Microscopic-macroscopic level densities for low excitation energies

Abstract

Level density $\rho (E,\mathbf{Q})$ is derived within the micro-macroscopic approximation (MMA) for a system of strongly interacting Fermi particles with the energy $E$ and additional integrals of motion $\mathbf{Q}$, in line with several topics of the universal and fruitful activity of A.S. Davydov. Within the extended Thomas Fermi and 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 $S$. For small shell-structure contribution one finds $\nu =\kappa /2+1$, where $\kappa$ is the number of additional integrals of motion. This integer number is a dimension of $\mathbf{Q}$, $\mathbf{Q}=\{N,Z,...\}$ for the case of two-component atomic nuclei, where $N$ and $Z$ are the numbers of neutron and protons, respectively. For much larger shell structure contributions, one obtains, $\nu =\kappa /2+2$. The MMA level density $\rho$ reaches the well-known Fermi gas asymptote for large excitation energies, and the finite micro-canonical combinatoric limit for low excitation energies. The additional integrals of motion can be also the projection of the angular momentum of a nuclear system for nuclear rotations of deformed nuclei, number of excitons for collective dynamics, and so on. Fitting the MMA total level density, $\rho (E,\mathbf{Q})$, for a set of the integrals of motion $\mathbf{Q}=\{N,Z\}$, to experimental data on a long nuclear isotope chain for low excitation energies, one obtains the results for the inverse level-density parameter $K$, which differs significantly from those of neutron resonances, due to shell, isotopic asymmetry, and pairing effects.

Comments: 24 pages, 4 figures, 1 table. arXiv admin note: substantial text overlap with arXiv:2109.01830