Gamma Rays Emitted In The Decay Of 31-Yr Hf-178(M2)

The spontaneous decay of the K-pi=16(+), 31-yr Hf-178 (m2) isomer has been investigated with a 15-kBq source placed at the center of a 20-element gamma-ray spectrometer. High-multipolarity M4 and E5 transitions, which represent the first definitive observation of direct gamma-ray emission from the isomer, have been identified, together with other low-intensity transitions. Branching ratios for these other transitions have elucidated the spin dependence of the mixing between the two known K-pi=8(-) bands. The M4 and E5 gamma-ray decays are the first strongly K-forbidden transitions to be identified with such high multipolarities, and demonstrate a consistent extension of K-hindrance systematics, with an inhibition factor of approximately 100 per degree of K forbiddenness. Some unplaced transitions are also reported.