Spin-dependent observables in surrogate reactions

Several quantities that can be observed in surrogate ratio methods (SRM) are investigated for nuclei of uranium region to see if they can be used to infer the spin-and-parity distributions that are crucially important to the validity of SRM. The evaporation neutron spectra and cascading γ-ray spectra are calculated by the Hauser-Feshbach theory, while fission fragment mass distributions are calculated by the fluctuation-dissipation theorem. It was found that the energy spectrum of cascading γ rays and those of evaporated neutrons become softer as the spin of the decaying nuclei increases. The multiplicity of emitted γ rays increases drastically as the spin increases. Both these effects are brought about by the fact that transition from high-spin states to low-lying levels are hindered. The mass distribution of fission fragments also shows a clear spin dependence, namely, the mass-symmetric fission is enhanced when the spin of the decaying system is large. This can be interpreted as an enhancement of mass division controlled by the liquid-drop energy relative to the shell correction part. The present results indicate that there are possibilities to use these observables to infer populated spin distributions in the surrogate reactions.