Giant resonances and neutron-nucleus total cross sections

Abstract

Optical model calculations of the total cross sections for neutron-nucleus scattering are examined in detail to determine whether Lane-Thomas-Wigner giant resonances in the strength functions ever cause corresponding maxima in the energy-dependence of the energy-averaged total cross sections. Although peaks of this nature do occur in certain partial-wave cross sections, non-resonant maxima of a completely different origin are present in other partial waves at nearby energies, so that the maxima observed in the total cross section are found to be complex superpositions of resonant and nonresonant single-particle effects. The nonresonant effect is dominant in the overwhelming majority of cases and causes the observed maxima to move to higher energy with increasing A for almost all elements. The principal exceptions occur at A ≈ 48 and A ≈ 96, where giant resonance peaks in the familiar 2d 5 2 and 3p 3 2 states may become large enough to visibly interrupt this trend. The energy dependence of the S-wave cross sections shows no resonance peaks whatever.