$K$-hindered decay of a six-quasiparticle isomer in ${}^{176}\mathrm{Hf}$

The structure and decay properties of high-$K$ isomers in ${}^{176}\mathrm{Hf}$ have been studied using beam sweeping techniques and the Gammasphere multidetector array. A new $\Delta K=8$ decay branch, from a $K^{\pi }={22}^{-}$, six-quasiparticle, isomeric ($t_{1/2}=43\mu$s) state at 4864 keV to the 20${}^{-}$ state of a $K^{\pi }={14}^{-}$ band, has been identified. The reduced hindrance factor per degree of $K$ forbiddenness for this decay is measured to be unusually low ($f_{\nu }=3.2$), which suggests $K$ mixing in the states involved. The deduced interaction matrix elements are discussed within the context of relevant $K$-mixing scenarios. The 3266-keV state, previously interpreted as a $K^{\pi }={16}^{+}$ intrinsic state, is reassigned as the $J^{\pi }={16}^{+}$ member of the band based on the $K^{\pi }={15}^{+}$ state at 3080 keV. The systematics of $f_{\nu }$ values as a function of the degree of forbiddenness is discussed in light of this change.

Figure 1

Partial level scheme of ${}^{176}\mathrm{Hf}$ showing levels above the $K^{\pi }={14}^{-}$ isomer populated in the decay of the $K$${}^{\pi }={22}^{-}$ isomer. For a discussion about the unobserved 37-keV transition from the 22${}^{-}$ state to the 20${}^{-}$ level of the $K^{\pi }={14}^{-}$ band, see text.

Figure 2

Double-gated coincidence spectra from out-of-beam data showing $\gamma$ rays from the decay of the $K$${}^{\pi }={22}^{-}$ isomer to (a) the $K^{\pi }={14}^{-}$ band via the new $K$-hindered branch (b) the $K^{\pi }={15}^{+}$ band; (c) a new 460-keV $\gamma$ ray in the $K^{\pi }={15}^{+}$ band (see text).

Figure 3

Half-life of the state feeding the $K$${}^{\pi }={14}^{-}$ band, obtained by fitting the time distribution of intensities of the 295-keV (15${}^{-}\to {14}^{-}$) transition, gated by the transitions above it, from six different time slices (see text for details).

Figure 4

Reduced hindrance factors ($f_{\nu }$) as a function of the degree of forbiddenness $\nu$ for $E$2 (circle) and $M$1 (triangle) transitions in ${}^{176}\mathrm{Hf}$.