Spin distribution in low-energy nuclear level schemes

The spin-cutoff parameter σ has been determined for experimental spin distributions at low excitation energies of 310 nuclei between F18 and Cf251 (more than 8000 levels with their spin). The results indicate a weak dependence on the mass number A of the spin-cutoff parameter σ2~A0.28, 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.