  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.
