Residual interactions and theK-mixing-induced fast decay of the three-quasiparticle isomer in171Tm

Abstract

The anomalously fast decay of a $19/{2}^{+}$ three-quasiparticle isomer in ${}^{171}$Tm was interpreted recently as an example of $K$ mixing induced by a very small mixing matrix element but a (random) close proximity to a collective state. To understand the source of the residual interaction we have generalized the projected shell model by introducing two-body octupole and hexadecupole forces into the Hamiltonian and expanding the model space with inclusion of specific three-quasiparticle configurations. It is found that the $K$ mixing is built up from small interactions transferred through numerous highly excited configurations that contain high-$j$ orbitals. While the chance near-degeneracy enhances the transition strength, the octupole correlation and Coriolis coupling produce the mixing matrix element.