Shell-structure and asymmetry effects in level densities

Level density $\rho(E,N,Z)$ is derived for a nuclear system with a given energy $E$, neutron $N$, and proton $Z$ particle numbers, within the semiclassical periodic-orbit theory beyond the Fermi-gas saddle-point method. We obtain $~~\rho \propto I_\nu(S)/S^\nu$, where $I_\nu(S)$ is the modified Bessel function of the entropy $S$, and $\nu$ is related to the number of integrals of motion beyond the energy $E$. For small shell structure contribution one obtains within the micro-macroscopic approximation (MMA) the value of $\nu=2$ for $\rho(E,N,Z)$. In the opposite case of much larger shell structure contributions one finds a larger value of $\nu=3$. The MMA level density $\rho$ reaches the well-known Fermi gas asymptotic for large excitation energies, and the finite micro-canonical limit for low excitation energies. Fitting the MMA $\rho(E,N,Z)$ to experimental data on a long isotope chain for low excitation energies, due to the shell effects, one obtains results for the inverse level density parameter $K$, which differs significantly from that of neutron resonances.