Estimations of level densities and radiative strength functions from experimental data and probable change in the properties of nuclei at E exc ⋍ 3–4 MeV

Abstract

Investigations of two-step γ cascades following thermal-neutron capture by heavy nuclei reveal that the density of excited levels is significantly smaller than that which is predicted by an exponential extrapolation like that of the back-shifted Fermi gas model. Data on two-step cascades allow one to determine in a model-independent way the most probable energy dependences of the level density and radiative strength functions, virtually over the whole excitation-energy interval below the neutron binding energy B n . Data for more than 30 heavy nuclei cannot be understood without assuming a considerable change in the nuclear properties at E exc ⋍ 3–4 MeV.