K. Tso

Time-scale and Branching Ratios in Sequential Multifragmentation

Abstract Experimental intermediate-mass-fragment multiplicity distributions are shown to be binomial at all excitation energies. From these distributions a single binary event probability can be extracted that has the thermal dependence p = exp [ -B T ] . Thus, it is inferred that multifragmentation is a sequence of thermal binary events. The increase of p with excitation energy implies a corresponding contraction of the time-scale and explains recently observed fragment-fragment Coulomb correlations.

Reducibility and Thermal Scaling in Nuclear Multifragmentation

Abstract Recent studies have revealed the existence of a number of reducibility and thermal scaling properties in nuclear multifragmentation. The probability of emitting n -fragments is found to be reducible to the probability of emitting a single fragment through the binomial expression. The resulting one fragment probability shows thermal scaling by producing linear Arrhenius plots. Similarly, the charge distributions associated with n -fragment emission are reducible to the one-fragment charge distribution. Thermal scaling is also observed. The reducibility equation contains a constant whose value, zero or positive, can be related to a univariant (two phase) or bivariant (one-phase) regime. The light fragment particle-particle angular correlations also show reducibility to the single-particle angular distributions as well as thermal scaling . A mass scaling associated with the angular correlations suggests emission from several small sources ( A ≈ 20). The limits of applicability of scaling and reducibility are discussed as well as their implications for the mechanism of multifragmentation.

Transition state rates and mass asymmetric fission barriers of compound nuclei 90,94,98Mo

Abstract Excitation functions were measured for complex fragments with atomic number Z = 5–25 emitted from the compound nuclei 90,94,98Mo produced in the reactions 78,82,86 Kr + 12 C . Mass-asymmetric fission barriers were extracted by fitting the excitation functions with a transition state formalism. The extracted barriers are several MeV higher on average than the calculations of the Rotating Finite-Range Model and substantially lower than predicted by the Rotating Liquid Drop Model. The symmetric fission barriers measured support the hypothesis of a congruence term that doubles for the fission of strongly indented saddle-point shapes. The excitation functions were analyzed to search for atomic number Z- and energy E-dependent deviations from transition-state-method predictions. All of the measured excitation functions can be scaled onto a single universal straight line according to the transition-state predictions. No Z- and/or E-dependent effects that could be attributed to transient effects are visible.

Stable Coulomb Bubbles

Coulomb bubbles, though stable against monopole displacement, are unstable at least with respect to quadrupole and octupole distortions. We show that there exists a temperature at which the pressure of the vapor filling the bubble stabilizes all the radial modes. In extremely thin bubbles, the crispation modes become unstable due to the surface-surface interaction. {copyright} {ital 1997} {ital The American Physical Society}

Scaling laws in 3He induced nuclear fission.

Nuclear Science Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720(February 8, 2008)Fission excitation functions of compound nuclei in a massregion where shell effects are expected to be very strong areshown to scale exactly according to the transition state pre-diction once these shell effects are accounted for. The factthat no deviations from the transition state method havebeen observed within the experimentally investigated exci-tation energy regime allows one to assign an upper limit forthe transient time of 10

Excitation functions and mass asymmetric fission barriers for compound nuclei 70,76Se

Abstract Excitation functions were measured for complex fragments with atomic number Z=5−20 emitted from the compound nuclei 70,76 Se produced in the reactions 58,64 Ni + 12 C. Mass asymmetric fission barriers were extracted by fitting the excitation functions with a transition state formalism. The extracted barriers were compared with those calculated from macroscopic nuclear models. The measured barriers for symmetric fission seem to support the hypothesis of a shape-dependent congruence energy, which doubles for fission of strongly indented saddle-point shapes. All of the measured excitation functions can be scaled onto a single straight line according to the transition state prediction.

Statistical interpretation of the correlation between intermediate mass fragment multiplicity and transverse energy

Multifragment emission following {sup 129}Xe+{sup 197}Au collisions at 30A, 40A, 50A, and 60A MeV has been studied with multidetector systems covering nearly 4{pi} in solid angle. The correlations of both the intermediate mass fragment and light charged particle multiplicities with the transverse energy are explored. A comparison is made with results from a similar system {sup 136}Xe+{sup 209}Bi at 28A MeV. The experimental trends are compared to statistical model predictions. {copyright} {ital 1999} {ital The American Physical Society}

Multifragmentation: Surface instabilities or statistical decay?

Boltzmann-Nordheim-Vlasov calculations show multifragmentation that seems to originate from surface instabilities. These instabilities are traced to a sheet instability caused by the proximity interaction. Experimental data, on the other hand, suggest that multifragmentation may be dominated by phase space.

Surface instabilities and nuclear multifragmentation

Central heavy-ion collisions, as described by a Boltzman-Nordheim-Vlasov calculation, form nuclear disks that break up into several fragments due to surface instabilities of the Rayleigh-Taylor kind. We demonstrate that a sheet of liquid, nuclear or otherwise, stable in the limit of infinitely sharp surfaces, becomes unstable due to surface-surface interactions. The onset of this instability is determined analytically. The relevance of these instabilities to nuclear multifragmentation is discussed.

Interpreting multiplicity-gated fragment distributions from heavy-ion collisions

In recent years, multifragmentation of nuclear systems has been extensively studied, and many efforts have been made to clarify the underlying physics. However, no clear consensus exists on the mechanism for multifragmentation. Is the emission of intermediate mass fragments (IMF: 3 {le} Z {le} 20) a dynamical process (brought on by the occurrence of instabilities of one form or another) or a statistical process (i.e. the decay probabilities are proportional to a suitably defined exit channel phase space)? Historically the charge (mass) distribution has played and still plays a very important role in characterizing multifragmentation. Since this subject`s inception, the near power-law shape of the charge and mass distributions was considered an indication of criticality for the hot nuclear fluid produced in light ion and heavy ion collisions. Here, the authors have studied different aspects of the charge distributions. The implications of the experimental evidence presented here are potentially far reaching. On the one hand, the thermal features observed in the n-fragment emission probabilities for the {sup 36}Ar + {sup 197}Au reaction extend consistently to the charge distributions and strengthen the hypothesis of the important role of phase space in describing multifragmentation. On the other hand, they have investigated charge correlation functions of multi-fragment decays to search for the enhanced production of nearly equal-sized fragments predicted in several theoretical works.