Spin distribution in low-energy nuclear level schemes

Abstract

The spin-cutoff parameter \ensuremath{\sigma} has been determined for experimental spin distributions at low excitation energies of 310 nuclei between $^{18}\mathrm{F}$ and $^{251}\mathrm{Cf}$ (more than 8000 levels with their spin). The results indicate a weak dependence on the mass number $A$ of the spin-cutoff parameter ${\ensuremath{\sigma}}^{2}~{A}^{0.28}$, and an even-odd spin staggering in the spin distribution of the even-even nuclei, with a strong enhancement of the number of states with spin zero. A modification of the spin-cutoff distribution formula is proposed in order to describe the even-even nuclei data. These findings are in good agreement with recent predictions of shell-model Monte Carlo calculations.