Article révisé par les pairs
Résumé : In the framework of the statistical model, the analytical approximation to the spin-dependent level density is improved by introducing in a new way the shell and pairing effects. The semi-classical approximation to the single-particle state density is used to describe the influence of the nuclear shell structure on the energy dependence as well as on the spin distribution of the level density. The pairing effect on the nuclear level density is introduced analytically within the BCS approximation in a very simple and accurate way. The new analytical level density formula appears to be in close agreement with the exact numerical shell-model calculation. A new level density formula taking into account the shell and pairing effects is proposed, generalized to deformed nuclei and shown to be in close agreement with experimental neutron resonance spacings.