Y. Lou

Production and characterization of hot nuclei in the reactions of 19 and 35 MeV/u 14N with 145Sm

Abstract Hot nuclei produced in the reactions of 261 MeV and 490 MeV 14 N with 154 Sm have been studied. The initial properties of these nuclei; excitation energies, angular momenta and temperatures, have been characterized through measurements of residue velocities, gamma ray multiplicities and α-particle energy spectra. Nuclei with excitation energies as high as 400 MeV and temperatures as high as 6 MeV are produced. Determinations of the variation of temperature with excitation energy for nuclei of A ⋍ 160 indicate that the apparent level density parameter a , defined as E ∗ |T 2 changes from A /8 at low energies to ≈ A /13 at 400 MeV excitation energy. Effective particle emission barriers suggest large shape fluctuations during the de-excitation cascade. At 35 MeV/u the variation of angular momentum transfer with linear momentum transfer in incomplete fusion reactions is in reasonable agreement with values calculated using a geometric overlap model.

Determination of the temperatures of hot nuclei from “first chance” emission spectra

Abstract The “first chance” spectra of light charged particles and neutrons emitted from 160Yb produced in the 60Ni+100Mo reaction have been isolated. The same initial temperature for the different evaporated particles and a decrease of the Coulomb barriers for the charged particles are observed. The inverse level density parameter K= A/ a reaches a value of 13.8±0.7 MeV at ETH≈236 MeV, confirming the decrease of the parameter a at excitation energies above 150 MeV.

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.

Entrance channel mass asymmetry and the decay of hot nuclei

Abstract The reactions 10 MeV/u 86 Kr + 76 Ge and 27 MeV/u 16 O + 150 Sm were used to form the same composite system with A ∼ 160 at about 300 MeV of excitation energy, as derived from measurements of both the linear momentum transfer and the neutron multiplicity. The comparison of the evaporative spectra of the α particles in coincidence with evaporation residues does not show evidence for effects related to the entrance channel of the reaction, such as those predicted by dynamical models.

The dependence of fission time-scales on fragment mass asymmetry in the 20Ne+165Ho reaction at 50A MeV

The pre-scission and post-scission multiplicities of alpha particles were determined in coincidence with fission fragments for four windows of exit channel mass asymmetry in the reaction 20Ne+165Ho at 50A MeV. The technique used employed a kinematic analysis of the energy spectra of alpha particles as a function of the emission angle with respect to the fission axis. Using a procedure of fitting the experimental energy spectra with those resulting from a Monte Carlo simulation program, the total multiplicity of alpha particles was separated into pre-scission and post-scission components. Using the pre-scission and post-scission multiplicities, neutron multiplicity measurements and other empirical observations, both the initial excitation energy and the excitation energy at scission of the compound nucleus were determined. The pre-scission times in the de-excitation of a highly excited 178W compound nucleus with initial excition energy of 570 MeV were evaluated using statistical model calculations. Only a small decrease in scission time was derived for the most asymmetric events studied. The possible contribution of very damped deep inelastic processes is discussed.