Fast decay of a three-quasiparticle isomer in Tm-171

Incomplete-fusion reactions have been used to study high-spin states in $^{171}\mathrm{Tm}$. Gamma rays and conversion electrons were measured using pulsed-beam conditions for enhanced isomer sensitivity. A ${K}^{\ensuremath{\pi}}=19/{2}^{+}$, three-quasiparticle isomer was identified, with a half-life of $1.7(2)$ \ensuremath{\mu}s. The faster than expected transition rates from the isomer can be understood as being due to a chance near-degeneracy, with mixing between the isomeric state and the ${I}^{\ensuremath{\pi}}=19/{2}^{+}$ member of the one-quasiparticle rotational band to which it decays. The implied mixing matrix element is $12(2)$ eV.