Energy dissipation

Abstract

The energies dissipated by ²³³Th, ²³⁹U and ²³⁶U nuclei in their descent from saddle-point to scission-point configurations are deduced from variations in the ratios of Cd-to-Ba and Ag-to-Ba yields measured as a function of neutron energy incident to the respective targets. Values of 1.5 ± 0.5, 4.5 ± 1 and 5 ± 2 MeV are deduced for the three fissioning systems, respectively. The method and assumptions made in deducing the energies are reviewed. The relatively small values of the energies are discussed in tends of other recent experimental and theoretical results.

Little or no evidence for shell effects has been observed heretofore in the fission of nuclides with mass and charge less than those of ²²⁶Ra. We have measured the total kinetic energy E_K release as a function of mass M in the fission of ²¹⁶Rn and ²²⁰Ra formed in the ²⁰⁹Bi + ⁷Li and ²⁰⁸Pb + ¹²C reactions at ≈18 MeV above the fission barriers. These measurements indicate an anomalously low E_K at symmetry. This phenomenon is attributed to a shell structure in the nascent fragments for neutron number N=66 at a large deformation (β=0.6). Structure in the E_K VS M distribution near M=136 is evidence for a spherical shell associated with N=82. The E_K measurements for the ²⁰⁸Pb + ¹²C reaction have been made at several bombarding energies. As the excitation energy is increased, E_K near symmetry increases and the structure in the E_K VS M distribution disappears, indicating a “washing-out” of the shell structures. Structure in the standard deviation of E_K with M distribution is further evidence of shell effects in these fissioning systems.

Work performed under the auspices of the Office of Basic Energy Sciences, Division of Nuclear Sciences, U. S. Department of Energy under contract number W-31-109-ENG-38.