Collective enhancements in nuclear level densities

Contributions to the nuclear level density from a deformed core and from surface vibrations are discussed. The influence of symmetries of the nuclear shape is highlighted by quoting and discussing analytic Fermi gas level densities for irregular, deformed and spherical shapes. A thorough evaluation of the rotational enhancement factor to the level density is carried out for 17 well deformed rare-earth nuclei. Counted experimental levels are compared to levels obtained from the combinatorial level-density model, applying the Folded-Yukawa potential with BCS quasiparticle paring. It is found that the phase space of the rotating core contributes fully to the level density at the low energies where reliable information of experimental levels exists. The analysis is inspired by recent thermal Shell Model Monte Carlo results, which are also included in the comparison. The situation at the neutron excitation energy is also discussed, together with the conditions for vibrational enhancement. Experiments aimed at investigating the fade-away of collective enhancements are briefly discussed.