Normal and anomalous K-hindered decays from four-quasiparticle isomers in ¹⁷⁶Lu

Two four-quasiparticle isomers, with Kπ = 12⁺ and (14⁺) and mean lives of 450(100) ns and 58(5) μs, have been identified in ¹⁷⁶Lu, at excitation energies of 1515 and 1588 keV, respectively. The 12⁺ isomer exhibits a large number of K-forbidden decay branches, populating the rotational sequences based on the Kπ = 7^- ground state, two Kπ = 8⁺ states, and a Kπ = 4⁺ state from the ν{7/2^-[514]}⊗π{1/2^-[541]} configuration. Most branches have decay rates that are consistent with normal K-hindrances except for the branch to the Kπ = 4⁺ band. It has an anomalously low hindrance factor, which is attributed to two-state mixing due to a near-degeneracy between the 12⁺ isomer and the 12⁺ member of this band. The implied mixing matrix element has a value of only 5 eV, showing explicitly that very small mixing matrix elements may be responsible for anomalous K-hindered decays.