Emission properties of fission fragments and their isomeric ratios

The properties of neutron emission from fragments formed in the spontaneous fission of ²⁵²Cf and in the thermal-neutron-induced fission of ²³⁵U are analyzed on the basis of the statistical model of nuclear reactions. Upon extracting the mean excitation energies of fission fragments from experimental data on the mean multiplicities of neutrons, the observables of neutron emission can be described over wide ranges of total kinetic energies and masses. The observed values of mean fragment spins are also reproduced. A method for calculating the isomeric ratios of the independent yields of fission fragments that is based on the cascade-evaporation model of excited-nucleus decay is employed to describe experimental data on ²³⁵U fission induced by thermal neutrons and on ²³⁸U fission induced by alpha particles. The effect exerted on the isomeric ratios for fission fragments by two different assumptions on the spin distributions of primary-fragment populations—the assumption of the distribution associated with rotational degrees of freedom and the assumption of the distribution associated with the internal degrees of freedom of fully accelerated fragments—is investigated.