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STUDIES OF HEAVY-ION REACTIONS AND TRANSURANIC NUCLEI
Progress Report for the Period
September 1, 2003–August 31, 2004
W. Udo Schröder
Principal Investigator
University of Rochester, Department of Chemistry
Rochester, New York14627-0216
September 2004
Prepared for
THE UNITED STATES DEPARTMENT OF ENERGY
UNDER GRANT NO. DE-FG02-88ER40414
NOTICE
This report was prepared and is published as an account of work sponsored by the United States government. Neither the United States nor the United States Department of Energy, nor any of their employees, nor any of their contractors, subcontractors, or their employees, make any warranty, express or implied, or assume any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed or represents that its use would not infringe privately owned rights.
I. Abstract
Progress is reported on
a) the experimental exploration of heavy-ion reaction mechanisms in exclusive 4 studies,
b) the theoretical modeling of heavy-ion reaction dynamics in terms of transport models,
c) the theoretical understanding of the thermodynamics of hot nuclei,
d) the experimental study of particle emission in p-induced spallation reactions,
e) the development of advanced electronic hardware and firmware.
Nuclear reaction mechanisms evolve in a smooth manner from the low-energy regime to the upper boundary of the Fermi energy domain. Dissipative orbiting and the emission of massive projectile and target like fragments are reoccurring characteristics of reactions studied now up to E/A > 60 MeV. Some of the new reaction phenomena associated with nuclear expansion have been identified as stabilizing elements in an energetic nuclear collision.
Cluster emission in peripheral and central heavy-ion collisions has been identified as a superposition of at least three different processes. Sequential evaporation of clusters from hot nuclei is now understood to result from the significant entropy gains associated with the enlarged surface areas of hot, expanded nuclei. This effect leads also to a relative increase in the stability of hot reaction primaries against particle decay and explains the survival of the primaries from a dissipative reaction. The entropy-driven expansion of hot, equilibrated nuclei is understood to be responsible for the observation of negative heat capacities and limiting temperatures in heavy-ion collisions. The same phenomenon affects the evolution of in-medium correlations as reflected in the effective nucleonic mass. The influence of nuclear expansion has been accounted for in a modified version of a statistical model code for nuclear decay. As in the description of the fission process, cluster emission represents a significant rearrangement of the nuclear mass that can approximately be modeled by a mass and density dependent retardation of statistical decay.
TABLE OF CONTENTS
I.Abstract ...... 3
Table of Contents ...... 4
II.Introduction ...... 6
III.Research Program ...... 13
A.Morphing of the Dissipative Reaction Mechanism...... 19
B.Characteristics of Projectile-Like Fragments in Dissipative 197Au+86Kr Reactionsat E/A=38.7 and 54.8 MeV 30
C.Bombarding-Energy Dependence of Light Charged Particle Emission in 197Au+86KrReactions at E/A=38.7 and 54.8 MeV 58
D.Mechanisms of Fragment Production in 209Bi+136Xe Reaction at E/A=28, 40, and 62 MeV 88
E.Projectile Splitting in 112Sn+56,62Ni Reactions at E/A=35 MeV...... 119
F.Numerical Modeling of Statistical Decay of Compound Nuclei at Equilibrium Density with Surface Entropy Effects 123
G.Thermostatic and Decay Properties of Excited Nuclei Studied within the Framework of Harmonic-Interaction Fermi Gas Model 168
H.Thermal Expansion Induced Retardation of Particle Evaporation from Excited Nuclear Systems 223
I.The Caloric Curve for Mononuclear Configurations...... 237
K.Stability of a BUU Ground State...... 261
L.CECIL-The Reaction 112Sn+112Sn at E/A=35 and 43.7 MeV...... 276
M.Nucleon and Composite-Particle Production in Spallation Reactions Studied with the Multi-Purpose Detector NESSI 282
N.Isospin Physics: Proposal for Upgrading CHIMERA...... 291
O.ICC8K – AN Intelligent CAMAC Controller, Part 2: Firmware and Tests of Functionality 296
IV.Publications and Activities ...... 313
A.Articles ...... 359
B. Invited Lectures and Presentations ...... 359
C.Contributed Papers at Professional Meetings ...... 360
D.Professional Activities ...... 361
V.Personnel ...... 362
Visitors...... 362
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II. Introduction
The research program of the University of Rochester Nuclear Chemistry group is supported by the United States Department of Energy under Grant DE-FG02-88ER40414. The group has active experimental programs at the Laboratorio Nazionali del Sudin Catania/Italy, at the COSY accelerator facility of the Forschungszentrum Jülich in Germany, andat the National Superconducting Cyclotron Laboratory of MichiganStateUniversity in Lansing/MI.
Research projects have been pursued jointly with nuclear science groups from the Universities ofCatania and Milan, LNS Catania, from Washington University (St. Louis), OregonStateUniversity(Corvallis),the German Hahn-Meitner-Institut Berlin,and theForschungszentrum Jülich, the French LaboratoryGANILin Caen, as well as with nuclear physics groups at the Universities of Warsaw and Cracow, Poland.
This report describes the research activities and results obtained by the University of Rochester Nuclear Chemistry group over the period from September 2003 through August/September 2004.
The main thrust of the group’s research has directed toward the exploration of heavy-ion reaction mechanisms, in particular the study of interesting new phenomena associated with surface cluster emission in different scenarios. The achieved progress in our systematic study of heavy-ion reaction mechanisms has been highly encouraging and points to even more interesting developments in the future. In contributions to this Annual Report on several heavy-ion systems, we have further established the features of the main components of cluster emission in such reactions at Fermi bombarding energies.Based on still scarce, incomplete, and tentative experimental evidence, one now distinguishes at least three different likely cluster emission processes in heavy-ion collisions:
1)Dynamical cluster production in elastic or inelastic projectile (or target) splitting or breakup;
2)Dynamical cluster emission during the breaking of a “neck” between projectile and target-like fragments in the reaction exit channel, or in the disintegration of a participant zone formed in the overlap of projectile and target;
3)Sequential, statistical cluster evaporation from projectile- and target-like fragments.
These processes should occur at different stages of a heavy-ion collision and should show different sensitivity to the projectile-target isospin asymmetry.
The process of sequential statistical cluster emission from hot, expanded systems (Process 3) has been subject of a major new and extensive experiment, we have carried out this past summer with the CHIMERA/ISOSPIN collaboration at LNS Catania. Results of this experiment (Sn+Sn), the first in the approved CECIL series, are discussed in this report.
In addition, we have further developed our understanding of the mechanical response of finite nuclei to significant heating. The results are surprising in that they do not only describe the propensity of hot, expanded nuclei to undergo cluster decay but also predict an unexpected increase in the overall lifetime and stability of hot nuclei, which renders observations of reaction dynamics in a new light.
Spallation reactions provide alternative means of producing hot nuclei, which undergo decay after the emission of a significant number of fast, pre-equilibrium particles. We have found that, although cluster emission has been observed in 2.5-GeV p-A reactions on heavy targets, their multiplicities are typically very small (<1), such that a detailed study of isotopic dependencies and energetics of cluster decay following spallation are difficult to achieve within the experimental parameters given.
Graduate and undergraduate students have become deeply involvedin the science underlying nuclear heavy-ion reaction mechanisms and decay modes by performing extensive simulation calculations with existing and modified computer codes. Their active involvement in the recent Catania experiment, in theoretical simulations, and in the development of advanced digital electronics models has been successful and very encouraging to all.The following provides a summary of the main aspects of our research accomplishments as documented by the different contributions to this Annual Report:
Important trends in the evolution of heavy-ion scattering and reaction mechanisms with bombarding energy and impact parameter have been reviewed. A remarkably smooth continuation of low-energy trends has been observed, justifying the term “morphing”. Some of the essential features of dissipative reactions that appear preserved at E/A = 50-62 MeV are dissipative orbiting and the presence and consequences of multi-nucleon exchange. The relaxation of the A/Z asymmetry with impact parameter is slow. Non-equilibrium emission of light particles and clusters is an important process accompanying the evolution of the mechanism. Evidence is presented for a new mechanism of statistical cluster emission from hot, metastable primary reaction products, driven by surface entropy. These results suggest a plausible reinterpretation of multi-fragmentation in terms that differ from conventional views of a liquid-gas phase transition.
A case in point is presented by the observed scenario working in the 197Au+86Kr reactions, which were studied atbombarding energies of E/A =38.7 and 54.8 MeV. The total reactioncross section, deduced from elastic scattering, as well as manyfeatures of projectile-like fragment (PLF) emission are foundto be directly extrapolated from low-energy systematics. Correlations between PLFdeflection angle and dissipated energy, characteristic ofdissipative orbiting, are still observed at 54.8 MeV per nucleon. Apparently, an expected change to repulsive nucleus-nucleus interactions has not (yet) taken place. These features are quantitatively well reproduced bycalculations with a group-developed reaction code based on the stochastic nucleon exchange model (NEM). They have beencombined with equilibrium-statistical model simulations of thesequential decay of primary PLF fragments, which is not considered in the primary reaction model. For example, the reconstructed meanatomic number of these primary PLFs is found to be nearly equal tothe atomic number of the projectile, as predicted by the NEM.It has been very encouraging to notice that the data are very sensitive to the underlying reaction mechanism. For example, simulation calculations based on a Boltzmann-Uehling-Uhlenbeck(BUU) model are not successful in reproducing the data. Overall,the reaction dynamics exhibit gradual changes with bombardingenergy and impact parameter.
The jointmultiplicity distributions of neutrons andlight charged particles (LCPs) measured with the unique SuperBall/Dwarf calorimeter provide interesting insights into the reaction mechanism. For the same two 197Au+86Kr reactions referred to above, these multiplicity distributions essentially overlap, althoughan approximately 1-GeV difference exists in available energy for the twobombarding energies. Statistical-model calculations for the decayof massive primary reaction fragments reproduce thisbombarding-energy invariant scaling of the joint multiplicitydistributions as functions of excitation energy. Explicit information provided by the experiment on the evolution of LCP emission patterns as functions of totalexcitation and bombarding energy suggest that statistical emissionfrom primary projectile-like fragments (PLFs) and target-likefragments (TLFs) are the dominant sources of LCPs in 197Au+86Kr reactions, assuming that “missing” energy is carried away by cluster emission from the primary fragments. The study provides additional evidence for slightly “morphed” dissipative reaction mechanism at Fermi bombarding energies.
Mechanisms of particle productions have also been studied for the heavy system 209Bi+136Xeat E/A = 28, 40, and 62 MeV. As for the 197Au+86Kr reactions, correlations between the kinetic energy and the deflection angleof projectile-like fragments feature the characteristic dissipative orbiting.The reaction cross section is dominated by dissipative binaryreactions of well-defined projectile- and target-likefragments. On the other hand, detailed information is now available for the 209Bi+136Xe system on a prompt cluster emission mechanisms associated with intermediate kinematics (“intermediate velocity source,” IVS). The Galilei-invariant velocitydistributions of various charged reaction products, show clearly presence and evolution with energy dissipation, of this third, intermediate-velocity source of emittedfragments. While the production of light-charged particles can beattributed mainly to evaporation from excited projectile- andtarget-like fragment, IVS mechanism appearsto be responsible for a large fraction of the observed intermediate-massfragment yields, for high and low excitation energies. A plausible scenario has such IVS clusters emitteddynamically in the overlap zone of the projectile and targetmatter distributions.
Some interesting information has been obtained from an exploratory experiment at LNS Catania on the 112Sn+56,62Ni reactions at E/A=35 MeV, which has mostly led to a better technical understanding of 4 devices at that laboratory. In the 112Sn+56,62Ni reaction, classes of events have been observed, in which correlations between atomic numbers of coincident fragment pairs are evident. The tentative explanation of this effect invokes the prompt splitting of Ni-like fragments in this reaction, reminiscent of earlier observations made by GSI groups at lower energies.
Progress has been made by the group also in the modeling of the statistical decay of hot, expanded nuclei. Including surface entropy effects and thermal expansion of hot nuclei, the statistical decay code GEMINI is able to predict significant branching ratios for cluster emission and fission. The code was modified to include these surface entropy effects, as well as a mass-dependent time delay in the decay process, well known from fission studies. Results from the modified and unmodified codes are presented and discussed, with emphasis on branching ratios for clusters and light particles.
These practical exercises are based on the realization that the expansion following thermal excitation of a nucleus changes significantly the number of states available for the various configurations of a hot nuclear system. Analytical and numerical studies of effects resulting from this phenomenon have been conducted in the framework of the Harmonic-Interaction Fermi Gas Model (HIFGM), combined with Weisskopf’s detailed-balance approach to statistical nuclear decay widths. It has been found that thermal expansion of nuclear matter leads to significant retardation of light-particle emission, extending the validity of the approach to a domain of total excitation energies in excess of E*/A=8 MeV.
The same HIFGM approach has been utilized to study the behavior of excited metastable nuclear systems, in the context of experimental signatures of multi-fragmentation (cluster decay) that are often attributed to hypothetical phenomena associated with a nuclear liquid-gas phase transition in nuclear matter. It is shown that signatures such as the existence of limiting temperatures for metastable systems, the existence of negative heat capacities, and the enhanced production of multiple clusters in heavy-ion collisions have a natural explanation within the HIFGF.
As an application of these model ideas, the caloric curve for mononuclear configurations is studied with emphasis on the dependence of the entropy of a hot nucleus on matter density and effective mass. It is shown that a plateau in the caloric curve is a consequence of decreasing density and the decay of correlations due to expansion, but does not indicate a phase coexistence of nuclear matter. At relatively low excitations, hot nuclei are metastable against binary fission, while at higher excitations instability with respect to multi-fragmentation sets in.
In attempts to study the evolution of nuclei modeled in the Boltzmann-Uehling-Uhlenbeck transport equation towards equilibrium, the properties of the BUU ground state have been investigated. Different initial density and associated momentum distributions have been employed to study the stability of the BUU ground state and the emission rates for particles emitted from this state. Temporal limits for metastability of this state have been established.
This summer, the first in a series (CECIL) of heavy-ion experiments, aiming at investigating cluster emission in heavy-ion reactions, has been undertaken at the LNS Catania, using the CHIMERA multi-detector. The specific goal of the project is to study experimentally the problem of nuclear multi-fragmentation, particularly theeffects of surface entropy, thermal expansion, and of a mass-dependent time delay. CHIMERA is a 4 detector that provides information on reaction products to be extracted by means of timeof-flight, ~E/E, and pulse shape discrimination techniques. The high granularity and particle identification capabilities of the detector will be primarily employed to study the decay of clusters from excited projectile-like fragments in peripheral 112Sn+112 Sn collisions at E/A=35 and 43.7 MeV. Illustrations are given of the data collected in the experiment.
The NESSI setup at the Forschungszentrum Jülich was used to measure neutron and charged-particle emission in proton-induced spallation reactions. Similar to our heavy-ion SuperBall/Dwarf setup, the NESSI detector provides event-by-event information on the deposited excitation energy. Data obtained for p-A reations on targets ranging from Al to U and proton energies between 0.8 and 2.5 GeV are compared to model predictions.
A proposal was generated for the upgrade of the CHIMERA multi-detector aiming at an improved A-Z particle identification. Successful tests of specific pulse shape identification methods suggest that a corresponding upgrade of the detector is feasible and warranted. A survey over some recent experimental results obtained with CHIMERA is also given.
Firmware has been developed for the ICC8K intelligent CAMAC controller designed and built earlier by the group. The module can be operated from a PC via the standard USB-2 port. The firmware consists of parts servicing the “master” Field Programmable Gate Array and on for the USB-2 controller IC. Tests demonstrated functionality with respect to all standard CAMAC operations. A Visual Basic GUI was developed to facilitate the use of the controller module in various performance tests.
During the Academic Year 2003/4, the Principal Investigator spent approximately 40% of his time to research and student training supported by this grant, increasing to 90% during summer of 2004. The Senior Scientist (J.T.) and the Research Associate (L.P, J.L.,) spent 100% of their efforts on these research projects (Dr. Ludwik Pienkowski until his departure in February 2004 and Dr. Jun Lu until July 2004, when he started a new position in a medical-imaging R&D team).
During summer of 2004, all graduate students and one undergraduate student worked on grant research. Mr. Mark Houck has worked full time on grant research and has had a very successful PhD experiment this last summer at LNS Catania. Ms. Iwona Pawelczak, who participated in that experiment, has finished her teaching assistantship and has since worked 100% on grant research. Mr. Zachary Chambers joined the group in Spring of 2004, has been partially supported by a teaching fellowship, but spent 20-20% on grant research during the academic year. He worked full time in grant research during summer of 2004 and has also participated in the Catania experiment.Mr. David Kostecke has worked on grant research this last year. However, battles with chronic health problems have limited his progress. He will be leaving the group later this year. We wish him all the best in his future endeavors.