Experimental manifestations of the effect of assumed breaking of nucleon cooper pairs in various nuclei

摘要

From a comparison of the absolute intensities of individual two-step cascades with the known intensities of their primary and secondary transitions of thermal neutron capture, the cascade and total population of similar to 100 levels have been determined for each of the following nuclei: K-40, Co-60, Ge-74, Cd-114, Sn-118, Te-124, Ba-137,Ba-138, Sm-150, Gd-156,Gd-158, Dy-165, Er-168, Yb-175, Hf-181, W-183,W-184,W-185,W-187, Pt-196, and Hg-200. These experimental results, as well as the intensities of two-step cascades to low-lying levels for the same nuclei, are reproduced most accurately when calculating using a level density with a pronounced steplike structure. In this case, the radiative strength functions of the primary transitions of compound state decays to levels in the region of such a structure are significantly strengthened in comparison with the strength functions for decays to upper and lower lying levels. The level populations of nuclei with any nucleon parity below approximate to 3-5 MeV can be reproduced only by taking into account a significant local strengthening of the radiative strength functions of secondary transitions to levels near kinks in the energy dependence of the level density, as well as a rather significant decrease for transitions to lower lying nuclear states. A simultaneous strong local change in the level density and radiative strength functions of all the cascade gamma transitions in a small region of nuclear excitation corresponds to the definition of a second-order phase transition.