B. D. Wilkins

Fission barriers and half-lives of the trans-radium elements

Abstract Spontaneous fission half-lives and fission barriers are calculated for the elements with 88 ≦ Z ≦ 108. The method is semi-empirical, based on the liquid drop model with modifications for the effects of nuclear structure. Spontaneous fission stability is predicted to decrease for nuclei just beyond the N = 152 neutron subshell, but should increase regularly for neutron numbers greater than 160. This effect should become quite strong as the N = 184 neutron subshell is approached, resulting in a number of experimentally observable isotopes in the mass region A ≈ 270−290. The results are discussed briefly with regard to problems of nucleo-synthesis as they pertain to the discovery of new isotopes and heavy element generation in stellar reactions.

Mass, charge and energy distributions in the very asymmetric fission of 249Cf induced by thermal neutrons

Abstract Fission yields of fragments with mass numbers A = 69–87 and fragment kinetic energies for A = 74–87 in the very asymmetric neutron induced fission of 249 Cf have been measured using the mass separator Lohengrin at the Institut Laue-Langevin in Grenoble. The chain yields observed in this very light mass region are considerably higher than predicted. An evaluation of the isotopic yields shows a strong increase of the proton odd-even effect from 7% to 17% with increasingly asymmetric fission. The neutron odd-even effect is increasing from 4% to 10%. These observations and a narrowing of the average nuclear charge distribution can be explained by a decreasing excitation energy with increasing asymmetry of the fission process.

Binary decay of 56Ni formed in the 32S

Fully energy-damped yields from the {sup 32}S+{sup 24}Mg reaction have been measured at center-of-mass energies of {ital E}{sub c.m.}=51.6 and 60.5 MeV with the use of an experimental arrangement where both of the resulting heavy fragments could be detected in coincidence. Energy, velocity, and angular distributions of the reaction fragments have been determined. The cross sections prior to secondary light-particle emission have been deduced for the breakup of the compound system into different mass channels. These data are discussed in terms of two possible reaction mechanisms: fusion followed by fission and deep-inelastic orbiting.

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.

Distribution of reaction strength in32S+182W collisions

The reaction /sup 32/S+ /sup 182/W has been studied at beam energies of 166, 177, 222, and 260 MeV with the goal of accounting for the distribution of the total reaction cross section among the main reaction channels. Cross sections as well as mass and energy distributions were measured over the entire angular range from 10/sup 0/ to 170/sup 0/ for elastic (which includes quasi-elastic), deep-inelastic, and fission-like products. Deep inelastic scattering accounts for a substantial fraction of the total reaction cross section at all energies, in particular near the barrier. This behavior is not predicted by current theories such as the extra push model. The angular distributions of fission-like products are more anisotropic than expected on the basis of the saddle point model at the two lowest energies, indicating a sizable contribution from quasi-fission processes, which is estimated from a quantitative analysis of this deviation.

Fusion evaporation-residue cross sections for sup 28 Si+ sup 40 Ca at E ( sup 28 Si)=309, 397, and 452 MeV

Velocity distributions of mass-identified evaporation residues produced in the {sup 28}Si+{sup 40}Ca reaction have been measured at bombarding energies of 309, 397, and 452 MeV using time-of-flight techniques. These distributions were used to identify evaporation residues and to separate the complete-fusion and incomplete-fusion components. Angular distributions and upper limits for the total evaporation-residue and complete-fusion evaporation-residue cross sections were extracted at all three bombarding energies. The complete-fusion evaporation-residue cross sections and the deduced critical angular momenta are compared with earlier measurements and the predictions of existing models. The ratios of the complete-fusion evaporation-residue cross section to the total evaporation-residue cross section, along with those measured for the {sup 28}Si+{sup 12}C and {sup 28}Si+{sup 28}Si systems at the same energies, support the entrance-channel mass-asymmetry dependence of the incomplete-fusion evaporation-residue process reported earlier.

Mass Yields in the Very Asymmetric Fission of 249Cf(nth,f)

Independent fission yields and fragment kinetic energies in the very asymmetric (A=87−72) fission of 249Cf have been measured using the mass separator LOHENGRIN at the Institut Laue-Langevin in Grenoble. The mass yields were found to be up to a factor of 100 higher than predicted in the US ENDF-V file. A preliminary evaluation of the isotopic yields shows a strong increase of the proton odd-even effect from 7 % to 15 %. This can be explained by a decreasing excitation energy in this very asymmetric region.

Distribution of reaction strength in /sup 32/S+/sup 182/W collisions

The reaction /sup 32/S+ /sup 182/W has been studied at beam energies of 166, 177, 222, and 260 MeV with the goal of accounting for the distribution of the total reaction cross section among the main reaction channels. Cross sections as well as mass and energy distributions were measured over the entire angular range from 10/sup 0/ to 170/sup 0/ for elastic (which includes quasi-elastic), deep-inelastic, and fission-like products. Deep inelastic scattering accounts for a substantial fraction of the total reaction cross section at all energies, in particular near the barrier. This behavior is not predicted by current theories such as the extra push model. The angular distributions of fission-like products are more anisotropic than expected on the basis of the saddle point model at the two lowest energies, indicating a sizable contribution from quasi-fission processes, which is estimated from a quantitative analysis of this deviation.

Nuclear reactions of /sup 197/Au with 11. 5- and 300-GeV protons. [Formation cross sections of >60 nuclides]

The formation cross sections of more than 60 nuclides produced in the reaction of 11.5- and 300-GeV protons with

Nuclear Reactions of Au-197 with 11.5-GeV and 300-GeV Protons.

^{197}\mathrm{Au}