Strong enhancement of level densities in the crossover from spherical to deformed neodymium isotopes

TL;DR

This study measures nuclear level densities in neodymium isotopes transitioning from spherical to deformed shapes, revealing a significant increase and saturation around mass 150, with theoretical models aligning well with experimental data.

Contribution

It provides the most comprehensive data set of level densities across a nuclear shape transition and explains the mass dependence through intrinsic and collective effects.

Findings

Level densities increase by a factor of ~170 at 7.5 MeV excitation energy.

Experimental data aligns closely with shell model Monte Carlo calculations.

Saturation of level densities occurs around mass 150.

Abstract

Understanding the evolution of level densities in the crossover from spherical to well-deformed nuclei has been a long-standing problem in nuclear physics. We measure nuclear level densities for a chain of neodymium isotopes $^{142,144-151}$Nd which exhibit such a crossover. These results represent to date the most complete data set of nuclear level densities for an isotopic chain between neutron shell-closure and towards mid-shell. We observe a strong increase of the level densities along the chain with an overall increase by a factor of $\approx 170$ at an excitation energy of 7.5 MeV and saturation around mass 150. Level densities calculated by the shell model Monte Carlo (SMMC) are in excellent agreement with these experimental results. Based on our experimental and theoretical findings, we offer an explanation of the observed mass dependence of the level densities in terms of the intrinsic single-particle level density and the collective enhancement.