M. Gui

Thermal properties and dynamics of hot nuclei

Abstract The heat capacities and the dynamics of hot medium mass nuclei have been explored by studies of light particle emission. The variation in heat capacity with temperature, expressed in terms of an effective Fermi gas level density parameter appropriate to the statistical description of nuclear de-excitation, corresponds to a decrease in a from A/8 to A/13 as the excitation energy increases from 1 to 2 MeV/nucleon. This result is compared to recent theoretical predictions. An apparent sudden change in the heat capacity observed at slightly higher excitation energies per nucleon is also discussed. Prescission and post-scission particle multiplicities and temperatures in coincidence with evaporation residues and fission fragments provide further information on the decay dynamics of hot nuclei. Possible means of isolating presaddle emission and of determining the temperature dependence of the fission barrier are discussed.

The determination of fission time scales from excitation function data

Abstract Fission excitation functions for 16 O+ 141 Pr, 165 Ho, 175 Lu and 197 Au, for 12 C+ 169 Tm and for 22 Ne+ 22 Ne+ 159 Tb have been analyzed to determine the excitation energy dependence of the fission probability. As excitation energy increases, this probability reaches a maximum and then decreases monotonically. The results provide new and detailed information on the time distribution of the fission probability and confirm that the bulk of the pre-scission emission occurs prior to the saddle point.

Dynamical and statistical properties of hot nuclei

Abstract Properties of medium mass nuclei having excitation energies of 1.5 to 6 MeV/u have been probed in measurements of the multiplicities and energy spectra of emitted particles and fragments. The particle spectra indicate a progressive reduction of both the level density parameter, a, and the emission barriers as the excitation energy is increased. The evolution of the dominant de-excitation modes, observed with increasing excitation energy, is found to be qualitatively consistent with the predictions of statistical models. There is, however, evidence that dynamics may play an important role in fragment emission processes. At energies of 6 MeV/u multifragment emission is an important process. Varying theoretical models predict such multifragment events but differ in their predictions of the probability of such events.

Light-particle evaporation as a function of neutron excess for medium-mass compound nuclei with Ex ≈ 2 MeV/u

Abstract Light-particle emission from the fission reactions of 63 Cu + 92,100 Mo and 20 Ne + 144,148,154 Sm has been studied using the Texas A&M Neutron Ball detector. Linearmomentuum transfers (LMT) were determined from fission-fragment-folding-angle measurements. An increase of neutron multiplicity with apparent linear-momentum transfer was seen. The initial excitation energies of the compound nuclei are estimated from the observed linear-momentum transfer and also reconstructed from the measured multiplicities and average energies of neutrons, protons and α-particles. The results of the two methods are in good agreement. The observed neutron multiplicities show a strong increase with increasing neutron-to-proton ratio of the composite system. The light-particle multiplicities are compared with calculated values using the statistical code GEMINI. The effect of the dynamic fission delay on the total light-particle multiplicities is explored and found to be small.

Dynamical aspects of intermediate mass fragment emission in the reaction of 32S + Ag at 3 A-MeV

Abstract The emission of intermediate mass fragments (IMFs) has been studied using the 4π array, AMPHORA. The energy spectra, the angular distributions, the multiplicities, and the charge distributions are studied inclusively as well as in coincidence with projectile-like fragments (PLFs) and other fragments. The low-energy component of the inclusive fragment spectra can be reproduced by a statistical binary decay code GEMINI, although the calculated absolute cross sections are an order of magnitude smaller than the experimental IMF cross sections. At intermediate angles the fragments are dominated by IMFs with Z ≤ 10 both for inclusive and coincidence events and the energy spectra of the fragments show hard components which cannot be explained as statistical emission. For this non-equilibrium component strong azimuthal angular correlations are observed in IMF-PLF and IMF-IMF coincidence events. Both the energy spectra and the azimuthal angular correlations of the non-equilibrium component are well reproduced by an extended classical dynamical model.

Mechanisms of non-equilibrium light-particle emission in 32S + Ag reactions at 30 A · MeV

Abstract Energy spectra of light charged particles (Z = 1,2) observed at θLP = 5°−150° in coincidence with fragments of Z = 5–14 at θF = ±9° are examined. The fragment-energy spectra show two components, one with a projectile-like velocity and another with a much lower velocity. Light particles in coincidence with fragments of projectile-like velocity show a very strong left-right asymmetry in the forward hemisphere. At very forward angles the asymmetry is essentially dominated by a recoil effect of the ejectile emitted from the fragment. The asymmetry at intermediate angles (27°⩽ θ ⩽ 60°) shows a strong dependence on the charge of the fragments and the asymmetry is similar among those for the composite particles, whereas the asymmetry is much smaller for protons. Light particles in coincidence with the lower-energy component of the fragment spectra show much less asymmetry. For such fragments α-particle energy spectra are reproduced without an intermediate-velocity source of high apparent temperature, whereas this high temperature source is needed for the proton spectra. These facts may indicate that the dominant emission mechanism of the intermediate-velocity protons and α-particles is different. Possible mechanisms for the production of these particles are discussed.

Dynamic restrictions on the de‐excitation of hot nuclei

Recent observations of particle emission in coincidence with heavy residues and with the fragments of binary decay processes provide information on both the statistical and the dynamical aspects of the decay of highly excited ‘‘Hot’’ nuclei. Evidence has been obtained for significant decreases in the nuclear level density with increasing excitation energy, for limiting temperatures which may signal a cluster phase in nuclear matter and for mass asymmetry dependent dynamic delays which invalidate simple statistical decay assumptions at intermediate excitation energies. A brief review of some such experiments is presented.

Statistical and Dynamical Aspects of Hot Nucleus De-Excitation

Recent experiments designed to explore fragment emission dynamics, time scales for fragment emission from highly excited nuclei, and multifragment emission processes are summarized. Reactions studied include 18.5 AMeV 136Xe + 48Ti, 30 AMeV 32S + Ag and 35 AMeV 40Ca + 40Ca. Nuclei with excitation energies of 3-6 MeV/u are produced in these reactions.