[en] Impact parameter dependent excited state populations of intermediate mass fragments are investigated for ³⁶Ar induced reactions on ¹⁹⁷Au at E/A=35 MeV. Population inversions, indicative of non-thermal excitation mechanisms, are observed in peripheral collisions characterized by low associated charged particle multiplicities. These population inversions disappear for collisions with larger associated charged particle multiplicities, consistent with a more complete thermalization for more complex final states. Discrepancies, observed in central collisions, suggest that the limit of local thermal equilibrium has not yet been observed. (orig.)

[en] The fragmentation of E/A=40 MeV ²⁰Ne projectiles into exit channels containing only p, d, t, ³He, α, ⁶He, ⁶Li, and ⁷Li fragments has been investigated. Relative shifts between the average longitudinal velocity of each fragment type were measured with ¹²⁰Sn and ¹⁹⁷Au targets. A post-breakup Coulomb acceleration component of these shifts was isolated. Its magnitude indicates that the projectile broke up while in near contact with the target. On the other hand, correlations between projectile fragments indicate that a significant fraction of these fragments are produced by the sequential decay of long- and short-lived unstable intermediates. A fully sequential decay process, although consistent with the experimental relative angle distributions, fails to reproduce the correlation function. Thus, a picture emerges of a prompt breakup step producing stable and unstable particles with the subsequent decay of the latter extending over a long time period

[en] Differential cross sections for evaporation residues and fission fragments for 35A, 100A, 130A and 155A MeV ¹⁴N on targets ranging from ¹⁵⁴Sm to ¹⁹⁷Au have been measured. The angle-integrated cross sections are larger than what might be expected. The fission fragment-fission fragment folding angle correlations for 35A, 100A MeV ¹⁴N and 25A MeV ¹⁶O on similar targets were also measured. The average linear momentum transfer has been deduced from both the fission angle correlation and from the fore-aft asymmetry of the fission angular distributions in the laboratory system. The data are all consistent with a picture where pre-equilibrium particle emission removes an increasing fraction of the orbital angular momentum as the bombarding energy increases. This allows a large range of partial waves to contribute to formation of a composite nucleus with a finite fission barrier. copyright 1996 The American Physical Society

[en] The impact parameter dependence of light charged particle (p,d,t,α) emission has been studied using an impact parameter selection based on coincident detection of residues or fission fragments. The energy spectra at twelve angles between 20 degree and 150 degree have been fit by a multiple moving source parametrization. The angle and energy integrated preequilibrium proton multiplicities decrease with increasing impact parameter in qualitative agreement with a Fermi jet calculation. The preequilibrium d/p and t/p multiplicities increase slowly with increasing impact parameter and are nearly identical at the two bombarding energies. The preequilibrium α/p ratio shows a less consistent dependency on impact parameter but decreases significantly with increasing bombardment energy. A calculation of the d/p and t/p multiplicity ratios with a transport model incorporating complex particle emission is quite successful in reproducing the absolute magnitude, impact parameter dependence, and bombarding energy dependence of the experimental total multiplicities. copyright 1998 The American Physical Society

[en] Multifragment decays of central collisions in ⁸⁴Kr+¹⁹⁷Au at E/A=70 MeV are studied. By utilizing a technique sensitive to the emission order of fragments, it is deduced that carbon fragments are emitted prior to beryllium fragments when these fragments have the same velocity. This observation is consistent with the cooling of a thermally decaying source. copyright 1996 The American Physical Society

[en] Multifragment decays of central collisions in ⁸⁴Kr+¹⁹⁷Au at E/A=35, 55, and 70 MeV are studied. The dependence of the extracted emission time on the velocity of the fragment pair is investigated. More energetic pairs manifest a stronger Coulomb interaction, indicating emission from a source of smaller spatial-temporal extent than less energetic pairs. This trend can be understood in the context of a statistical model that allows the source characteristics to evolve as the fragments are emitted

[en] The spatial-temporal extent of the emitting system for central collisions in ⁸⁴Kr + ¹⁹⁷Au at E/A = 35, 55, and 70 MeV is probed using fragment-fragment velocity correlation functions. Selection on fragment pairs of high velocity yields a stronger fragment-fragment Coulomb interaction than for inclusive fragment pairs. This result is consistent with fragment emission from a source of decreased spatial-temporal extent. The universality of this association and the effect of different selection criteria are explored. The sensitivity of the spatial-temporal extent deduced by the correlation function technique to measurement uncertainties, assumed source characteristics, and rotational effects is assessed. copyright 1996 The American Physical Society

[en] The dependence of intermediate-mass-fragment (IMF) element distributions on the multiplicity, N_IMF, of detected fragments has been measured for ⁸⁴Kr+¹⁹⁷Au collisions at E/A=35, 55, 70, 100, 200, and 400 MeV. The observed dependence can be parametrized as P(N_IMF|Z)∝P(Z)exp(-c·N_IMF·Z), where c is a beam-energy and excitation-energy dependent parameter. Previous work indicated this parameter is zero in the liquid-gas coexistence region and positive in the gaseous phase. In contrast, we observe both negative and positive values for c, revealing the meaning of this parameter to be less straightforward than previously assumed. The magnitude of c appears nonetheless to provide a nontrivial test of multifragmentation models. copyright 1997 The American Physical Society

[en] Charge and transverse energy distributions for intermediate mass fragments have been extracted for central ⁸⁴Kr+¹⁹⁷Au collisions at E/A=35-400 MeV. The slopes of the measured fragment charge distributions decrease monotonically with incident energy, consistent with the expectations for highly charged systems, but not with recent critical exponent analyses. Statistical model calculations, which reproduce the experimental trends, suggest that post-breakup fragment secondary decays alter significantly the observed charge distributions. Radial expansion velocities extracted from these calculations follow the systematics of Au+Au collisions. copyright 1997 The American Physical Society

[en] The relationship between observed intermediate mass fragment and total charged particle multiplicities has been measured for ⁸⁴Kr + ¹⁹⁷Au collisions at energies between E/A=35 and 400 MeV. Fragment multiplicities are greatest for central or near-central collisions. For these collisions, fragment production increases up to E/A∼100 MeV, and then decreases at higher energies