First search for $\alpha$ decays of naturally occurring Hf nuclides with emission of $\gamma$ quanta

The first ever search for $$\alpha $$ 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 $$\alpha $$ decay to the ground state of $$^{175}$$Yb was searched for thanks to the $$\beta $$-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 $$\lim T_{1/2}\sim 10^{15}{-}10^{18}$$ 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 $$T_{1/2}\ge 3.3\times 10^{15}$$ a.