Search for $\alpha$ decay of naturally occurring Hf-nuclides using a Cs$_2$HfCl$_6$ scintillator

Residual radioactive contaminants of a caesium hafnium chloride (Cs$_2$HfCl$_6$) crystal scintillator have been measured in a low background setup at the Gran Sasso National Laboratory of the INFN, Italy. The total alpha activity of the detector is at the level of 7.8(3) mBq/kg. The results of direct studies of the $\alpha$ decay of naturally occurring Hf isotopes that have been performed using the "source=detector" approach are presented. In 2848 h of data taking, the $\alpha$ decay of $^{174}$Hf was observed with T$_{1/2} = (7.0\pm1.2)\times10^{16} $ y.