Distributions of fission products from various low-energy fission reactions and the systematics of the odd-even fluctuations

Abstract

The dependence of the proton odd-even effect on the excitation energy of the fissioning nucleus and the mean kinetic energy of the fission fragments is examined in thermal neutron fission of $^{239}\mathrm{Pu}$ and in "fast" neutron fission of $^{235}\mathrm{U}$, $^{238}\mathrm{U}$, and $^{232}\mathrm{Th}$. The proton odd-even effect observed in thermal neutron fission is found to decrease with increasing excitation energy and/or with increasing atomic number of the fissioning even $Z$ nucleus. Possible factors responsible for the magnitude of the odd-even effect are discussed. An evaluation is made of a neutron odd-even effect, based on the observed mean proton odd-even effect, and consequently the mass distribution of fission fragments prior to neutron emission is calculated. A good fit is found between the calculated and the measured fragment mass yield curves and between calculated and measured neutron emission as a function of mass.NUCLEAR REACTIONS, FISSION $^{235}\mathrm{U}$($n,f$), $^{239}\mathrm{Pu}$($n,f$), $^{238}\mathrm{U}$($n,f$), $^{232}\mathrm{Th}$($n,f$), calculated odd-even effect; deduced systematics of the odd-even effect.