SHELL EFFECT ON ENERGY DEPENDENCE OF NUCLEAR LEVEL DENSITY

The energy dependence of the nuclear level density of Ni, Cu, As, Br, Sr, Y, Nb, Sn, Sb, I, Cs, Ba, La, Ce, Pr, Ho and Bi in the energy range 2-7.5 MeV was measured from inelastic neutron scattering. The observed level densities for the nuclei between the closed shells were well described by the Fermi gas model, and those for the nuclei at or near the shell closures were of non-Fermi gas form. The observed level densities of the above nuclei and of Co, Ag, In, Ta and Au were compared with results obtained by a direct counting of the nuclear levels based on the shell model and the following results were obtained. The appearance of level density forms different from the Fermi gas form is explained by the existence of large energy gaps between the major shells in the single-particle level scheme. There is a fairly definite correlation between the level density form and the degree of shell filling in the nuclear ground state. © 1969.