Magnetic dipole ${\gamma}$-ray strength functions in the crossover from spherical to deformed neodymium isotopes

TL;DR

This study uses advanced shell-model techniques to analyze magnetic dipole gamma-ray strength functions in neodymium isotopes, revealing low-energy enhancements and temperature-dependent modes related to nuclear deformation.

Contribution

It introduces a novel application of the maximum entropy method with different priors to reproduce low-energy gamma-ray strength enhancements in heavy nuclei.

Findings

Identification of low-energy enhancement (LEE) in M1 strength functions.

Demonstration of the importance of static fluctuations for LEE reproduction.

Observation of a finite-temperature scissors mode and spin-flip mode.

Abstract

We calculate the magnetic dipole $\gamma$-ray strength functions in a chain of even-mass neodymium isotopes $^{144-152}$Nd in the framework of the configuration-interaction (CI) shell model. We infer the strength function by applying the maximum entropy method (MEM) to the exact imaginary-time response function calculated with the shell-model Monte Carlo (SMMC) method. The success of the MEM depends on the choice of a good strength function as a prior distribution. We investigate two choices for the prior strength function: the static path approximation (SPA) and the quasiparticle random-phase approximation (QRPA). We find that the QRPA is a better approximation at low temperatures (i.e., near the ground state), while the SPA is a better choice at finite temperatures. We identify a low-energy enhancement (LEE) in the MEM deexcitation $M1$ strength functions of the even-mass neodymium isotopes and compare with recent experimental results for the total deexcitation $\gamma$-ray strength functions. The LEE is already seen in the SPA strength function but not in the QRPA strength function, indicating the importance of large-amplitude static fluctuations around the mean field in reproducing the LEE. Our method is currently the only one which can reproduce LEE in heavy open-shell nuclei where conventional CI shell model calculations are prohibited. With the onset of deformation as number of neutrons increases along the chain of neodymium isotopes, we observe that some of the LEE strength transfers to a low-energy excitation, which we interpret as a finite-temperature "scissors" mode. We also observe a finite-temperature spin-flip mode.