# Reduced transition probabilities for the gamma decay of the 7.8 eV isomer in $^{229}$Th

## TL;DR

This paper predicts reduced transition probabilities for the gamma decay of the $^{229}$Th isomer, highlighting the importance of nuclear structure effects like Coriolis mixing and quadrupole-octupole motion, which influence its lifetime and decay channels.

## Contribution

It introduces a detailed nuclear-structure model that accounts for fine interplay between nuclear motions, providing new insights into the isomer's decay probabilities and potential for nuclear frequency standards.

## Key findings

- Magnetic dipole transition probability is smaller than previous estimates.
- Electric quadrupole component may significantly affect internal conversion.
- Coriolis mixing and quadrupole-octupole motion are crucial for decay predictions.

## Abstract

The reduced magnetic dipole and electric quadrupole transition probabilities for the radiative decay of the $^{229}$Th 7.8 eV isomer to the ground state are predicted within a detailed nuclear-structure model approach. We show that the presence and decay of this isomer can only be accounted for by the Coriolis mixing emerging from a remarkably fine interplay between the coherent quadrupole-octupole motion of the nuclear core and the single-nucleon motion within a reflection-asymmetric deformed potential. We find that the magnetic dipole transition probability which determines the radiative lifetime of the isomer is considerably smaller than presently estimated. The so-far disregarded electric quadrupole component may have non-negligible contributions to the internal conversion channel. These findings support new directions in the experimental search of the $^{229}$Th transition frequency for the development of a future nuclear frequency standard.