Nuclear state and level densities of actinides with the shell-model Monte Carlo

Abstract

The study of heavy open-shell nuclei requires model spaces that are too large for the direct diagonalization approaches of conventional shell-model calculations. The shell-model Monte Carlo (SMMC) method enables calculations of the thermal properties of heavy nuclei in very large model spaces. Here, we extend the SMMC method to the actinides. Fifteen even-even and odd-mass actinides $^{232}$Th, $^{\textrm{234-239}}$U, $^{\textrm{240-243}}$Pu, $^{\textrm{246-248}}$Cm, and $^{250}$Cf are studied using a single-particle model space that is larger than one major shell each for protons and neutrons, with a total dimension of the many-particle space as large as $10^{32}$. We use spin projection methods to calculate nuclear level densities and average $s$-wave neutron resonance spacings, both of which are found to be in good agreement with recent experiments.

Figures (5)

• Figure 1: Comparison of the NSDs for actinides calculated in the SMMC (solid circles), HFB (solid lines) and the experimentally determined Ozen2025 BBF (\ref{['eq_bbf']}) (dashed lines). The red triangles are the NSDs at the neutron separation energies $S_n$ and the histograms describe experimentally known states with assigned spins up to 1.2 MeV Capote2009.
• Figure 2: The enhancement factors $K = \rho_{\textrm{SMMC}}/\rho_{\textrm{HFB}}$ of $^{234}$U, $^{236}$U, and $^{238}$U versus excitation energy $E_x$. The dashed line corresponds to $K=1$.
• Figure 3: Comparison of the SMMC NLDs (solid circles) with the Oslo method experiments Guttormsen2013Laplace2016Zeiser2019 (solid squares) for the same actinides as in Fig. \ref{['fig_NSD']}. The red triangles are the NLDs at the neutron separation energies $S_n$ and the histograms are experimentally known levels up to 1.2 MeV Capote2009.
• Figure 4: Same as in Fig. \ref{['fig_NSD']}, but for other actinides.
• Figure 5: Same as in Fig. \ref{['fig_NLDs']}, but for other actinides.