The role of seniority-zero states in nuclear level densities

At low excitation energies seniority-zero states dominate the level density of K=0 bands in deformed even-even nuclei, while they play no role at higher excitation energies. We describe the level densities in a Fermi-gas model as well as in a combinatorial level-density model and compare to detailed experimental data for some rare-earth nuclei. An explanation is provided for recent observations of an odd-even staggering in the spin-distribution function as an effect of r-symmetry of wave functions for deformed nuclei. The structure of 0+ states in deformed nuclei is discussed in the model and compared to data, stressing the role of the seniority quantum number. The Fermi-gas model is utilized to obtain an overview of the odd-even staggering phenomenon in other mass regions. Odd-even staggering in spherical nuclei, appearing in open-shell nuclei, is briefly discussed as caused by fermion exchange symmetry.