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This is to certify that the

dissertation entitled
“Projectile Fragmentation of Kryptcn Isotopes

at Intermediate Energies"

presented by

RPMAN PFAFF

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PROJECTILE FRAGMENTATION OF KRYPTON
IS OTOPES AT INTERMEDIATE ENERGIES

By

Raman Pfaff

A DISSERTATION

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

DOCTOR OF PHILOSOPHY

Department of Physics

1996

ABSTRACT

PROJECTILE FRAGMENTATION OF KRYPTON ISOTOPES AT
INTERMEDIATE EN ERGIES

By

Raman Pfaff

To gain a better understanding of the production of projectile-like fragmentation
products and exotic isotopes and to provide information on the stability of nuclei along the
path of the rapid-proton capture process, isotopic cross sections from the reactions
78Kr + 58Ni at 70 MeV/nucleon and 86Kr 4- 27A1 at 75 MeV/nucleon were measured at 0°
with the A1200 fragment separator at the National Superconducting Cyclotron Laboratory.
The particle stability of 69Br, which is a critical isotope to the path of the rapid-proton
capture process, was thoroughly probed during the fragmentation of 78Kr, and it appears to
be particle unstable. The experimental production cross section data are compared to
previous krypton isotope fragmentation data as well as to an intranuclear cascade code
developed for higher energies (> 200 MeV/nucleon) and a semiempirical parametrization
derived from high energy systematics. An effort was made to explore the dependence of
the N/Z ratio of the projectile on the observed isotopic distributions and to place tighter
limitations of the half-lives on several nuclei important to the rapid-proton capture

process.

To My Grandmother
Clara Kruh

iii

ACKNOWLEDGEMENTS

I’ll start out by thanking my advisor Walt Benenson for giving me the opportunity
to do research here at the NSCL and pursue my own research interests along the way. I
would also like to thank Dave Morrissey for helping me along my ‘fragmented’ research
path. Without these two individuals, I would not have made it this far.

I’ve met many graduate students during my years here and I can’t even begin to
name them all. However, there are several individuals I would like to mention. Mike
Fauerbach is the only true football fan left here at the lab, and he also helped me with
several aspects of my research. Carl Nelson helped to keep me motivated and I hope I’ve
done the same for him. John Johnson helped me out at the start of my long and winding
college career and hopefully our post college career won’t be quite that winding.

Finally, I’d like to thank my parents who gave me a great start to my education by
letting me read anything I could get my hands on, and Erin Shore who was kind enough to

put up with me during a rough year of a medical adventure.

iv

TABLE OF CONTENTS

LIST OF TABLES vii
LIST OF FIGURES viii
Introduction 1
1.1 Projectile Fragmentation ....................................... 1

1.1.1 Energy regimes of projectile fragmentation .................. 4
1.2 Purpose of the present work .................................... 6
Previous Research/Theory 10
2.1 Applicable Systematics ....................................... 11
2.2 ISApace Model ............................................. 19
2.3 Recent Research ............................................ 27

2.3.1 Krypton Fragmentation at 44 MeV/nucleon ................. 28

2.3.2 Krypton Fragmentation at 200 MeV/nucleon ................ 32

2.3.3 High Energy Krypton Fragmentation ...................... 36

2.3.4 Rapid-Proton Capture Process ........................... 41
Experimental Apparatus and Analysis 48
3.1 The A1200 Fragment Separator ................................ 48
3.2 Detector Setup .............................................. 49
3.3 Electronic Setup ............................................ 51
3.4 Isotopic Identification ........................................ 51

3.5 Parallel Momentum Distributions ............................... 56

Results and Discussion 60
4.1 Fragmentation of 86Kr Projectiles ............................... 60
4.1.1 Momentum Widths .................................... 60
4.1.2 Momentum Transfer ................................... 64
4.1.3 Isotopic Cross Sections ................................. 68
4.2 Fragmentation of 78Kr Projectiles ............................... 70
4.2.1 Isotopic Cross Sections ................................. 71
4.2.2 Memory Effect ....................................... 74
4.2.3 Implications for the rp-process ........................... 78
Summary 82
Measured Cross Sections 84
ISApace Input Parameters 91
List of References 92

vi

LIST OF TABLES

Table 4.1:Fitted slope parameters for individual elements from 86Kr fragmentation. . . . 66
Table A.1:Isotopic cross sections from 78Kr + 58Ni at 75 MeV/nucleon. ............ 85

Table A.2:Isotopic cross sections from 86Kr + 27A1 at 70 MeV/nucleon. ............ 89

vii

FIGURE 1.1:

FIGURE 2.1:

FIGURE 2.2:

FIGURE 2.3:

FIGURE 2.4:

FIGURE 2.5:

FIGURE 2.6:

FIGURE 2.7:

FIGURE 2.8:

FIGURE 2.9:

FIGURE 2.10:

FIGURE2.11:

LIST OF FIGURES

A simplistic picture of the projectile fragmentation process. ........ 2
Bar chart showing importance of in RNB experiments at the
NSCL. ................................................. 10
A plot used to determine a parametrization for the momentum
width from both target and projectile fragmentation residues. ...... 12
The plot used to determine the parametrization for the
momentum shift that fragments undergo during both target and
projectile fragmentation. ................................... 12
The experimental data originally used for the parametrization of
the memory effect ......................................... 17
Parametrization of the “memory effect” from Eq. 2.14. The data
used for this parametrization are shown in Figure 2.4. ............ 18
Nucleon-nucleon cross sections as a function of incident energy.. . . . 21
Nuclear density for 86Kr from the ISABEL code, a Yukawa
distribution and a square well density distribution ................ 23
Simple view of the energy diagram used to determine created
holes, captured particles, and free particles in the ISABEL code.. . . . 25
The momentum distribution for 72Ge31+ from the fragmentation
of 84Kr + 27A1 at 44 MeV/nucleon ............................ 29
The two—peak structure observed for several momentum
distributions from the reaction 86Kr + 27A1 at 44 MeV/nucleon. . . . . 29

Cross sections resulting from 84Kr + 27A1, 103Rh, 197Au at 44

MeV/nucleon ............................................

viii

.30

FIGURE 2.12:

FIGURE 2.13:

FIGURE 2.14:

FIGURE 2.15:

FIGURE 2.16:

FIGURE 2.17:

FIGURE 2.18:

FIGURE 2.19:

FIGURE 2.20:

FIGURE 2.21:

FIGURE 2.22:

FIGURE 2.23:

FIGURE 2.24:

FIGURE 2.25:

FIGURE 3.1:

FIGURE 3.2:

Selected yields from 44 MeV/nucleon krypton fragmentation
along with corresponding predictions from a high energy
fragmentation model ....................................... 31
Cross sections from krypton fragmentation at 200 MeV/nucleon.. . . . 32

Velocity shift for the fragmentation products resulting from
84Kr + 197Au at 200 MeV/nucleon ............................ 34

Parallel momentum widths for the intermediate energy
fragmentation products ..................................... 35

Typical momentum distribution observed during high energy
fragmentation - in this case 86Kr + 9Be at 500 MeV/nucleon ........ 36

Parallel momentum widths observed during high energy krypton
fragmentation of 86Kr + 9Be ................................. 37

Momentum shifts observed during the fragmentation of
86Kr + 9Be at 500 MeV/nucleon .............................. 38

The momentum shift for the fragments near the mass of the

projectile during high energy krypton fragmentation .............. 39
Cross sections for the reaction products resulting from high

energy fragmentation of 86Kr + 9Be at 500 MeV/nucleon .......... 40
The CNO and HCNO cycles ................................. 42

Proposed rp-paths for varied temperature and density conditions. . . . 43

Proposed path of the rp-process for the region applicable to the

present research ........................................... 44
Isotopic yields resulting from 78Kr + ”M at 65 MeV/nucleon. ..... 45
Yield from the fragmentation of 78Kr + Ni at 73 MeV/nucleon ...... 46
A schematic diagram of the A1200 fragment separator ............ 50

A schematic diagram of the electronic setup that was used during
the fragmentation experiments. .............................. 52

ix

FIGURE 3.3:

FIGURE 3.4:

FIGURE 3.5:

FIGURE 3.6:

FIGURE 3.7:

FIGURE 4.1:

FIGURE 4.2:

FIGURE 4.3:

FIGURE 4.4:

FIGURE 4.5:

FIGURE 4.6:

FIGURE 4.7:

FIGURE 4.8:

FIGURE 4.9:

Turning radius vs. horizontal position for PPACO at Dispersive
Image #2 that was observed during the 78Kr fragmentation
experiment ............................................... 53

A plot of AB vs. TOF observed during the reaction 78Kr + 58Ni.
The arrow indicates the “gap” where 69Br should be observed. ..... 54

Energy and time calibrations for the detectors during the 86Kr
fragmentation experiment ................................... 55

The AZ, and Q resolution obtained in the 86Kr fragmentation
experiment ............................................... 57

The momentum distribution of 80Br35+ with varied Gaussian fits.
The arrow indicates the projectile momentum. .................. 58

Parallel momentum distribution widths for isotopes observed
during the fragmentation of 86Kr. ............................ 61

Parallel momentum width from recent krypton fragmentation
experiments .............................................. 62

Momentum transfer vs. mass loss for the projectile-like
fragments observed during 86Kr fragmentation at 70 MeV/
nucleon. ................................................ 65

Isotopic cross sections determined from the fragmentation of
86Kr + 27A] at 70 MeV/nucleon. ............................. 69

Centroids of parallel momentum distributions for isotopes
observed during projectile fragmentation of 78Kr + 58Ni at 75
MeV/nucleon ............................................. 7 1

Mass distributions for the Z=30 to 2:38 isotopes recorded while
magnetic rigidity was optimized for observation of 69Br ........... 72

Isotopic cross sections determined from 78Kr + 58Ni at 75 MeV/
nucleon. ................................................ 73

Parametrization of the memory effect which shows data from
several krypton fragmentation experiments. .................... 76

Halflives calculated using the WKB approximation for 69Br and
73
Rb .................................................... 80

X

Chapter 1

Introduction

1.1 Projectile Fragmentation

In recent years the use of radioactive nuclear beams (RNB’s) has become a rapidly
moving and promising specialty of nuclear science, and the development and
implementation of RNB facilities has become a high priority at many research institutions.
There are only 263 stable isotopes in nature but estimates predict there may be more than
8000 radioactive isotopes which could be created and observed. These radioactive isotopes
are vital to the study of many aspects of nuclear physics including mass models, half-lives,
cross sections, shell structure, isomeric production rates, and astrophysical processes.
When planning such studies for the laboratory, target, projectile, and beam current
requirements must be explored in an effort to understand the practicality of the
experiment. For this aspect of the planning it is necessary to have reliable predictions for
the most basic results of the fragmentation process — most notably the expected cross
sections for the particles of interest.

In 1947 Serber [ser47] proposed the idea that peripheral highly energetic heavy-
ion reactions can be described as a two—step process in which each step occurs in clearly
separated time intervals. The first step describes the initial collision between the

constituents of the target and projectile nucleus and occurs rapidly, on the order of 10'23

1

2

seconds. This step can lead to highly excited objects (prefragments) which are usually
very different from the final observed fragments. Before detection, the prefragments lose
their excitation energy through the emission of particles (neutrons, protons, and small
clusters) and 'y-rays. This second step (deexcitation) occurs slowly relative to the first step
and typically occurs on the order of 10'16 to 10'18 seconds (depending on the excitation
energy of the prefragment). A simple portrayal of this process is shown in Figure 1.1 in
which the overlapping region of the target and projectile is sheared off, leaving an excited
prefragment. The prefragment then deexcites through statistical emission and becomes the

final observed fragment.

Due to the differences in the physical nature of the two processes, one usually uses
separate theoretical models to describe the two individual steps of the fragmentation. To
simulate the rapid first step of the nuclear reaction an intranuclear cascade model (INC) is

often used. The INC models treat the projectile—target interaction as simple nucleon-

Projectile Prefragment

  

Final Observed
Fragment

 
 

Target

FIGURE 1.1: A simplistic picture of the projectile fragmentation process.

3

nucleon collisions in a microscopic manner (further details will be discussed in the
subsequent chapter). After performing the INC calculation, one deals with excited
prefragments that are characterized by their atomic mass, proton number, excitation
energy, and their linear and angular momentum. These prefragments are usually far away
from their ground state and have a very short lifetime. To describe the deexcitation process
a statistical evaporation code is generally used. These codes were originally developed
(and successfully used) to describe compound nucleus reactions near the Coulomb barrier.
The basic properties of INC models and some details of the INC code used in the present
research will be described in Chapter 2 along with a discussion of the statistical
evaporation code. The results of the present research will be compared to the predictions
from the ISApace model [fau92], which uses the INC code ISABEL [yar79] in

conjunction with a statistical evaporation code PACE [gav80].

Use of the INC model together with the statistical evaporation code is a computer
intensive method that has been used to gain further insight into the physical properties
involved in the fragmentation process. Because it is often necessary to estimate cross
sections quickly during the preparation of an experiment, a relatively simple
parametrization is often a more practical method than a computer intensive solution.
Stimmerer et al. [sum90] used the data available in 1988 to develop the widely used EPAX
parametrization, which has proved reliable for a large portion of the high energy
radioactive nuclear beam research performed over the past decade. The parametrization
was developed from high energy data, and the only input parameters are the mass and
proton number of the projectile and target. The energy of the projectile is not considered to

be an essential input parameter due to the fact that EPAX was geared towards high energy

4

fragmentation in which the nucleon-nucleon cross section is relatively constant (limiting
fragmentation). However, recent advances in technology have allowed experiments to be
performed using intermediate mass projectiles at intermediate energies, in which energy
dependent factors could cause the EPAX parametrization to fail. The EPAX
parametrization and comparisons to the present experimental results will be discussed in

subsequent chapters.
1.1.1 Energy regimes of projectile fragmentation

The bombarding energies used for projectile fragmentation experiments have
changed with technological advances. The processes which occur at the various energies
differ tremendously. Low energy “fragmentation” was available for many years, and a
large amount of data has been accumulated [fle74]. At low energies (E/A S 20 MeV/
nucleon), several different reaction mechanisms contribute to the process. Reactions in
this energy regime are generally not considered “true” fragmentation and can not be
described by Serber’s [ser47] simple two—step process. The time of interaction is long, due
to the slow relative velocity between the target and the nucleus, and the Fermi momentum
of the individual nucleon constituents of the target and the projectile is greater than the
momentum of the nucleus itself. These factors can result in a combination of processes
(dependent on the impact parameter of the collision) including Coulomb scattering,
incomplete fusion, complete fusion, and compound nucleus interaction.

Reactions occurring at much higher energies (E/A 2 200 MeV/nucleon) are
considered to be “pure” fragmentation as was discussed by Serber. It should be noted that
the energy limits are not exact since transitions between dominant processes occur

gradually as a function of beam energy. In this situation the kinetic energy is on the order

5

of magnitude of the rest mass of the constituent nucleons, and the interaction time between
the target and the projectile is very short. Central collisions will result in a “shattering” of
the projectile into light particles and individual nucleons while distant interactions will
undergo Coulomb scattering and excitation. Peripheral reactions will exhibit “pure”
fragmentation in which the region of the projectile that overlaps the target during the
interaction will be torn off, leaving an excited prefragment. As accelerators improved over
the past few decades, many high energy experiments using heavy fragments were
performed [b0186, gol7 8]. In recent years exploration of fragmentation in the intermediate
energy regime (20 MeV/nucleon S E/A S 200 MeV/nucleon) has been carried out in an
effort to understand the transition from the high energy fragmentation to the complex
processes occurring in the low energy regime. This intermediate energy regime has proven
to be very interesting and challenging, to both the experimental procedures and theoretical

models.

The collection of systematic data in which cross sections and momentum
distributions were measured for a large collection of fragmentation products are still
limited, but growing. At the National Superconducting Cyclotron Laboratory (N SCL)
Souliotis et al. [sou9l, sou92] performed several experiments involving l4N beam on both
27Al and 181Ta targets at 75 MeV/nucleon and 18O + 27A] at 80 MeV/nucleon. Fauerbach
et al. [fau96] studied the fragmentation of 40Ar +9Be at 90 MeV/nucleon. The data
available for heavier elements are rather limited due to the complexity of the experiments,
from both the technological and analytical side. Until recently, beams comprised of
intermediate mass projectiles could not be easily produced at experimental facilities, and

analysis was difficult since good energy resolution was needed to separate isotopic charge

6

states which are prevalent in the intermediate and low energy regimes. Bazin et al. [baz90]
performed an experiment at GANIL near the low end of the intermediate energy regime
that used 84Kr + 27A1, 103Rh, 197Au at 44 MeV/nucleon. In an effort to add to the database
of information pertaining to intermediate energy fragmentation performed with
intermediate mass projectiles, two experiments were carried out at the NSCL; one
involved the fragmentation of neutron-rich 86Kr + 27A1 at 70 MeV/nucleon, and a second

experiment with proton-rich 78Kr + 58Ni at 75 MeV/nucleon.

1.2 Purpose of the present work

The primary goal of the present work was to gain a greater understanding of
projectile-like fragmentation in the intermediate energy regime, however, each experiment
had specific individual goals. For the fragmentation of the neutron-rich 86Kr +27A1
system, the cross sections and parallel momentum distributions were measured for many
nuclei near the beam mass, and the momentum dependence of the fragmentation products
was closely explored — most notably for nucleon pick-up products which occur rarely at
high energies. A parametrization was developed to explain the momentum shift observed
for fragmentation products that acquired as many as three additional protons in the
process. The results of the pick-up process data will be compared to previous experiments
which involved neutron pick-up [sou92]. The data from this experiment will also be
compared to previous neutron-rich krypton fragmentation experiments which were
performed at both higher and lower energies and with the predictions of the ISApace

model and the EPAX parametrization.

Fragmentation of the proton-rich 78Kr + 5 8N i system at 75 MeV/nucleon was

performed in an effort to provide further information on the stability of nuclei along the

7

path of the rapid-proton capture process (rp-process). The rp-process was first proposed by
Wallace and Woosley [wa181] who showed that heavy isotopes (up to A=100) could be
produced in astrophysical processes in which high temperatures and densities exist, such
as supernova shock waves, novae, and x-ray bursts [wor94, cha92]. The rp-process
proceeds via a sequence of proton capture and [3+ decays near and sometimes along the
proton drip line. Particle stability and half-lives are important in determining the rate and
actual path of the rp-process since it occurs during explosive processes in short time
periods (~ 10 - 100 5). When the rp-process path must pass through isotopes with long [3*
half-lives, the rp—process will be slowed or terminated. Mass models [jan88] differ on
predictions of the exact position of the proton drip line which prompted several
experiments that looked for possible termination points of the rp-process [rob90, moh91].
In recent years the odd Z isotopes of 65As and 69Br have been investigated as the most
likely termination points because the half-lives of 64Ge and 68Se, the proton capture
targets, are thought to be longer than the time scale of the explosion that provides the
proton flux. Evidence for the existence of 65As and 69Br (along with four other new
isotopes) was first reported by Mohar et al. [moh91]. A subsequent experiment measured
the half-life of several of the isotopes including 65As; however, 69Br was not observed
[win93, he195]. A recent experiment at GANIL [bla95] reported five new isotopes (60Ga,
64As, 69’70Kr, and 74Sr) which extended the experimentally observed proton drip line, but
no events were attributed to 69Br. The latter experiment had a flight path six times longer
than the one performed by Mohar et al. [moh9l], indicating that 6S’Br was not stable or had

a very short half life (< 100 us). To explore these possibilities the 78Kr fragmentation

8

experiment was performed such that it would be sensitive to nuclei with very short
(~ 100 ns) half-lives.

The present study of the proton-drip line nuclei also involved the measurement of
production cross sections of many proton-rich isotopes. The data from both the 78Kr and
86Kr fragmentation data allowed a parallel investigation of the so-called “memory” effect
[chu71] by comparison of the 78Kr to the 86Kr results. The memory effect deals with the
fact that fragments close to the mass of the projectile can “remember” the N/Z ratio of the
projectile. Prefragments with (very) high excitation energies are likely to produce final
products along a ridge parallel to the valley of B stability (e.g. Stephan et al. [ste91]) due
to evaporation of many nucleons. The final observed fragments that are far in mass from
the projectile will have no “memory” of the N/Z ratio of the projectile. Prefragments that
have low excitation energies will not lose many nucleons, will be much closer in mass to
the original projectile, and have an N/Z ratio similar to the projectile. The memory effect
parametrization is an attempt to describe the rate at which fragments will “forget” the N/Z
of the projectile. Isotopic cross sections from fragmentation reactions involving members
from both extremes of an isotopic chain can therefore provide crucial information on this
influence of the projectile N/Z ratio on the fragment charge dispersion distribution
(“memory effect”). Data from the experiment which utilized the very proton-rich 78Kr
projectile (N/Z ~ 1.17) are compared to data from the fragmentation of the very neutron-
rich krypton isotopes 86Kr (N/Z ~ 1.39) and an earlier experiment [ste9l] that involved

fragmentation of 8"’Kr (N/Z ~ 1.33).

9

The subsequent chapters will present the results from the two intermediate energy

fragmentation experiments which used proton- and neutron-rich krypton projectiles. The

following subjects will be addressed in detail:

isotopic cross sections for many projectile-like fragments

influence of the use of a Gaussian fit on the isotopic momentum widths
momentum shift parametrization developed for proton pick-up products
possible termination points of the rapid proton capture process

lifetime limitations placed on several proton drip-line nuclei

memory effect parametrization for use in the intermediate energy/mass regime
comparison of experimental data to the EPAX parametrization

comparison of experimental data to calculations using the ISApace code.

Chapter 2
Previous Research/Theory

Projectile fragmentation has become a widely used technique for production of
radioactive nuclear beams (RNB’s) at many facilities [mue93]. In the past decade, new
fragment separators have been constructed at Gesellschaft fiir Schwerionenforschung
mbH (GSI), The Institute of Physical and Chemical Research (RIKEN), Grand
Accélérateur National d’Ions Lourds (GANIL), and here at the NSCL [arm87, n0189,

she91] in an effort to take advantage of the technique of projectile fragmentation.

 

 

 

 

 

 

 

 

 

 

 

 

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8 10 12 14 16 18 20 22

N SCL PAC Meeting

FIGURE 2.1: Bar chart showing importance of in RNB experiments at the NSCL.

10

11

Figure 2.1 shows the rapid increase in the percentage of beam time used at the NSCL for
RNB research over the past several years and therefore the need for a good understanding
of the isotopic production cross sections from various beam and target combinations

which will be employed to produce the fragments of interest.

Two techniques are primarily used to predict the cross sections for intermediate
and high energy reactions, the EPAX parametrization and an intranuclear cascade (INC)
calculation (ISApace is currently a widely used INC code combined with an evaporation
code). This chapter will introduce much of the background material which led to the
development of the techniques and some recent research which inspired the present work
to be performed. The basics of the EPAX parametrization will be discussed in Section 2.1,
and Section 2.2 will do the same for the ISApace model. In Section 2.3 a recap will be
given for some of the previous research which was performed using krypton projectiles at
a variety of energies in an effort to study both projectile fragmentation and limits of

isotopic stability important to the rp—process.
2.1 Applicable Systematics

After more than two decades of target fragmentation experiments and a rising use
of projectile fragmentation to produce exotic isotopes, parametrizations were needed
which could describe three basic observables (the momentum width, longitudinal
momentum, and the yield of the fragments) for any target/projectile reaction so that
predictions could be made for future experimentation. In 1989 the “Morrissey
systematics” [mor89] included a parametrization of the momentum width and momentum
transfer of fragmentation products. It was shown that the formalism was applicable to both

target and projectile fragmentation which it should be — since the reactions are equivalent

 

 

2000 I I I I j Y r I T l T fl ‘I
. . l
: ° ° 0 o :
’8 l—- o 0 30
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FIGURE 2.2: A plot used to determine a parametrization for the momentum width
from both target and projectile fragmentation residues.

 

 

 

 

. I . . f I 4
o
400 — . .-
fo‘ . g :c .0 i
\ o
t. . ! '.l
o ooo o
E . Ila. I
‘A ‘o o o o 0
Q; 200 _ 9 .. . . 0‘
' I
’ o
.o o
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AA

FIGURE 2.3: The plot used to determine the parametrization for the momentum shift
that fragments undergo during both target and projectile fragmentation.

13

in different rest frames. Morrissey considered three different mechanisms for the
production of both target and projectile residue and showed (using conservation of

momentum) that the momentum width could be written as

o" = (50-4/ij (21}
where AA = Ainitial - Aobserved’ 0'0 is a constant, and the parallel momentum width is given
by a". Figure 2.2 shows a fit using a variation of Eq. 2.1 (where Prms = J3 - a") to a
collection of high energy data (both projectile and target fragmentation) available at that

time and it yields a value of 0'0 ~ 90 MeV/c which reproduces the data relatively well.

The longitudinal momentum transfer (for target fragmentation) was presented by

Morrissey as

<P{.> = mmg<B..>Bv/(v +1) (2.2)
where mtarg is the mass of the target, (Bu) is the average velocity along the beam axis, and
the kinematic factor of [iv/(7+1) depends only on the initial velocity of the beam.
Morrissey made the assumption that the mass of the primary residue is approximately
equal to the mass of the target (for target fragmentation). Figure 2.3 shows the longitudinal
momentum transfer plotted against the mass loss (AA = Amiga] - Aobsmed) for several sets
of data which were available at the time (both target and projectile fragmentation) and for
the relatively low AA values (where Eq. 2.2 would be applicable) a direct linear

relationship was observed where

@ ~ 8 MeV/c/u (2 3)
AA ° °

The slope of 8 MeV/c/u did a relatively good job reproducing the data available at the

time.

14

Using the parametrizations in Eq. 2.1 and Eq. 2.2, the momentum width and
transfer for the residue from high energy fragmentation reactions could be predicted
relatively well which leaves one important observable in need of a parametrization — the
cross section. One year after the publication by Morrissey, Siimmerer et al. [sum90]
published a paper in which over 700 experimental fragment cross sections available at that
time were analyzed — from both target and projectile fragmentation. The goal of the paper
was to determine an analytical form (often referred to as the EPAX parametrization) which
could predict the yield for any target/projectile reaction at high energy, and Siimmerer

points out that several parametrizations published at the time did not achieve that goal.

The cross section parametrization for a particular A and Z can be written as

follows:

o(A,Z) = Y(A) n exp(—R|23+A+Am-Z|U) (2.4)

where the first term represents a mass yield (the sum of the isobaric cross sections with
mass A), the n term is for normalization, and the exponential term represents the
distribution of elemental cross sections with a given mass around the maximum (Zp). The
width of the charge dispersion is controlled by the width parameter, R, and the exponent
U. The two A terms represent a shift in the peak of the charge dispersion due to the NZ
ratio of the projectile during projectile fragmentation or the target during target
fragmentation. The various terms are then broken down further to the point at which the
cross section is only a function of the mass of the target and projectile, and the mass and
charge of the isotope of interest. (Note that this is not dependent upon the energy of the
reaction since it was developed for high energy fragmentation.) The following breakdown

of the terms from Eq. 2.4 was developed by Siimmerer et al. by analyzing the data that

15

was available at the time and fitting the results with applicable functions. Over 85% of the
data present at the time could be reproduced within a factor of two using this
parametrization.
The Y(A) term is given as
Y(A) = ORP(A,)exp[—P(At)(At—A)] (2.5)
where At is the mass of the target, A is the mass of the observed isotope, P(At) is

formulated as

lnP(A,) = —7.57x10‘3A,-2.548, (2.6)

and the total reaction cross section term of GR is written as

OR = 450(Ap”3 + A,”3 — 2.38) mb (2.7)
where Ap is the mass of the projectile.

The exponential term contains the variables of R, U, and Zp which are strongly
correlated and Siimmerer et al. chose to fix the value of U and then determine the best
formalism for Zp and R. The U term was then given simply as

U: 2 for (Zp—Z)<0 and U: 1.5 for (Zp—Z)20 (2.8)
where Zp represents the peak of the charge distribution and can be written as
Zp(A) = ZB(A) + A (2.9)

and the right hand side of Eq. 2.9 is given by

 

A
z (A) = (2.10)
B 1.98 + 0.0155A2/3
and
2.041x10’4 - A2 ifA < 66
A = 2 (2.11)
2.703x10’ - A — 0.895 if A 2 66.

16

The width parameter of R is given as

In R(A) = —6.770x10"3A + 0.778 (2.12)

and it is pointed out that this variable can have a dramatic effect on the rates predicted for
exotic isotopes on the wings of the isotopic distributions. The normalization term 11 is then

given as:

n = J3 . (2.13)
1C

The final term from Eq. 2.4, that has not yet been discussed, is the Am or “memory
effect” term which was introduced by Siimmerer et al. to explain the “shift” in the charge
dispersion curve that occurs due to different N/Z ratios in the projectile during projectile
fragmentation or the target during target fragmentation. The majority of data that was
analyzed by Siimmerer consisted of target fragmentation where the N/Z ratio of the targets
was generally close to the valley of B stability, and experiments with targets (or
projectiles) far from the valley of stability did not occur until recently. Siimmerer used
very limited data to obtain a formalism for the memory effect, and those data are shown in
Figure 2.4. The neutron-rich data was obtained by Westfall et al. [wes79] and involved the
projectile fragmentation of 48Ca + 9Be at 212 MeV/nucleon, and the proton-rich data
implemented target fragmentation with the reactions of p+96Ru and p+96Mo at 1.8 GeV
[por64]. As can clearly be seen in Figure 2.4, the proton-rich data are extremely limited
and the neutron-rich data are “one-sided” and the value of ZP (needed in Eq. 2.8) could not
be directly determined from the data. The parametrization for AI“ from the limited data is
shown in Figure 2.5 by the solid curves (the hollow data points are the result of a

calculation — not actual data) and was described as

0 (11b)

Cross Section (mb)

FIGURE 2.4:

17

 

 

 

 

 

 

 

 

I I I I I I I I j I I I I I I I I I I T
_\ \l_
C
_ 0 Al ._
N F Ne Na M Si P
_ l l J l l l l l l l l l_l lg l I l l l I T
8 12 16 20 24 28
Neutron number
5 l f I I I I I fl I ' =
E E
101 5— ‘2‘
100 :r ‘E
E a
l- -t
10-1 E— :5-
E i
j- -
10" =— i.
10'3 E—
l
38

 

Fragment Charge Number (Z)

The experimental data originally used for the parametrization of the

memory effect.

18

 

1.0 ’

0,3 - neutron-rich nuclei o

Am / (z,-ZB)

 

 

 

 

1.0 '

0.8 ' proton-rich nuclei

0.6 '

Am / (Zt-ZB)

0.4 f

0.2 '

 

 

 

 

0.0 0.2 0.4 0.6 0.8 1.0
A/At

FIGURE 2.5: Parametrization of the “memory effect” from Eq. 2.14. The data used
for this parametrization are shown in Figure 2.4.

19

Am(A) = [c1(£)2+c2(£)4]AB(At) . (2.14)

where At is the target mass and AB (At) = Zt - ZB(At), in which Z[ is the target proton
number, and At is the target mass. Different values for the coefficients c1 and c2 were

determined for neutron- and proton-rich fragmentation and are given by

C1 = 0.4 and c2 = 0.6 for neutron-rich nuclei

. . (2.15)
CI = 0.0 and c2 = 0.6 for proton-rich nucler

and the solid curves in Figure 2.5 use these values.
2.2 ISApace Model

The basic idea behind all intranuclear cascade models is that high energy heavy-
ion reactions can be described as a sequence of independent nucleon-nucleon (N-N)
collisions between the constituents of the projectile and the target nucleus. Collisions
between the individual nucleons are treated as collisions between classical particles. In
other words: the colliding nucleons are not treated as a quantum mechanical system; rather
one assumes that the spacial coordinates and momentum of the nucleons is known, and
therefore their trajectories. All calculations in the code are performed using relativistic
kinematics, but, as was described above, the nucleons are treated in a classical sense. The
only quantum mechanical effect that is taken into account is Pauli-blocking. Describing
the colliding nuclei as a classical system can of course only be justified within certain
boundaries. The mean free path (2») between two subsequent nucleon-nucleon collisions
must be large relative to the range of the nuclear force (R), otherwise this effect on the
reaction dynamics must be taken into account. Assuming that the nuclear force is
established via the exchange of virtual mesons, a rough estimate of the range of the

nuclear force can be determined. The pion is the exchange particle responsible for the long

20

range part of the nuclear force. Using the mass of the pion and the AE - At uncertainty

relationship, we can estimate the mean lifetime of the virtual pion from

At . mncz ~ h, (2.16)
where mu is the rest mass of a pion, c is the speed of light, and h is the reduced Planck’s
constant. The range of the nuclear force can now be estimated by assuming that the pion
can not be propagating faster than the speed of light between its emission and absorption.

This leads to the determination of the range where:

R = c-Atz—h- (2.17)
me

and the limiting condition placed on the mean free path becomes

1» z—h—~ 1.5 fm. (2.18)

mnc

Another basic concept which stands behind all INC models is that the mean free
path between two subsequent collisions must be large relative to the de Broglie
wavelength of the colliding nucleons. This condition insures that the wave function of the
colliding nucleons is already in its asymptotic form before the next collision occurs and
enables one to neglect possible interference effects between subsequent collisions, as well
as the possibility of simultaneous collisions between more than two nucleons. The

condition we need to fulfill can be written as:

A » g ~ 3.4 fm (2.19)

where the projectile momentum of the present work was used.

To check if the two conditions placed on the mean free path are actually met in

heavy ion reactions, we need to get an estimate of the mean free path of nucleons inside

21

 

I I I IIIIIII I IIIIIIII

500— -

Ct (mb)

 

 

 

 

10 l l llllllll A llllllll

10 50 100 500 1000
Elab (MeV)

FIGURE 2.6: Nucleon-nucleon cross sections as a function of incident energy.

nuclear matter. In the following, effects related to being in the nuclear medium will be
neglected, e. g. the reduction of the nucleon mass due to its binding inside the nucleus. The
experimentally known total reaction cross sections (0') for collisions between unbound
nucleons from neutron-proton (n-p) and proton-proton (p-p) collisions can be combined

with the nuclear density (p) to make an estimate of the mean free path:

A = 1 ~1.7fm. (2.20)

p611, p

 

For the above estimate of the mean free path of nucleons inside nuclear matter the
saturation value of the nucleon-nucleon cross section of about 40 mb was used along with
a constant nuclear density of 0.15 fmns. From this crude estimate it is obvious that the
condition stated in Eq. 2.18 and Eq. 2.19, namely that the mean free path has to be much
larger than the range of the nuclear force and the de Broglie wavelength, are not exactly

met, and in fact they are actually of the same order of magnitude.

22

It is also important to look at the total N-N cross section as a function of incident
energy, and its influence on the mean free path of the nucleons. As can be seen in
Figure 2.6 [che68] the nucleon-nucleon cross section gets very large for low incident
energies. This behavior can be explained via the dominating pure s-wave scattering in this
energy regime. From Eq. 2.20 we can see that the mean free path gets very short and
quickly becomes smaller than the nuclear range. Therefore, one has to be very careful
when using INC models for incident energies below 100 MeV/nucleon. However, since
the present research is primarily interested in peripheral collisions leading to projectile-
like fragments, nature is on our side. The diffuseness of the nuclear surface leads to a
decrease in the nuclear density for peripheral reactions, thus increasing the mean free path
of the nucleons. This behavior offsets, at least in part, the increase in the total N-N cross
section at low incident energies. Figure 2.7 shows two approximations for the nuclear
density of the nucleus 86Kr. The constant density distribution shown in Figure 2.7 uses the
so called “folded Yukawa sharp cut-oft” density distribution [kra76] (where R=roAu3 and
a value of ro=1. 18 fin is used) while the INC code ISABEL [yar79] approximates this with

16 steps of constant densities. The agreement is good over the entire distribution.

As was mentioned earlier, INC models treat the projectile and target nucleus as an
ensemble of ‘free’ nucleons in which the nucleons undergo independent collisions during
a heavy ion reaction. It should be pointed out one more time, that the majority of INC
codes (including the ISABEL code used for the present work) use the free nucleon masses
and differential cross sections known from reactions between free nucleons to simulate the
heavy ion reaction. The calculations take the energy dependence of the N-N cross sections

into account as was shown in Figure 2.6. The angular distribution of the particles after the

23

collisions is determined by using the experimentally known differential cross sections at

the given energy. The azimuth angle is chosen randomly.

In principle, since the nuclear force is charge independent, one does not have to
distinguish between n-n and p-p collisions (after correcting for Coulomb effects),
however, the n-p (p-n) collisions must be treated separately. As is clearly shown in
Figure 2.6, the total reaction cross section for n-p (p-n) reactions at low energies is
significantly higher than the one for n-n (p-p). This effect can be explained by looking at
the relevant exchange particles for each type of collision. For the n-n (p-p) scattering
neutral pions (no) are the only allowed exchange particles, whereas for the scattering of
non-identical particles (n-p, p-n) the charged pions (1?, 1f) are also allowed exchange
particles along with the no’s, leading to three allowed exchange particles. The strength of

the interaction has to be weighed according to the number of exchange particles.

 

 

   

 

 

 

 

020 I T I T f I I I I I I I I
_ I I I q
1- dashed curve ”folded Yukawa" distribution 1
.. histogram ISABEL "stepped" distribution .
_ solid line square well nuclear density _,
0.15 '— ‘ --I
A '- -
c9 _ .
g 0 10 '— _
a f .
0.05 L- _
r- d

0.00 l l l l l I l l
0 2 4 6 8
r (fm)

FIGURE 2.7: Nuclear density for 86Kr from the ISABEL code, a Yukawa distribution
and a square well density distribution.

24
The ISABEL code used for this work takes the following six collisions into

account:
N + N —-) N + N elastic scattering
N + N -) A + N inelastic scattering
A -—> 11: + N isobar decay
7: + N —> A pion absorption
A + N —> N + N isobar absorption

Al + N1 —> A2 + N2 isobar-nucleon exchange.

As was previously mentioned, the cross sections for the firsttwo types of collisions are
taken from experimental systematics, and the cross section for the isobar absorption and
isobar-nucleon exchange are taken to be the same as the elastic scattering. The cross
section for the isobar decay is obtained by detailed balance [ber88].

Another important variable that needs to be determined by the INC code is the
excitation energy of the prefragments. It should be noted here that the ISABEL code with
the option of a uniform Fermi gas distribution was used for the present work, and it has
been shown [fau92a] that this leads to the best reproduction of experimental data. The
excitation energy is determined from the energies of ‘holes’ created in the Fermi sea
during the collision and the energies of particles which are captured by the nuclear
potential. Particles with an energy below a certain “cutoff” energy are considered captured
(or trapped) by the nuclear potential. This cutoff energy is isospin dependent and is
determined as follows: 1) for neutrons the sum of the Fermi energy and twice the neutron
binding energy and 2) for protons it is given by the maximum value of either the sum of
the Fermi energy and twice the proton binding energy or the sum of the Fermi energy, the
proton binding energy and the Coulomb energy. Figure 2.8 shows a simplified view of the

energy diagram for clarification.

25

 

 

 

 

 

 

FIGURE 2.8: Simple view of the energy diagram used to determine created holes,
captured particles, and free particles in the ISABEL code.

All nucleons will be “tracked” by the ISABEL code as long as they are inside the
overlap volume of the two colliding nuclei, independent of their energy. Nucleons outside
of the overlap volume will only be tracked until a) their energy drops below the cutoff
energy - in which case they are considered captured, or b) they leave the projectile or
target volume, and they are then considered free particles. For details of these processes

please refer to [che68]. The excitation energy is therefore given by:

* holes trapped
E = Z Eh(i) + Z Et(j) (2.21)
i i
and the linear momentum of the prefragments is similarly given by the sum over the Fermi
momentum of the holes created in the Fermi sea and the sum of the momenta of the

captured particles:

_ holes trapped
P = 25.0» 2 13.0). (2.22)
i i

26

We have now discussed the first step of the heavy ion reaction, which is simulated
via an INC code, leading to so called prefragments. These prefragments are characterized
by their mass, charge, excitation energy, as well as their linear and angular momentum.
Since the prefragments are highly excited, they are far away from their ground state and
have rather short lifetimes. The prefragments reduce their excitation energy via the
evaporation of neutrons, protons, alphas, and sometimes even more complex clusters of
nucleons, fission processes, as well as via the emission of y—rays. To simulate these
processes, one usually uses so called statistical evaporation codes. These codes have been
successfully used to describe compound-nucleus systems, produced in fusion-evaporation
reactions close to the Coulomb barrier, and since the nuclear excitation energies involved
in fusion-evaporation reactions are of the same order of magnitude as the prefragments
produced in peripheral heavy ion collisions, this seems to be a reasonable step. However,
this assumption can only be justified if the produced prefragments reach a statistical
equilibrium before they decay. Comparison of the de—excitation calculations to

experimental data [gav80] seems to prove that this assumption is fulfilled.

The basic concept of the statistical evaporation codes goes back to an idea of Niels
Bohr from 1936 [boh36]. It assumes, that the de-excitation of the excited nucleus depends
on the statistical weight of all possible (allowed) decay channels and their transmission
probability. The entrance channel, or in other words “how the system became excited,”
does not matter. This of course implies that the system has reached a thermal equilibrium
before it decays — which can lead to problems if one wishes to use statistical models for
extremely high excited systems which are produced in violent central collisions. In this

situation, the lifetime of the system might become too short to equilibriate all inner

27
degrees of freedom. Another basic assumption of the statistical model is that of the

sequential decay. It assumes, that if more than one decay occurs, they happen one after the
other. It’s also assumed that the decaying nucleus has enough time between the sequential
decays to reach thermal equilibrium again. For the calculations performed in the present

work, the well established and widely used code PACE by Gavron [gav80] was utilized.
2.3 Recent Research

The intermediate energy regime is generally considered to range from 20 MeV/
nucleon to 200 MeV/nucleon, and the results from two experiments performed with
krypton projectiles near the opposite ends of that energy regime were recently published.
These experiments showed that different reaction processes are occurring at the two ends
of the intermediate energy regime, which is not unexpected since a transition must occur
between the low and high energy regimes. An experiment performed by Stephan et al.
[ste9l] at 200 MeV/nucleon led to results similar to those obtained in high energy
fragmentation. Bazin et al. [baz90] performed an experiment at 44 MeV/nucleon which
had significantly different results and seemed to indicate that the primary mechanism
involved at that energy was deep inelastic transfer and not (high energy) fragmentation. A
third experiment involving krypton fragmentation at 500 MeV/nucleon was later carried
out by Weber et al. [web94] in an effort to understand the energy at which the transition to
true fragmentation is taking place. Recent experiments involving krypton fragmentation

were also performed in an effort to explore the rp-process.

The “specific” experiments [moh91, bla95] did not involve systematic studies of
the cross sections, momentum distributions, and momentum transfer, but they closely

studied the fragmentation products near the proton drip-line. Some of the results and

28

conclusions from the systematic measurements, along with the rp—process related work
will be presented here in order to provide some background which will be helpful for

understanding the motivation for the present research.
2.3.1 Krypton Fragmentation at 44 MeV/nucleon

D. Bazin et al. [baz90] performed an experiment which involved a primary beam
of 84Kr at 44 MeV/nucleon impinging on several targets (27Al, 103Rh, 197Au) using the
GAN IL facility. The data provided a mapping of the isotopic yields and momentum
distributions from the three different targets and covered a wide range of masses and
elements (6 5 Z S 38). Typical momentum distributions from this experiment are shown in
Figure 2.9. The arrow indicates the momentum per nucleon of the beam, and as expected
the momentum distribution of the observed fragment falls slightly below this level,
however, it should be pointed out that several of the distributions of the low Z fragments
from the 84Kr + 27A] reaction showed a two-peak structure in which one peak was located
at a velocity greater than the projectile velocity, while the other was well below that value.
This effect is shown in Figure 2.10. The two-peak structure is indicative of forward and
backward emission of light fragments in an asymmetric fission like process — which
certainly is not present during high energy fragmentation. The cross sections resulting
from the fragmentation at 44 MeV/nucleon are shown in Figure 2.11, and the author
points out several features that are evident in the figure which show that two reaction
mechanisms are contributing to isotopic yield: 1) There is a noticeable contribution of
yield near the N/Z of the projectile which is significantly different for the various targets
that were used, and 2) a large contribution of yield is far from the N/Z ratio of the

projectile and falls closer to the valley of B-stability. The later seems to indicate that highly

29

 

350 I T I r I I T j I I r I I

.1“ 250 P

200-

150 -

Counts (min

 

 

 

 

230 240 —350 260 270 280 290 300
p (MeV/c/u)

FIGURE 2.9: The momentum distribution for 72Ge3‘1+ from the fragmentation of
34Kr + 27A] at 44 MeV/nucleon.

 

 

 

 

 

30 ' -
"Fe
E 20 -
.2 .
8
U 10 "
O L l +
180 220 260 300 340
MeV/c/u

FIGURE 2.10: The two-peak structure observed for several momentum distributions
from the reaction 86Kr + 27A] at 44 MeV/nucleon.

 

 

 

 

 

103 2n 1: 4 63 |:!: 41-Ge l:::-. '1
‘V';! ':vv 1' "vvv“'o
2 2' '3. .: V'A' .AV V'A‘.
10 " :' v. "' .: v':."' -:v v ‘: "
v I
101 I- :' .ub :‘ 'd- :‘ 'VA-t
1 - 35KH27A1 l: 9 86KI+ 1031111 ' ' 36K” 1“Wm
33 60 62 64 66 68 70 72 62 64 66 68 70 72 74 65 70 75
'a r I I I I U I fir I
= 4 As Se Br ...-
. lo I db up .-I
.0 .:I .- .|:I-I.l. .I-‘
a 1."‘,“:-- ct- .:“ ‘A‘ .. «In .z‘A‘ AA“ ‘- d
V V V' ‘ I 1" A - A
E 102 " :1" 'v. 1‘. " -::' "v, ‘A .0 .::v""' ‘ ‘
o v .d ‘ ‘ In.‘
I-I I. v ‘ ’1 It
>~ A -
100 I' 1 1 1 u 1 1 1 1 ‘P 1 1 1 ‘
70 75 80 70 75 80 85 75 80 85
I I I r I' I .I I U I I I I I U U I I I I
103 ~Kr _.- "Rb ... w Sr .
I I . I
1— fl . “ 4.- .. .1- . I ' . u
101» .'u“‘“ -- - u‘n“ -- -' -
A '7 . ' A . A '
'- - A 'V' ‘_ .‘A __ . ‘ ‘ A ‘ ‘ d
101 J‘ ' ‘un- ‘ ‘.--. A ‘ .1
A A

 

 

 

 

 

74767880828486 76788082848688788082848688

Atomic Mass

FIGURE 2.11: Cross sections resulting from 84Kr + 27Al, 103Rh, 197Au at 44 MeV/
nucleon.

excited prefragments are responsible for the production of the observed fragments, while
the difference caused by altering the targets seemed to indicate that an incomplete fusion
process was the primary reaction mechanism (as had been previously described in work
involving 22 MeV/nucleon 84Kr + 197Au, 108Ag [luc87]). The experimental cross sections
for 2:10, 20, and 30 are shown in Figure 2.12 and are compared with theoretical values
from a high energy model that was often used at that time (see [baz90] for complete
details). It is quite apparent that the code used could not reliably predict the experimental

results.

Although it was not mentioned in the published article, one can use the typical

momentum distribution that was shown in Figure 2.9 to get an estimate of the longitudinal

31

86Kr + 103Rh at 44 MeV/nucleon

 

 

 

 

 

 

 

Z=10 " Z=20 " ‘
103 - -- .
A I h 1
.3 .(p I
E 102 ' J” ‘
D. 1’ 1
a J '0 J
v I
'c 101 ' " '7’ ‘
Fr ' f ‘
w I. \‘
100 ’ "’ "
0 Experiment
* " “ 9 Calculation ‘
18 20 22 24 26 28 40 45 50 65 70 75

Atomic Mass

FIGURE 2.12: Selected yields from 44 MeV/nucleon krypton fragmentation along
with corresponding predictions from a high energy fragmentation
model.

momentum width (referred to as 0'0, see discussion of the Goldhaber model in Chapter 4
for further details). The value of 0'0 ~ 95 MeV/c is typically found from high energy
fragmentation, but the data at 44 MeV/nucleon yield a value of co ~ 130 MeV/c which is
significantly larger than expected.

The cross sections and momentum distributions observed during fragmentation at
44 MeV/nucleon showed drastic differences from the predictions of codes used to describe
high energy fragmentation at that time. The author concluded that no real fragmentation
occurred at this energy and that the transition to “pure” fragmentation needed to be
explored in an effort to reliably predict the cross sections of exotic nuclei and pointed out

that an experiment would shortly be performed at 200 MeV/nucleon.

32
2.3.2 Krypton Fragmentation at 200 MeV/nucleon

An experiment by Stephan et al. [ste91] measured the isotopic cross sections and
velocity distributions for the fragmentation products from 84Kr + 197Au at 200 MeV/
nucleon. A wide variety of isotopes ranging from Z=11 to #37 was measured at 06° and
1.5’. This energy is considered to be at the lower edge of the high energy regime, and the

data was expected to exhibit the typical properties of high energy fragmentation.

The experimentally measured cross sections are shown as a contour plot in
Figure 2.13 along with predicted “ridge lines” that were obtained through the use of a) a

participant spectator model [gos77] (see [ste91] for the details), and b) an INC model. In

 

............................

.......

.........

............

...........

.....

    

;§;;‘§"j;;f;;§ , ..... -r'*-—- x10 surface energy

“32:52?sissississfsf‘“Kr +' A" 200 Win}?-

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

iiit 1.1!.
25 30 35 40 45 50 55

FIGURE 2.13: Cross sections from krypton fragmentation at 200 MeV/nucleon.

33

order to reproduce the experimental cross sections with the participant-spectator model the
value of the prefragment excitation energy had to be drastically increased (by a factor of
10) to have the theoretical ridge line approximate the ridge line of the experimental data.
This clearly indicated that the prefragments had a much greater excitation energy than had
previously been assumed during high energy target fragmentation (from which most
models had been deve10ped). A large portion of the previous research had been target
fragmentation or had been performed with either light (A < 40) or heavy (A > 100)
projectiles, and it was becoming apparent that intermediate energy/intermediate mass
fragmentation could not be easily understood with the current models. The increased
excitation energy was discussed in a previous work [01179], and a frictional interaction was
added to the standard surface energy term which had been used with the clean cut
abrasion-ablation model. Stephan points out that this frictional term could not account for

the tenfold increase in prefragment excitation energy.

The INC based model (in this case Stephan used the ISABEL INC code followed
by a deexcitation code called LOT 0 [got91]) did a relatively good job reproducing the
ridge line of the experimental data, and the INC code did predict relatively high excitation
energies for the prefragments. However, although the difference is small, it can be seen in
Figure 2.13 that the experimental distribution is slightly shifted to the proton-rich side of
the predicted values near the mass of the projectile and then shifts to the neutron-rich side

of the predictions for the low Z fragments.

The velocity shift of the fragmentation products is shown in Figure 2.14. Using the
formalism of Morrissey (Eq. 2.3) it can be shown that their experimental data closely

follows the trend given by -(P,’,) ~ 11.7 MeV/c - AA, where the constant value of 11.7

34
MeV/c is slightly higher than the value of 8 MeV/c which was expected from the

systematics of Morrissey. Also shown in Figure 2.14 is the predicted line from the INC
code which does follow the general linear trend of the experimental data and although the
velocity shift is greater than the theoretical values developed from high energy models, the

overall linear trend is well reproduced.

The parallel momentum widths of the fragmentation products are shown in
Figure 2.15 along with comparisons to the Goldhaber model (see Chapter 4 for further
details) and the INC based model. For the Goldhaber model, the constant value
of Go ~ 90 MeV/c (representative of the longitudinal momentum width) is typically
observed during high energy fragmentation, and the 200 MeV/nucleon data resulted in a

value of 60 ~ 95 MeV/c , which is comparable to the value from high energy

 

0,010 .......... WA ... , ........ .1 .

  
 
 
 
 
   

 

 

 

I V [T v v v
tintegedA/ZFZ .o '
‘ I
I ' 1 " .
P o a ‘
a 4 .
0.000 - ' 5 _
A r .
an. > .
3 50'05622 j
0.004 -.
. —- Intranuclear cascades ‘
0.002 _,
. " Kt + ”At. 200 MoV/u «
i 9111 "' 0.6. 1
0000 1. ...1. .....1UJLHL..AIIAIL..L.1.1411111..,,
O 10 20 30 40 50
AA

FIGURE 2.14: Velocity shift for the fragmentation products resulting from
84Kr + 197Au at 200 MeV/nucleon.

35

fragmentation. In the systematics presented by Morrissey [mor89] it is mentioned that 0'0
should not be sensitive to the reaction mechanism. At low energies, where fragmentation is
not the principle reaction mechanism, it has been shown [bor86] that 60 decreases as the
incident energy decreases, while a wide range of high energy fragmentation data has

shown a relatively constant value.

The data consisting of cross sections, momentum shifts, and momentum widths
obtained at 200 MeV/nucleon seemed to be fairly representative of high energy
fragmentation. The data taken at 200 MeV/nucleon was much closer to true fragmentation
compared to the 44 MeV/nucleon data, and it was evident that further exploration of
intermediate energy regime was needed in order to gain a greater understanding of the

reaction processes occurring during the fragmentation of intermediate mass projectiles.

 

     
 

 

 

 

1000 i ,
Lint.eger(A/2)-Z
0 l
I 2
0 3
a 4
’5 v 5
p
>
0.)
a 500 h
E l -------- Goldhaber (a.-85ueV/c) I
9- ---- Experimental systematics
— Intranuclear encodes
i J
“K: + ”A11 200 MeV/u J
9|» '3 0.6.
o I 1 _l l l l l I
O t 2 3 4 5 6 7 8

FIGURE 2.15: Parallel momentum widths for the intermediate energy fragmentation
products.

36

 

 

 

 

 

 

I v I v I I V I
-‘% ifs? i
’6 3 8‘’Ge 1
§ ‘ i
0 -I q
z - .
\

“g 1'5 :1

~ 2 i

6 .. a

< q .-

\ 4 .
2

0.1-: 1

z 1

1 I _' 1 I ' I "

-1000 -500 o 500 1000

q" (MeV/c)

FIGURE 2.16: Typical momentum distribution observed during high energy
fragmentation - in this case 86Kr + 9Be at 500 MeV/nucleon.

2.3.3 High Energy Krypton Fragmentation

The results from the two intermediate energy fragmentation experiments could not
be completely reproduced by the theoretical models. An experiment involving high energy
fragmentation of 86Kr + 9Be, 59Co, 181Ta at 500 MeV/nucleon was performed at GSI by
Weber et al. [web94]. At this energy it was expected that the EPAX parametrization and

the ISApace model could reproduce the experimental data.

Shown in Figure 2.16 is a typical momentum distribution from the 86Kr + 9Be
fragmentation which is quite Gaussian (the “tail” often seen at low and intermediate
energies is not apparent at high energies). Using the momentum distributions the
longitudinal momentum widths (6) were determined. In Figure 2.17 the widths are shown

along with the calculated values from the Goldhaber model (solid curve) and the

37
systematics developed by Morrissey [mor89] (dashed curve). The Goldhaber model seems

to overpredict the width for the fragments near the mass of the projectile, while the
Monissey systematics accurately reproduce the experimental data near the projectile, but
overpredicts the width of the light fragments. This is explained by two effects: 1) the
Goldhaber model predicts the width for the initial reaction fragments (prefragments)
rather than the final observed fragments which have undergone evaporation and 2) the
Morrissey systematics were developed from target fragmentation with light ions and thus
should not be extrapolated for large mass differences between the projectile and observed

fragments.

The momentum shifts are shown in Figure 2.18 and the dashed line indicates the
Morrissey systematics of (PI) ~ constant - AA , where a constant of 8.8 MeV/c was the

best fit (recall that the Morrissey systematics predicted a value of ~ 8 MeV/c). The linear

 

BOO-

diq") (HOV/C)
g

N
a
?

 

 

 

0

 

o '15'2'o'abftb'5b'sh'7b'ob'so
Mass Number

FIGURE 2.17 : Parallel momentum widths observed during high energy krypton
fragmentation of 86Kr + 9Be.

38

relationship corresponds well to the experimental data, but closer investigation of the
individual isotopic lines near the Z of the projectile showed significantly different slopes
(relative to the 8.8 MeV/c overall trend) ranging from 14 MeV/c for 2:36 to 58 MeV/c for
Z=30. This effect is shown in Figure 2.19 and was not observable for the isotopes with
Z S 28. The author points out that this effect could indicate that the proton-rich fragments
are produced in the first step of the fragmentation process and the second evaporation step
does not have a large influence on those products — thus the proton-rich isotopes have a
small momentum transfer relative to the neutron-rich fragments in the individual isotopic

chains.

The cross sections at 500 MeV/nucleon are shown in Figure 2.20 along with the
predicted yields from the EPAX parametrization (dot-dash) [sum90], a geometrical

abrasion model (dashed) [gai91, sch93], and the ISApace code (histograms) [fau92]. As

 

600- °' ..... o -

-<Pf> (MeV/cl

 

 

 

 

v v I I I V I V r i I T I ‘V
0 to 20 30 £0 50 60 70 80 90
Mass Number

FIGURE 2.18: Momentum shifts observed during the fragmentation of 86K: + 93c at
500 MeV/nucleon.

39

was seen in the krypton fragmentation at lower energies, and some recent high energy
projectile fragmentation, the theoretical predictions are not a true representation of the
experimental data. The EPAX parametrization does not reproduce the ridge line or the
width of the isotopic distributions. The abrasion model does a relatively good job of
reproducing the data near the mass of the projectile but underpredicts the yield of the
proton-rich fragments with Z < 34. The excitation energy per abraded nucleon was
doubled relative to the original model in an effort to reproduce the data (that is shown in
Figure 2.20). The ISApace model does a relatively good job reproducing the data but also

underpredicts the yield of proton-rich fragments as did the abrasion model.

It is evident that the models can not yet reproduce recent experimental results with
intermediate mass fragmentation, and further data is certainly required so that the models
can be modified. However, even without accurate knowledge of the momentum transfer
and momentum width, much research on projectile fragmentation has occurred in an effort

to explore the rapid-proton capture process.

 

300- .

ND

-<P;> (MeV/c)

 

 

 

 

Mass Number

FIGURE 2.19: The momentum shift for the fragments near the mass of the projectile
during high energy krypton fragmentation.

 

 

 

 

 

 

 

 

 

 

 

 

Mass Number (A)

FIGURE 2.20: Cross sections for the reaction products resulting from high energy
fragmentation of 86Kr + 9Be at 500 MeV/nucleon.

41
2.3.4 Rapid-Proton Capture Process

The rapid proton capture process (rp-process) was first proposed by Wallace and
Woosley [wa181] as an extension to the well studied carbon-nitrogen-oxygen (CNO) and
the hot carbon-nitrogen-oxygen (HCNO) processes [cha92, wor94]. These processes are
shown diagrammatically in Figure 2.21. During the myriad of astrophysical events (novae,
supernovae, x-ray bursts, Thome-Zytkow objects, etc.) which can occur, a wide variety of
temperatures and densities can be present. A large amount of research has been performed
(both experimentally and theoretically) to explore the binding energies, half-lives, and
reaction cross sections for the isotopes involved in the CNO and HCNO processes.
Detailed calculations of the reaction networks for these relatively low temperature and low
density processes have been carried out. When the temperature and density increases, the
HCNO cycle can break free from its cyclical behavior and begin to increase to higher
masses through the key reaction 15O(oz,'y)19Ne(p,'y)20Na. The path that the rp-process
follows (rp-path) from this point onward shows tremendous differences depending on the
temperature and density present at the time and the overall burning time of the
astrophysical process. Several predicted paths for various burning conditions are shown in

Figure 2.22 for which the calculations were carried out for Z S 36 [wor94].

During conditions of high density and high temperature, a large amount of
uncertainty is introduced to the rp-path due to stability and half-life issues pertaining to
nuclei on or near the proton drip-line. In this temperature/density regime, the rp—process
consists of a sequence of proton captures that occur until the point at which the proton drip
line is reached and the nucleus will then B-decay, and the sequence will continue. The rp-

process near possible termination points as was proposed by Wallace and Woosley is

42

F K W \

 

 

G 9
9-]
ll
.0
N

I
I
I
I
I
I E
I
I
I
I

 

 

K CNO cycle J ;.

\ HCNO cycle /

 

 

 

 

FIGURE 2.2]: The CNO and HCNO cycles.

shown in Figure 2.23. This process is “terminated” when the B-decay half-life of a nucleus
along the path is significantly longer than the burning time of the astrophysical event or
proton capture leads to an unstable species. In recent years the search for the possible
termination points has concentrated on 65As, 6SiBr, and 73Rb. The isotope of 73Rb is
generally considered to be proton unstable (S1) = - 590 i 270 keV), but the stability of 65As
(Sp = - 70 :I: 250 keV) and 69Br (S1) = - 180 :1: 300 keV) is questionable as can be seen from
the proton separation energies (using data from the most recent nuclear mass tables
[aud93]). A large amount of research was conducted to search for proton dominant decay

of 65As and 6931‘, but no evidence for this was observed [rob90, win93].

43

T9= 0.3
p = lOag/cma

    
 

24 26 28

22

    

2 Ho
H
O 2
N———)
'r,- L5
p - Io‘sgxcm3

24 2. 2.

FIGURE 2.22: Proposed rp-paths for varied temperature and density conditions.

 

 

 

   

 

 

 

 

‘ .cs.) ~.,'..’. .- .

I“ \. p, “‘4
| in... 6.5:", .( v. 9697;?!) ,I' V .- 223.3
:1. 5:": .5. 1: $143.» l..\ .2 \‘v/
I :0. . 4v. >.< > we or .6 .z-z;
{.z .-: :39, ”inf: .Q'er»: 5% r .A‘
' .1 x :9". 49.».

{we

 

 

 

 

 

 

N 68 69 71°}
565 66 69
64 68

 

 

 

 

 

 

1.9....- 1‘15. cur."
‘ , ,_ ¢._,;§:§;;.§:;t .;.‘.y2,.5$:-’:""
. 2::- ~ 2?" ' 66 67
6‘ ’ 6 I , 2- . «:4: r, .
. '. ~ . .ux- -
' .A .. . .

 

 

 

 

_ 64 Ga
Zn

 

: 56
55

54 55 [5g 57 .Ni

 

 

 

 

 

 

 

 

 

 

 

 

FIGURE 2.23: Proposed path of the rp-process for the region applicable to the present
research.

45
In one of the first experiments performed with the A1200 [she9l], Mohar et al.

[moh9l] saw the first evidence for many new nuclei along the proton-drip line which were
important for information related to the rp-process. The experiment provided clear
evidence for the existence of 65As, along with several possible events which could be
attributed to 69Br (see Figure 2.24 where first observed nuclei are indicated by arrows). No
evidence for 73Rb was seen, and it was concluded that 72Kr was the termination point for
the rp-process. In a subsequent experiment [win93] an attempt was made to measure the
half-lives for many of the nuclei first observed nuclei by Mohar et al. [moh91] and the
measurements allowed determination of the half-lives of 61Ga, 63Ge, and 65As which

provided vital information for the theoretical network calculations used in determining

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

105 105
B 104 104 Ge
C: 103 103
g 103 102
U 101 1o1 \7
“510° w°|flm1|1|
59 60 61 62 63 64 65 66 60 61 62 63 64 65 66 67
E 105
r-Q 104
E 104 As 3 Br
:3 103 10
102
Z 102
101 <7 101 {7
1°° In] I l l J 1°° l l L Ll
63 64 65 66 67 68 69 7o 67 66 69 7o 71 72 73 74

Mass Number

FIGURE 2.24: Isotopic yields resulting from 78Kr + 58Ni at 65 MeV/nucleon.

46

outcomes of the astrophysical processes. However, no evidence for 69Br was obtained
during this experiment. An experiment recently performed at GANIL [bla95] further
extended the known limits of the proton-drip line in an attempt to study the nuclei along
the rp-path. Fragmentation of 78Kr + Ni at 73 MeV/nucleon was employed in an effort to
maximize the possible rate of 69Br. The yield for various isotopes is shown in Figure 2.25,
and it is apparent that no evidence for the existence of 69Br was observed. A limitation was
placed on the possible half-life for 69Br in this experiment which had a flight path over six
times the length of the one performed by Mohar et al. and it was pointed out that this
could be an indication of a very short half-life of 69Br. Since the stability of 69Br is critical
to the understanding of the rp-process, the present work was carried out with a relatively
short flight path and also mapped the momentum distributions and measured the cross
sections of the isotopes produced in the reaction 78Kr + 58Ni in which evidence of 69Br

was first observed.

 

 

 

10l :- l

Counts

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Proton Number (Z)

FIGURE 2.25: Yield from the fragmentation of 78Kr + Ni at 73 MeV/nucleon.

47

In an effort to gain a greater understanding of the processes occurring during
intermediate energy projectile fragmentation, to explore possible termination points of the
rp-process, and to determine the applicability of high energy calculations to the
intermediate energy regime two experiments were performed which involved the
fragmentation of both neutron- and proton-rich krypton nuclei. The experimental
procedure and set-up are described in the next chapter and are followed in subsequent

chapters by the results that were obtained.

Chapter 3

Experimental Apparatus and Analysis

3.1 The A1200 Fragment Separator

The measurements were performed at the National Superconducting Cyclotron
Laboratory using a ~ 0.1 pnA 86Kr beam at 70 MeV/nucleon delivered by the K1200
cyclotron for the neutron-rich fragmentation and a ~ 45 ppA 78Kr beam at 75 MeV/
nucleon for the proton-rich fragmentation. The beam was incident on a 4 mg/cm2 27Al
target placed at the medium acceptance target position of the A1200 mass separator
[she9l] in the former experiment and a 102 mg/cm2 58Ni target in the latter. The angular
acceptance for fragments was A6 = 34 mrad and MI = 30 mrad centered around 0° in the
86Kr fragmentation experiment and the acceptance for fragments was A6 = 20 mrad and
A4) = 40 mrad centered around 0° for the 78Kr fragmentation. The momentum acceptance
for both experiments was Ap/p = 3%. The thickness of the target in the 86Kr fragmentation
experiment was chosen to limit the broadening of the fragment momentum distributions
induced by energy and angular straggling in the target, rather than to optimize the
production rate of specific fragments. This enabled a detailed study of the isotopic parallel
momentum distributions. The target for the 78Kr fragmentation was chosen to optimize the
production rate for the bromine isotopes in an effort to explore the mass region relevant to

possible termination points of the rp-process thoroughly. In both experiments, the
48

49

magnetic rigidity of the separator was varied in overlapping steps of 2% covering a range
that would allow identification of a wide variety of isotopes in the regions of interest (Bp =
2.40 to 2.85 Tm for 86Kr fragmentation and Bo = 2.27 to 2.49 Tm for the 78Kr
fragmentation). Four beam monitor detectors mounted around the target position in a
Cloverleaf pattern enabled normalization of the beam current for data taken at different

rigidity settings.
3.2 Detector Setup

The two fragmentation experiments used similar detector setups as is shown in
Figure 3.1. In order to measure possible short halflives during the 78Kr fragmentation
experiment, a second silicon telescope was placed 7.5 m downstream of the Final
Achromatic Image position (not shown in figure). For isotopes with a half life on the order
of 100 us, a reduced isotopic count rate would be observed in the second silicon telescope
relative to the first due to the decay that would occur over the final 7.5 m of additional

flight path.

For the 8‘SKr fragmentation experiment, the velocity of each particle was
determined from the time-of-flight (TOF) measured between two 8 mg/cm2 plastic
scintillators separated by a 14 m flight path. The timing scintillators were located at the
Dispersive Image #1 and the focal plane of the A1200. During the 78Kr fragmentation
experiment the TOF of the reaction products was measured between an 8 mg/cm2 plastic
scintillator (located at Dispersive Image #1) and the front—most detector of either silicon
telescope (flight path of 14 m or 21.5 m) through the use of a time pick-off signal. The
position and angle of reaction products were measured at both the second dispersive image

and at the focal plane with two pairs of X-Y position sensitive parallel plate avalanche

50

  
   

Dispersive Image #1 Final
Achromatic
Target Position Dispersive Image #2 Image

 
   

/‘3 .....
o”
\ Detecwr '
krypton beam Stop Time 2 PPACs " 2 PPACs

from K1200 X,,Yl,0,¢ TOF Detector
Cyclotron Si Telescope

X..Y2,AE.B...

Start Time

FIGURE 3.1: A schematic diagram of the A1200 fragment separator.

counters (PPAC's) [swa94] separated by approximately 40 cm. For each particle, the
position information at the second dispersive image PPAC was used together with NMR
measurements of the A1200 dipole fields to determine its magnetic rigidity Bp, which is

linearly related to the particle momentum via the equation

B :2, 3.1
P q ()

where q is the particle charge. Reaction products reaching the focal plane were implanted
into a four-element silicon detector telescope, consisting of two 300 um AE detectors
followed by two 1000 pm thick E detectors in the 86Kr experiment (the 78Kr experiment
was composed of detectors with thicknesses of 100 um, 75 pm, 500 um, and 1000 um).

All silicon telescope detectors had an active area of 300 m2.

51

3.3 Electronic Setup

Both fragmentation experiments were performed with a standardized A1200
electronics configuration. A schematic diagram of the electronic modules is shown in
Figure 3.2. The TOF was measured with a TAC between the timing scintillator at Image
#1 and the timing detector located in the focal plane (either a second timing scintillator or
the first silicon detector time pick-off). The four signals (left, right, up, and down) from
each PPAC were read by ADC’s. The energy signals from the silicon detectors were sent to
ADC’s and the time signal from the front most silicon detector was read by a TDC. The
master gate was constructed using valid signals from the two AE detectors and a NOT
BUSY CPU signal. Additional software gating insured that each energy signal had a valid
PPAC signal by checking the validity of BIT 2 and BIT 3. A total of 30 detector events
were recorded for every valid master gate. The data acquisition system (both hardware and
software) that was used to read the ADC’s, TDC’s, QDC’s, and write the data to tape was

the standard NSCL data acquisition system [fox89].
3.4 Isotopic Identification

By transporting charge states of the primary beam through the A1200 at different
rigidity settings the relationship between the magnetic rigidity and the horizontal position
at the second dispersive image was calibrated. An example of this can be seen in
Figure 3.3. The momentum for each of the primary beam charge states is then determined
through the use of Eq. 3.1. Using the momentum of the charge states, an energy
calibration for each silicon detector is initially determined by comparing the ADC channel
for the silicon detector to the amount of energy that should have been deposited in the

detector (through the use of the formalism found in Hubert et al. [hub89]). The total

52

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

a: :22: V. sew
‘ F FAN +—— TAC srop
D OUT — TPC
BIT]
PPAC SA ADC
DOWN "A g SCALER
D we
PPAC SA ADC
UP WA g SCALER
D we
PPAC SA —"""— ADC
RIGHT TFA r— C SC ALER
F
D TDC
BIT 2 (Dispersive Image)
PPAC sA , ADC g >— BIT 3 (Focal Plane)
LEFT WA g SCALER
D TDC
A51 AMP c SCALER
FAST TDC
F
W D TAC START
SCALER SCALER
FAN
m ma SA _ ADC g cm F B" 4 [mo START (COMMON)
—I C _
AMP Fm _ F __ sew N m, STOP (COMMON)
AMP D _ 'I'DC CPU D om ADC STROBE
TAC sum 31%; @— ooc moss
m: SA ADC MASTER —@— BIT STROBE
[E'— AMP fl 'I‘FA ——'c‘ GATE
5 — SCALER
P_
52 PRE SA —"”— ADC
AMP WA C
F SCALER
D

 

\ C SCALER
CY CLgAI'RON I TDC
D TAC

 

 

 

FIGURE 3.2: A schematic diagram of the electronic setup that was used during the
fragmentation experiments.

53

p vs x for PPACO 73Kr fragmentation

 

3.16 I I I r1 II II I I I I I I I I I I I I I I I r11 1 I

C o q

f 4

3.14 - o 1

A 3.12 - O _
é . O .
Q . I
3.10 - _

n o ..

3.08 — O _
3.06 _I I I I I [J I_LIJ I I I [I I LII I I 1_Ll I I I 1.

 

 

 

1000 1500 2000 2500 3000 3500 4000
PPACO x channel

FIGURE 3.3: Turning radius vs. horizontal position for PPACO at Dispersive Image
#2 that was observed during the7 8Kr fragmentation experiment.

kinetic energy is then determined by summing the energy loss in all four silicon detectors.
Using the obtained values of AE, total kinetic energy, TOF, and magnetic rigidity, it is
possible to determine unambiguously the mass (A), proton number (Z), and charge state

(Q) of individual isotopes using standard relationships:

z = a+b./AE(7— 1) (3.2)

 

 

_3__.105 (T In). TKE Bp (3 3)

=931.5 MeV Bp(y—l) '
_ Q39 1

A 3.10587 T m (3'4)

where a and b are constants determined for each detector, TKE is the total kinetic energy
with dimensions of MeV, and Bo is the magnetic rigidity with dimensions of Tesla-meter,

and B and 7 are the standard relativistic parameters ([3 = v/c and y = l/A/l -—BZ)

54

11‘.
N
l |
D)
O\

 

SI

.. 3g. ‘fi:§:.%fi;..u;}*A 3,3. ..,__ 1w '

energy loss (arb. units)

‘7

fl.

 

\I

time of flight (arb. units)

FIGURE 3.4: A plot of AE vs. TOF observed during the reaction 78Kr + 58Ni. The
arrow indicates the “gap” where 69Br should be observed.

55

 

 

 

 

 

 

 

 

 

 

 

 

 

    
   

  
  

   
 

 

 

 

 

 

 

 

 

8m : Y 1' I V I Y Y I I I W I I I V V I V V Y I T 1 V I V V I 1 -fi' Y I I I V I Y—r I V Y 1 I d
700 3 AEl Calibration i 8002 A132 Calibration :
600 :— -: - «
.- q )- d
A : : 1 a
z, 500 :— ‘5 E 600 e ‘1
E 400: Y=I+bx+cx2 _L‘ a : Y=a+brt+crt2 j
m E a 9.5623 3 m - a -11.922 -
300 :~ -: 40° : 1

: b 0.51048 : _ b 0.50777
200 I— c 655716—06-2 200 -_ c -5.368e-06 _.
100 ; 11’ 0.99993 5 : 111 0.99997 1
0 : I 1 l I l L l I l l l l l l I l l l l l l l l l l A I l l l : 0 I. l l I l J. l l l l L L 1 1 l J J l I l d

0 400 800 1200 1600 0 500 1000 1500 2000
ADC Channel ADC Channel
m III I I I W T T—I I T—r T I Y T Y—FT‘T T W T r7 I V I 1: 1m p Y I I I I I I I I V V I I V I I I V d
.. 1- -4
3500 :— E1 Calibration E 12001 E2 Calibration j
Z I ~ 4
3000 E 5 1000 g 1
2500 I- ‘3 5‘ C 3
g E Y=11+bit+cx2 E g 800 T Y=I+bx+cx2 "I
2000 T '1 v C - I
: a 319.62 : m 600 __ a 13212 _
m 1500 r b 1.109 1 b 0.32653 3
1000 3 c 458266-06 5 400 E“ c 290486-06 1
5_ R2 0.99776 j _ RI 0.99872 _‘
500 : 200 _ 1
0 : l l I I l I l l l I l l I J l I I l l l l l I l l l l l l l I l l L: {J L l l l l l_l_l l l l l l l l L l l l I l l -
0 500 1000 1500 2000 2500 3000 3500 0 1000 2000 3000 4000 5000
ADC Channel ADC Channel

 

   
  
   
  
 

 

    

,,,,
134 — TOF Calibration -

. Y=a+bx+cx2 .

132 L a 173.54 _‘

”g I b 0.029772 2
g 130:. c 4.58% .:
r— E 111 0.99492 I
4

128 - 4
126*- ‘-

FL I l l l l l 1 l l l l 1 l I I l l l l l I l I] 1 1 1-

 

 

 

1000 1050 1100 1150 1200 1250 1300
TDCChannel

FIGURE 3.5: Energy and time calibrations for the detectors during the 8‘SKr
fragmentation experiment.

56

determined from the TOF of the fragment. The two AE measurements provided redundant
proton number (Z) determinations. The Z calibration was done using the 2-dimensional
AE vs. TOF data for a given magnetic rigidity. A sample AE vs. TOF plot in which the
magnetic field was optimized to observe 69Br is shown in Figure 3.4. The figure indicates
the N=Z line and the krypton isotopic line. By selectively gating on a wide variety of
isotopes in the AE vs. TOF plot, it is possible to obtain good energy and time calibrations
which span the range of the observed data. Energy and time calibrations for the 86Kr

experiment are shown in Figure 3.5.

The charge of each particle is then determined with Eq. 3.3. The good charge
resolution enabled an integer value of Q to be used when determining the mass number
(A) with Eq. 3.4. Using these equations, the present measurements had resolutions
(FWHM) of AA ~ 0.004, AZ ~ 0.01, and AQ ~ 0.01 (shown in Figure 3.6), which allowed
clear separation of all the fragments. With the AZ, and Q of each event determined, it
became possible to explore the momentum distributions and cross sections of the

fragmentation products.

3.5 Parallel Momentum Distributions

Given the number of events for each individual isotope at a given magnetic rigidity
(which corresponds to a particular momentum through the use of Eq. 3.1), a figure
consisting of the number of events (or events/sec) plotted against the momentum of the
individual particle is constructed. The number of events at the individual rigidity settings

were normalized relative to each other with the information provided by the beam monitor

Yield (a.u.) Yield (a.u.)

Yield (a.u.)

FIGURE 3.6:

 

L
d
-
_
—
d
_—
c4
—1
-
—
—
‘
c-I
-1
-

150

100

Illllllllllll

50

llllllllllllllllll

 

 

 

 

 

 

 

 

 

Lilli

 

 

 

 

 

 

.1 . . .Ulllil

5O 60 70
Atomic Mass (A)

 

on
O

 

llllllTTllllllllllll

1 00
5O

 

 

 

    

 

1 |1111l1111l
20 25 30 35

Atomic Number (Z)

 

500

rTllIlllllllllTlllll

llllw

20 5 30
Atomic Charge (Q)

100
50

 

 

 

 

 

 

 

 

 

 

 

 

l Illlllll lllllllll IIIL

llllljlll lllllllll l 11F

 

 

 

 

 

 

 

 

The AZ, and Q resolution obtained in the 86Kr fragmentation
experiment.

58

 

  
  
  
   
    

     
    

 

 

 

 

5 I I I I I I I I I I I I I I I I I I I T I I I I
0- I l 1 l 1 .
" 86K _ .4
_ r + A1, E—70 MeV/nucleon . _
r ,i‘K 7
40 F- ‘ 1 '—
. . . - ' -l
“U _ Gaussran fits usmg ‘ _
c: ~ . 4
8 _ various cut—off .. . J
30— -. . _
3’, _ percentages '1. -
\ _ 1. .
(I) _ O z ............ , - .
:1 20— 50 z .- ; —
o » : - -
U 1- 80 7O ————— '. -.
_ 9 -
_ 100 7a ‘ """" .- .
10— _
L .
— I . .1
1- / .I -
0 "1"11'1’ 1 l 1 J 1 1 l 1
320 330 340 350 360 370 380

p/ A (MeV/C)

FIGURE 3.7: The momentum distribution of 80Br35+ with varied Gaussian fits. The
arrow indicates the projectile momentum.

detectors. Since the particle momentum is parallel to the momentum of the projectile this
type of figure provides the parallel momentum distribution for each individual isotope (as

an example, parallel momentum distribution for 80Br35+ is shown in Figure 3.7).

The parallel momentum distribution of each isotope was fitted with a Gaussian
function from which the parallel momentum width and the mean momentum transfer were
obtained. The isotopic yield was extracted by integrating the Gaussian fit over momentum
space, however, the projectile-like fragment momentum distributions at this energy are
generally asymmetric, with a tail on the low-momentum side of the momentum
distribution. The effect of this asymmetry on the present data was explored through the use
of “cut-off percentages” on the low-momentum side of the distributions. For the 86Kr

experiment the Gaussian functions were fitted to the data with a least-squares technique

59

from varying percentages of the peak height on the low momentum side to the end of the
high momentum side of the distribution. A typical momentum distribution from the 86Kr
data is shown in Figure 3.7 along with Gaussian fits done with varying cut-off percentages
on the low momentum side of the distribution. The figure indicates that the width of the
distribution becomes narrower as the cut-off increases toward the top of the peak. Previous
papers have used a variety of cut-off percentages during the data analysis and there is no
standardized percentage that is typically used. The 50% cut was selected as the best
representation of the 86Kr data for subsequent discussion, and the errors introduced by the
varied cut-off percentages will be discussed along with the results. The 78Kr experiment
was performed with a relatively thick target and the momentum distributions were far
more Gaussian in shape than was the case for the 86Kr data. The choice of cut-off
percentage for the 78Kr data did not have a noticeable effect on the fits, and no cut-off was

applied to the low momentum side of the 78Kr data.

After determining a Gaussian fit for each isotope, the momentum shift was
obtained by comparing the centroid of the Gaussian fit in momentum space to the

momentum of the primary beam (indicated by the arrow in Figure 3.7).

The cross section for each isotope was determined by integrating the Gaussian fit
over momentum space to obtain the total yield for each individual isotope. The beam
current measurement (from the four beam monitor detectors and a measurement made
periodically with a Faraday cup inserted at the entrance of the A1200) provided
information about the total number of projectile particles incident upon the target. After
correcting the isotopic yield of each element for the acceptance of the A1200 fragment

separator, the cross section was then determined from standard formulae.

Chapter 4
Results and Discussion

4.1 Fragmentation of 86Kr Projectiles

4.1.1 Momentum Widths

In the Goldhaber model [gol74], the removal of independent nucleons from the
projectile results in a Gaussian momentum distribution. The width of this distribution is

given by the expression

 

(4.1)

 

where AF is the fragment mass, Ap is the projectile mass, and 0'0 is a reduced width related

2
Fermi

to the Fermi momenta of the individual nucleons (0'02 = p /5 ). This model
reproduces the parallel momentum distribution widths of the PLF’s relatively well for high
energy fragmentation, and for light ion fragmentation at intermediate energies [gol74].
However, the experimental value of 60 is typically found to be smaller than the values
predicted using known Fermi momenta. Several arguments have been presented to explain
the observed narrowing of the width. One possibility is that Pauli exclusion effects could

lead to a decreased width [ber81], while another argument [web94] points out the fact that

the width predicted by the Goldhaber model is relevant to the prefragments - not the final
60

61

 

 

 

 

 

P TT""I""I""|"‘
600:- “Kr + A], 13:70 MeV/nucleon .—
3 _
> 400
o
E
9‘:
b -
200 _
q
I
O

70 75 80 85
Mass Number (A)

FIGURE 4.1: Parallel momentum distribution widths for isotopes observed during
the fragmentation of 86Kr.

observed fragments. The prefragments will undergo a particle evaporation step leading to
mass loss but little increase in momentum width (given isotropic evaporation), thereby
yielding momentum widths smaller than expected. The present measured values of the
parallel momentum widths in the projectile rest frame are shown in Figure 4.1 (error bars
represent the statistical error added in quadrature with the uncertainty due to cuts on the
tail of the momentum distributions). The best fit to our data using the Goldhaber model
with 00 = 124 MeV/c is indicated by the solid line. By interpolating results obtained in a
quasi—elastic electron scattering measurement [mon71], we obtain a value of pFemi = 260
MeV/c for 86Kr which translates into a 0'0 ~ 116 MeV/c. Also shown in Figure 4.1 are the
results from calculations with the ISApace code [fau92] fit with the Goldhaber model

(dotted line). This approach yields a reduced width of 0'0 ~ 100 MeV/c which is also

62

 

 

 

 

 

:I :.I IleTTlIIIIIIIIIIIIIIIIIII:
140— -—
’8 r— -1
\ :EJ :
>
a.) 120— _
2 Z Z
V
O : l :
b 100_—‘ o _
I 0 :
BO-IIIIIIIIJJIIIIIILJIIII lllllll-
o 100 200 300 400 500 600

Projectile energy (MeV/nucleon)

FIGURE 4.2: Parallel momentum width from recent krypton fragmentation
experiments.

smaller than the value obtained with our data. The dot-dash line in Figure 4.1 shows a

calculation based on the systematics of Morrissey [mor89] that were obtained from

parametrization of high energy fragmentation data. The parallel momentum width in this

parametrization is given by

G" = const ’ «1AA (4.2)

where AA = AP - AF and the constant is generally between 85 - 100 MeV/c. The best fit to
our data requires a constant of 120 MeV/c, which is again larger than the typical value
determined from high energy fragmentation. Values of the reduced width obtained from
other experiments involving krypton fragmentation at various energies [web94, ste9l,
baz90] are summarized in Figure 4.2. The values of the reduced width for both the 200
MeV/nucleon and 500 MeV/nucleon data were obtained by applying a best fit with the
Goldhaber model while limiting the fit to the fragment range covered in the present work.
The experiments performed at higher energies (84K: at 200 MeV/A [ste91] and 86Kr at

500 MeV/A [web94]) have best fit values of 00 ~ 100 MeV/c and 0'0 ~ 90 MeV/c,

63

72Ge31+ momentum distribution shown in

respectively. After applying a 50% cut to the
Figure 4 of Bazin et al. [baz90] a value of 00 ~ 130 MeV/c is estimated for the 44 MeV/
nucleon 86Kr + 27A] data. Applying an 80% cut to the low energy side of the momentum
distribution yields a value of 0'0 ~ 105 MeV/c. Figure 4.2 shows that the reduced width
observed during heavy-ion fragmentation exhibits a definite broadening as the projectile
energy is lowered into the intermediate energy regime. This is in contrast to fragmentation
involving light and intermediate mass projectiles where the general trend shows a

relatively constant reduced width until the intermediate energy range is reached, where the

reduced width begins to decrease smoothly [mur83].

The large parallel momentum distribution widths observed in the present data are
probably due to the coexistence of competing reaction mechanisms in the intermediate
energy regime. Apart from the ‘pure’ fragmentation component (that completely
dominates at higher energies), both the low-energy tail typical of more dissipative
processes [got91] and the broadening associated with nucleon pick-up reactions during the
formation of the prefragment [sou92] will increase the distribution width. The value of 00
~ 124 MeV/c was obtained with a cut-off at 50% of the peak height on the low momentum
side of the momentum distribution. It should be noted that as the momentum distributions
begin showing tails on the low momentum side for experiments performed in the
intermediate and low energy regimes, various assumptions have to be made about the
effect of the low energy tail. Qualitative decisions have been made on how much of the tail
to include and this, of course, can lead to different interpretations of data. In the present
data, Gaussian fits including the entire tail yield a value of 60 ~ 130 MeV/c while placing

the cut-off at 80% of the peak height yields a value of 60 ~ 115 MeV/c. Performing a fit

64

exclusively with the high energy side of the momentum distribution (i.e. a 100% cut-off)

yields a value of 0'0 ~ 105 MeV/c, although the fit is rather qualitative (see Figure 3.7).

4.1.2 Momentum Transfer

Morrissey [mor89] has defined average parallel momentum transfer (PI’I) as the
product of the projectile mass, the average measured velocity of the fragment B" , and a
kinematic factor of Bay/(7+1), and has shown that a linear relationship exists between
(PII) and the product of the total number of nucleons removed from the projectile AA,
with a slope parameter of 8 MeV/c (see Section 2.1). This dependence was derived from
high energy fragmentation data. The average parallel momentum transfer (Pfi) for the
individual elements is shown in Figure 4.3 together with the results of a calculation with
the ISApace code [fau92] (denoted by asterisks) which shows good agreement with the
present data. The statistical error is smaller than the plotted points. Changing the Gaussian
fit cut-off percentage on the momentum tail has a relatively small effect on the
determination of the parallel momentum transfer, introducing an uncertainty of ~ 2% (the
shift of the Gaussian centroid can be seen in Figure 3.7) which was added in quadrature to
the statistical uncertainty. The solid lines in Figure 4.3 are the result of a linear fit to all the
fragment data with Z S 36. The resulting slope parameter of 8.8 MeV/c (represented by
solid lines) is in good agreement with the overall trend of the fragmentation products (Z S
36), while the pick-up products differ significantly from this trend. Recent measurements
[ste9l] at an energy of 200 MeV/nucleon have a similar slope for the overall fit to the
fragmentation products while the slopes of the individual elements exhibit deviations from
this trend. As is evident from Figure 4.3, the slopes of the individual fragmentation

elements (2 S 36) are slightly steeper than the 8.8 MeV/c average slope, where linear best-

65

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66

fit slope parameters for the individual elements change monotonically as shown in
Table 4.1. This effect indicates that a larger excitation energy is required to produce the
proton-rich isotopes which then causes a greater downshift in momentum relative to the
neutron-rich isotopes. Recent data from krypton fragmentation at 500 MeV/nucleon
[web94] showed a very similar effect, with the slope parameters of the PLF’s being larger
than 8.8 MeV/c. However, the slope parameters in the high energy krypton fragmentation
[web94] increase strongly as the proton number decreased from Z = 36 (14 MeV/c) to Z =
30 (58 MeV/c). An increased slope parameter for individual elements was also observed in
xenon fragmentation at 790 MeV/nucleon [fri93] in which case the data yielded a slope of

~ 16 MeV/c for the near-projectile elements.

Table 4.1: Fitted slope parameters for individual elements from 86Kr fragmentation.

Proton number (Z) Slope Parameter

 

33 11.6 :1: 0.4
34 12.2 :1: 0.3
35 12.1 i 0.2
36 12.5 :t 0.2
37 10.7 :t 0.2
38 11.9 :I: 0.4
39 5.5 :1: 0.8

More interesting are the parallel momentum transfers of the charge pick-up
products which deviate significantly from those of the fragmentation products. The
formation of the pick-up products (in this case with Z > 36) is assumed to take place
during the initial prefragment formation when individual protons are acquired from the
target nucleus. The final observed fragment is then formed through neutron evaporation.

Another possible production mechanism is the excitation of a A—resonance that can

67

change the isospin of a projectile nucleon. However, the ISApace calculations indicate that
for 86Kr fragmentation at 70 MeV/nucleon this effect can account for no more than 0.5%
of the total charge pick-up products. Using momentum conservation arguments similar to
those used by Souliotis et al. [sou92], we can determine the momentum shift arising
purely from picking up protons. Initially the projectile has a momentum pp with a mass
number Ap and the proton in the target has a momentum of pt. After the collision the
prefragment has a momentum of ppp and a mass number of Ap + AAt where AAt is the
number of nucleons removed from the target. Assuming the evaporation is isotropic, the
average final fragment velocity (VF) will be identical to the average velocity of the
prefragment, and it can then be shown that the velocity shift due to picking up the target

nucleons is given by

 

AB = 1 . AA‘ [p ._(&)] (4.3)
" moc Ap-I-AAt Fem“ AP

which corresponds to a shift in the parallel momentum transfer of

 

’ AP bl Pp
P offset - AP + A A” + 1AA‘[(A—P)_ pt] (44)

where B and 'y are the relativistic parameters from the initial projectile velocity and
moc = 931.5 MeV/c. This offset is then added to the parallel momentum transfer yielding

a parametrization of the form

MeV
c

 

where the best-fit slope parameter of 8.8 MeV/c (the slope of the fragmentation products)
was held fixed. Fitting the present charge pick-up data to this parametrization and

adjusting the momentum of the picked-up proton(s), we obtain a value of pt ~ 220 MeV/c.

68

The results are shown with dashed lines in Figure 4.3. The value obtained for the proton
momentum indicates that the protons acquired during the pick-up process are moving
parallel to the projectile with approximately their maximum Fermi momentum. This effect

has previously been observed in reactions involving neutron pick-up [sou92].

4.1.3 Isotopic Cross Sections

The absolute cross sections for the Z = 33 (arsenic) through Z = 39 (yttrium)
isotopes were obtained by integrating the Gaussian fits of the parallel momentum
distributions over momentum space and are shown in Figure 4.4. The error bars indicate
the statistical uncertainty added in quadrature to the uncertainty due to altering the fitting
limits on the tail of the momentum distribution. Inclusion of the tail of the momentum
distribution increases the cross sections by ~ 8%. The absolute beam current was
~ 0.1 pnA, however, the overall normalization is only accurate within a factor of two. The
relative cross sections are much more precise than this. The absolute normalization
uncertainty is not included in the error bars. The solid histograms represent the calculated
cross sections from the ISApace model [fau92], which is considered to be valid between a
few hundred MeV/nucleon and a few GeV/nucleon. ISApace uses the Yariv-Fraenkel
ISABEL intranuclear cascade code [yar79] to model the prefragment formation step
followed by a modified PACE evaporation code [gav80] to calculate the deexcitation step.
The magnitudes and shapes of the predicted isotopic cross section distributions are in
remarkable agreement with the experimental data for the fragmentation products (Z S 36).
In contrast, the measured cross sections of the charge pick-up products are greater than
predicted by ISApace, and this difference grows as the number of acquired protons

increases (although the shapes of the distributions are reproduced rather well).

69

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Figure 4.4 also shows cross sections predicted from the semi-empirical
parametrization (EPAX) established by Siimmerer et al. [sum90] (dotted histograms).
Cross sections predicted by the EPAX code are generally higher than the observed cross
sections for the neutron-rich isotopes while the proton-rich isotopes are underpredicted.
This indicates that the evaporative step of the whole process contributes significantly to
the proton-rich side of the isotopic chains, as expected for high excitation energies in the
prefragments. Both ISApace and EPAX underpredict the cross sections of the charge pick-
up products indicating that, as expected, the high-energy models are not able to reproduce

the experimental data for the charge pick-up products.

The ISApace model has previously been used to predict the cross sections for 500
MeV/nucleon 86Kr fragmentation as discussed by Weber et al. [web94]. In this case, the
ISApace calculation predicted the cross sections fairly well, while the EPAX calculation
showed a similar underprediction for proton-rich nuclei and overprediction of the neutron-
rich nuclei that is observed for the current data at 70 MeV/nucleon. It was suggested that
the Gaussian shape of the charge dispersion used in the EPAX formula needs to be

modified in order to reproduce the data (see Eq. 2.4).
4.2 Fragmentation of 78Kr Projectiles

The parallel momentum distributions of a number of reaction products were
monitored on-line and fitted with a Gaussian function. The centroid values were then used
to identify the most appropriate magnetic rigidity setting for the observation of 69Br. The
centroids (in terms of magnetic rigidity) for the isotopes covering Z = 24 to 38 are shown
in Figure 4.5 where the horizontal dashed lines show the range of magnetic rigidity

covered during this experiment. The general trends exhibited by the reaction products and

71

 

2-55 P I I V Y I 1 1 r r I r r r ‘r T .

2.50 L _‘
I.

2.45

2.40

Bp (Tesla—meter)

2.35

2.30

 

 

 

 

2.25 h I I I I I I I I I I I I I I 1

Atomic Mass

FIGURE 4.5: Centroids of parallel momentum distributions for isotopes observed
during projectile fragmentation of 78Kr + 58Ni at 75 MeV/nucleon.

in particular that of the bromine isotopes, show that this rigidity range would have

permitted the observation of 6S’Br if it were stable.

4.2.1 Isotopic Cross Sections

Figure 4.6 shows the mass spectra for isotopes with atomic numbers 30 5 Z S 38
obtained at a fixed magnetic rigidity setting optimized for observation of 69Br. The
absence of 69Br is clearly observed in the bromine mass spectrum, whereas other T2 = -1/2
nuclei are present. The asterisk symbols in Figure 4.6 indicate several events that can be
attributed to 606a and 70Kr, confirming the recent identification of these isotopes by Blank
et al. [bla95]. The measured isotopic cross sections, determined by integrating the
Gaussian functions over momentum space after correcting for the acceptance of the
A1200, are shown in Figure 4.7. Also shown in Figure 4.7 are the cross sections calculated
from both the EPAX parametrization [sum90] and the ISApace model [fau92]. Both codes

were originally developed for high energy (or ‘pure’) fragmentation (FJA > 200 MeV/

72

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nucleon), but recent experiments have shown their applicability for reactions involving
intermediate-mass projectiles at intermediate energies [pfa95, fau96]. A comparison of the
(absolute) experimental cross sections with the EPAX parametrization and the ISApace
code shows several overall features. The EPAX code noticeably underpredicts the
formation of proton pick-up products (Z > 36), a not unexpected feature considering this
parametrization was developed from high-energy fragmentation in which pick-up
reactions seldom occur. The ISApace code is able to reproduce relatively well the single-
proton pick-up, but the predicted cross sections for reaction products that have acquired
more than one proton (Z > 37) start to fall off dramatically. The magnitude of the predicted
cross sections from both EPAX and ISApace agree relatively well for the reaction products
below krypton (Z < 36), although the predicted distributions are more neutron-rich than

the experimental cross section distributions (Z 2 30).

4.2.2 Memory Effect

Together with the results of previous experiments which involve fragmentation of
neutron-rich krypton projectiles (86Kl' and 84Kr [pfa95, ste9l]), the data from the 78Kr
fragmentation experiment can provide additional insight into the influence of the projectile
NfZ ratio on the fragment charge dispersion distribution for reactions in the intermediate
energy/intermediate mass regime. In order to include this effect properly in their semi-
empirical fragmentation product cross section code, Siimmerer et al. [sum90] developed a
parametrization that took into account previous observations from (target) fragmentation
experiments; (i) the maxirna of fragment charge distributions always lie on the neutron-
deficient side of the valley of B stability, (ii) for targets/projectiles close to B-stability, the

most probable charge of a fragment isobaric chain is only dependent on fragment mass,

75

and (iii) the size of the memory effect is different for neutron- and proton-rich projectiles.
Chu et al. [chu71] had described this effect as

ZP(A) = Z5(A) +A (4.6)
where ZP(A) is the most probable charge and the B-stable charge ZB(A) can be

approximated by the smooth function (thus avoiding shell effects) [mar71]

A

z (A) = .
B 1.98 +0.0155 - A”3

 

(4.7)

The A term, which describes the difference between experimentally obtained values of Zp
and 213’ was parameterized by Siimmerer et al. [sum90] using the form

A _ { 2.041x10‘4 - A2 ifA < 66

2 (4.8)
2.703X10— - A — 0.895 if A 2 66.

To describe the additional shift in the charge distribution maxima (2,) that is caused by the
N/Z ratio of the target/projectile (depending on whether target- or projectile-like residues
are studied) an extra “memory effect” term Am was added:

Zp(A) = ZB(A) + A + A1m . (4.9)
A fit to the (scarce) experimental data available at the time (see Section 2.1) led to a

parametrization for Am in the form

Am(A) = [algal + c2(l-Ԥ)4]AB(A,) (4.10)

where A, is the target mass and AB (A,) = Z, - ZB(A,), in which Z, is the target proton
number and A, is the target mass. Different values for the coefficients c1 and c2 were
determined for neutron- and proton-rich fragmentation as the memory effect appeared to
be smaller for fragmentation of proton-rich targets/projectiles compared to neutron-rich

systems. Figure 4.8 illustrates the dependence of the memory effect Am on the ratio of A/

76

AI) (where A is the fragment mass and Ap is the projectile mass) for the most abundantly
produced final fragment of each isobaric chain (the so-called “ridge line”) from the
present experiment. The ridge lines are shown also from two other experiments with more
neutron-rich krypton isotopes: 86Kr fragmentation at 70 MeV/nucleon [pfa95] and 84Kr
fragmentation at 200 MeV/nucleon [ste91]. The hollow symbols are indicative of proton
pick-up products. Also indicated in Figure 4.8 (by the dashed line) is the curve
representing the parametrization of Eq. 4.10 for the 78Kr fragmentation (the
parametrization for the reactions involving 84Kr and 86Kr are not shown on the plot, but
exhibit similar agreement to the data as that for the 78Kr fragmentation data). It is
apparent that the memory effect for intermediate energy/intermediate mass fragmentation

behaves differently than expected from the high energy data. Both the data from the

 

  
   
  

I 78Kr + 58Ni, '75 MeV/nucleon
. 84Kr + 1971111, 200 MeV/nucleon
0 86Kr + 27111, '70 MeV/nucleon
— — ”Kr EPAX parameterization

current parameterization for:
(solid lines)

I T I I I

 

 

 

; A) 78Kr
_2 1 1 1 1 I 1 1 1 1 I 4 1 1 1 1
0.2 0.4 0.6 0.8

A/Ap

FIGURE 4.8: Parametrization of the memory effect which shows data from several
krypton fragmentation experiments.

77

current proton-rich fragmentation of 78Kr as well as the data from the neutron-rich
fragmentation of 8“Kr and 86Kr show a much steeper dependence on the mass ratio than
the parametrization. Recent measurements with 129Xe and 136Xe beams at 790 MeV/
nucleon showed a similar trend for the proton-rich projectile and the reaction products
from the neutron-rich projectile (136Xe) as they deviated from the standard
parametrization [fri93]. Using a formalism similar to Siimmerer et al. [sum90], the
memory effect from the three intermediate-energy krypton fragmentation experiments can

be described by

Am(A) = [e,(£t)4+e2(£;)]AB(A,) (4.11)

with values of c, = 1.55 and 02 = -0.425. The modified parametrization was determined by
performing a least square fit (with two n-th order polynomial terms) to the experimental
data. The c2 coefficient becomes negative to account for the fact that the proton-rich
fragmentation data dips below the Am = 0 line (this effect was also observed in the limited
data (see Figure 2.4) used by Siimmerer et al. [sum90]). The parametrization shows that
fragments far from the projectile approach the valley of B stability (Am/AB(A,) ~ 0) and
those near the projectile mass are close to the N/Z ratio of the projectile (Am/AB(A,) ~ 1).
This modified parametrization does a good job reproducing the experimental data and is
indicated by solid curves in Figure 4.8. (Because the 86Kr fragmentation experiment
[pfa95] was concentrated on fragments near the Z of the beam, this data is limited to Z 2
33.) It should also be noted that, in contrast to the two other data sets which were
measured around 0°, the 84Kr [ste91] data was obtained at angles of 06° and 15°. The fact
that the 84Kr ridge line in Figure 4.8 begins to curve downward for Z < 20 indicates that

parts of the parametrization used in this analysis are not applicable near and below argon

78

(Z = 18), as was discussed by Siimmerer et al. [sum90]. Charge pick-up products (Z > 36
in this case), which are rarely produced from high energy fragmentation, are commonly
observed at intermediate energies. The memory effects for the pick-up products observed
in previous krypton fragmentation experiments [ste91, pfa95], as indicated in Figure 4.8
by unfilled symbols, seem to follow closely the general trend of the fragmentation
products (Z S. 36). This fact, together with the observation that the overall curvature of the
memory effect is steeper than the standard parametrization, is a strong indication that the
prefragments are produced by processes other than the “pure” fragmentation that occur in
high energy reactions. This assumption is also supported by the relatively large pick-up
product cross sections that were observed in the current experiment and the 86Kr

fragmentation [pfa95].

4.2.3 Implications for the rp-process

From the isotopic cross sections shown in Figure 4.7, it is possible to estimate the
number of 69Br events that should have been observed. Assuming an exponential decrease
in cross section near the proton drip line (as is predicted by the EPAX parametrization
[sum90]), ~ 300 counts of 69Br should have been observed as determined from the
number of 7OBr events that were identified. This estimated number of events that should
have been observed can, together with the short flight path (~14 m from production target
to the focal plane silicon telescope), be used to place an upper limit on the half-life of
69Br of 24 ns. Most mass models predict 69Br to be only slightly proton unbound. In the
1993 Atomic Mass Tables [aud93] the value of Sp: -180 i 300 keV is found from the
listed binding energies of 69Br and 688e. Assuming that the proton is emitted from a pm

state (as is the case in the mirror nucleus 69Se), the proton penetrability WKB

79

approximation indicates a half-life of ~ 103 seconds which implies that the main decay
mode is BVEC with an estimated half-life on the order of 100 ms [tak73]. For the WKB
approximation a normalized Wood-Saxon nuclear potential was used in conjunction with
the centrifugal, spin-orbit, and Coulomb terms as was described by Winger et al. [win93].
The recent GANIL experiment [bla95] limited the 693r half-life to 100 ns or less which
corresponds to being proton unbound by at least 450 keV. The current tighter limit on the
69Br half-life of 24 ns or less indicates that this nucleus is proton unbound by at least 500
keV. The present experiment also gives some information on 73Rb. Because of its non-
observation in a wide variety of measurements over a number of years [moh9l, bla95,

aur77, hen94], this isotope should be considered to be particle unbound.

The systematics in Figure 4.5 show that the magnetic rigidity range covered in the
present experiment would also have included 73Rb. Using the EPAX parametrization and
the observed number of 74Rb events, approximately 75 73’Rb events should have been
observed, yielding an effective upper limit of 30 ns for the half-life of 73Rb. In the case of
73Rb, the majority of mass models predict this isotope to be proton unbound. The value of
8p = -590 :1: 270 keV determined from binding energies in the 1993 Atomic Mass Tables
[aud93] yields a proton emission half-life of ~ 700 ns (using the WKB approximation).
The present data limits the half-life of 73Rb to less than 30 ns and assuming the emitted
proton comes from the f5,2 state (since the mirror nucleus is 73Kr) indicates that 73Rb is
unbound by at least 680 keV. Under the previous assumption that 69Br was particle stable

[cha92], the rp-process was generally thought to proceed via

688e(p,r)69Br(13*)70Kr(13+)7°Br<p.7)7‘Kr<B*)7‘Br(p.7)72Kr.

80
In view of their recent results (regarding the particle instability of 69Br), Blank et al.

[bla95] have proposed an alternative rp-process path:
688e(B+)68As(p. )69Setp. )7°Br<p. )7'Kr(B+>“Br(p. >72Kr.

The most significant modification is that the rp-process must now wait for the decay of
68Se which has a long half-life (1.6 min) relative to the assumed burning time (~ 10 sec) of
the astrophysical processes in which the rp-process is likely to proceed to the high mass
region (A > 70). In processes with extended burning times (~ 100 sec) [wor94, cha92], the
rp-process can slowly continue to 72Kr which slows the process due to the fact that 73Rb
is unstable and 72Kr has a 17.2 second half-life. Many of the rp-process calculations
[wor94, cha92] are extended to ~ 1000 seconds to explore the astrophysical efi'ects of an

extended burning time, and in this situation the rp-process could slowly proceed to masses

 

Halflife (sec)

 

 

 

I I I I I l I I J I L I L I I I J I I I I I
0.3 0.4 0.5 0.6 0.7 0.8 0.9

Emitted Proton Energy (MeV)

I I I I I I I

FIGURE 4.9: Halflives calculated using the WKB approximation for 69Br and 73Rh.

8 1
higher than 72Kr. Of the five new isotopes reported by Blank et al. [bla95], two (60Ga and

64As) could alter the rp—process path as it approaches “Se. The other three isotopes (of
69Kr, 70Kr, and 74Sr), however, have no influence on the modified rp-path due to the
“bottle-necks” caused by the instability of 69Br and 73Rh. Although no evidence for 64As,
69Kr, and 74Sr was seen in the present experiment (assuming an exponential decrease in
cross section and the number of events attributed to 65As, 70Kr, and 75Sr, no counts of
64As, 69Kr, and 74Sr should have been observed), the modified rp-process path presented

by Blank et al. [bla95] seems to reflect the current experimental evidence accurately.

Chapter 5

Summary

The present work involved systematic measurements of fragment yields from the
intermediate energy projectile fragmentation of 78Kr and 86Kr isotopes, and several major
points were presented: 1) The EPAX parametrization and the ISApace code can be used
with mixed results in the intermediate energy/intermediate mass regime, as is shown in
Figure 4.4 and Figure 4.7; 2) when the final observed fragments have acquired one or
more protons, their parallel momentum shift can be explained through the use of
momentum conservation when the Fermi momentum of the individual protons is
considered; and 3) with the results of the present work and some other recent experiments
involving intermediate energy krypton fragmentation, modifications to the memory effect
parametrization were made. These three points are pertinent for future planning of
radioactive nuclear beam experiments - most notably the cross section predictions since it
is important to have a reasonably accurate estimate for the time required to produce the
required number of a particular exotic isotope. The present work also helped add to the
growing database of systematic studies of momentum distributions and cross sections in
this energy/mass regime, and the additional information will help in the modification of

the EPAX parametrization and the ISApace code.
82

83

The experiment on the projectile fragmentation of 78Kr helped supply systematic
cross section data but, it also had the specific goal of determining the termination point of
the rapid-proton capture process - most notably the particle stability of 69Br. The data
shown in Figure 4.6 showed that 69Br is not particle stable, and this will cause the
termination of the rp-process. Tight limits were placed on the half-lives of both 69Br and
73Rb (both were considered important to understanding the rp—process) and this data will
now have to be incorporated into astrophysical network calculations.

Further research in this area is still necessary in order to gather additional
systematic measurements of cross sections and momentum distributions from a variety of
target/projectile combinations at varying energies in the intermediate energy/intermediate

mass regime.

Appendices

Appendix A

Appendix A

Measured Cross Sections

The following tables list the experimental cross sections for the individual
observed isotopes in the reaction 78Kr + 58Ni at 75 MeV/nucleon (Table A. 1) and the reac-
tion 86Kr + 27A] at 70 MeV/nucleon (Table A2). The uncertainty listed is statistical in
nature. Further details on other uncertainties can be found in the text along with plots of

this data (See Figure 4.4 on page 69 and Figure 4.7 on page 73).

84

85

Table A.l (con’t).

 

Table A.l: Isotopic cross sections from 78Kr + 58Ni at 75 MeV/nucleon.
Z A Cross Section (mb) Stat. unc. (mb)
21 44 3.85E-02 8.8E-03
21 45 4.73E-02 7.0E-03
21 46 2.91E-02 5213-03
21 47 1.21E-02 3.5E-03
21 48 2.29E-02 1 . 1E-02
22 45 9.50E-03 3.4E-03
22 46 5.33E-02 7.8E-03
22 47 6.76E-02 7.4E-03
22 48 6.53E-02 8.3E-03
22 49 1.04E-01 2.7E-02
22 50 6.46E-01 4.5E-01
22 51 7.55E-03 3.6E-03
22 52 3. 10E-03 2313-03
23 47 4.56E-03 2.4E-03
23 48 1.99E-02 3.8E-03
23 49 5.37E-02 3.1E-03
23 50 1.47E-01 5.4E-03
23 5 1 1. 17E-01 4.9E-03
23 52 4.95E-02 3.8E-03
23 53 1.23E-02 2.8E-03
24 49 6.52E-03 3. lE-03
24 50 4.54E-02 5.6E-03
24 5 l 1.86E-01 4.7E-03
24 52 5.33E-01 8013-03
24 53 4.73E-01 1.0E-02
24 54 1.60E-01 6.7E-03
24 55 2.39E-02 3.7E—03
25 51 7.36E—03 1.8E—03
25 52 1.04E-01 4.8E-03
25 53 9.01E—01 1.0E-02
25 54 1.99E+00 1.7E-02
25 55 1.15E+00 1.6E-02
25 56 4. 17E-01 1.4E-02
25 57 4.38E-02 5.2E-03
26 53 1.46E-02 2.0E-03
26 54 4.37E-01 7.7E—03

86
Table A.1 (con’t).

Cross Section (mb) Stat. unc. (mb)

 

26
26
26
26
26
27
27
27
27
27
27
27
28
28
28
28
28
28
28
29
29
29
29
29
29
29
29
30
30
30
30
30
30
30
30
3 l
3 l
3 l
3 l

3.14E+00 1.9E-02
4.86E+00 2.7E-02
3.72E+00 3.6E-02
7.74E-01 2.0E-02
5.99E-02 6.7E-O3
4.72E-02 3.3E—03
1.50E-1-00 1.5E-O2
1.23E+Ol 5.9E-02
1 .82E+01 8.2E-02
8. 12E+00 6. lE-02
1.04E+00 2.3E-02
1.01E—01 9.0E-O3
1 .65E-01 4.7E-03
3.76E+00 2.7E—O2
1.93E-I-01 7.0E-02
2.73E+01 9.3E-02
1.10E+01 7.3E-02
1.47E+00 2.6E-02
1.30E-01 1.0E-02
3 .SOE-Ol 8. 1E-O3
5.56E+00 3.3E-O2
3.19E+01 1.2E-01
3.87E+01 1.4E-01
1.29E-I-01 8.4E-02
1.73E+00 3.5E-02
l .90E-01 1 .2E-02
1.54E-02 1.8E-03
2.07E-02 2.5E-03
7. 19E-01 1 .4E-02
8.26E+00 4.3E-02
4.21E+01 1.5E-01
4.30E+01 1.5E-01
1.55E-I-01 1.0E-Ol
5.87E-01 1.6E-02
1.53E-02 1.4E-04
2.54E-02 5.0E-03
6.54E-01 1.4E-02
1. 10E+01 8.5E-02
3.40E+01 1.5E-01

87
Table A.l (con’t).

Cross Section (mb) Stat. unc. (mb)

 

31
31
31
31
31
32
32
32
32
32
32
32
32
33
33
33
33
33
33
33
33
34
34
34
34
34
34
34
35
35
35
35
35
35
36
36
36
36
36

3.88E+01 1.7E-01
2.69E+01 1.7E-01
2.66E+00 6.6E-02
3.07E-01 9.0E-03
2.77E-02 2.6E-03
1.05E-01 8.4E-03
2.03E+00 2.8E-02
1.40E+01 7.3E—02
4.09E+01 1.4E-Ol
4.72E+01 1.6E-01
2.08E+01 1.1E-01
4.43E+00 4.1E-02
5.04E-01 1.3E-02
8.37E-02 7.3E-O3
1.45E-I-00 2.3E-02
1.04E+01 6.5E-02
3.12E+01 1.2E-01
3.95E-I-01 1.4E-01
1.94E+01 9.0E-02
4.68E+OO 3.6E-O2
9.60E-01 4.5E-02
6.71E-02 6.2E-O3
1 .20E+00 2.5E-02
8.30E+00 5.9E-02
2.61E+01 1.0E-01
3.29E+01 1.1E-01
1.60E-101 6.5E-02
5.49E+00 4.3E-02
3.58E-02 4.7E-03
7.14E-01 1.8E-02
5.87E-1-00 5 .2E-02
1.87E+01 8.3E-02
2.49E+Ol 8.5E-02
1.51E-I-01 5.4E-02
2.80E-02 4.2E-O3
5.75E-Ol 1.6E-02
3.78E-l-00 4.1E-02
1.28E+01 6.4E-02
1.93E+01 6.1E-02

88
Table A.1 (con’t).

 

Z A Cross Section (mb) Stat. unc. (mb)
37 74 7.81E-03 1915-03
37 75 1.32E—01 8.5E-03
37 76 1.23E+00 2.6E-02
37 77 3.06E+00 4.8E-02
37 78 2.14E+00 2.2E-02
37 79 3.59E-01 7.3E-03
38 77 1.86E-02 155-02
38 78 5.03E-02 5.9E-03
38 79 1.55E-01 9.1E-03
38 80 4.69E-02 4.0E-03

89

 

Table A.2: Isotopic cross sections from 86Kr + 27A] at 70 MeV/nucleon.
Z A Cross Section (mb) Stat. Unc. (mb)
33 68 7.57E-02 1.9E-02
33 69 1.46E+00 1.4E—01
33 70 4.99E+00 1.9E-01
33 71 8.92E+00 3.0E-01
33 72 1.38E-1-01 3.0E-01
33 73 1.58E+01 4.1E-01
33 74 1.09E-1-01 5.8E—01
33 75 8.58E+00 3.7E-01
33 76 7.66E+00 5.0E-01
34 69 3.87E-02 1.0E-02
34 70 5.25E-01 7.8E-02
34 71 1.31E-1-00 1.7E-01
34 72 3.41E+00 1.7E-01
34 73 7.60E-l-00 2.4E-01
34 74 1.43E-1-01 3.4E-01
34 75 1.67E+01 3.7E-01
34 76 1.83E+01 8.4E-01
34 77 1.34E+01 5513-01
34 78 1.08E+01 1.5E-I-00
35 71 2.62E-02 8.9E-03
35 72 3.05E-01 5.5E-02
35 73 8.18E-01 8113-02
35 74 2.35E+00 1.5E-01
35 75 5.85E+00 2.2E-01
35 76 1.17E+01 3.8E-01
35 77 1.75E+01 3.5E-01
35 78 2.45E+01 1.1E+00
35 79 2.1 1E+01 6.9E—01
35 80 2.30E+01 7.8E-01
36 73 1.26E-02 5.5E-03
36 74 1.86E-01 4.6E-02
36 75 5.80E—01 6.6E-02
36 76 1.55E+00 1.2E-01
36 77 4.38E+00 2.3E-01
36 78 9.69E+00 3.5E-01
36 79 1.73E+01 9.0E-01
36 80 2.68E+01 8.1E-01
36 81 3.68E+01 9.1E-01

90
Table A2 (con’t).

Cross Section (mb) Stat. Unc. (mb)

 

36
37
37
37
37
37
37
37
37
37
37
37
38
38
38
38
38
38
38
39
39
39
39
39

82
75
76
77
78
79
8O
81
82
83
84
85
79
80
81
82
83

85
81
82
83

85

3.40E+01 1.2E+00
1.13E-02 5.9E-03
4.79E—02 3.4E-02
1.04E-01 3.8E-02
8.97E-01 6.8E-02
2.41E+00 1.1E-01
5.97E+00 3.2E-01
1.26E+01 3.7E-01
1.55E+01 6.4E-01
1.76E+01 1.2E+00
1.96E-I-01 8.4E-01
1.49E+01 8.1E-Ol
2.84E-02 7.4E-03
4.21E-01 4.6E-02
8.03E-01 1.0E-01
1.32E+00 1.0E-Ol
2.35E+00 1.9E-01
1.73E-1-00 1.5E—Ol
1.32E+00 1.6E-01
7.79E-02 2.0E-02
1.13E—01 2.7E-02
1.76E-01 5.0E-02
1.32E-01 3.6E-02
3.58E-02 1.1E-02

Appendix B

Appendix B

ISApace Input Parameters

The following is the input file used for the ISApace [fau92] calculation for the
73Kr + 58Ni at 75 MeV/nucleon reaction. The input for the 86Kr calculation looked similar
other than the target and projectile masses and proton numbers.

$PUT
CSA = 0.0000000E+00, 1006.500 . 58.00000 . 28.00000 .
10.10000 . 10000.00 ,9*0.0000000E+00.

AP = 78.00000 ,
ZP = 36.00000 ,
YP = -99.00000 ,

[RAND = 1234567,
CUTOFA = ll*0.0000000E+00.

MODE = 73.

NP = 1,

KNP = 2,

GAPL = 1.100000 .
IPAULI = l,

ISONSW = 0,
GAMISO = 0.0000000E+00,
[DEN = 7.

JKEY = 3,

MPRIN = 0,

VAP = 0.0000000E-1-00,
VPION = 0.0000000E+00.
NZMAN = 6000000,

NTDEL = 3.
RCAS = 3.000000 .
TIMEF = 20.00000 .
KASKEW = 0.
NOPOT = 0.

DX =-2.500000 , 1.000000 ,5*0.6000000 , 1.000000 .
DXP = -2.500000 , 1.000000 , 0.9000000 , 0.6000000 .
0.3000000 ,2*O.2000000 ,2*0.3000000 ,7*0.4000000 .
BDD = 0.0000000E+00,

NMIN = 4

SEND

91

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[arm87]

[aud93]
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[baz90]

[ber8 1]
[ber88]

[bla95]
[boh36]
[bol86]
[bor86]

[cha92]

[che68]

[chu7 1]

[fau92]

[fau96]

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