Weak $K$ hindrance manifested in $\alpha$ decay of the ${}^{178}\mathrm{Hf}{}^{m2}$ isomer

An experiment has been performed to detect the $\alpha$ emission mode in ${}^{178}\mathrm{Hf}{}^{m2}$ isomer decay and a partial half-life of $(2.5\pm 0.5)\times {10}^{10}$y was measured. It was concluded that $\alpha$ decay is strongly retarded by the centrifugal barrier arising due to the high spin of this isomeric state. Additional analysis shows, however, that the $K$-hindrance in this $\alpha$ decay is relatively weak, despite the strong manifestation of spin-hindrance.

Figure 1

$\alpha$ decay scheme for ${}^{178}\mathrm{Hf}$ nuclei in the ground and isomeric states. The $Q_{\alpha }$ values were deduced from the nuclear mass tables [4] and the level energies and angular momenta from Ref. [5]. The presently measured absolute intensity is given for $\alpha$ decay of the $m2$ level to be 1.2 × ${10}^{\mathrm{-9}}$.

Figure 2

Geiger-Nuttall systematics of $\alpha$ decay half-lives for nuclei in the range of $Z=60\text{–}82$.

Figure 3

Spin hindrance $F(\ell )$ versus angular momentum taken by the emitted $\alpha$ particle (bottom panel) and the reduced hindrance $f$ (top panel). Three points marked as ${}^{212m}$Po correspond to different branches of its $\alpha$ decay.