Nuclear shell model and level density

The accurate knowledge of the nuclear level density is crucial for understanding the nuclear structure and for numerous applications including astrophysical reactions. In this review paper, we discuss the shell-model description of the nuclear level density, the use of the statistical moments method and underlying physics. The level density found with the moments method is shown to agree with the results of the exact diagonalization of the Hamiltonian matrix. The statistical approach is also compared to other standard methods for deriving level densities. The role of specific interaction matrix elements is reviewed in connection to the behavior of the level densities as these evolve. Chaotization and thermalization processes, collective enhancement and phase transitions are discussed with changing strengths of specific groups of two-body interaction matrix elements. The popular phenomenological constant temperature model is compared to the moments method and the effective temperature parameter of the model for different isotopes is discussed.