the experimental end-station for
those secondary nuclides isolated in this experiment ......................................... 25

Table 4 Total number of decays recorded by the implantation detector for each of the
isotopes of interest. Also given is the calculated number of decays from the
implantation rate and scaler measured dead-times ............................................ 36

Table 5 PMT slopes for the neutron bar array as deduced from the precision time
calibrator data ...................................................................................... 38

Table 6 Calibration delayed neutrons of '6C and 17N given by energy and branching
ratio .................................................................................................. 39

Table 7 Centroid channels of the neutron peaks utilized in the calibration of the neutron
bar array ............................................................................................. 41

Table 8 Centroid channels of the prompt beta peaks utilized in the calibration of the
neutron bar array ................................................................................... 42

Table 9 Parameters derived from the delayed-neutron energies and their centroid
channels necessary to calibrate the neutron bars .............................................. 44

Table 10 Summary of the calibration ﬁts for each of the array bars. The parameters (1
and offset are generated from the M and B results ............................................ 46

Table 11 Data points used to generate an efﬁciency calibration for the neutron bar
array. The Monte-Carlo points are from reference [93Ha] ................................. 48

Table 12 Delayed-neutron lines observed during the decay of '9N. The TOF is given
along with the neutron energies in both the lab and Center of Mass frames .............. 51

Table 13 Delayed-neutron branching ratios for those neutron lines observed in the
neutron bar array during the decay of ' N ...................................................... 51

Table 14 Summary of the delayed neutrons observed during the decay of 20N. Listed
are the centroid channels and the corresponding neutron energy in both the lab and

Center of Mass frames ............................................................................ 53

Table 15 Calculated branching ratios for the delayed-neutron lines observed by the
neutron bar array during the decay of 20N ...................................................... 54

xiii

Table 16 Calibration Parameters for the lithium detectors in the seven bit time domain.
The inverse velocity of a neutron in (ns/m) is equal to the TDC channel times the slope
plus the offset ...................................................................................... 57

Table 17 Calibration points utilized to determine the total efﬁciency of each lithium
detector. The source neutrons are from the decay of 16C and 17N .......................... 57

Table 18 Calculated value of the geometric efﬁciency and best-ﬁt value of the scaling
parameter for each of the lithium detectors .................................................... 60

Table 19 Branching ratio determination for delayed neutrons observed by the lithium
detectors during the decay of 19N. Also tabulated are the parameters necessary for the
branching ratio calculation along with the associated errors ................................. 63

Table 20 Gamma rays attributed to background ............................................. 71

Table 21 Gamma ray assignments for transitions observed during the beam-off period
of the 19N production runs ........................................................................ 76

Table 22 Gamma ray assignments for transitions observed during the beam-off period
of the 20N production runs ........................................................................ 76

Table 23 Cross table for 60Co. For N gamma rays emitted at “column” energy there are
N times “cell value” gamma rays emitted at “row” energy in coincidence ............... 78

Table 24 Cross table for 207Bi. For N gamma rays emitted at “column” energy there are
N times “cell value” gamma rays emitted at “row” energy in coincidence ............... 78

Table 25 Cross table for 134CS. For N gamma rays emitted at “column” energy there
are N times “cell value” gamma rays emitted at “row” energy in coincidence ........... 78

Table 26 Off-line source calibration data for determining the efﬁciencies of the Ge
detectors. The emboldened columns are the data necessary to generate a calibration
curve ................................................................................................ 81
Table 27 On-line data pertinent to the efﬁciency calculation of the Ge detectors ....... 82
Table 28 Ge detector efﬁciencies determined using the coincidence technique ......... 83

Table 29 Gamma-ray intensities for transitions assigned to the beta decay of 19N.... . ..87

Table 30 Literature values for the de-excitation branching of the 3945 keV excited state
of '90 ................................................................................................ 88

Table 31 Calculated intensities of the de-excitation gamma rays from the 3945 keV
excited state of '90 ................................................................................ 89

xiv

Table 32 Final gamma-ray intensities for those gamma rays attributed to the decay of

"’N ................................................................................................... 90
Table 33 Calculated relative intensities of gamma rays emitted during the decay of 20N.
The intensities are relative to the 1674 keV line that is set to 100 .......................... 91
Table 34 Absolute intensity of gamma rays ascribed to the decay of 20N ................ 92
Table 35 Intensity of the gamma-ray lines attributed to the decay of 20N ................ 92

Table 36 Gamma-ray transitions identiﬁed as being a result of 19N beta decay to bound
excited states of '90 ............................................................................... 93

Table 37 Summary of beta decay strength to individual bound levels in 190 from the
decay of 19N ........................................................................................ 95

Table 38 Summary of observed delayed-neutron transitions from l9N by energy ....... 96

Table 39 Gamma rays characteristic of de-excitation in '80. The half-lives of the
gamma rays are listed along with the intensity of the lines assuming the i9N parent. . ..97

Table 40 Values for the (B+n) feeding strength to each of the pertinent l80 excited
states ................................................................................................ 98

Table 41 Neutrons in coincidence with selected 180 gamma rays. Listed are those
neutrons that may be in coincidence on the bases of the gated spectrum .................. 99

Table 42 Cross table for the assignment of intermediate states of 19O populated during
the decay of '9N .................................................................................. 102

Table 43 Gamma-ray transitions identiﬁed as being a result of 20N beta decay feeding
bound excited states of 20O ..................................................................... 103

Table 44 Bound excited levels of 20O populated during the decay of 20N. Listed are the
branching ratios for the beta transitions as determined from the measured gamma-ray
data ................................................................................................ 104

Table 45 Energy and branching ratio data for delayed neutrons observed during the
decay of 20N. Also given is the Pn value for delayed neutrons emitted above 1.1 MeV in
energy ............................................................................................. 105

Table 46 Calculated log(ft) and B(GT) values for the possible beta transitions in the
decay of 19N. The transition probablility is the branching ratio divided by 100. . . . 109

XV

Table 47 Comparison of the '90 energy levels from this experiment with predictions
made using the shell model. The subscripts on the energies indicate the choices for the
I90 level due to the uncertainty in the neutron decay ....................................... 111

Table 48 Calculated log(ft) and B(GT) values for speciﬁc decays assigned to the decay
of 20N. The transition identiﬁer segregates beta decay to bound states from the delayed

neutron emitting level ........................................................................... 112

Table 49 Summary of transitions observed during the decay of 19N that cannot be
deﬁnitively assigned to intermediate levels in '90 .......................................... 121

Table 50 Summary of transitions observed during the decay of 20N that cannot be
deﬁnitively assigned to intermediate levels in 20O .......................................... 124

xvi

CHAPTER 1

INTRODUCTION

1.1 GENERAL INTRODUCTION

The advent of radioactive ion beam technologies has opened numerous areas of
inquiry for practitioners of nuclear science. Until recently, investigation of the atomic
nucleus and its properties had been restricted to the 300 or so nuclides in the valley of
B-stability as opposed to the some 7000 nuclides that are predicted to be particle bound.
The new experimental tools that permit the production and isolation of radioactive
species have empowered theorists and experimentalists to engage in studies that would
not be possible in their absence. Among the many ﬁelds that have been broached in the
near past are studies of nuclear structure, nuclear reactions, the nature of astrophysical
cycles, and the cosmological processes postulated to have initiated the universe.

The degree to which we understand the nucleus is often measured by the ability
of theoretical models to predict fundamental properties such as isotopic masses, half-
lives, B-decay systematics, delayed particle emission, and reaction strengths. Structure
models, such as the shell model, are continually tested and reﬁned by experimental data
that restrict or ﬁx the input parameters; and models of stellar and cosmological
nucleosynthcsis are Similarly affected by both decay and reaction measurements. What
follows is a brief summary of the importance of studying the decay of 19N and 20N by

experiment for their ability to illuminate speciﬁc aspects of nucleosynthesis and their

relevance with regard to the predictions and reﬁnement of the structure models in this
mass region.
1.1.1 STRUCTURE

Measuring the beta decay properties of a nuclide provides information about the
quantum ground state of that isotope and states in the daughter that are populated during
the decay. Though the energy spectrum of the emitted beta particles is continuous,
observation of delayed particles (gamma rays, protons, neutrons, alphas) whose Spectra
are discreet typically permits quantitative determination of the daughter level fed by the
beta decay.

If the relative energy of the parent and daughter ground states are known then
the energy of the populated level can be deduced from the secondary particle. This then
allows for the Q-value of the beta decay to be calculated by difference. Further, if the
strength of the speciﬁc transition, also known as the branching ratio, can be measured
quantitatively then the energy of the transition coupled with the strength of the
transition can be utilized to calculate the log(ft) value which is the logarithm of the
partial half-life times a spectral correction factor and proportional to the nuclear matrix
element. Knowing the log(ft) value facilitates the assignment, or at least a restriction,
on the spin and parity of the intermediate level in the daughter that was fed by the beta
transition.

The most salient features predicted by models of nuclear structure, particularly
the Shell model, are the energy and quantum properties of excited states in nuclei.
Although there are other properties, the spins, parities and energies are the most easily

tested by experiment and, for the most part, are quantities upon which other inferred

properties are based. The utility of the shell model is that the model can be applied to
systems that may not be accessible to experimental inquiry. In order to inspire
conﬁdence in such extrapolation, it is necessary for the model to be proven valid in
systems that are testable by experiment.

For the speciﬁc cases of studying '9N and 20N, the decay measurements will
provide insight into the structure of 19O and 200 respectively. By restricting the Spin
and parity of the excited levels populated during the decay, the predictions of the shell
model for these nuclides will be tested. Since these isotopes are relatively light the shell
model should accurately describe the properties in this region prior to extrapolation to
more exotic species.

1.1.2 NUCLEOSYNTHESIS

The generation of elements from the basic nuclear constituents is-believed to
have occurred during the early stages of the so-called big bang and within the interior of
stellar objects — which is an ongoing process. The process is driven by the fusion of
seed nuclei with the simplest objects; neutrons, protons and in some cases, alphas.
Many of these reactions and their cross sections are known for the fusion of stable
isotopes with the lighter particles, however, most of the reaction cross sections for
capture by unstable species have not been measured.

To make the network calculations for the production of elements, and hence the
isotopic abundances, it is necessary to know or estimate the reaction cross sections for
many radioactive Species with the simple species. This is true of both the primordial
and stellar nucleosynthcsis networks. Although it would be desirable to measure the

reaction cross sections for all nuclei with the low mass subset, such voluminous,

comprehensive assessments are not currently required. Typically, Speciﬁc reactions are
targeted as being particularly important to measure experimentally while other, less
perturbing reaction rates may be estimated systematically. One reaction cross section
that has been propounded as important for both inhomogeneous models of the big bang
(IMB) and stellar nucleosynthcsis is the neutron capture on 18O producing '90 and
gamma rays.

The standard model of the big bang predicts that elements beyond lithium will
not be produced with any appreciable strength. The IMB model, where the number of
neutrons is larger than protons, predicts that elements up to and beyond mass (A=20)
may be generated [94Ra]. The production of elements beyond neon in this model
proceeds through the generation of 180 via the l4C(ow) and l7O(n,'y) reactions. Here
mass may either be recycled through the 18O(p,0t)l5N reaction or mass may be built up
through the 18O(n,y)'90 reaction. For material to be processed out of the CNO range
into the mass (A=20) region, the strength of the '80(n,y) reaction must exceed that of
the 18O(p,ot) reaction [96Me]. Thus, 180 represents a potential end-point for the
accumulation of mass in the IMB scenario; rendering the '80(n,y)'90 a quantity of
primary importance to measure.

With regard to network calculations in astrophysics: the 180 seed nuclide is
utilized in the mass build-up occurring in the helium core burning Stage of asymptotic
giant branch stars. In these objects, 180 is manufactured from alpha capture on 14N and
processed through to 22Ne by the 18O(ot,y) reaction. These stars are propounded to be
the main locales for the slow neutron capture process (S-process); where the source of

the neutrons is the 22Ne(0t,n) reaction. If the 18O(n, ) reaction strength is relatively

high, the neutron production (and thus s-process rates) will be dramatically altered. The
reason being two-fold: ﬁrst, if the l8O(n, ) reaction competes well with the '80(a,y)
then neutron production will be quenched by removing the 22Ne seed from the 22Ne(0t,n)
reaction, and second, a strong '80(n,y) will remove neutron material from the system
that would otherwise initiate the S-process on different seed nuclei. Consequently, an
experimental insight into the '80(n,‘y) reaction is of critical importance as an input
parameter for the network calculations of this stellar environment.

Information may be gained about the l8O(n,'y)'90 reaction by measuring the
decay properties of '9N. By measuring the beta branching strength of '9N to levels in
'90 and calculating the log(ft) values, the spins and parities of the populated levels can
be restricted. From these level properties, at least those levels above the neutron
emission threshold, the resonant neutron capture strength of the 18O(n,'y) reaction can be
calculated.

1.2 LITERATURE SEARCH - 19N,2"N

Numerous publications appear in the literature reporting experimental Studies of
the decay modes of '9N and 20N. In fact, there have been half-life studies,
measurements of the total neutron emission probability, and an experiment designed to
record the y-ray Spectra following the decay of 19N. However, rather than clarifying the
decay properties of these nuclides, the published quantities are disparate - rendering the
inferred decay modes somewhat uncertain. What follows is an overview of the
published experimental studies relevant to the decay properties of these neutron rich

nuclides.

1.2.1 PRIOR MEASUREMENTS OF 19N DECAY PROPERTIES

The energy level scheme of the mass A=l9 system pertinent to the decay of 19N
is Shown in ﬁgure 1 [95Ti] with the level energies given in MeV relative to the ground
state of 19O. The scheme indicates that the B-decay Q-value is about 12.5 MeV (see
also [93Au]), while the maximum energy available for B-delayed neutron emission is
about 8.5 MeV. In addition, B-delayed alpha particle emission is energetically possible
by about 3.5 MeV and, although this particular channel has not been observed
experimentally, Similar decay channels have been observed during the decay of other
nitrogen isotopes [93Bu, 94Do].

There are eight known levels in the 190 daughter above the ground state that are
bound with respect to neutron emission. Therefore, any beta decay feeding to these
levels will result in the emission of one or more de-excitation y-rays. What is more,
there are some forty known levels in '90 above the neutron emission threshold through
which B-delayed neutron emission is, at least, energetically possible. Given that there is
so much energy available for B-delayed neutron emission and that there are so many
levels through which such a transition may occur, it should not be surprising that B-
delayed neutron emission comprises a signiﬁcant fraction of the decay strength of this
nuclide.

Experimental study of the decay of '9N began at GANIL in 1986 with an
experiment by Dufour er al. where B-delayed 'y-ray Spectroscopy was performed
[86Du]. The results of the study found the half-life of 19N to be 0.32(10) seconds and
they were also able to identify three y-ray transitions which they ascribed to levels in

'90. The characteristic energies of the three gamma lines were found to be 96.0, 709.2,

12 .54

 

3.957

 

180—1—0

11.25 ( )

10.21 7/2—
9.93 ( ) E

9.43 ( ) _______
9.253 ( )

9.022 ( )
8.916 ( )

8.561 ( )

8.247 ( )
8.048 ( )

7.242 ( )
6.988 ( )

6.583 ( )
6.4058 ( )

6.1916 ( )

5.7046 7/2+
5.5035 ( )

5.1484 (EB/2+)
5.007 (3/2)
"7025 5/2+ :

4.4025 (3/2)
4.1093 3/2+

3.2316 (1/2-) _‘

3.0674 (3/2+)

2.3715 9/2+

0.096 3/2+

11.58 ( )

10.66 7/2—

9.98 ( )
9.9 7/2-
9.56 ( )
9.324 ( )

9.064 ( )
8.923 ( )

8.591 ( )
8.45 ( )

8.132 ( )

7.508 ( )

7.118 ( )
6.903 ( )

6.4662 (7/2)
6.2693 7/2—
6.1169 3/2+

5.54 3/2+
5.384 (9/2)

5.082 1/2-
4.9683 (5/2)

4.582 3/2—
43281 (3/2)

3.9449 3/2—

3.1535 5/2+

2.779 7/2+

1.4717 1/2+

0 5/2+

8.996

 

15C+or

(__ 190

Figure 1 Energy level diagram of the mass 19 system pertinent to the decay of 19N.

and 3137.8 keV with relative intensities of 100(10), 63(21), and 76(21), respectively.
The energies imply a cascade from the 3.9449 MeV level through the 3.2316 and
0.0960 MeV levels, however, the errors on the intensities confound any serious attempt
to determine speciﬁc branching strengths to individual levels in the 190 daughter.

Indeed, the Dufour result is in itself an enigma because other studies [70Fi,7lHi]
where the 3.9449 MeV level has been populated (through transfer reactions) suggest
that this state relaxes t0 the ground state, 0.0960 and 1.4717 MeV excited states with
approximately the same probability [95Ti]. The text of Dufour et a1. indicates that their
experiment was sensitive to transitions in this energy range - thus the absence of these
lines in their report is puzzling.

There have been two experiments conducted in the past to measure the total
neutron emission probability (Pn) of 19N. The ﬁrst was performed at GANIL by
Mueller et a1. [88Mu], and the second by Reeder and collaborators at LAMPF [91Re].
The GANIL study re-measured the half-life to be 0.210(+200 -100) seconds and found
Pn to be 0.33 (+34 -1l), while the Reeder group determined the half-life to be 0.329(19)
seconds and the total neutron emission probability to be 0.624(26) of the decay strength.
Though the two measurements of the Pn value agree within error, the Mueller result has
a large uncertainty and, what is more, the Reeder value has since been retracted by
private communication [970i] and in conference proceedings [94Re]. The latter report
gives an updated analysis and reports a half-life of 0.255(10) seconds, and the revised
PIn as 0.487(21) of the total decay Strength. Consequently, the only remaining refereed

value in the literature for the PH strength of '9N is that reported by Mueller.

To date there has been no experiment to make a Spectroscopic measurement of
delayed neutrons emitted by 19N. The only Spectroscopic evidence appears in a
publication by Ozawa and collaborators in 1995 [9502] where l9N appears as a
contaminant in their experiment designed to measure the delayed neutron spectrum of
19C. They were able to attribute one peak in the neutron TOF spectrum as being due to
the decay of 19N. This neutron appeared at an energy of 0.59 MeV in the lab and was
attributed to the l9N ﬁ-decay populating the 4.582 MeV state in '90. Their
interpretation was based on the change in intensity of this peak as a function of time
relative to the other observed neutrons.

1.2.2 PRIOR MEASUREMENTS OF 20N DECAY PROPERTIES

The data in the literature pertinent to the decay of 20N is less by comparison to
l9N. The portion of the mass (A=20) system for the decay of 20N is Shown in ﬁgure 2
with the level energies given in MeV relative to the ground state of 200. There have
been two measurements of the half—life and two measurements of the PI, value of its
decay. Once again, the work was done by the Mueller and the Reeder collaborations
[88Mu, 91Re]. The Mueller result for the half-life is 0.100(+30 —20) seconds and the Pn
is 0.53(+11 -7), while the Reeder publication of 1991 lists the half-life as 0.142(19)
seconds and the total neutron emission probability as 0.661(50) as a fraction of the total
decay strength. Again, the Reeder values were updated in 1995 [94Re] and the new
values are given as 0.129(8) for the half-life and 0.520(33) for the Pn value. There is
no data available with respect to y-ray emission following the B-decay, and no

publication involving a spectroscopic measurement of delayed neutrons emitted by 20N.

1.2.3 LITERATURE SEARCH SUMMARY

In total, there have been Six half-life measurements of '9N. However those with
the better statistical accuracy do not agree to within two Standard deviations. The value
of the Pn strength that remains in the literature is largely uncertain, and the delayed 'y-ray
data do not agree with other, related measurements on the excited states of '90. The 20N
experiments have better agreement in half-life and Pn value but independent
conﬁrmation of the Mueller result [88Mu] is still desirable. There is no spectroscopic
or branching strength data available for 20N regarding beta, delayed neutron or ‘y-ray
emission. A summary of the experimental data currently in the literature, regarding the

decay of 19N and 20N, is given in table 1.

Table 1 Literature search for experimental measurement of the 19N and 20N decay.

 

 

QUANTITY 19N 20N REF.

QB (MeV) 12.54 17.97 93Au
Half-Life (ms) 329 (19) 142 (19) 91Re
210 (+200)(-100) 100 (+30)(-20) 88Mu

235 (32) - 88Sa

300 (80) - 88Du

320 (100) - 86Du

255 (10) 129 (8) 94Re

Neutron Emission 62.4 (26) 66.1 (50) 91Re
Probability (%) 33 (+34)(-11) 53 (+1 l)(-7) 88Mu
48.7 (21) 52.0 (33) 94Re

Decay y—Rays (keV) 96, 709, 3138 N/A 86Du

 

-10-

17.97< )

 

 

 

 

 

 

 

 

 

20
10.13 2+ N
—— 9.770 0+
8.962(0+):— _
8.554 4+ 8804 3
7.608 7.754 4+: 73;; g)—
,9©+ 7.252 5———— ' < ‘)
ﬂ
—— 6.555(2 )
5.614(3—)
—— . 7 +
5.394 2+_____ 23% 3+
5.002( )— 4.850 4+
4.456 0+
—~———— 4.072 2+
3.570 4+
—— 1.674 2+
0 0+
200

Figure 2 Energy level diagram of the mass 20 system relevant to the decay of 20N.

-11-

1.3 SHELL MODEL PREDICTIONS FOR THE DECAY OF 1“N

In 1988, following the appearance in the literature of the Dufour experimental
results [88Du], a shell model study of the ”N decay was published by Warburton
[88Wa]. Essentially, the paper included calculations of the B(GT) strength for each Of
the allowed and ﬁrst forbidden transitions to levels in '90. In this model the ground-
State spin and parity of '9N was assumed to be (1/2-), which is consistent with the
known spin and parity of the other odd-mass nitrogen isotopes, and the program
OXBASH was used to determine the Spin, parity and energy of the '90 levels. The
salient features Of the Shell model results are as follows. First, only about 7% [6%] of
the beta strength populates y—emitting allowed [ﬁrst forbidden] ﬁnal states in 190.
Second, that the fraction of the decay leading to neutron emission should be about 0.87
(or 87%) of the total decay strength. Third, that the half-life of '9N should be about
0.54 seconds. And fourth, that less than 5% of the allowed strength populates levels in
'90 above the (15 C + or) threshold.

Taking the theoretical and experimental results together, it may be concluded
that the calculated half-life over-predicts the Observed half-life by about a factor of two,
and that the total B-delayed neutron emission probability, P“, is predicted to be
signiﬁcantly higher than that which has been measured. Also, the model predicts
signiﬁcant B—strength to bound levels that would yield (either directly or through
cascade) the 1376 keV y—ray characteristic of the transition between the 1472 and 96
keV levels in l9O — a transition that was not observed in the Dufour experiment [88Du].

Warburton comments that, if the calculated B(GT) values to the allowed states

under-predict the actual values by and order of magnitude, the missing transition

-12-

strength would by reduced to a degree where the experimental study [88Du] may not
have been sensitive to it. Increasing the B(GT) values in this way would also bring the
model half-life in line with experiment, however, the Pn value under those conditions
would be reduced to about 0.3 or 30% of the total branching strength. Indeed,
Warburton summarizes that the predicted Pn value is very large given the energy range
and number of bound states available in 19O, and suggests that experimental

measurement of the total neutron and y—ray emission probabilities may be the only way

to measure the absolute B-strengths to low lying levels in '90.
1.4 EXPERIMENTAL GOALS

In light Of the Shortcomings and potential errors in the measured values of the
l9'20N decay modes and the general disagreement between experiment and theory in the
case of 19N, an experiment to re-measure the decay properties of 19N and 20N was
proposed at the National Superconducting Cyclotron Laboratory. The goals of the
experiment were to thoroughly establish the decay properties of these nuclides by
making spectroscopic measurements of the delayed neutrons and y—rays emitted during
their decay. From these measurements, the predictions of the shell model may be
compared and contrasted with experiment, and the importance Of the 18O(n,'y) reaction

in both the IMB and stellar nucleosynthcsis networks can be estimated.

-13-

CHAPTER 2

EXPERIMENTAL SETUP

2.1 PRODUCTION OF SECONDARY RADIOACTIVE ION BEAMS

The only naturally occurring sources of the isotopes of interest are the hot, dense
environments associated with stellar objects. When this is taken together with the fact
that the half-lives of '9N and 20N are reportedly less than one second [95Ti], it should
not be surprising that there are no terrestrial samples of these nuclides upon which
experiments may be performed. As a result, a facility capable of artiﬁcially generating
these isotopes was required.

One facility capable of producing ”N and 20N in yields commensurate with the
experimental goals is the National Superconducting Cyclotron Laboratory at Michigan
State University. In this study, the necessary radioactive ion beams (RIBS) were
generated by fragmenting an intense primary beam of 22Ne in a beryllium target. The
primary beam of 22Ne was accelerated in the K1200 Cyclotron and extracted at an
energy of 80 MeV/A. The 22Ne beam then struck a thin beryllium foil (564mg/cm2)
resulting in the production of a wide variety of secondary fragments near or below mass
(A=22).

The nuclides of interest were selected, sequentially, using the A1200 fragment
mass analyzer [91Sh]. The fragmentation products were separated on the basis of their

mass to charge ratio (All) by the ﬁrst set of A1200 dipoles. The Speciﬁc nuclides were

-14-

further separated from other reaction products by passing them through an achromatic
wedge (241 mg/cm2 Al equivalent) and the second pair of A1200 dipoles. The wedge
induced momentum Shifts, based on the atomic charge of the recoil products, allowing
unambiguous separation among nuclides at the ﬁnal focus of the A1200.

Once puriﬁed, the RIBS were transported, using various electromagnetic
devices, to the low background experimental end-station located in the N3 vault at the
NSCL complex. Generally speaking, the transmission Of the secondary beam from the
A1200 focus to the experimental end-station was about 80 percent over the course of
the study. For this experiment, the primary beam was pulsed to allow for efﬁcient and
conclusive data recording. The beam-on and beam-off intervals were selected on the
basis of the half-life of the nuclide being transported to the experimental end-station.
The beam pulsing sequences for each nuclide Studied are given in table 2.

Table 2 Beam-On and Beam-Off pulsingtimes for each nuclide investigated.

 

Nuclide Experimental Runs Beam-On Time Beam-Off Time
17N 17 - 23 8 s 12 s
16C 46 - 50 1.5 s 3 s
l9N 59 - 90 1 s 3 s
20N 93 - 105 0.5 s 0.8 s
20N 106 - 116 0.375 8 0.375 s

 

2.2 EXPERIMENTAL END STATION

The radioactive beams of 16C, l7N, '9N, and 20N initially passed through a
parallel plate avalanche counter (PPAC) located at the inlet of the 92” chamber and
immediately upstream of the main detector array. The PPAC was most useful during

the preliminary beam tuning as it provided position information on the incoming beam.

-15-

Downstream of the PPAC, the RIBS passed through a thin Zirconium window
separating the beam-line vacuum (10'6 torr) from the end station that was at atmospheric
pressure. The detection system consisted of four dependent sub-systems that will be
referred to as the implantation system, neutron bar array, 6Li array, and Germanium
pair.

2.2.1 IMPLANTATION SYSTEM

After transmission through the PPAC, the ion beam entered the ﬁrst detection
sub-system called the implantation system. The implantation system consisted of two
transmission mounted, silicon surface barrier detectors, 8 series of aluminum degrader
foils, and a thin plastic scintillator. The silicon detectors were 300 pm thick and had
surface areas of 300mm2. The scintillator was made of BC412 plastic and had the
dimensions of 2.5cm x 2.5cm x 0.3cm deep. The two silicon detectors were placed one
on the upstream side, and one on the downstream Side of the plastic scintillator. The
upstream silicon was referred to as the dE detector, the downstream silicon as the
VETO detector, and the plastic scintillator was referred to as the START detector. The
components of the implantation system and the detector designations are shown in
Figure 3.

During the beam-on intervals, the beam passed ﬁrst through the dE detector, was
degraded in the aluminum foils, and then implanted into the START scintillator. The
dE detector provided a useful real-time monitor of the purity of the secondary beam,
while the dE detector in concert with the START detector and the VETO detector
provided data on the fraction and purity of the secondary beam actually implanted

within the scintillator.

-16-

During the beam-off intervals, the scintillator generated a signal caused by
transmitted beta particles from the decay of the implanted species. The coincident
signals from the photomultiplier tubes (PMT’S) were used for half-life measurements,
the determination of the total number of decay events, and also provided a start signal

for the delayed neutron time-of—flight spectroscopy.

plantation Detector
Energy Degrader “
\ \ § .,.. _,,--"'

, ---- Delta E Detector Photomultiplier Tube

 
  
 
 

Figure 3 Schematic diagram of the implantation detection system.

2.2.2 NEUTRON BAR ARRAY

The second detector system utilized in this study was the so-called Neutron Bar
Array. The neutron bar array was originally constructed by Dr. Richard Harkewicz
during the period of his graduate study at Michigan State University [92Ha]. This array
consists Of sixteen plastic scintillator bars, composed of BC412 plastic, with the

approximate dimensions 157cm x 7.6cm x 2.54cm thick. The scintillator bars were

shaped such that the radius of curvature of every bar is equal to one meter. The sixteen
neutron bars are housed in an aluminum frame with the bars arranged such that they are
concentric with a Single point in space. At this point, the START detector is situated to

enable a time—of-ﬂight technique for the determination of delayed neutron energies.

    

Implatation
Detector

."'
."

."'
,.

Kapton Window
Figure 4 Schematic diagram of the Neutron Bar Array.

The beta decay in the implantation scintillator provides a common start while a
neutron event detected within an array bar provides the individual stop. The time
difference between the events yields the time-of-ﬂight and, knowing the ﬂight path, the
neutron energy. The neutron bar array was selected for its ability to efﬁciently detect
delayed neutrons in the energy range between one MeV and ten MeV. A schematic

diagram of the neutron bar array is Shown in Figure 4.

-18-

2.2.3 “Li GLASS SCINTILLATORS

To detect beta-delayed neutrons emitted with energies lower than the threshold
of the neutron bar array, the detector end-station contained three inorganic scintillators
containing enriched quantities of 6Li. The detectors were right-cylinders with radii
equal to 2.54 cm. Two Of the three were 2.54 cm deep while the depth of the third
crystal was equal to 1 cm. The three detectors were placed about the START
scintillator without Shadowing the bars of the array, and with as much separation as
possible to minimize scattering among the devices. The two thicker 6Li glass
scintillators were placed about 25cm from the source activity while the thinnest detector
was Situated about 10cm from the source activity.

The 6Li glass scintillators also provided spectroscopic information on delayed
neutron emission using a TOF technique. Again, the implantation scintillator provided
a start signal from the beta decay and the detection of a neutron by a 6Li glass
scintillator gave an individual stop Signal.

2.2.4 HIGH PURITY GERMANIUM DETECTORS

To detect gamma rays emitted during the decay of the isotopes of interest, the
detector end-station included two, high purity germanium crystals. The detectors were
120% and 80% efﬁcient relative to a standard sodium iodide detector and were placed
as close as possible to the source activity given the physical constraints of the neutron
bar array and its supporting apparatus. The detectors were situated upstream of the
source activity to minimize the effect of the opening angle with respect to the beam-
pipe. The angle between the detectors and the implantation position was about eighty

degrees. The more efﬁcient (Ge#l) and the less efﬁcient (Ge#2) were placed at about

-19-

17cm and 22cm away from the source activity, respectively. Figure 5 is a schematic
representation of the end station showing the implantation system, the neutron bar array,

6Li detectors and germanium detectors.

 

Alminm Imhmim “Ii-Gas
Mair Mr Stimuls

 

mm

51

l‘hnmlhs —->

 

 

 

  

 

 

 

 

 

 

 

 

 

 

 

 

6 .
51161.1( AD snot-101m) SI? 1

 

 

 

Figure 5 Schematic diagram of the entire detector end-station (top view).

2.3 ELECTRONICS AND DATA ACQUISTION

Data taking for the experiment employed standard electronics modules available
within the NSCL electronics pool for all systems including the CAMAC portion. The
beam pulsing signals were supplied by a programmable dual gate generator where a
m Signal was input to the cyclotron RF control. The phase of one of the RF

electrodes was shifted by this signal and the beam was cut-off in the accelerator.

-20-

For both the beam-on and beam-off intervals, 8 master gate was caused by a
coincident event in the two PMTS attached to the implantation scintillator. This event
in conjunction with the beam-status bit (on or off) triggered read instructions for the
electronics for the detectors of interest. The neutron detectors were disabled during the
beam-on period, as was the real-time clock, however, the germanium detectors were
enabled during both the beam-on and beam-off intervals. Figures 6-8 Show the
electronics set-up utilized in this experiment.

The data were recorded on an event-by—event basis and written to 8mm tape for
subsequent ofﬂine analysis. The in-house SARA analysis code was utilized for both
online and ofﬂine purposes along with the associated XAMINE display package. The
data taking was divided into runs of approximately two hours duration to ensure
manageability of the ﬁles during analysis.

2.4 SECONDARY BEAM PURITY

In this experiment, four radioactive ion beams were generated by the
fragmentation of 8 22Ne beam in a beryllium foil. Two of the beams were produced for
calibration purposes (16C and ”N) and two of the beams were the isotopes of interest for
the experiment ('9N and 20N). The purity of the secondary beams was established at
two points. The ﬁrst was during beam tuning through the A1200 fragment mass
analyzer (which is independent of the user deﬁned set-up in the experimental vaults)
and the second was by analysis of the data recorded downstream by the implantation

system at the end-station.

-21-

PPRC UP. DONN, LEFT. RIGHT swu

RDC

 

BERN PULSINO 9ND REAL-TIME CLOCK

 

STOP RBI-TIE CLOCK

 

 

 

 

 

 

 

REF“ 0" M l - ID "5111121 DETECTOR 91111 011' LOGIC
11811 arr mTE T0 "5111121 DETECTOR 91111 EFF” 10131:
1111 [ms GEiERﬂTiR
NIH/TTL 01110112011 FELLUHER

 

 

RESTf-RT REFl-TllE CLOCK

Figure 6 Electronics diagrams of the PPAC, Real-Time clock and RF pulsing
control elements of the end-station.

-22-

Neutron Bars

 

 

9141—1111
1091

.1- - 9“" Iﬂ—I—m
2' 7.1.—

—‘ 90181

0 m
m m ®
w 1111.55.
001111011

BLi Scintillators

  

 

 

_I

Figure 7 Electronics diagrams for the neutron bars, ‘Li detectors and germanium
detectors.

-23-

STRRT DETECTOR

DELRY— ODC

     

SCOT-ER

DELRY DISC. TDC STOP

 

 

 

FROH RIGHT

 

 

CPU STQRT

 

 

FIFO All COTE

 

Figure 8 Electronics diagrams for the components of the Implantation system.

-24-

During beam tuning, the purity of the secondary product of interest is
determined by monitoring both the fragment TOF through the separator, and the energy
loss of the fragments in two silicon PIN detectors (typically 300 microns thick). The
Optimum settings are selected for variables such as the momentum acceptance of the
spectrometer, the width and breadth of the momentum slits, the thickness of the
achromatic wedge, the thickness of the production target and, occasionally, the
composition of the target itself. Prior to transmitting the secondary beam to the desired
experimental vault, a purity run is performed and the data stored for later reference.

For this experiment, the secondary beams were also monitored for purity at the
end-station using the implantation system. The yield and composition of the secondary
beam entering the vault is determined by the dE detector and the RF time-of-ﬂight from
the cyclotron while the yield and composition of the beam actually implanted into the
start detector is determined from the dE vs. RF time-of-ﬂight vetoed by the second
Silicon detector located downstream of the implantation scintillator. The results of the
purity analysis for both the Spectrometer analysis and end-Station response is Shown in

table 3 for the four secondary beams of l6C, l7N, 19N, and 20N.

Table 3 Beam purity at the A1200 focal plane and the experimental end-station
for those secondary nuclides isolated in this experiment.

 

SECONDARY A1200 IMPLANTED CONTAMINANT FRACTION

 

BEAM % PURE % PURE %
16C 99.3 (9) 9970(6) ”N, 18N 0.7
17N >999 9975(4) N/A N/A
'9N 99.5(10) 995(1) 17C 0.5
”N 98.8(17) 94.4(21) 18C 1.1

 

-25-

From the data collected it is clear that the calibration beams of "’C and 17N, as
well as the ”N nuclide of interest, are better than 99% pure, while the secondary 20N
beam is about 95% pure. The lower purity of the 20N beam is not unexpected due to the
nature of the process through which 20N is produced from a primary beam of 22Ne. The
reaction required a nucleon transfer or a charge exchange with the target material and
hence the momentum distribution of this particular isotope covers a larger range. As a
consequence, there is a greater probability of momentum overlap with other secondary
products and thus the purity is reduced relative to the fragmentation products.

The implantation rate taken together with the sealer values for the dead-time
during the beam-on and beam-off intervals permits a determination of the total number
of decays for each of the isotopes of interest that would have been observable during the
beam-off period. The results of these calculations are given later in table 4 in

comparison to the more rigorous approach described in section 3.4.2.

-26-

CHAPTER 3

EXPERIMENTAL RESULTS

3.1 OVERVIEW OF ANALYSIS PROCEDURE

The analysis of the experimental data can be grouped into three parts. The ﬁrst
is the analysis of the beta particle emission leading to evaluations of the half-lives and
to the total number of decays of the isotopes of interest (NB)- The second is with respect
to delayed neutron emission by analyzing the data recorded in the neutron bar array and
the 6Li inorganic scintillators to determine the delayed-neutron energies and intensities.
The third is an analysis of the gamma-rays emitted during the decays as recorded by the
germanium detectors.

In this chapter, the information that can be gleaned strictly from the
experimentally recorded data is presented. What follows is a detailed analysis of the
methods by which the efﬁciencies of the detection systems were established, the
calculations for the intensities of the delayed-neutron and gamma-ray lines, the
determination of the decay half-lives of 19N and 20N, and the calculations for the total
number of decays observed for each of the isotopes. The order of presentation is ﬁrst,
the beta decay, second the delayed-neutron emission and third, the gamma-ray analysis.

In Chapter 4, the experimental results are interpreted to yield the decay schemes
of 19N and 20N. To do so, the gamma-ray data and delayed neutron results are viewed in
concert with the idea of accounting for gamma-ray cascades and to consider the

possibility of delayed neutron and gamma coincidences.

-27-

3.2 HALF-LIFE MEASUREMENT OF ”N

The ﬁrst priority in the experimental Study of ”N was to determine the half-life
of the nuclide. Due to the high secondary yield of 19N the data acquisition experienced
a high degree of dead time. To minimize the contribution of this effect to the half-life
determination, the primary 22Ne beam was attenuated by a factor of about 30 during the
run in which the half-life was measured.

The beta decay of I9N populates states either in '90 or 18O depending on whether
the exit channel is pure beta or delayed neutron emission. AS such, the half-life ﬁt to
the recorded data must include a term for the decay of 19O. Fortunately, the half-life of
19O (26.9 seconds) is well established [95Ti] and is also roughly two orders of
magnitude longer than the l9N parent. This large difference, taken together with the fact
that the total time of the half-life run was much longer than either half-life (~2 hours),
permits a simplifying assumption where the two decays can be treated as independent
quantities and the feeding term characteristic of secular decay modes may be
eliminated.

The time sequence for the pulsing of the primary beam during the half-life run
was set such that the beam-on period (implantation) was one second and the beam-off
period (data acquisition) was three seconds. The half-life determination was made on
the basis of the Spectrum collected as a function of the real-time clock (Ortec model
RC014) during the beam-off periods. The full—scale time of the clock was set to sixteen
seconds.

The delayed neutron daughter, '80 is stable and therefore a secular component

for this decay is not necessary. Figure 9 shows the half-life ﬁt to the data collected in

-28-

the implantation scintillator for the decay of 19N. Shown are the raw data (black), the
l9N component (blue), the background due to 190 (black), and the aggregate ﬁt to the
Spectrum (red). The result of the ﬁt indicates that the half-life of 19N is 299 i3 $16

milliseconds. This result is in close agreement with references [88Du, 91Re].

 

3000 ”N Half-life

   

COUNTS
N
o
o
o

1 000

 

 

 

 

O

100 200 300
CLOCK CHANNEL (2048 = 16 seconds)

Figure 9 Fit to the half-life spectrum of 19N.

3.3 HALF-LIFE MEASUREMENT OF ”N

The production rate of 20N from the primary 22Ne beam was orders of magnitude
lower than that realized in the 19N experiment. AS a result, there was little or no dead
time associated with monitoring the decay of 20N. The half-life ﬁt included a term for
the decay of both 200 and 20F, the daughter and granddaughter, but once again these
half-lives are orders of magnitude longer than that of 20N — permitting the same

simplifying assumption as with the 19N case.

.29-

During the 20N runs, the beam-off time (where the real-time clock is active) was
changed from 0.8 seconds to 0.375 seconds to improve the efﬁciency for recording 20N
events (see table 2). Both series, long and short respectively, permit half-life
determinations for 20N. Figure 10 shows the ﬁt to the long beam-off data with the 20N

component (blue), the sum of the daughter contributions (black) and the aggregate ﬁt to

 

 

 

 

 

 

the data (red).
3000
20N Half-life
_ Beam Off = 0.8 seconds
a, 2000
l— .
2
D .
o .
0 1000 -
o I l L 1 1 1 l l l 1 1 -
0 200 400 600 800

CLOCK CHANNEL (2048 = 2 seconds)
Figure 10 Fit to the half-life spectrum of 20N.

The results of the long and short beam-off time data for the half-life of 20N are
118 $6 $10 and 124 i7 i10 milliseconds, respectively. These results are in reasonably
good agreement with the previously reported measurements in the literature [94Re].

3.4 TOTAL NUMBER OF EVENTS

The total number of decay events for each nuclide was determined by the

response of the implantation scintillator to beta particles. The total number of events is

-30-

a necessary quantity to calculate the efﬁciency of the neutron detectors and to determine
the absolute branching ratios for the ”N and ZON decay channels. The speciﬁc
procedure used to solve for the total number of decays was to integrate the half-life
spectrum collected for each nuclide.

3.4.1 DEAD TIME CONSIDERATIONS

Due to the high yield of secondary fragments, the data acquisition system
experienced considerable dead-time. To account for the effect of dead-time
analytically, the procedure outlined by Knoll [89Kn] and Leo [94Le] for non-
paralyzable dead-time was employed. In this technique, the relationship between the
actual and observed activity can be described by the equation:

m=n/(1+nt) [1]
Where In is the observed activity, n is the actual activity and I is a dead-time related
constant. It is noteworthy that the parameter, I, will depend on the properties of the
particular nuclide under scrutiny (half-life, implantation rate, daughter properties) thus 1:
is not general and must be solved for each secondary product of interest.

To elucidate an accurate value for the parameter, 1:, subset half-life spectra were
generated by applying tight gates around the peaks of the well-separated neutron
transitions recorded by the neutron bar array, and reﬂecting back onto the half-life
spectrum. This protocol maximized the signal/background ratio and also ensured that
the resultant half-life spectrum was as pure as possible - containing only one
radioactive component. The caveat to this technique is that the dead-time formula is no
longer strictly applicable. This is due to the fact that 13 (though it is a constant over all

channels) is dependent on the total instantaneous activity and not on any subset.

-31-

However, the fraction of counts missed due to the computer being busy will be a
constant at each channel for all subsets of the total half-life spectrum. Mathematically,
the value of:

1 / ( 1 + n I) [2]
will be a different constant at each channel for all gated and ungated half-life spectra.
Clearly, n, the actual number of counts at any channel, varies among the gated subsets,
thus the solution for I will vary based on the inverse of n. However, since at channel
zero (or any speciﬁc channel for that matter) the fraction of counts missed by the
computer is the same for all subsets of the total half-life spectrum, the product of the
value of I with the initial activity will be the same for all subsets. For our purposes, it
is more convenient to solve for the activity-times-I constant (AIcon) at channel zero,
and for the initial gated activity for each subset of the total half-life histogram and
compare across subsets for reliability.

For the calibration beams of '6C and ”N, the protocol was as follows: ﬁrst, the
neutron—gated half-life spectra were ﬁt with MINUIT and MINOS where the decay
constant was ﬁxed to the known half-lives and AIcon and the initial activities were
allowed to vary freely — as was the background. Second, the total half-life spectrum
was ﬁt in MINUIT/MINDS in a similar way except that the value of AIcon was
restricted to be within the range deﬁned by the solution of the ﬁts to the gated spectra.
Third, from the minimized ﬁt, the total number of decays for each isotope was
extracted. For the ”N and 20N nuclides, this same procedure was followed except that,
in addition to ﬁxing the half-lives to the literature or independently determined values,

the procedure was repeated in its entirety with the half-lives permitted to vary freely in

-32-

the minimization. As an example, ﬁgure 11 shows the aggregate delayed neutron

spectrum of 16C along with the neutron gates used to reﬂect back onto the half-life

 

 

 

 

 

 

 

spectrum.
10000
160
1000
E
D 100
O
U
10
1 L l a a l A A a L l a a a A l L
250 500 750 1 000 1250

TIME-OF-FLIGHT (100 p8)

Figure 11 Delayed-neutron spectrum of 1“C showing the gates used to reﬂect back
onto the half-life spectrum for the dead-time analysis.

The gated half-life spectra were ﬁt using the equation given by Knoll [89Kn]
and Leo [94Le] where the quantity n was of the form:

n = A, Exp[-M CH] + A2 Epotz CH] + constant [3]
where M was ﬁxed to the known decay constant of “SC in inverse channels, A; is the
initial activity of .6C in counts/channel, A2 and 1.2 were parameters included to allow for
the possibility of a second radioactive component, and CH is the channel of the half-life
spectrum. In addition to these parameters, the value of the total initial activity times the

dead-time parameter I (AIcon) was allowed to vary freely for each gated spectrum.

-33-

Figure 12 shows the neutron-gated half—life spectra and the best-ﬁt curves to those data

 

 

 

 

 

 

 

 

as determined in MINUIT/MINDS.
LA A H.L. gatedonBOBkeVneutron
1500 -" - ‘ A
,.... « ‘ - Mlnoe fit to 808 keV neutron H.L.
A a D H.L. gated on 1715 keV neutron
“-11 — Mlnoe fit to 1715 keV neutron H.L
3 1000 A‘, 1
c _ In .M , s
a - 4| I t “.0:
8 u- L £515.1ng I ‘3.”
5‘ . ‘ . ..A
-PiL - L: .i"'..‘ ' - A AVA ‘V, \.‘ u ‘
5m . bk; I—.—,'l. ‘ A “
II I .3: “I“ ‘- "-‘v, 3‘
. _ -. ‘3‘?“ EI:__‘- 13"- :A’ (‘0‘
o 1 1 l a a l a a
0 60 120 180
Clock Channel

Figure 12 Best fit curves to the neutron-gated half-life spectra of 16C.

From these ﬁts, the value of AIcon is deduced to be O.64(5) for the '6C data,
which is the weighted mean of the best-ﬁt results for this parameter in
MINUIT/MINDS. This value of AIcon was input into the parameter list for the ﬁt to
the total half—life spectrum and permitted to vary between the bounds deﬁned by the
one-sigma range. Again, the half-lives of '6C and the beta daughter 16N were ﬁxed to
their literature values, while the initial activities and a constant background were
permitted to vary freely. Figure 13 shows the total half-life spectrum collected for the

'6C calibration runs and the best ﬁt to those data using the procedure outlined above.

-34-

 

 

 

 

 

 

 

 

1 60000 .
l

_ 0 Raw halt-lite data - 16C

120000 -
— MlnultMlnoe beet tlt
E
3 80000 -
40000 P
p
l-
t
0
0 60 120 180

Clock Channel

Figure 13 Fit to the full half-life spectrum recorded by the implantation
scintillator during the decay of 16C.

The best-ﬁt value of AIcon was 0.642(5), which is within the range deﬁned by
the gated data. Integrating the '6C component with the dead time included leads to a
total number of '6C decays recorded in the experiment as 125(5) x 107 events.
3.4.2 SUMMARY

A similar procedure was followed for all isotopes of interest. In all cases, the
reduced chi-squared of the ﬁts was better than 1.15 and the value for AIcon derived
from the gated spectra reﬂected the value obtained for the total half-life data. Table 4
lists the ﬁnal values for the total number of decays of each of the observed isotopes in
this experiment. Also given in table 4 are the results for a determination of the total
number of decays of the primary isotopes as calculated from the implantation rates and

the scalar dead-times.

-35-

Table 4 Total number of decays recorded by the implantation detector for each of
the isotopes of interest. Also given is the calculated number of decays from the
implantation rate and scalar-measured dead times.

 

 

 

Secondary Beam Isotope Total # of Decays Total # of Decays
from
From Analysis Implantation
1“’C “’C 125(5) x 107 1.30) x 10"
'°C l6N 3.5400) x 105 N/A
”N l7N 2.30) x 107 2.4(5) x 107
l9N 19N 4.64(10) x 107 5.3(12) x 107
”N "’o 4.4(2) x 107 N/A
20N 20N 7.60(10)x 105 7.4(21) x 105
35 NEUTRON BAR ARRAY

The sixteen detectors of the neutron bar array were utilized to determine the
energy of delayed neutrons emitted during the decay of the nuclides of interest and also
the intensity or strength of the observed branches. Therefore, the individual detectors
required calibration for both energy and detection efﬁciency. Since neutron energies
were determined using a time-of-ﬂight technique, the energy calibration is effectively a
time calibration for the neutron bars. To calibrate the array in time, two techniques
were combined: ﬁrst, the slope of the responses from the TDC modules were calibrated
using a precision time calibrator and second, the known delayed neutron lines of "5C
and 17N were monitored to deduce the TDC offset. For the efﬁciency calibration, the
decay of 16C and 17N provided transitions with known intensities over a range of

neutron energies while Monte-Carlo simulations allow for the determination of the

-36-

array’s efﬁciency for higher energy neutrons. Each of these techniques is discussed in
the following sections.
3.5.1 TIME CALIBRATION OF NEUTRON BAR ARRAY

The energy of neutrons emitted following the beta decay of HSC, 17N, 19N, and
20N were determined using a time-of-ﬂight technique. As a consequence, it was
desirable to ﬁnd the time response of the time—to—digital-converters (TDCs) accurately
and independent of the detectors’ intrinsic responses. This was accomplished using a
precision time calibrator (Ortec model 462) where the calibrator start was split and sent
to the implantation detector CFDs, while the calibrator stop was input, sequentially, to
the CFD channel of each photomultiplier tube (PMT) in the experimental set-up. The
period of the time calibrator was set to ten nanoseconds and the range was set to 320
nanoseconds. These were chosen knowing that the TDC modules’ full scales were set
by hand to be approximately 200 nanoseconds. Due to the fact that each bar of the
neutron array contained two PMTS, the time response of both TDC channels was
determined independently. A representative spectrum obtained using the time calibrator
is shown in ﬁgure 14.

This technique was used to determine the slope of the time response of each
photomultiplier tube utilized for time-of-ﬂight measurements. In total there were 32
such PMTS - two for each of the sixteen neutron bars. Table 5 gives the result of the
TAC calibrator measurement for the slope of the PMTS.

The slopes determined by this method were incorporated into pseudo parameters

within the SARA analysis code. The event time for a detected neutron in one of the

-37-

500
400 -

l.
300 -
200 -

 

COUNTS

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1 00 1 1 1 l 1 1 1 l 1 1 1 l 1 1 1
400 800 1200 1 600

CHANNEL

 

2000

Figure 14 Spectrum collected by one of the neutron array bars from the precision
time calibrator.

Table 5 PMT slopes for the neutron bar array as deduced from the precision time
calibrator data.

 

 

 

PMT SLOPE PMT SLOPE
Designation Channels/us Designation Channels/us
BAR 01 R 9.960(4) BAR 01 L 9.891(2)
BAR 02 R 9.899(5) BAR 02 L 9.910(3)
BAR 03 R 10.002(l) BAR 03 L 10.275(2)
BAR 04 R 10.278(3) BAR 04 L 10.317(2)
BAR 05 R 10.506(2) BAR 05 L 10.541(3)
BAR 06 R 10.354(3) BAR 06 L 10.376(2)
BAR 07 R 10.357(4) BAR 07 L 10.511(5)
BAR 08 R 10.300(4) BAR 08 L 10.262(4)
BAR 09 R 10.3l3(2) BAR 09 L 10.133(2)
BAR 10 R 10.317(3) BAR 10 L 10.054(4)
BAR 11 R 10.253(3) BAR 11 L 10.230(2)
BAR 12 R 10.603(5) BAR 12 L 10.601(3)
BAR 13 R 10.609(4) BAR 13 L 10.61 1(5)
BAR 14 R 10.571(3) BAR 14 L 10.553(4)
BAR 15 R 10.578(4) BAR 15 L 10.611(3)
BAR 16 R 10.336(4) BAR 16 L 10.766(4)

 

-33-

array bars was deﬁned as the average TDC channel recorded by the two phototubes of
each array bar relative to the start signal of the implantation detector. The incorporation
of the slopes into the pseudo code results in a histogram for each bar where the channels
reﬂect a time duration of 100 picoseconds.

From these histograms having slopes of 100ps per channel, the energy/time
response of the array bars was determined, on-line, by observing the delayed neutron
emission from known sources. The decay of 1"C and 17N are known to exhibit beta
delayed neutron emission. The energies of the delayed neutrons emitted from these
sources serve as useful on-line calibration beams for the neutron bar array. Table 6
gives the delayed neutron lines from these sources and the reference from which they

are drawn.

Table 6 Calibration delayed neutrons of 1“C and 17N given by energy and
branching ratio.

 

SOURCE ENERGY CM ENERGY LAB INTENSITY REFERENCE

 

keV keV Percent
l7N 407.6 384 38.0 03) 76A1
17N 1245.4 1172 50.1 (13) 76A1
l7N 1808.2 1702 6.9 (5) 76A1
1“’C 859 808 84.4 (17) 9311
“SC 1829 1715 15.6 (17) 9311

 

Secondary beams of '6C and '7N were generated in a manner completely
analogous with i that described in section 2.1 and monitored under experimental
conditions identical to those of the 19N and 20N runs. The exception being that the beam

pulsing sequence was adjusted according to the half-lives of the respective nuclides.

-39-

The spectra collected for 16C and 17N delayed neutrons were analyzed on a bar-by-bar
basis to determine the peaks’ centroids as a function of neutron energy.

To determine the centroid of the calibration peaks, the slope-adjusted spectra
were ﬁt to functional forms containing peak and background components. The peak
shape was approximated by a combination of gaussian and Breit-Wigner functions.
Below the centroid the shape of the peak is gaussian while at channels above the
centroid the shape is Breit-Wigner. This functional form proved to be the most reliable
estimation of the response function of the array detectors. The background term was
simply a broad asymmetric gaussian plus a constant to account for random events and

neutron back scattering from the end-station environment.

 

1 25
1 00
75
50
25
0 .
700 900 1 1 00 1 300 1 500

TDC Channel

Figure 15 Delayed-neutron spectrum collected by one of the array bars during the
decay of “C.

Counts

 

 

 

 

 

As an example, ﬁgure 15 shows the delayed neutron spectrum collected in one
of the array bars during the decay of '6C. The ﬁt to the data is indicated in red where

the best ﬁt was determined by minimizing the total Chi-squared using the program

-40-

MINUIT from the Cem library. Typically, the reduced Chi-squared was between 0.9
and 1.2 for the ﬁts to the individual bars for both the 16C and 17N data.

Table 7 gives the results of the ﬁts to all of the calibration neutron peaks.
Tabulated are the centroids of the calibration lines (in TDC channels) as a function of

delayed-neutron energy.

Table 7 Centroid channels of the neutron peaks utilized in the calibration of the
neutron bar array.

 

 

Array 1715 keV peak 808 keV peak 1702 keV peak 1172 keV peak
Bar Centroid Centroid Centroid Centroid
1 754.9 1009.8 759.1 874.7
2 753.7 1011.7 757.5 874.6
3 776.8 1034.3 781.6 898.5
4 745.9 1005.6 749.0 864.5
5 814.4 1081.2 819.7 937.9
6 796.6 1060.3 803.0 919.9
7 810.6 1069.0 816.6 931.8
8 756.4 1013.5 760.4 877.5
9 737.1 997.3 740.4 858.5
10 752.1 1009.8 757.5 873.1
11 718.7 975.3 725.5 838.3
12 776.7 1036.2 782.0 897.5
13 748.1 1004.4 751.7 868.0
14 732.5 989.1 735.4 853.4
15 729.1 981.9 732.7 851.4
16 773.3 1033.4 778.1 895.1

 

Unfortunately the individual array bars were not efﬁcient enough at low energy to
detect the 384 keV neutron from the decay of '7N. However, in addition to the known

neutron transitions, the plastic bars were able to record events due to the beta particles.

-41-

These peaks provide additional calibration points in the time spectrum of each array
detector. Table 8 gives the MINUIT result for the centroid of the prompt beta transition

on a bar-by-bar basis.

Table 8 Centroid channels of the prompt beta peaks utilized in the calibration of
the neutron bar array.

 

 

Array B-Prompt '6C B-Prompt l7N
Bar Centroid Centroid
1 234.0 234.6
2 232.6 233.4
3 256.2 257.9
4 226.3 226.6
5 294.0 295.2
6 276.2 277.3
7 292.4 293.0
8 238.7 239.4
9 221.4 220.8
10 233.9 234.5
1 1 200.2 200.6
12 258.3 258.5
13 229.4 229.7
14 210.2 210.8
15 201.8 202.3
16 247.7 248.7

 

To calibrate the individual detectors, the neutron and beta energies were
converted into velocities with the dimension of metres/(100 picoseconds). The unusual
unit was chosen to take advantage of the known 100 picosecond wide bins of the PMT
slope corrected histograms. For the neutrons, the velocity was determined from the

general equation:

_42-

E = ‘/2 mBzcz [4]
where E is the laboratory energy listed in table 6. For the neutrons, no relativistic
correction was included as the energies were low enough to treat them classically. For
the B-particles, the energy used to calculate the velocity was for the most probable
energy in the distribution. This was taken to be 0.33 of the B-decay energy - yielding
most probable B energies of 1.265 and 1.522 MeV for 17N and 16C, respectively. These
were deduced by weighting the known decay branches by their normalized probability
and summing over the possible transitions. The calculated velocity of the representative
[3 particles, unlike the neutron case, included a relativistic correction.

The inverse of the calculated particle velocity was ﬁt to the observed histogram
channel using the general form:

l/v = 1/d - (TDCChannel - Oﬁset) [5]
where v is the velocity (m/100ps), d is the distance between the position of the source
activity and the array bar (metres), and the offset is measured in channels or lOOps.
Expanding the equation leads to a simple linear relationship:

UV 2 l/d-TDCChannel—l/d-Oﬁset [6]
or equivalently:

y = M 6.4-3 171
By way of example, table 9 lists the particle energies, their calculated velocities, and
respective peak centroids for the determination of the parameters M and B (leading to d

and Offset) for one of the sixteen array bars.

-43-

Table 9 Parameters derived from the delayed neutron energies and their centroid
channels necessary to calibrate the neutron bars.

 

 

Particle Energy Velocity 1/v Peak Centroid
Type MeV m/ lOOps lOOps/m Channel
Neutron 1.702 0.00180 554.2 759.1
Neutron 1.172 0.00149 667.8 874.7
Neutron 1.715 0.00181 552.1 754.9
Neutron 0.808 0.00124 804.3 1009.8
“5C [3 1.522 0.02823 35.4 234.0
l7N B 1.265 0.02742 36.5 234.6

 

These data, speciﬁcally columns four and ﬁve of table (above) were ﬁt to a line yielding
best ﬁt values for M and B along with their associated interpolation errors. Figure 16
shows the data and the ﬁt for neutron bar number one where the centroid is plotted on

the X—axis and the inverse velocity is plotted on the Y-axis.

 

 

  

 

 

 

A 900
E .
3
§ .
5 600 '
E‘ u
g -
g 300 -
3 _ y = 0.9894x - 195.97
3 F12 = 1
> .
S.
o 1 1 1 1 1
200 500 800 1100
Centroid Channel (100ps)

 

 

 

Figure 16 Plot of inverse velocity versus centroid channel for the calibration of the
neutron bar array.

From the best-ﬁt values of M and B determined on a bar-by-bar basis, values for the
parameters (1 and Offset can be calculated. When known, the constants d and Offset can
be incorporated into new SARA pseudo parameters that will result in histograms that
have normalized the time-of-ﬂight to a ﬂight path of one metre. The channel number
will be a measure of the inverse velocity in units of (lOOps/metre) or, equivalently,
time-of-ﬂight in hundreds of picoseconds. What is perhaps most important is that,
following this transformation, the time-of—ﬂight spectra of the individual array bars are
gain matched, if you will, which allows for a cumulative neutron TOF spectrum to be
recorded. This is advantageous generally, but in particular it ensures that the
experiment as a whole is as sensitive to lower-energy delayed neutrons as possible.
Table 10 lists the values of the best-ﬁt parameters and the calculated values of d and
Offset for each of the sixteen array bars.
3.5.2 NEUTRON BAR ARRAY - EFFICIENCY CALIBRATION

The time calibrated spectra of the individual bars were summed to generate the
ﬁnal delayed neutron spectra with the caveat that bar #15 was excluded due to a failure
of one of the phototubes during the experiment. The summed spectra collected for each
of the calibration isotopes are shown in ﬁgures 17 and 18. Included in the ﬁgures are
the raw data, the individual peak contributions (blue), the two-component background
(black), and the aggregate ﬁt to the data (red). The best ﬁt is the minimum in the total
x2 which was determined using the minimization package MINUIT. The error in the ﬁt
for each parameter was calculated within the MINUIT routine using the associated
MINOS package that removes the parameter correlation and hence delivers a more true

evaluation of the individual parameter errors.

-45-

Table 10 Summary of the calibration fits for each of the array bars. The
parameters (1 and Offset are generated from the M and B results.

 

 

Array Bar M Value B value Distance (1 Offset
(from ﬁt) (from ﬁt) metres Channels
1 0.989(2) — 196(1) 1.01 1(2) 198(1)
2 0.987(2) -1930) 1.013(2) 196(1)
3 0.988(2) 217(2) 1.012(2) 220(2)
4 0.989(2) -187( 1) 1.011(2) 189(2)
5 0.981(4) 252(3) 1.019(4) 257(4)
6 0.983(3) 235(3) 1.018(4) 239(3)
7 0.990(2) 254(2) 1.010(2) 256(2)
8 0.992(3) 200(2) 1.008(3) 202(2)
9 0.992(5) -l81(3) 1.008(5) 183(3)
10 0.991(2) -196(l) 1.009(2) 197(2)
11 0.991(2) -l63(l) 1.009(2) 164(1)
12 0.989(2) 219(2) 1.01 1(2) 221(2)
13 0.992(1) -1920) 1.008(1) 193(1)
14 0.986(2) -1720) 1.014(2) 174(1)
15 0.980(4) -162(3) 1.020(4) 165(3)
16 0.978(2) 206( 1) 1.022(2) 21 1(1)

 

-46-

 

 

\-

 

 

 

1 P . J 4 L l

300 600 900 1200
Time-of-Flight (100 ps)

Figure 17 Aggregate TOF delayed-neutron spectrum recorded by the neutron bar
array during the decay of 16C.

 

 

 

 

 

 

 

10000
17"

1000
.9

§ 100
o
O

10 [\
1 7 1 1 1 1 1 1 .

300 500 700 900 1 100
Time-of-Flight (100 ps)

Figure 18 Aggregate TOF delayed-neutron spectrum recorded by the neutron bar
array during the decay of 17N.

-47-

To generate an efﬁciency calibration curve, the known transitions of the

calibration isotopes (”Q 17N) were integrated. Knowing the individual branching ratios

for the delayed neutron transitions [93Ti, 76A1] and the total number of parent decays,

several calibration points for the total efﬁciency were deduced. For higher energy

neutrons, the efﬁciency of the array has been approximated using a Monte-Carlo

method [93Ha]. Table 11 gives the calculated efﬁciency points from the on-line data

and ﬁgure 19 shows the ﬁnal efﬁciency calibration curve including the Monte-Carlo

data and the spline ﬁt. The error in the efﬁciency determination has been shown to be

about ten percent of the extracted value [93Ha].

Table 11 Data points used to generate and efficiency calibration for the neutron
bar array. The monte-carlo points are from reference [93Ha].

 

 

Source En (keV) Branch (%) Parent Peak Area Efﬁciency
Decays Total
l7N 384 38.0 (13) 2.3(1)E7 6.1(5)E3 6.98(55)E-4
l7N 1172 50.1 (13) 2.3(1)E7 3.33(4)1~:5 2.89(10)E-2
”N 1702 6.9 (5) 2.3(1)E7 5.07(32)E4 3.20(31)1~:2
16c 808 84.4 (17) 1.25(5)E7 1.50(3)E5 1.42(5)E-2
“5c 1715 15.6 (17) l.25(5)E7 8.51(16)E4 4.37(49)E—2
Monte Carlo 1800 N/A N/A N/A 3.35(34)E—2
Monte Carlo 3000 N/A N/A N/A 3. l6(32)E-2
Monte Carlo 4500 N/A N/A N/A 2.86(29)E-2

 

-43-

 

0.05

 

  
  

 

 

 

 

 

 

 

 

>. .
(z, 0 04 : +

9.1 ' : J1

‘E’ 0.03 5 w {J
h 0 02 E —SPLINE FIT

..l ' : I 17N DELAYED NEUTRONS

E 0_01 I. I 16C DELAYED NEUTRONS

O I :1 MONTE CARLO

'- 0. 111l111l111l111

0 1000 2000 3000 4000 5000
NEUTRON ENERGY (keV)

Figure 19 Calibration curve for the total efficiency of the neutron bar array.

353 NEUTRON BAR RESULTS — l9N AND 2"N

The delayed neutron spectra of the isotopes of interest (”M 20N) were recorded
under experimental conditions identical with those of the calibration beams of "3C and
'7N. The spectra collected by the individual bars were summed to generate cumulative
delayed neutron spectra suitable for integration. The raw data were ﬁt to peaks having
the same functional form as with the calibration spectra and were minimized using the
MINUIT/MINDS package.
3.5.4 19N DELAYED NEUTRON SPECTRUM

Figure 20 shows the cumulative neutron TOF spectrum collected by the neutron
bar array during the decay of 19N. The background components of the ﬁt are shown in
black, the individual peaks are shown in blue, and the sum of the components is shown

in red. The T OF scale is given in bins having 100 ps width.

-49-

1 0000

 

 

 

 

 

 

 

a, 1000
'E
3
O
0 100
10
250 1000 1250

50Time-of-Flight (100 ps)
Figure 20 Delayed-neutron time-of-flight spectrum collected by the neutron bar

array during the decay of 19N.

As is evidenced by the ﬁgure, the delayed neutron spectrum of ”N is a rich one.
The best-ﬁt to the data indicates that there are eleven delayed-neutron lines with eight
of these being reasonably well-separated, and three appearing as shoulders on larger
peaks. From the best-ﬁt centroids and knowing the time of ﬂight scale, the energy of
the neutron lines may be calculated. Table 12 lists the peak centroids, and calculated
delayed neutron energy in both the laboratory and center-of-mass reference frames.

From the integrated areas of the eleven observed neutron peaks, the total
efﬁciency calibration, and the sum of the parent beta decays, the branching ratios of the
transitions can be calculated. Table 13 lists the 19N observed delayed neutrons by
energy, the values of the parameters necessary for the branching ratio calculation, and

the corresponding branching ratios.

-50-

Table 12 Delayed neutron lines observed during the decay of 19N. The TOF is
given along with the neutron energies in both the lab and center of mass frames.

 

 

Peak Designation Time of Flight Neutron Energy Neutron Energy
( 100 ps) Lab. (MeV) C.o.M. (MeV)
1 335.24 465(8) 491(8)
2 371.55 379(6) 400(6)
3 424.68 290(5) 3.06(5)
4 441 .01 269(4) 2.84(4)
5 471.07 236(4) 249(4)
6 499.75 209(4) 221(4)
7 659.86 120(3) 127(3)
8 699.93 1.07(3) l.13(3)
9 814.76 079(3) 083(3)
10 944.24 059(3) 062(3)
1 1 1066.70 046(3) 049(3)

 

Table 13 Delayed neutron branching ratios for those neutron lines observed in the
neutron bar array during the decay of 19N.

 

 

Neutron E.1 (MeV) Peak Parent Total Branching
Peak COM Area Decays Efﬁciency Ratio (%)

1 491(8) l.l9(3)E4 4.64(10)E7 2.85(29)E-2 090(10)
2 400(6) 5. 13(32)E3 4.64(10)E7 3.00(30)E-2 037(4)
3 3.06(5) 2.91(36)E4 4.64(10)E7 3.18(32)E-2 1.97(32)
4 2.84(4) 1.44(3)E5 4.64(10)E7 3.22(33)E-2 9.6(1 1)
5 249(4) 5.34(12)E4 4.64(10)E7 3.29(33)E-2 3.5(4)
6 221(4) 3.50(78)E4 4.64(10)E7 3.36(34)E-2 2.2(4)
7 1 .27(3) 2.60(62)E4 4.64(10)E7 2.96(30)E-2 1 .9(2)
8 1 . 13(3) 1.88(2)E5 4.64(10)E7 2.50(25)E—2 16.2(17)
9 083(3) 1.25(7)E4 4.64(10)E7 1.33(14)E-2 2.0(3)
10 062(3) 6.48(29)E3 4.64(10)E7 6.51(66)E-3 2.2(3)
1 1 049(3) 1. 14(4)E4 4.64(10)E7 2.84(29)E-3 8.7(10)

 

-51-

3.5.5 20N DELAYED NEUTRON SPECTRUM

The data collected by the neutron bar array during the decay of 20N was summed
in a manner consistent with the previously detailed analyses. Unfortunately, the
production of 20N from a primary beam of 22Ne requires a particle transfer or charge-
exchange mechanism rather than the more probable nucleon removal. As a result, the
intensity of the 20N secondary beam was dramatically reduced relative to the other
secondary isotopes studied. Because of this reduction in intensity, the data collected by
the neutron bar array was compressed by a factor of two to better examine the features
of the plot and to improve the ﬁts to the spectrum.

Once again, the individual bar data were summed to produce and aggregate
delayed-neutron time-of-ﬂight spectrum for the decay of 20N. The time bins, in this
case, had widths of 200 ps. Figure 21 is the aggregate neutron bar spectrum and shows
the raw data, background terms (black), individual peaks (blue) and the cumulative ﬁt
to the Spectrum (red). It is noted that this spectrum has the y-axis on a linear‘scale
rather than on a logarithmic scale as for the previous spectra.

The cumulative ﬁt from MINUIT supports a delayed neutron spectrum
consisting of eight transitions. Six of these are well separated in energy while the other
two appear as shoulders of larger peaks. From the best-ﬁt centroids, the energy of the
delayed neutrons can be calculated knowing that each channel represents 200 ps in time.
Table 14 lists the 20N peak centroids, and the corresponding energies in both the

laboratory and center of mass reference frames.

-52-

250

 

200
:2 150
a
8 100 f
0
50 . . MM.

100 200
TlME-OF-FLIGHT (200 ps)

Figure 21 Delayed neutron spectrum recorded by the neutron array during the

decay of 20N

Table 14 Summary of delayed neutrons observed during the decay of 20N. Listed
are the centroid channels and the corresponding neutron energy in both the lab

and center of mass frames.

 

-.Q'.‘Lt l— s a...

300

1"’N NEUTRON EAR SPECTRUM

 

 

 

Peak Designation Centroid Neutron Energy Neutron Energy
lch = 200 ps Laboratory (MeV) C.o.M. (MeV)
1 171.9 4.41(7) 4.64(7)
2 188.1 368(6) 388(6)
3 201.4 321(5) 338(5)
4 228.4 2.50(4) 2.63(4)
5 250.3 208(4) 2. 19(4)
6 290.6 l.55(3) 163(3)
7 321.7 126(3) 1.33(3)
8 343.6 1.1 1(3) l.l6(3)

 

From the integrated peak areas, the measured number of parent 20N decays, and

the efﬁciency calibration, the branching ratio for each transition was calculated. Table

 

15 is a summary of the branching ratio calculation for 20N delayed neutrons that

includes the values of the parameters necessary to elucidate the decay strength.

Table 15 Calculated branching ratios for the delayed neutron lines observed by
the neutron bar array during the decay of 20N.

 

 

Neutron En (MeV) Peak Parent Total Branching
Peak COM Area Decays Efﬁciency Ratio (%)
1 1 . 16(3) 900(30) 7.6(1)E5 2.65E-02 4.5(5)
2 1.33(3) 600(38) 7.6(1)E5 3.12E—02 2.5(3)
3 1.63(3) 2200(57) 7.6(1)E5 3.47E-02 8.3(9)
4 219(4) 3087(56) 7.6(1)E5 3.37E-02 12.1(13)
5 2.63(4) 329(33) 7.6(1)E5 3.26E-02 1.3(2)
6 338(5) 486(25) 7.6(1)E5 3.12E-02 2.1(3)
7 388(6) 624(26) 7.6(1)E5 3.03E—02 2.7(3)
8 4.64(7) 102( 19) 7.6(1)E5 2.88E-02 0.5( 1)

 

3.6 “Li INORGANIC SCINTILLATORS

In addition to the neutron bar array, the experimental set-up included three 6Li
glass scintillators to detect delayed neutron events. The TDC modules for these devices
were calibrated in a manner completely analogous with the procedure outlined for the
bars of the array. The slopes of the TDC channels were determined by the use of a
precision time calibrator set to 10ns intervals and a 320ns range.

To complete the calibration, the decay of the isotopes 16C and 17N were
monitored by the glass scintillators. Figures 22 and 23 Show the spectra collected by
one of the three 6Li detectors (detector #3) in the time domain for each of the '6C and
‘7N calibration beams. Included in the plot is the best ﬁt to the data assuming a
gaussian peak shape below the peak centroid and a Briet-Wigner shape above the

centroid. The error bars are the statistical errors of the points only. It is noted that, due

-54-

to the low total efﬁciency of the 6Li devices, the spectra are compressed to seven hits to

better illuminate the salient features of the presented time-of-ﬂight plots.

 

1 00
“U #3 Spectrum Of 1'C
3 75
§ 50
O
U

25
0

 

 

20 40 60 80 1 00
Channel (7 Bit Full Scale)
Figure 22 Delayed neutron spectrum collected in "Li detector #3 during the decay

of 16C.

 

150

6L1 #3 Spectrum of "N

Counts
01
O

 

 

0

 

20 40 60 80 100
Channel (7 Bit Full Scale)

Figure 23 Delayed neutron spectrum collected in 6Li detector #3 during the decay
of N.

-55-

As with the neutron bars, a calibration curve was constructed on the basis of the
observed centroid channel of the delayed neutrons and their known energies. The
parameters of the ﬁt were the same as outlined in section 3.5.1 and will not be repeated
here. Figure 24 shows the calibration curve for one of the 6Li detectors (#3) generated
using the compressed data. The y-axis value is inverse velocity in (ns/m) and the x-axis

is the TDC channel of the neutron peak centroid.

 

   

 

 

 

130 _
105 E
’E‘ E
3, 80 L
§ ; 111: = 3.3289*CH - 35.014
r R2=0.9992
55 L-
l.
30 I. 1 l 1 l 1
20 30 40 50
TDC Channel

Figure 24 Time calibration curve for 6Li detector #3 from the 16C and 17N decays.

The procedure was repeated for each of the three 6Li detectors in the seven-bit
time domain. The ﬁnal values of the slope and offset parameters relating the TDC
channel to the inverse velocity are given in table 16. The inverse velocity in (ns/rn) will
be equal to the channel multiplied by the slope parameter plus the offset parameter. The

energy of the neutron in MeV will be 5227.083 times the squared velocity in ns/m.

-56-

Table 16 Calibration parameters for the lithium detectors in the seven bit time
domain. The inverse velocity of a neutron in (us/m) is equal to the TDC channel
times the slope plus the offset.

 

 

Detector Slope Offset
6Li #1 3.631 -36.406
6Li #2 7.651 -114.15
‘11 #3 3.329 25.014

 

3.6.1 EFFICIENCY CALIBRATION

To determine the efﬁciency of the 6Li glass detectors as a function of energy, the
four delayed neutrons emitted by the calibration isotopes, 17N and 16C were integrated in
each of the spectra. From these data, the known branching ratios of the neutron lines,
and the total number of decays extracted from the half-life spectra, a measure of the 6Li
glass detectors’ efﬁciencies at the corresponding energies was calculated. Table 17
gives the measured total efﬁciency as a function of neutron energy for the three 6Li

detectors.

Table 17 Calibration points utilized to determine the total efficiency of each
lithium detector. The source neutrons are from the decay of 16C and 17N.

 

 

15., keV 6Li #1 e—total 61-1 #2 e—total 61-1 #3 e—total
384 1.6805) E4 2.6206) E-4 135(9) E-4
808 4.06(39) E—5 6.65(61) E-S 3.13(31) ES
1172 3.81(35) E-S 7.96(49) E-5 3.01(22) 13-5
1715 3.39(62) E-S 5.70(94) E-S 3.08(58) E—5

 

The neutron capture cross section of (Li is well known over a large range of

incident neutron energies and they have been tabulated in the common nuclear

-57-

databases [OONN]. The molar composition of the inorganic material was supplied by

the manufacturer [973i] and the isotopic fraction of 6Li was also made available. From

the known neutron-capture cross section and the physical characteristics of the

scintillators, the intrinsic efﬁciency of the three 6Li detectors was calculated following

the procedure outlined in [81Fr]. The center of mass energies were converted to lab

energy for the cross-sectional data and the intrinsic efﬁciency curve was scaled to ﬁt the

measured total efﬁciencies supplied by the calibration beams of '6C and 17N. Figure 25

shows the scaled intrinsic efﬁciency for 6Li detector #3 and the data to which the cross-

sectional curve was scaled.

 

 

 

 

 

 

1.E-03
6Lure

>1
0
C
.2
.2
Ei'LEEO4
5
O f
h h

1.E‘05 111111 111111

100 600 1100 1600
Lab Energy (keV)

Figure 25 Calibration curve for the total efficiency of 6Li detector #3.

2100

The scaling factor derived for the three 6Li glass scintillators should be

representative of the detectors’ geometric efﬁciencies. The solid angle coverage of the

-58-

detectors was calculated from the measured physical geometry and crystal dimensions.

The necessary parameters to calculate the geometric solid angle are shown in ﬁgure 26.

Hperture

H

“ﬁ—H

2 D

 

Source

Figure 26 Parameters necessary to determine the geometric efﬁciency of an
aperture.

Here, A is the detector radius (2.54cm), Z is the linear source-to-aperture distance, and
D is the distance between the source activity and the edge of the aperture. From these
values, and assuming that the source/detector separation is much larger than the source
spread, the geometric solid angle (GP) may be calculated from the expression:

G = l/2(1—Z/D) [8]

P
Table 18 lists the geometric solid angle for each of the 6Li detectors calculated
in this manner along with the scaling parameter for the efﬁciency ﬁts based on the

reported cross-sectional data. The error in the scaling parameter reﬂects the change in

its value necessary to produce a A12 of one unit in the ﬁt.

-59-

Table 18 Calculated value of the geometric efficiency and best fit value of the
scaling parameter for each of the lithium detectors.

 

 

Detector Geometric Efﬁciency Scaling Factor
611 #1 4.1102) x 10‘3 3.5707) x 10'3
‘11 #2 l.46(8) x 10‘2 1.5703) x 10'2
61.1 #3 264(7) x 10'3 2.89(25) x 10'3

 

Of note here is the good agreement between the solid angle estimates from the
physical geometry and the best-ﬁt value for the scaling parameter, particularly for
detectors two and three. The agreement provides a greater measure of conﬁdence in the
total efﬁciency derived for the 6Li glass scintillators and thus for the branching ratio
determinations for the l9N isotope of interest.

3.6.2 l”N RESULTS — “Li DETECTORS

Unfortunately, the intensity of the 20N secondary beam taken together with the
total efﬁciency of the 6Li detectors resulted in too few events recorded by the devices to
be of use. As a consequence, the results presented here are restricted to those for l9N.

The goal behind including the 6Li detectors was to enable the observation of low
energy delayed neutrons from the decay of 19N. To accentuate the low energy peaks in
the 6Li spectra, the time calibration was utilized to re-bin the data into the energy
domain. The following three ﬁgures 2729 show the spectra collected by each Of the
6Li detectors during the decay of 19N where the data have been converted into an energy
scale and redistributed into bins of 40 keV width. Also shown are the ﬁts to the peaks
where the peak shapes used were Simple gaussians and the background is of the form

l/xz.

 

  

 

 

 

120 C ' .
Udetector #1
100
(I)
§ 80
o 60 3 . ‘ ‘
o 23:.
'400 41 'i‘\_
800 1200 1600
Energy (keV)

Figure 27 Energy spectrum of delayed neutrons recorded by ‘Li detector #1
during the decay of 19N.

 

 

 

 

 

1 60 *
“Ll detector #2

1 20
.‘2
§ 80 A
O
o -

40 —J=—‘ 3.
‘ I k“ $-
0 s 1 l 1 1 1
800 1200 1600

Energy (keV)

Figure 28 Energy spectrum of delayed neutrons recorded by 6Li detector #2
during the decay of 19N.

-61-

 

120

   

1 00 6Li detector #3
'2', 80 J ' .
§ 60
O ,
(J

\

 

   

 

—

 

i Al
1 L _,

1 200 1 600

40 " \
g -- all‘
0 . We. .
O 00 800

4

Energy (keV)
Figure 29 Energy spectrum of delayed neutrons recorded by ‘Li detector #3

during the decay of 19N.

From the integrated area of the peaks, the detection efﬁciency calibration, and
the total number of '9N decay events, the branching ratio can be deduced for the
transitions observed in each of the three devices. The dominant sources of error in the
branching ratio determination come from the integrated area of the peaks and the total
efﬁciency. Due to the uncertainty in the shape of the 6Li energy spectrum background,
the error was estimated from a series of ﬁts to the data with different background shapes
including exponentials, lines, and functions of the form l/xz. All ﬁts were done in
MINUIT/MINDS and the large uncertainties in the peak areas reﬂect differences in the
best—ﬁt values of the peak parameters over the various functional forms of the
background.

The error in the efﬁciency comes from two sources; ﬁrst, the error in the scaling

parameter imparts an error of nearly ten percent in all three cases and, second, the error

-62-

in the centroid energy from the ﬁts drives an additional uncertainty in the efﬁciency,
particularly for low energy neutrons where the slope of the efﬁciency curve is
extremely steep. Consequently, taking the energy error into account yields large
fractional errors for the ~280 and ~460 keV transitions. Table 19 gives the necessary

parameters and the calculated branching ratios for the delayed neutrons by detector.

Table 19 Branching ratio determination for delayed neutrons observed by the
lithium detectors during the decay of 19N. Also tabulated are the parameters
necessary for the branching ratio calculation along with the associated errors.

 

 

 

 

Detector Neutron Peak Area 19N Total Efﬁciency Branching
Energy (keV) Decays (Total) Ratio (%)
6Li #1 280(80) 331(115) 4.64(10)E7 4.9(13)E-4 l.5(7)
6Li #1 450(40) 449(1 15) 4.64(10)E7 l .01(16)E-4 9.6(29)
6Li #1 1040(40) 305(100) 4.64(10)E7 3.86(41)E-5 17.0(59)
6Li #2 270(80) 483( 155) 4.64(10)E7 8.6(14)E-4 1.2(4)
6Li #2 460(40) 649( 155) 4.64(10)E7 1.63(24)E-4 8.6(20)
6Li #2 1070(40) 590(195) 4.64(10)E7 6.61 (69)E-5 19.2(67)
6Li #3 280(80) 306( 80) 4.64(10)E7 3.93(65)E-4 1.7(6)
6Li #3 440(40) 306(80) 4.64( 10)E7 8.5(19)E-5 7.8(27)
6Li #3 1040(40) 267(50) 4.64(10)E7 3. 13(34)E-5 18.4(40)

 

The branching ratio results for the corresponding transitions among the three 6Li

detectors are in excellent agreement. The weighted means of the measurement yield
branching ratios of 1.4(22) %, 8.6(10) % and 18.2(23) % for the delayed neutrons
having laboratory energies of about 280 keV, 460 keV and 1050 keV respectively.
3.7 GAMMA RAY DATA

The experimental end-station included two HPGe detectors to monitor gamma-
rays emitted during the decay of the isotopes of interest. The detectors were energy

calibrated with in-house sources of I34Cs, 207Bi, and 60Co - all of which have known

-63-

decay schemes and well-established gamma cascades. The detectors were energy
calibrated over the full 8192 channel spectra.

The in-house sources were monitored by afﬁxing the source’s solid support
backing to the implantation scintillator. However, unlike the radioactive beam runs, the
implantation scintillator was removed from the trigger and replaced by a NaI detector
"start" signal. This substitution for the implantation scintillator "start" permitted
conclusive identiﬁcation of the gamma ray causing the "start" signal in the electronics
which later facilitated a determination of the HpGe detectors’ efﬁciency at various
known energies. The NaI detector was placed directly beneath the implantation
scintillator to provide the maximum possible solid angle coverage which averaged out
angular correlations. As an example of the spectra collected in the HPGe detectors
using the calibration sources, Figure 30 shows the response of one of the gamma-ray
detectors to the 60C0 activity.

The peaks observed in the Germanium detectors were ﬁt to symmetric gaussian
shapes plus linear backgrounds. Using these data, a calibration line for each of the two
germanium detectors was determined by plotting the centroid channel versus the known
transition energy. The data points were ﬁt to a line using EXCEL. The raw data and
the results of the straight line ﬁts are shown in ﬁgure 31. The slope and offset for the
energy calibration of Gel are 0.4919(2) keV/CH and ll.51(31) keV, respectively, while
the slope and offset for the full-scale energy calibration of Ge2 are 0.7309(3) keV/CH

and 9.02(29) keV.

 

1 0000

1 000

 

100

Counts

 

10

 

 

 

1 1 l 11 l 1 l 1 l

0 2000 4000 6000 8000
Ge #2 - Channel

Figure 30 Gamma-ray spectrum collected in Ge #2 from the 60Co calibration
source.

 

 
  
  

 

 

 

2500 _
2000 E
g : Ge2 (keV) = 0.7309*CH + 9.02
3 1500 5
a Z
3 1000 -
5 Ge1 (keV) = o.4919*c1-1 + 11.51
500 :
o h 1 L 1 l 1 1 1 l 1 1 1 l 1 1
0 1 000 2000 3000 4000

Channel
Figure 31 Energy calibration curves for Ge#l and Ge#2.

-65-

3.7.1 GAMMA-RAY SPECTRA OF THE ISOTOPES OF INTEREST

The experiment proceeded by implanting secondary beam activity within the
start detector for a set time followed by a beam—off interval where the real-time clock
was in play. The germanium detectors, however, were supported by an electronics
algorithm that permitted data to be collected during both the implantation period and the
beam-off period. This was done to allow for the possibility of observing gamma rays of
lower intensity. For the branching ratio determinations outlined in section 4.1, the
included data are restricted to the beam-off interval as this is the portion of the sequence
from which the beta activity may be conclusively extracted.

Figures 32 through 39 show the spectra collected in the germanium detectors
during the beam-off sequences of the data runs for the four secondary beams (”Q 1‘,N,
19N, 20N). The spectra are plotted on a logarithmic scale of counts versus energy in keV

as calculated from the calibration formulae indicated in ﬁgure 31.

-66-

1000

 

Ge#1 16C

100

Counts

10

   

1

 

'Ml I1 I w w

 

.Il.

 

 

 

 

 

 

 

 

 

2000 3000
EWO‘OV)
Figure 32 Gamma-ray spectrum collected by Ge#l during the decay of 16C.
1000
Ge#2 1“c
100
E
8
1O ‘
N H "' - u
, III ll‘ 1 II I * II." II. H. l J M l. H
0 1000 2000 3000 4000

Energy (keV)

Figure 33 Gamma-ray spectrum collected by Ge#2 during the decay of 16C.

-67-

1000

 

Ge# 1 "N

100

10 I
- - lllll
1000

0

Counts

    

2000 3000
Energy (keV)

Figure 34 Gamma-ray spectrum collected by Ge#l during the decay of 17N.

 

 

1000 ~
Ge#2 "N

Counts

 

 

o 1000 2000 3000
Energy (keV)

Figure 35 Gamma-ray spectrum collected by Ge#2 during the decay of 17N.

-68-

 

 

 

 

 

 

100000
Ge #1 19"

10000

1000
g
5

100

10

1 L 1 1 l 1 1 L L 1 1 1
0 1000 2000 3000 4000

Energy (keV)
Figure 36 Gamma-ray spectrum collected by Ge#l during the decay of 19N.

 

1ooooo
Ge#2 "N

10000

 

Counts

 

 

 

Energy (keV)

Figure 37 Gamma-ray spectrum collected by Ge#2 during the decay of 19N.

-69-

1000

 

Ge#1 20N

100

 

Counts

 

 

 

 
 

 

10 , 1
i I 1 I
1 H 1
, 111111111 1| 1 1| 1 1 1 I .l I l.
0 1000 2000 3000 4000

Energy (keV)

Figure 38 Gamma-ray spectrum collected by Ge#l during the decay of 20N.

 

  

 

 

 

 

 

 

 

 

 

 

 

 

 

1mm~
Gb#2 20N
100
3
§
8 1
10. 1
. x 1' 11“
; ‘ ' 1|
, 111111111111 1. 1111111 . . L -1
0 1mm 2mm am» «no

Energy (keV)
Figure 39 Gamma-ray spectrum collected by Ge#2 during the decay of 2"N.

-70-

3.7.2 BACKGROUND GAMMA RAYS

Gamma-ray lines that appeared in the spectra of any three of the four isotopes of
interest were assigned as being due to background. Table 20 lists, by detector, the
transitions satisfying this criterion. As expected, there is a direct correspondence
between the two detectors for the gamma rays identiﬁed in this way. The exception to
the background assignment, of course, is the 511 keV transition which is not necessarily

due to background exclusively.

Table 20 Gamma rays attributed to background.

 

 

 

Ge # 1 Ge # 2
Energy (keV) Energy (keV)
352 351
420 419
442 441
5 l l 5 l l
569 571
605 606
844 840
1014 1014
1273 1273
1367 1368
1808 1809
3.7.3 RATIO OF EFFICIENCIES

To determine the detection efﬁciency ratio of Gel to Ge2, and to detect
systematic problems that may have developed over time in the germanium detectors, the
ratio of counts in Gel to Ge2 for non-background transitions was calculated. The ratio
of Gel to Ge2 intensities should be a constant for all gamma transitions originating at
the implantation position provided that both germanium detectors were stable over the

course of the experiment. Furthermore, if constant, the ratio of the counts is equal to

-71-

the ratio of the total detection efﬁciencies. Figure 40 shows a plot of the ratio of counts
in Gel to counts in Ge2 (during the beam-off period) versus the energy of the gamma-

ray line for those transitions observed during the 19N production runs.

h

 

(A)
I

 

 

Ge#1 I Ge#2 (Beam Off)
N

 

 

 

1 h
_ Ratio = 2.224(53)
o 1 1 1 l 1 1 1 l 1 1 1 l 1 1 1
0 1 000 2000 3000 4000
Energy (keV)

Figure 40 Ratio of counts detected in Ge#1 to counts in Ge#2 during the beam-off
period of the 19N experimental runs.

The ﬁgure suggests that the ratio of counts is indeed a constant. The value of the
constant is 2.225(53) determined from a weighted linear ﬁt to the data yielding a best-ﬁt
slope consistent with zero. Thus, the total detection efﬁciency of Gel is equal to this
constant multiplied by the total detection efﬁciency of Ge2 over the energy range
indicated.

3.7.4 GAMMA RAY SEGREGATION
As noted earlier, the germanium detectors were active during both the

implantation and beam-off intervals. By taking the ratio of counts detected at each

-72-

gamma line over all times to the counts detected during the beam-off period, it was
possible to segregate the observed gamma rays into groups. This was useful for
assigning gamma rays as being due to the decay of the parent or the decay of the
daughter.

The implantation and beam-off periods were set to maximize the counts detected
from the isotope of interest. However, there may be signiﬁcant total activity due to the
beta decay of the daughter. The intensity of counts detected over all times to beam-off
will not be the same for the daughter(s) as it will be for the isotope of interest — due to
the difference between the half-lives. Thus, if the statistics are good enough, a plot of
the counts over all times to beam-off counts should ﬁlter gammas due to the daughter’s
decay from those due to the implanted species. Figure 41 shows an example of this for
”N where the ratio of counts in Ge2 for all times to beam-off is plotted against
transition energy.

From ﬁgure 41 it appears that four of the observed gamma rays (blue) are not
from the same source as the others (red). This is not unexpected as, in addition to
gamma rays from the decay of '9N, there should also be gamma rays present following
the beta decay of the 19N beta daughter, '90. This result was useful for the assignment
of previously unobserved gamma rays.

3.7.5 GAMMA RAY HALF-LIFE MEASUREMENTS

In addition to the detection ratios, the gamma rays were assigned on the basis of
their half-life. In most cases, the intensity of the transitions was sufﬁciently strong to
permit an analysis by this technique, however, it is noted that for the weakest transitions

this method was not conclusive due to statistics and to the unfortunate fact that the

-73-

general background, whether due to Compton edges or betas or..., tended to have a

half-life consistent with the dominant source implanted.

 

 

 

 

"a? 2 —
1n Ii I I
a _ i i Ii ' 'l
55
‘3: 1 _ I III
(5

o 1 1 l I I 1 l 1 1 1 l 1 1 L

0 1 000 2000 3000 4000

Energy (keV)

Figure 41 Ratio of counts detected in Ge #2 over all times to beam-off period
during the decay of 1 N.

Unlike the determination of the beta half-lives, the approach taken here was not
rigorous. To circumnavigate the dead-time, the half-life spectra were ﬁt over the ﬁnal
80% of the clock channels rather than utilizing the more complicated dead-time/‘t
formalism outlined in section 3.4.1. Fortunately, the high purity of the secondary
beams and the dramatic difference between the half-lives of the parent and daughter
isotopes permitted a conclusive separation of gamma rays even using this less rigorous
approach. It is necessary to note that the best-ﬁt value of the half-lives from the gamma

gating were systematically longer than those from the beta decay analysis — not

-74-

surprisingly. As an example, ﬁgure 42 shows two representative half-life spectra from

gamma rays observed during the implantation of a l9N secondary beam.

 

1 0000

  
 
  

 

1000 in
a, ll Gated on ~97 keV
'E gamma ray
3 IlGated on ~198 keV
G gamma ray

 

 

 

100

Tm = 301(6) msﬁuI-l
Ilium!

1o 20 30 4o 50
Clock Channel

Figure 42 Gamma-ray gated half-life spectra for two transitions observed in Ge
#1.

 

 

3.7.6 GAMMA RAY ASSIGNMENTS 1"N, 20N SECONDARY BEAMS

The gamma gated half-life spectra and the ratio of counts analysis facilitate the
assignment of gamma rays to speciﬁc sources and as originating from speciﬁc levels.
From the half-life data, the identity of the beta-decaying isotope may be extracted and
from the energy of the gamma ray, the speciﬁc transition can be determined by
comparison with daughter levels and in some cases, known transition energies given in
the literature. Table 21 give the assignment of the observed, non—background gamma

rays for the ”N secondary beam runs and table 22 for the 20N secondary beam runs.

-75-

Table 21 Gamma ray assignments for transitions observed during the beam-off
period of the 19N production runs.

 

 

R.I.B. 'y-Ray B-Decay y—Ray E-initial E-ﬁnal
keV Source Source level level

”N 96 ”N ”o 96 0
”N 110 ”o ”F 1 10 0
”N 197 ”o ”F 197 0
”N 511 - - - -
”N 821 ”N ”o 4456 3634
”N 1357 ”o ”F 1554 197
”N 1374 ”N ”o 1472 96
”N 1444 ”o ”F 1554 110
”N 1472 ”N ”o 1472 0
”N 1554 ”o ”F 1554 0
”N 1633
”N 1652 ”N ”o 3634 1982
”N 1938 ”N 18o 3920 1982
I9N 1960 I9N 190 _ _
”N 1980 ”N 18o 1982 0
”N 2473 ”N ”o 3945 1472
”N 2623 ”N ”o - -
”N 2822 ”N ”o - -
”N 3136 ”N ”o 3232 96
”N 3338 ”N ”o - -
I9N 3434 19N 190 _ _
”N 3849 ”N ”o 3945 96
”N 3945 ”N ”o 3945 0

 

Table 22 Gamma ray assignments for transitions observed during the beam off
period of the 2”N production runs.

 

 

R.I.B. 'y-Ray B-Decay y—Ray E-initial E-ﬁnal
keV Source Source level level

”N 96 ”N ”o 96 0
”N 110 ”o ”F 1 10 0
”N 197 ”o ”F 197 0
”N 1057 ”o ”F 1057 0
”N 1 127
”N 1357 ”o ”F 1554 197
”N 1374 ”N ”o 1472 96
”N 1444 ”o ”F 1554 110
”N 1634 ”F ”Ne 1634 0
”N 1674 ”N ”o 1674 0
”N 1896 ”N ”o 3570 1674

 

-76-

3.7.7 EFFICIENCY CALIBRATION - STANDARD SOURCES

The efﬁciency of detectors Gel and Ge2 at speciﬁc gamma-ray energies was
established utilizing both on-line and off-line data. The bulk of the efﬁciency
determination arises from standard sources that were placed at the implantation position
and monitored by the germanium detectors with a sodium iodide serving as the trigger
for the electronics. This is the off-line portion.

The technique involved selecting one gamma ray in the Nal detector and
calculating the number of gamma rays at a different energy that would be emitted in
coincidence with the ﬁrst one. The sources used were '34Cs, szi, and 60Co which all
have gamma cascades as a part of their decay scheme [96Fi]. The following cross
tables, for each of the isotopes (tables 23-25), detail the coincidence strength for gamma
ray pairs in a cascade. The column headers indicate the energy of the gamma ray
detected by the Nal trigger. The row headers indicate the energy of a second gamma
ray that is in coincidence with the trigger — available to be recorded by one of the
germanium devices. The table value is the fraction of the column-energy counts that
will be emitted at the row energy. The efﬁciency of each germanium detector at a
particular energy was determined by gating on one of the gamma rays detected in the
Na] crystal. The trigger peak was integrated to determine the total number of start
Signals generated by the trigger gamma ray. As an example, ﬁgure 43 shows the
relevant portion of the Nal spectrum collected for the 60Co calibration source and the

ﬁts to the two peaks.

-77-

Table 23 Cross-table for 60Co. For N gamma rays at "column" energy there are N
times "cell value" gamma rays emitted at "row" energy in coincidence.

 

 

”Co 6 1173 1332
1173 0 0.99945
1332 1 0

Table 24 Cross-table for 207Bi. For N gamma rays at "column" energy there are
N times “cell value” gamma rays emitted at "row" energy in coincidence.

 

 

 

”7131 1770 1442 1063 898 328 570
1770 o 0 0 0 0 6.91e-2
1442 0 0 0 l l 7.10e-6
1063 0 0 0 0 0 8.42e-1
898 0 9.94e-1 0 0 0 0
328 0 5.50e-3 0 0 0 7.1e-6
570 1 5.50e-3 l 0 l 0

 

Table 25 Cross-table for 13"le. For N gamma rays at “column” energy there are
N times “cell value” gamma rays emitted at “row” energy in coincidence.

 

 

”4C3 1365 802 569 1039 475 796 1168 563 605

 

 

l 365 0 0 0 0 0 0 0 0 3.10e-2
802 0 0 0 0 0 0 8.57e-l 8.57e-l 7.30e-2
569 0 0 0 0 0 1.81e-1 0 0 l.58e-2
1039 0 0 0 0 0 0 0 0 1.00e-2
475 0 0 0 0 0 0 1.43e-l 1.43e-1 1.20e-2
796 0 0 0 0 0 0 0 0 8.69e-l
l l 68 0 1.77e- l l 0 1.77e-1 0 0 0 0

563 0 8.23e-1 0 0 8.23e-l 0 0 0 9.00e-2
605 l 8.23e-l l l 8.23e-l l 0 l 0

-73-

 

80000

 
 
   

 

 

 

3 60000 E
§40000 .
0 20000 /
o .
1100 1200 1300 1400 1500

Nal Channel

Figure 43 Gamma-ray spectrum of 60Co collected by the Nal start detector.

The energy of the trigger gamma ray, in this case 1332 keV, was designated the
column of the appropriate cross—table. The energy of a gamma ray detectable by either
germanium in coincidence with the trigger was found in the cross-table (by row).
Multiplying the integrated number of events in the trigger peak by the value in the cell
deﬁned by the row and column energies determined the total number of transitions at
the row energy that could possibly have been detected by the germanium detectors.

Setting a gate around the Nal peak and reﬂecting back onto the individual
germanium spectra allowed for a measurement of the coincidence spectrum. The peak
in the germanium spectrum corresponding to the row energy was then integrated to
determine the intensity at that energy. As an example, ﬁgure 44 shows the spectrum

accumulated by Ge2 while gated on the 1332 keV peak in the NaI detector.

-79-

 

1 200

 

 

 

 

Ge #2 - ”Co spectrum gated on 1332 keV
peak In Nal start detector

0, 900

E Ey = 1173 keV

: 600

8

300
o L l l l l 1 l l n l
0 1 000 2000 3000 4000
Energy (keV)

Figure 44 Gamma-ray spectrum collected in Ge # 2 gated on the ”Co 1332 keV
line in the Nal start detector.

The efﬁciency of the speciﬁc germanium detector at the row energy is then
equal to the integrated area of that peak in the germanium detector divided by the
number that could possibly have been detected under these conditions. Table 26 lists
the results of the efﬁciency determinations for Gel and Ge2 using this technique.
Tabulated are the calibration source, the Nal peak energy and integration, the Gel
(Ge2) peak energy and integration, fraction as taken from the cross-tables, and
calculated Gel (Ge2) efﬁciency.

3.7.8 EFFICIENCY CALIBRATION - ON-LINE SOURCES

In addition to the off-line source measurements, there are data available within
the experimental runs that provide a measure of the germanium detector efﬁciencies at
various energies. The decay of '7N provides two known gamma-ray lines for which the
branching is known [93Ti] and the decay of '90 provides ﬁve gamma-ray lines with

known branching ratios [95Ti]. From the integrated half-life spectra, the total number

-30-

of decays due to each isotope may be determined. Taking the individual branching
ratios and the total number of decays in concert yields the number of gamma rays
emitted at the speciﬁc energies. The efﬁciency of the germanium detector at this
energy is then simply the observed intensity of the line divided by the total number

emitted. Table 27 lists the quantities necessary to deduce the germanium detector

efﬁciencies at the tabulated energy.

Table 26 Off-line source calibration data for determining the efficiencies of the Ge
detectors. The emboldened columns are the data necessary to generate a
calibration curve.

 

 

 

 

Source Nal gate Counts Gel Peak Counts Fraction Efﬁciency
(keV) (keV) (x-table) Ge # l
600) 1 173 4.74E+O6 1332 1 1294 1 2.38E-03
60C0 1332 4.29E+O6 1173 10795 0.99945 2.5on3
20731 570 1.201~:+07 1063 22077 8.42E-Ol 2.193-03
20731 570 1.203+07 1770 1663 6.91E-02 2013-03
20731 1063 5.28E+06 570 17629 1 334303
20731 1770 3.413+05 570 1044 1 3.06E-03
134Cs 795 1.25E+06 604 4131 1 3313-03
134CS 795 1.25E+06 569 722 1.81E-Ol 3.193.03
134Cs 605 2.05E+06 563 704 9003-02 3.81E-03
134Cs 605 2.05E+06 796 5374 8.69E-01 3013-03
Source Nal gate Counts Ge2 Peak Counts Fraction Efﬁciency
(keV) (keV) (x-table) Ge # 2
“Co 1 173 4.74E+06 1332 5222 1 1.10303
60C6 1332 4.29E+O6 1173 5055 0.99945 1.18E-03
20731 570 1.203+07 1063 1 1 177 8.42E-01 1.11303
20731 570 1.203+07 1770 801 6.9lE-02 9703-04
20731 1063 5.28E+06 570 9238 1 1.75303
20731 1770 3413.05 570 581 1 1.70303
'34Cs 795 1.25E+O6 604 2076 1 1.66E-03
'34Cs 795 1.25E+06 569 364 1.81E-01 1.61E-03
'34Cs 605 2.05E+06 563 318 9003-02 1.723-03
'34Cs 605 2.0SE+O6 796 2506 8.69E-Ol 1.403-03

 

-8]-

Table 27 On-Line data pertinent to the efficiency calculation of the germanium
detectors.

 

 

E7 Branch # of Bs # of 78 Gel Ge2 eGel eGe2
keV % (/ 1e7) Counts Counts x1000 x1000
19O —> ”15"“ + [3-

110 2.5400) 4.4(2) l.12(6)E6 3.1(1)33 N/A 2.7705) N/A

197 95.9(21) 4.4(2) 4.2(3)37 9.96(4)E4 3.64(2)E4 2.3602) 0.815(5)

1357 50.4(11) 4.4(2) 2.2(2)37 5.57(3)34 2.54(2)34 2.5103) 1.14(6)

1444 264(6) 4.4(2) 1.16(6)E6 2.84(8)E3 1.22(5)33 2.4404) 1.050)

1554 139(3) 4.4(2) 6.1(4)E5 1.77(7)33 7.1(4)32 2.8908) 1.16(9)
l7N —>"O*+[3-

871 334(50) 2.3(1) 7.7(12)35 2.58(6)E3 1.15(4)1~:3 3.36(53) 1.5004)

2184 034(6) 2.3(1) 7.8(15)E4 150(14) 7.2(10)E1 l.92(44) 0.92(21)

 

3.7.9 EFFICIENCY CALIBRATION - GERMANIUM COINCIDENCES

In addition to the on-line calibrations described in the previous section, it was
possible to generate efﬁciency calibration points for Ge#1 at low energy using a
coincidence technique. The energy levels of the '90 and ”F daughters are well known
and some of the properties of the decay of each level are also known or may be inferred.
For example, the observation of a 1376 keV line during the decay of ”N indicates the
transition from the 1472 keV excited state in 19O to the 96 keV excited state. It must be

true that for every 1376 keV gamma ray there is a corresponding emission of a 96 keV

-82-

gamma ray. Therefore, the efﬁciency of Ge#1 at 96 keV can be determined by gating
on the 1376 keV line in Ge#2 and reﬂecting back onto the Ge#1 energy spectrum. The
efﬁciency of Ge#1 at 96 keV will be equal to the number of observed 96 keV counts in
the gated spectrum divided by the total number of 1376 keV counts in Ge#2. The
statistics of such an analysis are necessarily poor due to the multiplicative aspect of the
germanium efﬁciencies. However, in the absence of another method to measure the
efﬁciency of at least one of the germanium detectors at 96 keV, the poor statistics will
have to be absorbed. Table 28 gives the results of the germanium coincidence analysis

and the calculated efﬁciency at the points taken.

Table 28 Germanium detector efficiencies determined using the coincidence
technique.

 

 

Ge#2 Peak Ge#1 Peak Ge#1 Peak Ge#2 Peak Efﬁciency at
Gating (keV) Energy (keV) Area Area Ge#1 Energy
3849 96 l4(3.7) 6604(84) 2.12(57)E-3
1376 96 18(4.3) 10412(109) 1.73(41)E-3
1357 197 82(9.1) 29080(175) 2.82(32)E-3
1444 1 10 2(1 .4) 1384(72) 1 .45(102)E-3
2473 96 1 1(3.3) 6401 (85) 1.72(52)E-3
2473 1376 16(4) 6401(85) 2.50(63)E-3

 

3.7.10 EFFICIENCY CALIBRATION - SUMMARY
Taking the data acquired from the on-line and off-line sources in concert,
efﬁciency curves for detectors Gel and Ge2 were constructed. Figure 45 shows the

comprehensive raw efﬁciency data for detector Ge2.

-83-

 

0.002

 

 

 

 

 

> 1-
~ 3’ =1
.2 b an ﬂ i
5 0.001 : I | a *
T43 _ IOn-Ilne (17M, 190)
'2 . _ 1 DOﬂ-llne(6OCo, 20731, 134Cs)
0.000 -

 

O 500 1000 1500 2000 2500
Energy (keV)

Figure 45 Data points taken for the efficiency determination of Ge#2.

The data above 500 keV were ﬁt to a functional form given in [88De]. The equation of

the ﬁt is given as:

e = 605/Ea)“ [9]
where e is the efﬁciency, c is a dimensionless variable parameter, E is the gamma-ray
energy in keV, E0 is a constant set equal to 1 keV to eliminate the energy dimension,
and 5 is a constant based on the volume of the germanium crystal that is given by:

6 = 2.14—0.629logV [10]
Where V is the volume of the germanium detector in cm3. In this experiment, the
volume of detector Gel was 542 cm3 and the volume of Ge2 was 411 cm3 where these
quantities were deduced from the speciﬁcation sheets provided by the manufacterer

[OORo]. Figure 46 shows the same calibration data for Ge2 and the best ﬁt to the

efﬁciency data using the above formulae.

-34-

 

0.002

    
  

 

 

 

 

 

>. .
g c = 0.039701)
.2
.2 '
8: 0.001 -
Ill _ I f '
E 1 I Calibration Points
H b
'2 _ ' —BestFltL1ne _
0000 LalnllllnlnlnlllLlllnnn

 

O 500 1000 1500 2000 2500
Energy (keV)

Figure 46 Best fit to the efficiency calibration data for Ge#2.

Since there is only one variable in the ﬁt and therefore no correlation problem,
the error on parameter c was deduced by the change in its value necessary to produce an
increase in the total chi-squared of one unit. This error is valid over the interpolation
region ranging from about 560 keV to 2180 keV in energy. To estimate the error
associated with extrapolating the ﬁt to higher energies, the high efﬁciency at 560 keV (c
+ 0') and the low efﬁciency at 2180 keV (c — 6) were ﬁt to the same functional form.
This was repeated for the low efﬁciency at 560 keV (c — 0') and the high efﬁciency at
2180 keV (c + 0'). The two curves diverge above the interpolation range and hence give
signiﬁcantly higher error on the efﬁciency in the high-energy extrapolation region.
Figure 47 shows the efﬁciency data for Ge2, the best-ﬁt line to the data, the error range
in the interpolation region, and the error range in the extrapolation region. It is noted
that the percent error in the interpolation region is roughly 8% of the efﬁciency whereas

at 3200 keV, the percent error has increased to approximately 14% of the efﬁciency.

-35-

 

0.002

 

 

 

 

 

 

 

>4 .

0

C .-

.2

.2 L

E 0.001 . I

E .

O

P b

0.000 n l 1 l n 1
O 1000 2000 3000

Energy (keV)

Figure 47 Interpolation and extrapolation error bars for the efficiency
determination of Ge#2.

The calibration data points cover a range having an upper limit of 2200 keV or
thereabouts. As a consequence, the efﬁciency calibration for the germanium detectors
is suspect for the higher energy gamma rays (>3.0 MeV) that were observed from the
decay of 19N (see ﬁgures 36, 37). The efﬁciencies at these gamma ray energies would
be extrapolations over an extremely large span. However, the reliability of the
extrapolation was checked at a somewhat higher energy by comparing the extrapolation
against data recorded by the same germanium detectors during a different NSCL
experiment [OOPr] having a calibration point at 2615 keV. The shapes of the calibration
curves above 500 keV and including the 2615 keV point were found to be in excellent
agreement. This agreement between the experiments improved the conﬁdence for

efﬁciency extrapolation up to at least 2600 keV.

-86-

3.7.11 GAMMA RAY INTENSITIES — ”N

The branching ratios for gamma ray emission as a function of energy can be
calculated for those transitions that were assigned as following the beta decay of '9N.
The integrated area of the gamma lines divided by the detector efﬁciency leads to the
total number of gamma rays emitted over the course of the data runs. This value
divided by the total number of decays (from the integrated half-life spectra) of the
respective isotopes gives the intensity of the lines per decay of the parent isotope. Table
29 gives the energy of the gamma line, number of decays observed in each of Gel and

Ge2, the efﬁciency of each detector at that energy, the total number of source decays,

and the calculated intensity of each gamma line.

Table 29 Gamma-ray intensities for transitions assigned to the beta decay of 19N.

 

 

Energy Gel Ge2 B-Source eGel eGe2 17 Gel Iy Ge2
(keV) Counts Counts (x 1e-7) (x 1000) (x 1000) (x 1000) (x 1000)
96 25155 N/A 4.64( 10) 1.86(51) N/A 291(80) N/A
821 1220 579 4.64(10) 2.75(22) 1.42(11) 10(1) 9(1)
1374 12455 5571 4.64( 10) 2.22(20) 1. 10(9) 12 1( 13) 109(11)

1472 824 330 4.64( 10) 2.15(18) 1.06(9) 8(1) 7(1)
1652 1426 635 4.64(10) 2.05(15) l.01(7) 15(2) 14(2)
1938 845 394 4.64( 10) 1.9l(14) 0930) 10(2) 9(2)
1980 15043 6735 4.64(10) 1.89(14) 0.92(7) 172(15) 158(14)
2473 7946 3513 4.64(10) 1.73(15) 0.82(8) 99(10) 92(10)
3136 3617 1602 4.64( 10) 1.56(19) 0.72(9) 50(7) 48(7)
3849 8454 3614 4.64(10) N/A N/A N/A N/A
3945 1249 61 1 4.64( 10) N/A N/A N/A N/A

 

-37-

3.7.12 INTENSITIES OF HIGHER ENERGY l9N GAMMA RAYS

For the case of the 3232 keV gamma line, there is no alternative to the efﬁciency
determination other than the extrapolation. Therefore, the entry in table 29 for the 3232
keV line is based on that portion of the analysis even though it is signiﬁcantly higher in
energy than the more reliable interpolation range. However, the 3945 keV level in '90
has been populated in the past and the subsequent de-excitation of this level has been
monitored [70Fi, 71Hi]. From the independence hypothesis, the de-excitation of a level
should not be a function of the manner by which it is populated — provided there is no
polarization component. Therefore, since the de-excitation gamma rays and their
intensities have been recorded in the past, this provides an opportunity to determine the
intensities by other means. Table 30 lists the data from the literature relevant to the

relaxation of the 3945 keV level.

Table 30 Literature values for the de-excitation branching of the 3945 keV excited
state of 19O.

 

Initial Level Final Level Gamma Energy Branch (%) Branch (%)

 

(keV) (keV) (keV) [70Fi] [7 1 Hi]
3945 0 3945 24(8) 33(8)
3945 96 3849 48(8) 39(8)
3945 1472 2473 28(4) 28(4)

 

The detection efﬁciency for the 2473 keV line is considered reliable due to the
fact that this energy is below 2615 keV which was the highest energy point taken as a

calibration line for the germanium detectors. Accepting the efﬁciency at 2473 keV as

-33-

valid (and thus the calculated intensity), and utilizing the relative de-excitation strength
of the 3945 keV level from the literature, extrapolating the efﬁciency of the germanium
detectors for the 3945 and 3849 keV levels is obviated. Table 31 gives the calculated
intensities for the 3945 and 3849 keV levels on the basis of the literature data and the

subsequent beta decay branching ratio to the 3945 keV level.

Table 31 Calculated intensities of the de—excitation gamma rays from the 3945
keV excited state of 19O.

 

 

 

 

Ge #1 17 2473 keV 17 3945 17 3849
(ﬁxed) Calculated Calculated
Ref. [70Fi] 99(10) 85(32) 170(41)
Ref. [71Hi] 99(10) 117(35) 138(37)
Ge #2
Ref. [70Fi] 92(10) 79(30) 158(39)
Ref. [71Hi] 92(10) 108(33) 128(35)

 

The compilation by Tilley et al. of the previously published experimental
results, states that the de-excitation values given in [71Hi] are considered to be more
reliable that that of reference [70Fi]. Due to the lack of conﬁdence in the efﬁciency
extrapolation, the value quoted for the intensity of the de-excitation gamma rays at 3945
and 3849 keV from the bound excited state at 3945 keV will be that derived on the basis
of the experimental results in [71Hi]. Table 32 lists the ﬁnal values for the absolute

intensity of the gamma rays ascribed as being due to the decay of 19N.

-39-

Table 32 Final gamma-ray intensities for those gamma

rays attributed to the

 

 

decay of 19N.

Energy 17 Gel Iy Ge2
(keV) (x 1000) (x 1000)
96 291(80) N/A
821 10(1) 9(1)

1374 121(13) 109(11)

1472 8(1) 7(1)
1652 15(2) 14(2)
1938 10(2) 9(2)
1980 172(15) 158(14)
2473 99(10) 92(10)
3136 50(7) 48(7)
3849 138(37) 128(35)
3945 117(35) 108(33)

 

3.7.13 GAMMA RAY INTENSITIES — "’11

Due to the low production rate of 20N relative to the other RIBS, the potential for
missing gamma rays during the beam off period was substantial. To limit this effect,
the relative intensities of the gamma rays ascribed to the decay of 20N were determined
using the data collected in the germanium detectors over all times. The inherent
assumption in this portion of the analysis was that the relative efﬁciency of the
germanium detectors as a function of energy is not affected by monitoring over all
times.

Table 33 lists the peak areas, efﬁciencies, and calculated relative intensities for
gamma rays observed in each of the germanium detectors over all times and assigned to
the decay of 20N. The intensities are relative to the 1674 keV line which is set to one

hundred.

-90-

Table 33 Calculated relative intensities of gamma rays emitted during the decay of

20N. The intensities are relative to the 1674 keV line that is set to 100.

 

 

E'y Ge #1 Ge #2 e-Ge #1 e-Ge #2 17 Ge #1 Iy Ge #2
keV Counts Counts (x 1000) (x 1000) Relative Relative

96 968(32) N/A 1.86(51) N/A 88(24) N/A
1374 234(39) 93(32) 2.22(20) 1 . 10(9) 18(3) 15(5)
1674 1206(45) 565(26) 2.05(15) 100(7) 100(8) 100(8)
1814 74(13) 48(15) 1.97(20) 0.96(8) 6(1) 9(3)
1895 174(37) 74( 12) 1 .93(18) 094(8) 15(4) 14(3)
2394 155(26) 59(10) l.76(16) 0.84(9) 15(3) 12(3)
2782 47(19) 12(5) 1 .65(15) O.78(9) 5(2) 4(2)

 

The results for the relative intensities of the lines are in close agreement between

the two germanium detectors. The gamma lines at 96 and 1374 keV are characteristic

of transitions in 190. These gamma rays indicate beta delayed neutron emission from

20N leading to excited levels of 190 rather than to the ground state.

To determine the absolute intensities, the stronger peaks that were integrable in

the beam-off spectra collected by the germanium detectors were analysed. This permits

the calculation of the absolute intensity at these energies due to the fact that it is during

this time period when the beta decay of 20N can be monitored quantitatively. Also, the

half-life gated on the 96 and 1374 keV lines as they appear in the beam-off spectra

conﬁrm them as being due to the decay of 20N. Table 34 gives the absolute intensities

for some of the 20N gamma ray lines using this method.

-91-

Table 34 Absolute intensity of gamma rays ascribed to the decay of 20N.

 

Ey Ge #1 Ge #2 20N [3— e-Ge #1 e-Ge #2 17 Ge #1 17 Ge #2

keV Counts Counts /105 (x1000) (x1000) (x1000) (x1000)

 

96 347(21) N/A 7.6(1) 1.86(51) N/A 246(69) N/A

1374 74(16) 40(9) 7.6(1) 2.22(20) 1.10(9) 44(11) 48(12)
1674 421(23) 205(16) 7.6(1) 2.05(15) l.00(7) 270(25) 270(29)
1895 32(8) N/A 7.6(1) 1.93(18) 0.94(8) 22(6) N/A
2394 57(15) N/A 7.6(1) l.76(16) 0.84(9) 43(12) N/A

 

Taking the relative gamma intensities in concert with the smaller sub-set of
measured absolute intensities, the absolute intensities for all of the lines for both
detectors can be calculated. Since the 1674 keV line has the lowest error, this line’s
intensity was utilized to calculate the absolute intensity of the others. Table 35 gives

the ﬁnal evaluated intensities of the gamma lines due to 20N.

Table 35 Intensity of the gamma-ray lines attributed to the decay of 20N.

 

 

By 17 Ge #1 17 Ge #1 17 Ge #2 17 Ge #2 17
keV Measured Calculated Measured Calculated Final
(x 1000) (x 1000) (x 1000) (x 1000) (x 1000)
96 246(69) - - N/A 246(69)
1374 44(1 1) - 48( 12) - 46(9)
1674 270(25) - 270(29) - 270(19)
1814 - 16(4) - 24(9) 18(4)
1895 22(6) - - 38(10) 26(6)
2394 43(12) - - 32(10) 37(8)
2782 - 13(6) - 11(5) 12(4)

 

CHAPTER 4

INTERPRETATION

4.1 INTERPRETATION OF "N GAMMA RAYS

To determine the branching ratios for the beta decay of 19N to bound excited
states of 19O, the transitions listed in table 32 of section 3.7.12 were assigned as either
being due to transitions between states in '90 or between states in 180. Table 36 lists
those transitions assigned to 190 along with their intensities and the characteristic levels

of the transition. The intensities are given per 1000 parent decays.

Table 36 Gamma-ray transitions identified as being a result of 19N beta decay to
bound excited states of 1”O.

 

 

Energy '90 Initial ‘90 Final 17 Gel Iy Ge2
(keV) Level (keV) Level (keV) (x 1000) (x 1000)
96 96 0 291(80) N/A
1374 1472 96 121(13) 109(11)
1472 1472 0 8(1) 7(1)
2473 3945 1472 99(10) 9200)
3136 3232 96 50(7) 48(7)
3849 3945 96 138(37) 128(35)
3945 3945 0 1 17(35) 108(33)

 

From these data, it is possible to construct an energy level diagram illustrating
the observed transitions. This will show which gamma rays must be in coincidence

with each other and permit a determination of which levels are fed directly from the

-93-

beta decay and which gamma rays are consequences of cascades. Figure 48 shows the
known energy level scheme for the ground state and bound excited states of 19O. The

levels are given in keV relative to the ground state at zero and are adapted from

reference [95Ti].

3945 3X2-

 

/3232 (1/2-}
3154 5/2+
‘3057 <3/2+>

2779 ?/2+
23?2 9/2+

 

 

 

 

 

 

 

14?2 1X2+
98 3f2+

 

 

 

 

 

 

 

0 5/2+ V L
19
0

Figure 48 Partial energy level diagram of 1’0 showing the bound-state transitions
observed in this experiment during the decay of 19N.

To deduce the branching ratios for the beta decay of 19N to bound states of '90,
the intensities of those transitions that are not fed by a cascade remain unchanged.
Those transitions that may be due to both beta feeding and as a result of one or more
gamma-ray cascades require that their intensities be adjusted for the cascade feeding. In
this case, the only transitions that may be due to both beta feeding and cascade feeding

are the 1472—0, 1472-96, and 96—gs transitions. Unfortunately, the experimental

-94-

conditions prevented a determination of the germanium detector efﬁciency near 96 keV
for detector Ge#2. Consequently, there can be no redundancy for the calculation of the
beta feeding to the 96 keV level during the decay of 19N.

From the observed intensities and the derived decay scheme, the branching
ratios for pure beta feeding to bound excited states of 19O can be calculated. The sum of
the three transitions proceeding from the 3945 keV level is a measure of the beta
strength to this level. The intensity of the 3232-1472 transition is a measure of the beta
decay branching ratio to the 3232 keV level, and the sum of the 1472—gs and 1472-96
transitions, less the feeding through cascades, is a measure of the beta decay to the 1472
keV excited state. Similarly, the intensity of the 96 keV gamma ray less the cascade
feeding from multiple sources will be a measure of the beta branch to the 96 keV level.
Table 37 gives the calculated direct beta-decay branching ratios to bound excited states
of 190 where soluble. The last row of the table gives the total branching ratio to bound

excited states of [90 above and not including the 96 keV level.

Table 37 Summary of beta decay strength to individual bound levels in 190 from
the decay of 19N.

 

 

Beta Decay Level Branching Ratio (%) Branching Ratio (%) B.R. (%)
Populated Directly Germanium #1 Germanium #2 Mean Value
3945 3/2— 35.4(52) 32.9(51) 34.2(37)
3232 (1/2-) 5.0(10) 4.8(7) 4.9(6)
1472 1/2+ O.30(18) 0.24(16) O.27(12)
96 3/2+ < 4.3 N/A <4.3
Ground State 5/2+ N/A N/A N/A
TOTAL B.R. 40.7(54) 38.0(53) 39.4(38)

 

-95-

The branching ratio determination for direct beta feeding to the '90 ﬁrst excited
state at 96 keV is given as an upper limit. The branching ratio calculated from the 96
keV line’s intensity less the feeding is consistent with zero. The upper limit represents
subtracting the minimum possible cascade contribution (mean less one sigma) from the
maximum possible intensity (mean plus one sigma) of the 96 keV line. This experiment
was not sensitive to the beta decay of I9N feeding the ground state of '90 directly.
4.2 INTERPRETATION OF 1"N DELAYED NEUTRONS
Between the neutron bars and the 6Li detectors, the experiment was able to
record twelve beta-delayed neutrons emitted during the decay Of 19N. Table 38

summarizes the Observed delayed neutrons and their calculated branching ratios.

Table 38 Summary of observed delayed-neutron transitions from 19N by energy.

 

 

Neutron Energy (MeV) 3.R. (%) from Array B.R. (%) from 611 glass
491(8) 0.9000) -
400(6) 037(4) -
3.06(5) 1.97(32) -
2.84(4) 9.6111) -
249(4) 3.5(4) —
221(4) 22(4) -
127(3) 190) -
l.13(3) 16.2(17) 18.2(23)
083(3) 20(3) -
062(3) 22(3) -
049(3) 8.7(10) 8.6(10)
030(8) - 1.4(3)

 

-96-

TO determine the beta branching to speciﬁc excited levels of 19O that
subsequently emit a neutron, it is necessary to determine if the neutron emission
populates the '80 ground state or one Of the excited states. There is about 8.5 MeV
available for delayed neutron emission and over twenty excited states in '80 that could
possibly be populated following the emission of a neutron from an excited level Of 19O.

The analysis Of the gamma-ray data collected by the germanium detectors
showed that there were four gamma rays characteristic Of transitions in '80. These
gamma rays could be indicative Of beta-delayed neutron emission populating excited
states Of 18O or they could simply be due to the beta decay Of an l8N contaminant
though there is no evidence Of this (see table 3). If the '80 gamma rays are due to 19N
delayed neutron emission then the corresponding half-lives of the gamma rays should
be equal to the half-life Of 19N (~300ms) and if the 18O gamma rays are due to the beta
decay of 18N then the gated half-lives will reﬂect the half-life of 18N (~620ms). Table
39 gives the measured half-lives Of the four 180 gamma rays and the intensity of the
lines for the measurements made by each of the germanium detectors. The intensities

of the gamma rays are given per 1000 l9N decays.

Table 39 Gamma rays characteristic of de-excitation in 18O. The half-lives of the
gamma rays are listed along with the intensity of the lines assuming the 19N parent.

 

 

By E, level Ef level Half-Life 17 17 17
keV 18O l8O (ms) Ge #1 Ge #2 Mean
1982 1982 0 300(10) 172(15) 158(14) 166(10)
1938 3920 1982 273(17) 10(2) 9(2) 10(2)
1652 3634 1982 305(14) 15(2) 14(2) 15(2)
821 4456 3634 364(45) 10( 1) 9(1) 10(2)

 

-97-

On the basis Of the measured gamma-ray half-lives, it may be concluded that the
observed 180 gamma rays are indeed due to the decay Of 19N and thus reflect delayed
neutron emission to excited states Of '80. Accounting for the cascade feeding in the
same manner as outlined in section 4.1, and incorporating level schemes from [95Ti],
the branching ratios for delayed neutron emission populating the speciﬁc excited states
Of '80 may be calculated. Table 40 gives the total strength of the (B+n) feeding Of the

'80 excited levels.

Table 40 Values for the (B + n) feeding strength to each of the pertinent 18O
excited states.

 

 

18O Excited State Populated Beta + Neutron Feeding Strength
(keV) (% of '9N decays)
1982 13.7(1 1)
3634 0.1(3)
3920 1.2(3)
4456 15(4)

 

To determine the beta feeding to speciﬁc levels in '90 prior to the neutron or
gamma ray emission, it is desirable to ﬁnd which delayed neutrons are in coincidence
with the '80 gamma rays that were observed. An effort was made to measure, directly,
which neutrons and gammas correspond to the same transition by gating on the gamma
rays of table 40 and reflecting back onto the (summed) delayed neutron spectrum
recorded by the neutron bar array. Table 41 gives the results Of the coincidence

measurement for each Of the gamma rays gated.

-98-

Table 41 Neutrons in coincidence with selected 18O gamma rays. Listed are those
neutrons in coincidence and those neutrons that may be in coincidence on the basis
of the gated spectrum.

 

 

Gamma-Ray Gate Neutron in Coincidence Neutron in Coincidence
(keV) (keV) Deﬁnite (keV) Possible
1982 2840 490
830 2210
1270
1653 1270
1938 830
821 None None

 

On the basis Of these results, it can be concluded that the 830 keV neutron is
emitted from a level in '90 that populates the 3920 keV excited state Of 180 due to the
fact that this neutron appears to be in coincidence with both the 1938 and 1982 keV
gamma lines. Also, the 1270 keV neutron appears to be in coincidence with the 1653
keV gamma ray — suggesting that the beta delayed neutron populates the 3634 keV
excited state or perhaps the 4456 keV excited state. It is not possible to distinguish
between the two given the statistics of the (821 keV gamma ray gated) neutron
spectrum. Also, the strong 2840 keV neutron is in coincidence with the 1982 keV
gamma ray suggesting that this neutron is emitted by a level in 19O that populates the
ﬁrst excited state of '80. The 2210 and 490 keV neutrons may or may not be in
coincidence with the 1982 keV gamma line. The low statistics of the coincidence
spectra prevent conclusive assignment of these neutrons to the gamma ray.

4.3 DELAYED NEUTRON ASSIGNMENTS l9N
From the delayed neutron branching ratios, Observed '80 gamma rays and the

measured neutron + gamma ray coincidences, a cross table for the possible delayed

-99-

neutron transitions may be constructed (Table 42). The cross table cells represent the
energy of the intermediate 190 level populated by the beta decay and emitting the
neutron. The columns deﬁne the level of '80 populated following the emission of the
neutron. The energies are given in keV for all entries and the cell values are relative to
the ground state of 190. Those cells that are blank have been eliminated as possible
transitions either on the basis of the total energy (greater that that available for delayed
neutron emission) or on the basis of the neutron branching ratio relative to the 18O
gamma-ray intensities. Since there is no evidence of any 180 levels being populated
other than those listed in table 41, the cross table (42) will be limited to those excited
states plus the l8O ground state.

Of the twelve delayed neutron transitions, only ﬁve can be conclusively
assigned to speciﬁc levels in 190. These delayed neutron energies and corresponding
'90 level are underlined in the cross table. The remaining seven cannot be ﬁxed as they
may proceed through different intermediate states on the basis of the measured data.
From the delayed neutron branching ratios and the deﬁnitive assignments it can be
concluded that the small possible branch for delayed neutrons populating the 3634 keV
excited level in 18O is either in fact zero (with which the measurement is consistent) or
is a result of a delayed neutron that was not measured in our experiment. Figure 49
Shows the transitions due to the beta decay of ”N that can be conclusively identiﬁed on
the basis of the data recorded in this experiment. The energies are given in keV and are
relative to the ground state of 19O. The excited levels in 18O are relative to the ”0

ground state.

-100-

12.54 (1/2-)

18N

$83 () [1.9(2)]

8779 () [9.8(11)]

870? () [2.06)]
4456 1- [15(4)] .

3920 2+ 11.21311
3834 o+ [0.113)I“"""""

 

 

 

 

 

1932 2+ [13.711111'
5082 1/2- [8.71101]

4582 312- [2.28)]

 

o (3957) o+ [II/111ml

 

3945 3/2- [34.2(37)]

 

18 0+” 3232 (1/2-) [4.905)]

 

 

1472 1l2+ [O.27(12)]

 

 

 

 

S 3l2+ [(4.3]
0 5/2+ nun]

 

 

 

 

190

Figure 49 The beta decay of 19N showing the transitions that can be ascribed to
19N on the basis of the experimental data.

-101-

Table 42 Cross table for the assignment of intermediate states of 19O populated
during the decay of 19N.

 

 

E-Neutron E-'9O level E-lgO level E-‘gO level E-‘90 level E-'9O level

C.o.M. (”‘o gs) ('801982) ('80 3634) (”*o 3920) (”304456)
4910 8867 10849

4000 7957 9939

3060 7017 8999

2840 8112

2490 6447 8429

2210 6167 8149

1270 268;
“—0 M

8—9 m

9L0 ﬂ

490 4447 6429

300 4257 6239

 

4.4 INTERPRETATION OF 20N GAMMA RAYS
To determine the branching ratios for the beta decay of 20N to bound excited

states of 20O, the transitions labeled in table 32 of section 3.7.12 were assigned as either

-102-

being due to transitions between states in 20O or between states in '90. Table 43 lists

those transitions indicative of beta decay to bound States of 200.

Table 43 Gamma-ray transitions identified as being a result of 20N beta decay
feeding bound excited states of 20O.

 

 

Gamma Energy 200 Initial 200 Final Intensity
keV Level Level (x 1000)
1674 1674 O 270(19)
1814 5387 3570 18(4)
1895 3570 1674 26(6)
2394 4072 1674 37(8)
2782 4456 1674 12(4)

 

From these data it is possible to construct and energy level diagram illustrating
the observed transitions among the known 200 levels. AS with the I9N data, this will
make it possible to determine the cascade feeding and thus lead to an assessment of the
branching ratios for the feeding of speciﬁc levels in 200 during the beta decay. Figure
50 shows the gamma transitions in 200 observed in this experiment.

From the observed intensities and the derived decay scheme, the branching
ratios for pure beta feeding to Speciﬁc bound levels in 200 can be calculated. Table 44
gives the result of the branching ratio determination for the decay of 20N to bound

excited levels in 20O.

-103-

5304 2+

4850 4+

 

538? 0+
5234 2+
5002 {)

445B 0+
4072 2+

3570 4+

15?4 2+

0 0+

Figure 50 Partial energy level diagram of 20O showing the bound-state transitions
observed in this experiment during the decay of 20N.

Table 44 Bound excited levels of 20O populated during the decay of 20N. Listed are
the branching ratios for the beta transitions as determined from the measured

gamma-ray data.

 

20O Excited Level Populated Directly

Branching Ratio of Direct B-Feeding

 

Energy of Level (keV) (Percent)
5387 0+ 1.8(4)
4456 0+ 12(4)
4072 2+ 37(8)
3570 4+ 0.8(8)
1674 2+ 19.5(22)
G.S. 0+ N/A

Total Branching Ratio 27.0(46)

 

-104-

4.5 INTERPRETATION OF 20N NEUTRON SPECTRUM

Analysis of the delayed neutron spectrum collected by the neutron bar array
suggests eight beta delayed neutrons emitted by the 20N isotope. Table 45 lists the
energy of those delayed neutrons and their corresponding branching ratios. Taking the
data together, the delayed neutron emission probability from 20N is 34.0(40) percent of
the decay strength for delayed neutrons above 1.1 MeV in energy. The low rate and
decreasing detection efﬁciency of the array below this cut off prevent measurement of

delayed neutrons at lower energy.

Table 45 Energy and branching ratio for delayed neutrons observed during the
decay of 20N. Also given is the P“ value for delayed neutrons emitted above 1.1
MeV in energy.

 

 

Energy of 20N Delayed Neutron Branching Ratio

(Center-of-Mass MeV) (Percent)
1.16(3) 45(5)
1.33(3) 25(3)
1.63(3) 83(9)
219(4) 12.1(13)
2.63(4) 13(2)
338(5) 2-1(3)
388(6) 270)
4.64(7) 05(1)
TOTAL 34.0(40)

 

The gamma-ray spectra collected during the decay of 20N showed two gamma

rays at 96 and 1374 keV - which are characteristic of transitions in 19O. The ﬁrst a

-105-

transition from the 96 keV ﬁrst excited state to the ground state and the second a
transition from the 1472 keV second excited state to the 96 keV level. These gamma
rays were Shown to have half-lives consistent with that of 20N indicating that there is
beta delayed neutron emission from 20N populating excited levels in l90 rather than the
ground state. The branching ratios for these transitions, with cascade feeding taken into
account, are 20.0(70) and 4.6(9) percent for the 96 and 1374 keV transitions,
respectively.

With regard to delayed neutron emission from 20N, it is not possible to assign
beta decay branching ratios to Speciﬁc excited levels of 200 above the neutron emission
threshold. This is due to the following considerations: ﬁrst, several possibilities exist
for the ﬁnal '90 level populated rendering the evaluation of the level energy
inconclusive, second, the implantation rate of 20N coupled with the detection
efﬁciencies prevent neutron-gamma coincidence measurements that could restrict the
200 level energy, third, only seven 200 excited levels above the neutron emission
threshold are known with but four of these being above the lowest experimentally
observed neutron energy, and fourth, the extent of the above-threshold levels is below
10.5 MeV relative to the ground State of 200 while the Q—value is about 18 MeV.

The one exception to this is the 2.19 MeV delayed neutron peak. The intensity
of this transition is Strong enough that the coincidence between it and the 96 keV should
have been observed. The spectrum, however, does not indicate a coincidence between
the two - suggesting that the delayed neutron emission populates the ground state of 190
rather than the 96 keV excited state. Furthermore, one of the known 200 levels is at

9.77 MeV (~2.l7 MeV above the neutron emission threshold) which is consistent in

-106-

 

energy with the 2.19 MeV neutron observed. Thus, this transition iS tentatively
assigned as proceeding through the 9.77 MeV level in 200 with a branching ratio of
12.1(13) percent. Figure 51 shows a portion of the mass 20 level scheme where the
transitions resulting from the decay of 20N that can be deﬁnitively assigned are
presented. The energies of the levels in 20O are given in keV relative to the 200 ground

State and the excited levels of 19O are iven in keV relative to the 19O ound state.
g gr

-107-

2011

9770 0+ [12.1(13)]

 

1472 112+ [4.8(9” -—

 

 

S 3I2+ [20.0(70)]
0(7808) 512+ [mu

 

 

190+n

 

”7 0+ [1.8(4)]

4456 0+ [1.2(4)]

 

4072 2+ [3.701)]

 

 

3570 4+ [0.8(8)I

 

 

 

 

1674 2+ [19.5(22)]

 

 

0 0+ [llll]

 

200

Figure 51 The beta decay of 20N showing the transitions that can be conclusively
identified on the basis of the experimental data.

-108-

4.6 19N SHELL MODEL CONSIDERATIONS

From the energies of the 190 intermediate levels that may be populated directly
in the l9N beta decay and the measured branching ratios of the transitions, the log(ft)
values of the speciﬁc transitions can be calculated. The log (ft) values provide insight
into the nature of the decay (allowed etc...) and also lead to a determination of the
B(GT) transition Strength. The log (i) values are taken from the literature [71Go] while
the t value is simply equal to the half-life of 19N in seconds divided by the measured
branching ratio of the Speciﬁc channel expressed as a transition probability (BR/ 100).
The B(GT) strength is equal to (6177/ft).

Table 46 lists all transitions observed in this experiment (delayed neutron as
well as beta decay to bound states of 19O), the experimentally derived transition
strength, the log (ft) values and the calculated B(GT) strength. The column labeled
“transition identiﬁer” gives the energies of the delayed neutrons for the (B+n)
transitions while “to bound” indicates a transition to the bound 190 level listed in the
second column. For those cases where the delayed-neutron emitting state is not
conclusive, all possibilities are Shown based on the delayed neutron energy.

Shell model calculations to predict the Gamow-Teller beta decay strengths of
19N to levels in 190 were calculated by Warburton in reference [88Wa]. In the model,
the even parity States of 190 were calculated within the (Os)"’(0p)'2(2s,ld)3 model space,
the l9N states were assumed to arise from the (0s)4(0p)”(2s,1d)4 model space, and the
odd parity ‘90 levels required consideration of both the (08)4(0p)“(28,1d)4 component
and the (OS)4,(Op)'2(25,ld)2(0f,1p)l component to account for all possible 1130)

excitations. The model uses a modiﬁed Millener-Kurath interaction to calculate the

-109-

odd-parity levels in 19O and the Chung-Wildenthal or Universal (281d) interaction for

the low-lying even-parity levels.

Table 46 Calculated log(ft) and B(GT) values for the possible beta transitions in
the decay of 19N. The transition probability is the branching ratio divided by 100.

 

 

Transition 190 level Transition Log (ft) B(GT)
Identiﬁer Populated Probability
(B+n) 4910 8867 0.009 4.65 1.37E-01
(B+n) 4910 10849 0.009 3.20 3.87
(B+n) 4000 7957 0.0037 5.49 2.00E-2
(B+n) 4000 9939 0.0037 4.39 2.52E-l
(13+n) 3060 7017 0.0197 5.13 4.55E-2
([3+n) 3060 8999 0.0197 4.25 3.45E-1
(B+n) 2840 8779 0.0964 3.68 1.28
(B+n) 2490 6447 0.0349 5.08 5.09E-2
(B+n) 2490 8429 0.0349 4.30 3.07E-1
(B+n) 2210 6167 0.0224 5.37 2.65E-2
(B+n) 2210 8249 0.0224 4.57 1.67E-1
(B+n) 1270 9683 0.0189 3.87 8.32E-1
(B+n) 1130 5087 0.1618 4.83 9.18E-2
(B+n) 830 8707 0.0202 4.40 2.45E—1
(B+n) 620 4577 0.0215 5.83 9.04E-3
(B+n) 490 4447 0.0869 5.27 3.33E—2
(B+n) 490 6429 0.0869 4.70 1.24E-1
(B+n) 300 4257 0.014 6.11 4.78E—3
(B+n) 300 6239 0.014 5.55 l.74E-2
to bound 3945 0.342 4.79 9.95E-2
to bound 3232 0.049 5.81 9.64E-3
to bound 1472 0.0027 7.39 2.54E-4
to bound 96 0.043 6.42 2.33E-3
to bound 0 n/a n/a n/a

 

Table 47 compares the transitions observed in this experiment with the
predictions of the shell model calculations of Warburton. The table, where necessary,
identiﬁes transitions by subscript that cannot be conclusively ascribed to the speciﬁc
level in I90. Tabulated are the branching ratio, log(ft), B(GT), and level spins. The

B(GT) values have been multiplied by a factor of 1000.

-110-

Table 47 Comparison of the 19O energy levels from this experiment with
predictions made using the shell model [88Wa]. The subscripts on the energies
indicate the choices for the 19O level due to the uncertainty in the neutron decay.

 

 

 

 

This Experiment Shell Model Predictions
Ex M BR% Logft B(GT) Ex J1: BR% Lﬁft B(GT)
96 3/2-1- <4.3 6.42 2.3 96 3/2+ 0.49 7.30
1472 l/2-1- 0.27 7.39 0.25 1472 1l2+ 4.88 6.06
3232 l/2,3/2 4.9 5.81 9.6 3232 1/2- 1.1 6.77 1.0
3945 l/2,3/2 34.2 4.79 99.5 3945 3/2- 6.2 5.86 8.5
4257) l/2,3/2 1.4 6.1 l 4.8
44472 l/2,3/2 8.7 5.27 33.3
4577 l/2,3/2 2.2 5.83 9.0 4582 3/2- 3.4 5.97 6.6
5087 l/z,3/2 16.2 4.83 245.0 5082 1/2- 39.2 4.77 103.6
61673 '/2,3/2 2.2 5.37 26.5
6239. l/2,3/2 1.4 5.55 17.4
64292 l/2,3/2 8.7 4.7 124.0
64474 l/2,3/2 3.49 5.08 50.9
6755 3/2- 12.7 4.75 109.3
70175 I/2,3/2 2.0 5.13 45.5
71 19 3/2- 4.3 5.08 50.6
7509 3/2- 12.2 4.49 200.1
7622 1/2- 1.3 5.42 23.3
7843 3/2- 3.7 4.87 82.7
79576 l/2,3/2 0.37 5.49 20.0
8196 3/2— 5.4 4.55 172.6
82493 l/2,3/2 2.24 4.57 167.0
84294 l/2,3/2 3.49 4.30 307.0
8505 1/2- 3.6 4.57 166.9
8506 3/2- 2.5 4.74 1 l 1.2
8707 l/2,3/2 2.02 4.40 245.0
8779 l/2,3/2 9.64 3.68 1280
88677 l/2,3/2 0.9 4.65 137.0
89995 l/2,3/2 1 .97 4.25 345.0
9683 l/2,3/2 1.89 3.87 832.0
99396 l/2,3/2 0.37 4.39 252.0
10849, l/2,3/2 0.9 3.20 3870
4.7 BETA DECAY OF 2"N

For the beta decay of 20N feeding bound excited levels of 20O and the one
delayed neutron transition assignment, it is possible to determine both the log(ft) and

B(GT) values for the Speciﬁc transitions. The Q value of the 20N ground state to 20O

-111-

ground state transition is known to be 17.97 MeV. From this quantity, the branching
ratios and the half-life measurement, the log(ft) and B(GT) values were calculated.

Table 48 gives the results of these calculations.

Table 48 Calculated log(ft) and B(GT) values for specific decays assigned to the
decay of 2"N. The transition identifier segregates beta decay to bound states from
the delayed neutron emitting level.

 

 

Transition 200 level Transition Log(ft) B(GT)

Identiﬁer keV Probability (x 1000)
(13+ n) 2190 9770 0.121(13) 4.7 123

To bound 1674 0.195(22) 5.9 8

To bound 3570 0.008(8) 7.1 0.5

To bound 4072 0.037(8) 6.3 3

To bound 4456 0.012(4) 6.8 1

To bound 5387 0.018(4) 6.4 2.5

 

The log(ft) values are consistent with ﬁrst forbidden beta transitions. This leads
to the conclusion, Since 0+ and 2+ levels are populated, that the spin and parity of the
20N ground State is low (1- or 2-) and most likely equal to (l-).

4.8 ASTROPHYSICS RESULTS l”N

To deduce the probability and relative strengths of astrophysical reactions, and
thus the rates of isotopic production, the most important quantity to determine, on a
reaction-by-reaction basis, is the reaction rate per particle pair, <O‘v>. This quantity
incorporates, into the reaction rate, changes in the cross section as a function of energy
where the energy, in itself a temperature dependent distribution, is based on the

particular astrophysical environment. Mathematically, <O'V> is given by the expression:

-112-

< 0v > = (8/7r;t)(kr)‘3’2 foam/5 exp(— E/kT)dE [11]

where O' is the cross section, v is the velocity, u is the reduced mass, k is the Boltzmann
constant, and T is the temperature of the system [88Ro, 95Ra, 67Fo].

The above expression is completely general though, typically, reaction cross
sections are separated into direct capture and resonant capture components. The
separation is permissible due to the fact that all of the non-trivial differences between
the direct and resonant components resides in the 6(E) term of the integral. Thus,
rewriting the total cross section O(E) as a sum of the O'(E) components affects the
equation only by expanding the integral into a sum of component integrals. The total
cross section <O’V>, then, may be expressed as [950z, 67Fo]:

<O'V> = <ov>dc + <ov>res [12]
where the subscripts dc and res refer to direct capture and resonant capture,
respectively.

4.8.1 DIRECT CAPTURE PART OF THE “‘O(n,y)"o REACTION RATE

For neutron induced astrophysical reactions, such as the (my) reaction that is of
interest here, determining the direct capture component to the overall rate is somewhat
simpler than for rates involving charged particles. The reason this is so is due to the
fact that penetrability considerations, generally associated with coulomb interactions or
tunneling phenomena, are either non-existent or greatly reduced where neutrons are
involved. For example, the absorbtion of (3:0) or S-wave neutrons by a target nucleus
is not hindered by a barrier to its penetration (penetrability = 1). Of course, even higher

order neutron partial waves (€>0) do not experience a Coulomb barrier, however, they

-ll3-

are weakly hindered by an angular momentum barrier which is a function of the
system’s reduced mass and the interaction energy. Even so, the angular momentum
barrier is effectively much lower than the barrier to charged particles and furthermore,
can be approximated by a simple mass and energy relation. For p-wave (i=1) neutrons,
the penetrability is approximated by:
P1 2 [ll/2 E3/2 [13]
The direct capture cross section <O’v>dC can be solved as a Maclaurin series
expansion in E”2 about zero energy. The result in terms of <O’v>dc takes on the form

[67Fo, 88Ro]:

S ’(0)

<O'V> . = 5(0) 1+0.3312—T"2+0.06463S (0)
dc 5(0) 9

S (0)

 

7;] [14]

where the constant S and its derivatives are parameters of the expansion and can be
determined experimentally [67Fo], the derivatives of S are with respect to Em, and T9 is
the temperature in billions of degrees Kelvin.

For our particular reaction of interest, the '80(n,y)190 reaction, the values of the
constant S and its derivatives are tabulated in [94Ra]. Using these values, the direct
capture cross section <O’v>dc can be calculated for the l8O(n,'y)l9O reaction. The result
yields:

< 012 >1. :- 4.60-10‘2'T9 cm3/sec [15]
for temperatures of (To > 0.01). However, it is noted that the values of the constants
given in [67Fo] are the arbitrary estimates made by the author. These estimates were
made prior to the measurement of the thermal neutron absorption cross section of I80

and are reported with an uncertainty of a factor of two.

-ll4-

In addition to the method of calculating the direct capture cross section as above,
the direct capture cross section <O’v>dC may be approximated by the sum of the s-wave
(6:0) and p—wave (€=1) components such that [94Ra]:

<ov>dC = <O’V>dc.,~ + <O‘v>dc,p [l6]

Determining the s-wave part of the total cross section <6V>dcg takes advantage
of the SO-called l/v law where it holds that the product Onvn is a constant over all
energies [88Ro]. Therefore, knowing the cross section at any energy provides
knowledge of the cross section at all energies for the S-wave part of the interaction.
Since the thermal neutron absorption cross section is known for 18O [94Ra], the S-wave

component <O’v>dc,S can be found from the expression [67Fo]:

<ov>dm = 2.20-10"9o(rhenna1) cm3/Sec [17]

taking the experimental value of O(thermal) to be 1.60 x 10'4 barns, the calculated 8-
wave portion of the direct capture cross section becomes:

< 0v >2... = 3.5210723 cm3 /sec [18]

To determine the p-wave component of the direct capture cross section,

<Ov>dc,p, the expression:

<ov> = 1787-10465], cm3/sec [19]

dr'.p
from Rauscher et a1. [94Ra] was used. In this expression, the Sn term is the S-factor for

l’2). This term incorporates all of the

p-wave neutrons given in units of (barn MeV'
Strictly nuclear properties of the cross section into one expression where the cross

section and the S-factor are related by:

0(5) = 5,045)” 1201

-115-

The approximation for the penetrability of p-wave neutrons (equation 13) has been
folded into the expression. The value of Sn for the l8O(n,’y)'9O reaction has been

measured experimentally and is listed in table 7 of reference [94Ra]. Given that:

S = 52.4 ”barns/MW"2 [21]

II

the p-wave component of the direct capture cross section can be calculated as:

<ov>m = 9.36-10‘2'T9 cm3/Sec [22]

Thus, the total direct ca ture cross section <O’v>dc for the l8O(n, )190 reaction as
P 7

calculated from the sum of the s-wave and p-wave components is then:

<0»), = <0v> +<ov>w = 3.52-10'23+9.36-10'2'T9 cm3/sec [23]

(16.5
The large p—wave cross section relative to the s-wave is most probably due to direct p-
wave capture into the ﬁrst excited, 96 keV level (30*) of 190.

Because the parameters given in [67Fo] are arbitrarily assigned and because the
parameters listed in [94Ra] are determined through experiment, the value of <t'5v>dc
calculated in the above equation is considered more reliable than that of equation 15.
Thus, the result generated by equation 23 will be the value incorporated into the
expressions for the total reaction cross section from here forward.

4.8.2 RESONANT CAPTURE PART OF THE 18O(n,y)“'o REACTION RATE

In addition to the direct capture of neutrons by a nuclide, there is the possibility
of neutron absorption through resonances in the reaction product. The strength of
resonant capture depends on the relative energy of the level with respect to the unbound
system, the temperature or energy distribution of the incident neutrons, the resonant

widths of the entrance and exit channels and, the spin/parity of the resonance and target

-ll6-

nuclide. The functional form of the equation used to determine the resonant part of the

cross section, <O’v>,,.,, is given as [9SOz, 88Ro, 94Ra, 67Fo]:

<ov> = 2557-10430”),A‘3’219‘3’2Exp(-11.605£,/T,) chsee [24]

FE.)

where A is the reduced mass in atomic mass units, E, is the center of mass energy of the
resonance in MeV, and (my)r is the resonance strength in MeV.

The resonance strength ((1)7)r is the product of a Statistical factor and the gamma
width of the resonance (1‘7). Mathematically, the value is given by:

(cur), = <21,+1)/1<2JT+1)<2JP+1)1 - I“ [251

7

Where the J values are for the resonance, target nuclide ground state, and projectile, and
the gamma width (1}) is in units of eV. For the case of the l8O(n,'y)'9O reaction rate, the
target Spin is 0 ('80 ground state spin), the neutron projectile spin is '/2 (incident
neutron), and the gamma width is expected (1“,) to be less than 1 eV from the
systematics [93En, 9502]. Consequently, the value for (my), can be calculated to be:

((07), s (21,+1)/2 . 10*5 MeV [26]
with Jr being the Spin of the resonance in '90 absorbing the incident neutron.

To assess the upper limit on the resonance contribution to the 18O(n,'y)190
reaction proceeding through the states observed in this experiment, all observed
neutrons were assumed to populate the ground state in '80. This minimizes the
resonance energy and thus maximizes the potential contribution to the reaction. Also,
the beta transitions were assumed to be of the allowed type (which is consistent with the
log(ft) values) and the ground state spin of 19N was assumed to be l/2-, which is

consistent with the other known odd-mass nitrogen isotopes. Assuming the allowed

-ll7-

decay and the spin of the 19N ground state, the spin and parity of the intermediate levels
in 190 are restricted to being either ‘/2- or 3/2-, with 3/2- being the choice that will
maximize the resonant contribution (see equation 26).

On the basis of these assumptions, the measured energies of the delayed
neutrons, and with reference to equation 24, the strength of the '80(n,y)'90 reaction rate
through each excited level of '90 can be reduced to a functional form having but one
parameter — the temperature. Equation 27 gives the functional form of the resonant
contribution for the delayed neutron emitting states observed in this experiment.
<crv>m = 5.55-10’” 7,7"2 x
[Exp(—3.48/ T9) + Exp(—5.69/ T9) + Exp(—7.20/T9) + Exp(—9.63/ T9)

+ Exp(—13.l 1/T9) + Exp(-l4.74/T9) + Exp(—25.65/T9) + Exp(—28.90/T9)

+ Erp(—32.96/T,) + Exp(-35.51/T,) + Exp(-46.42/T,) + Exp(—56.98/T,)] [27]

Clearly, the last several terms will not contribute much to the total resonant
capture strength due to the large negative exponent. This is a consequence of the high
relative energy of the state with respect to the ('80 + 11) energy.

4.9.3 COMPARISON OF DIRECT AND RESONANT CAPTURE STRENGTHS

The direct capture strength <O’V>dc is given as a function of temperature in
equation 23 while the resonant capture strength is given as a function of temperature in
equation 27. A plot of the two components of the '80(n,y)'9O reaction over all
temperatures will illuminate the relative probability for the two branches. Figure 52
shows such a plot where the direct capture component is shown in red and the resonant
capture component is Shown in blue. Also shown is the temperature range normally

associated with the inhomogeneous big bang model [94Ra].

-118-

 

 

 

-15 l

 

 

 

   

 

 

 

 

 

 

7C :

5 :

=3 -20 r-

3 :

8 _25 E —Dlrect Capture

o ..

g -30 E —Resonant Capture

O)

O

— _35 1 1 1 1 1 1 1 1
0.1 1.0 10.0

Temperature ( x 109 K)

Figure 52 Comparison of the direct capture and resonant capture cross sections
for the 18O(n,'y)“l'O reaction as derived from this work.

The important feature of the plot is that the direct capture cross section
dominates the resonant capture cross section over most of the temperature range of
interest for the inhomogeneous big bang model. In fact, the direct capture component is
many orders of magnitude stronger up to a temperature of about 0.4 T9. Above a
temperature of 0.8 T9 however, the resonant capture is comparable to the direct capture
and thus could be the dominant component of the reaction rate at the upper edge of the

postulated temperature range of the IMB model.

-119-

CHAPTER 5

SUMMARY AND CONCLUSIONS

5.1 SUMMARY AND CONCLUSIONS — 1"N

In this experiment, quantitative measurement of the ”N decay was made
including a determination of the half-life, beta decay to bound excited states of 190 and
delayed neutron emission to levels in '80. The half-life of 19N was determined to be
299 (i 3 i16) milliseconds; which is in agreement with reference [91Re].

Seven gamma rays characteristic of '90 were observed accounting for
39.4(38)% of the total decay strength. The gamma rays suggest direct beta feeding to
three, possibly four, bound excited levels of '90. The levels populated are the 3945
(3/2-), 3232 (1/2-), 1472 (1/2+) and possibly the 96 (3/2+) level, with branching ratios
of 34.2(37), 4.9(6), O.27(12) and 4.3(UL), respectively. The results of the gamma ray
spectra are not consistent with those published in [88Duf] as the 709 keV gamma ray
reported there was not observed in this experiment.

Twelve delayed neutrons were observed by the neutron bar array and the 6Li
glass detectors. The neutrons ranged in energy from 300 keV up to nearly ﬁve MeV
and in branching ratio from 0.37(4)% up to 16.2(17)% of the total decay strength. In
addition, gamma rays from 180 were determined to be from the ”N source suggesting
delayed neutron emission to bound excited states of [80 rather than direct population of
the ground state. The total delayed neutron emission probability for 19N was found to

be 50.9(62)% of the decay strength; which is in agreement with reference [94Re]. The

- 120-

branching ratio for delayed neutron emission to bound excited states of '80 was found
to be l6.5(2l)% of the total. Figure 53 Shows the beta feeding to speciﬁc levels in 190,
where possible, and table 49 Shows the transitions that cannot be assigned on the basis

of these experimental data.

Table 49 Summary of transitions observed during the decay of 19N that cannot be
definitively assigned to intermediate levels in 19O.

 

 

Transition Type Neutron Energy (COM) Branching Ratio (%)
(B + n) 300 keV 1.4(3)
(B + n) 490 keV 8.7(10)
(B + n) 2210 keV 2.2(4)
(B + n) 2490 keV 3.5(4)
(B + n) 3060 keV 1.97(32)
(B + n) 4000 keV 0.37(4)
(B + n) 4910 keV 0.90(10)

 

A comparison of the shell model predictions with the data recorded in this
experiment iS not particularly favorable. The model derived half-life is a factor of two
larger than experiment and, with the exception of the lower energy levels being
comparable in energy, the delayed neutron emitting excited States do not appear to track
between experiment and theory even when the transition uncertainty is included in the
mix. It is hoped that consideration of these new data will facilitate improvements in the
model parameters to better approximate the reality of this nuclear system.

From the observed neutron energies and their log(ft) values, a functional upper

limit on the resonant neutron capture cross section of '80 was derived. The resonant

-121-

4458 1- [1.5(4)] .
3920 2+ [1.2(3)]

 

 

12.54 (1/2-)

19”

$83 () [1.9(2)]

8778 () [9.801)]
8707 () [2.0(3)]

 

3834 11+ 10.11311“—

 

 

 

1882 2+ [13.7(11)]

 

5082 1/2- [8.700)]

/— 4582 312— [2.213)]

 

0 (395?) 0+ [Hill]

18 0+“

Figure 53 The beta decay of

 

7 3845 3/2- [34.2(37)]

 

3232 (112-) [4.8(6)]

 

 

1472 1/2+ [O.27(12)]

 

 

 

 

S 3l2+ [(4.3]
0 5I2+ [Ila]

 

 

 

 

19!]

l9N showing the transitions that may be conclusively

identified from the experimental data.

-122-

cross section was compared with the direct capture cross section and found to be orders
of magnitude lower, over most of the temperature range of interest, for inhomogeneous
big bang models of nucleosynthcsis. It is noted, however, that at about (T9 = 0.8), the
upper limit of the resonant part is comparable to the direct capture cross section.

From the beta decay log(ft) values and the known spin and parity of the '90
levels populated, the ground state spin and parity of '9N is restricted to be either (1/2-)
or (3/2-). This result is consistent with the common-sense assignment of (1/2-) that is
characteristic of all other odd-mass nitrogen isotopes where the ground state properties
are known. It is noted that this experiment was not sensitive to the 19N ground-State to
ground-state decay, or to delayed alpha emission which is energetically possible.
5.2 SUMMARY AND CONCLUSIONS - z”N

As with l9N, the 20N data permitted re-evaluation of the beta decay half-life,
measurement of the delayed neutron spectrum, and observation of beta feeding to bound
excited states of the daughter 200. The half-life of 20N was found to be 121 (:1: 5 1'10)
milliseconds; which is in agreement with the results published in [94Re, 88Mu].

Analysis of the gamma ray data illuminated ﬁve transitions characteristic of 200
indicating direct beta feeding to bound levels of the daughter. From the gamma ray
energies and the inferred cascades the beta decay can be demonstrated to populate four,
possibly ﬁve, bound excited levels of 20O. The populated levels are the 5387 (0+), 4456
(0+), 4072 (2+), 1674 (2+) with branching ratios of 1.8(4), 1.2(4), 3.7(8) and 19.5(22)
percent, respectively. The ﬁfth possible level is at 3570 (4+) with a branching ratio of

0.8(8) percent (which is also consistent with zero percent). From these spin/parity

-123-

values and the calculated log(ft) of the beta decay, the ground state of 20N is most
consistent with a (1-) Spin and parity. assignment.

The neutron bar array permitted identiﬁcation of eight delayed neutrons ranging
in energy from 1.16 to 4.46 MeV and ranging in branching ratio from 0.5(1) to 12.1(13)
percent of the total decay strength. The probability for neutron emission is calculated to
be 34.0(40)% of the total decay strength for neutrons above 1.1 MeV in energy. From
the gamma ray data, two transitions characteristic of '90 are observed and assigned as
following directly from the decay of ZON- These are the 96 keV and 1374 keV lines and
their observation suggests delayed neutron emission from 20N populating excited levels
of '90 rather than the ground state.

Due to the lack of information regarding higher energy levels in 20O, and the
uncertainty in the decay characteristics, it is only possible to assign one beta transition
to a speciﬁc neutron-emitting level of 200. Figure 54 shows the beta feeding to speciﬁc
levels in 200, where possible to assign, and table 50 shows the transitions that cannot be

assigned on the basis of these experimental data.

Table 50 Summary of transitions observed during the decay of 20N that cannot be
assigned to speciﬁc intermediate levels of 20O.

 

 

Transition Type Neutron Energy (COM) Branching Ratio (%)
(B + n) 1160 keV 4.5(5)
(B + n) 1330 keV 2.5(3)
(B + n) 1630 keV 8.3(9)
(B + n) 2630 keV 1.3(2)
(B + n) 3380 keV 2.1(3)
(B + n) 3880 keV 2.7(3)
(B + n) 4640 keV 0.5(1)

 

-124-

 

‘20”

8770 0+ [12.1(13)]

 

1472 112+ [4.8(9)] -—

 

 

S 372+ 120.0(70)]
0(7808) 512+ [I’ll]

 

 

190+n

 

5387 0+ [1.8(4)]

 

4458 0+ [1.2(4)]

 

4072 2+ [3.7(8)]

 

 

3570 4+ [0.8(8)]

 

 

 

 

1874 2+ [18.5(22)]

 

 

0 0+ [I70]

200

Figure 54 The beta decay of 20N showing the transitions that may be conclusively
identified from the experimental data.

-125-

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-128-

 

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