Spin/Parity Dependent Level Density

Abstract

It is shown that the Constant Temperature (CT) model of nuclear level density is a direct consequence of a symmetrized Poisson distribution of nuclear level spacings. The standard CT model describing the total level density is shown to be fatally flawed due to discontinuities at the Yrast energies, the onset of new $J^{\pi}$ sequences, that disrupt the exponential formula and cause the back shift parameter to become nonphysically negative. A new CT-JPI level density model is proposed with a constant temperature and separate back shift parameters for each $J^{\pi}$ sequence. The CT-JPI model is also constrained to reproduce the spin distribution predicted by Ericson's spin distribution function at the neutron separation energy. A fitting procedure is described for determining the temperature $T$, back shifts $E_0(J^{\pi})$, and spin cutoff parameters $\sigma_c$ from nuclear structure and resonance data. The CT-JPI model is demonstrated to successfully predict the level densities for a wide range of spins and parities for 46 nuclear with Z=7-92. In variance with earlier predictions the spin cut-off parameters show no mass dependence and instead substantial variation at all mass regions.