Parameters of Distribution of the Primary Gamma-Transition Intensities Following Resonance Neutron Capture and Some Properties of Compound Nuclei $^{157,159}$Gd

Abstract

The re-analysis of the published experimental data on the primary gammatransition intensities following neutron capture in different groups of neutron resonances in 156,158 Gd has been performed. There are determined the most probable values of sum of E1 and M1 primary transitions, numbers of excited by them levels of both parities, ratios of radiative strength functions k(M1)/k(E1), dispersions of deviations of random values of intensities from the average and ratios of mean intensities of primary transitions to levels J=5/2 with respect to analogous data for J=1/2 and 3/2 (capture of the 24 keV neutrons) in narrow excitation energy intervals.All the data on level density and sums of radiative strength functions confirm the presence of clearly expressed step-like structure in level density below 3 MeV and general trend in change in strength functions as changing primary gamma-transition energy. Variations of distribution dispersions and, especially, ratio k(M1)/k(E1) (or k(E1)/k(M 1)) at changing excitation energy point to strong change in structure of these nuclei above 1.0-1.5 MeV.As in neighboring isotopes 156,158 Gd, the shape of energy dependence of k(M 1) + k(E1) considerably differs as changing nuclear mass. This can be due to dependence of gamma-decay process on structure of neutron resonance and/or levels excited by gamma-transitions. The dilemma appeared can be solved only in direct experimental search for structure of neutron resonances in region of their energy of about two nucleon pairing energy in nuclei of corresponding mass.