Semiempirical nuclear level density formula with shell effects

A new semiempirical nuclear level density formula is proposed, which takes into account the influence of nuclear shell structure on level densities and the excitation energy dependence of shell effects. The ground state shell and pairing energies enter directly into this formula, which involves three mass-independent parameters characterizing the average single-particle level density near the Fermi level and the wavelength of shell oscillations. The present formulation is shown not only to give a good fit to the experimental data on neutron resonance spacings of spherical nuclei, but also to provide a reliable extrapolation to higher excitation energies. The present analysis has also brought out the need to include the dependence of level density parameter a on the surface to volume ratio of nuclei. The analysis of the data for deformed nuclei with the present formulation does not indicate an enhancement of the level densities of the magnitude suggestive of a rotation degree of freedom completely decoupled from intrinsic degrees at the excitation energy equal to neutron binding energies