Rare alpha decays of natural Hf isotopes

A small series of four caesium hafnium chloride (Cs (Formula presented) (Formula presented) HfCl (Formula presented) (Formula presented), CHC) crystal scintillators has been measured in a low-background setup at the Gran Sasso National Laboratory of the INFN, Italy. After characterization of the detectors, new measurements concerning the rare alpha activity of natural Hf isotopes have been done. Based on cumulative data collected over 85.23 d, a new half-life value for the (Formula presented) (Formula presented) Hf (Formula presented) (Formula presented) decay has been obtained: (Formula presented) (Formula presented) ) (Formula presented) (Formula presented) year. This result is consistent with that reported earlier, which was obtained using one of the CHC crystals employed in the present work, albeit within a different experimental setup. The combined value is T (Formula presented) (Formula presented) = (2.95 (Formula presented) (Formula presented) 0.16) (Formula presented) (Formula presented) year. These measurements have resolved the long-standing discrepancy between the previously available experimental values and theoretical predictions. The experimental limits on the half-lives for (Formula presented) (Formula presented) decay of other natural Hf isotopes have been derived in the range of (Formula presented) (Formula presented) year (90% C.L.). The limit for the (Formula presented) (Formula presented) decay of (Formula presented) (Formula presented) Hf was set for the first time.