Strong absorptions of thermal neutrons caused by virtual states

Abstract

Abstract Cross sections of compound nucleus (CN) formation by S-wave neutron scattering, which began to be studied in a pioneering work by Feshbach et al., are reanalyzed by the optical model plus the method of Jost function (JFM), which is one of modern theories to handle bound and unbound continuum states in a unified manner. We have calculated the CN cross-section for a thermal neutron scattering (En = 25.3 meV) by about 300 stable targets. The enhancements in the CN cross section at the specific mass number region, A ∼ 10, 50 and 160, which are well-known results in the previous optical model analyses, are reproduced by the present calculation. The JFM calculation is applied to investigate the origin of the enhanced CN cross-section, and the poles in the scattering matrices (S-matrices) are systematically explored in the complex momentum plane. In JFM, the S-matrix poles representing the virtual states, which are different from the usual resonances described by the Breit-Wigner formula, appear around the origin (zero energy) in the momentum plane at the mass region with the enhanced CN cross-section. The present analysis of the S-matrix means that the virtual state strongly enhances the CN cross-section. We also discuss the present results in connection to giant resonances and compound nuclear resonances, which were regarded as the established theories to understand the enhanced absorption of the thermal neutron in the previous studies.