  The physical conditions for the emergence of the extremely low-lying nuclear
isomer $^{229m}$Th at approximately 8 eV are investigated in the framework of
our recently proposed nuclear structure model. Our theoretical approach
explains the $^{229m}$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 behaviour close to the line of isomer-ground state
quasi-degeneracy 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.
