First search for α decays of naturally occurring Hf nuclides with emission of γ quanta

The first ever search for α decays to the first excited state in Yb was performed for six isotopes of hafnium (174, 176, 177, 178, 179, 180) using a high purity Hf sample of natural isotopic abundance with a mass of 179.8 g. For ^179 Hf, also α decay to the ground state of ^175 Yb was searched for thanks to the β -instability of the daughter nuclide ^175 Yb. The measurements were conducted using an ultra-low-background HPGe-detector system located 225 m underground. After 75 days of data taking no decays were detected but lower bounds for the half-lives of the decays were derived on the level of lim T_1/2∼ 10^15-10^18 a. The decay with the shortest half-life based on theoretical calculation is the decay of ^174 Hf to the first 2^+ 84.3 keV excited level of ^170 Yb. The experimental lower bound was found to be T_1/2≥ 3.3× 10^15 a.