# Theoretical predictions for the magnetic dipole moment of $^{229m}$Th

## TL;DR

This paper presents a new nuclear model prediction for the magnetic dipole moment of $^{229m}$Th, aligning closely with recent experimental measurements and offering insights into nuclear structure and transition probabilities.

## Contribution

The authors develop a coupled collective quadrupole-octupole and single-particle nuclear model that accurately predicts the isomer's magnetic dipole moment, improving upon previous Nilsson model predictions.

## Key findings

- Predicted isomer magnetic dipole moment of -0.35μ_N matches experimental -0.37(6)μ_N.
- Model overestimates ground state dipole moment by 40%.
- Provides insights into parity mixing, Coriolis effects, and gyromagnetic ratio g_R.

## Abstract

A recent laser spectroscopy experiment [J. Thielking et al., Nature (London) 556, 321 (2018)] has determined for the first time the magnetic dipole moment of the 7.8 eV isomeric state $^{229m}$Th. The measured value differs by a factor of approximately 5 from previous nuclear theory predictions based on the Nilsson model, raising questions about our understanding of the underlying nuclear structure. Here, we present a new theoretical prediction based on a nuclear model with coupled collective quadrupole-octupole and single-particle motions. Our calculations yield an isomer magnetic dipole moment of $\mu_{ IS}= -0.35\mu_N$ in surprisingly good agreement with the experimentally determined value of $-0.37(6)\mu_N$, while overestimating the ground state dipole moment by a factor 1.4. The model provides further information on the states' parity mixing, the role and strength of the Coriolis mixing and the most probable value of the gyromagnetic ratio $g_R$ and its consequences for the transition probability $B(M1)$.