Deviations from the Statistical Description of Neutron Level Spacing Distributions and Stabilizing Effects of Nuclear Shells in Positions of Nuclear Excited States

Abstract

It is well known that the statistical model gives reliable description of properties of nuclear states at high excitation, say, in heavy nuclei. Statistical model is based on the complexity of nuclear forces, that results in the great number of nuclear models at present. Combined consideration of the existing experimental data on the level positions (E*) in many nuclei (in the broad scope of nuclei) shows rather complex distributions. But only analysis of experimental data on excitation levels can give the answer to the question if the energies are random or they are not fully independent (in different nuclei and shells). In principle it may be the case that the comparison of the spectra of different nuclei (including highly excited states) will show systematic deviations from statistical distributions, for instance, from Wigner distribution, in case of the neutron levels of a given nucleus. This paper is a review of studies of deviations of the level spacings in complex nuclei from statistical distributions carried out to date. The major part of the paper (two first sections) deals with analysis of l)hyperfine structure — of distinguishing effect of certain spacings in neutron resonances of heavy nuclei (energy of the order of tens of ev).