Radioactive contamination of ZnWO4 crystal scintillators

The radioactive contamination of ZnWO4 crystal scintillators has been measured deep underground at the Gran Sasso National Laboratory (LNGS) of the INFN in Italy with a total exposure 3197 kg h. Monte Carlo simulation, time-amplitude and pulse-shape analyses of the data have been applied to estimate the radioactive contamination of the ZnWO4 samples. One of the ZnWO4 crystals has also been tested by ultra-low background γ spectrometry. The radioactive contaminations of the best ZnWO4 samples are estimated to be less than 0.002 mBq/kg (228Th and 226Ra), the total α activity is 0.18 mBq/kg. The β active 65Zn and the α active 180W have been detected in ZnWO4 crystals. The effect of the re-crystallization on the radiopurity of the ZnWO4 crystal has been studied. The radioactive contamination of the ceramic components of the set-ups used in the crystals growth has been checked by low background γ spectrometry. Some ideas for future improvement of the radiopurity level of ZnWO4 crystal scintillators are briefly discussed.