The physical conditions for the emergence of the extremely low-lying nuclear isomer ${}^{229m}\mathrm{Th}$ at approximately 8 eV are investigated in the framework of our recently proposed nuclear structure model. Our theoretical approach explains the ${}^{229m}\mathrm{Th}$-isomer phenomenon as the result of a very fine interplay between collective quadrupole-octupole and single-particle dynamics in the nucleus. We find that the isomeric state can only appear in a rather limited model space of quadrupole-octupole deformations in the single-particle potential, with the octupole deformation being of a crucial importance for its formation. Within this deformation space the model-described quantities exhibit a rather smooth behavior close to the line of isomer–ground-state quasidegeneracy determined by the crossing of the corresponding single-particle orbitals. Our comprehensive analysis confirms the previous model predictions for reduced transition probabilities and the isomer magnetic moment, while showing a possibility for limited variation in the ground-state magnetic moment theoretical value. These findings prove the reliability of the model and suggest that the same dynamical mechanism could manifest in other actinide nuclei giving a general prescription for the search and exploration of similar isomer phenomena.

• Received 20 July 2020
• Revised 9 December 2020
• Accepted 22 December 2020

DOI:https://doi.org/10.1103/PhysRevC.103.014313

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Open access publication funded by the Max Planck Society.

Published by the American Physical Society