Radioactive contamination of ZnWO4 crystal scintillators

Abstract The radioactive contamination of ZnWO 4 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 ZnWO 4 samples. One of the ZnWO 4 crystals has also been tested by ultra-low background γ spectrometry. The radioactive contaminations of the best ZnWO 4 samples are estimated to be less than 0.002 mBq/kg ( 228 Th and 226 Ra), the total α activity is 0.18 mBq/kg. The β active 65 Zn and the α active 180 W have been detected in ZnWO 4 crystals. The effect of the re-crystallization on the radiopurity of the ZnWO 4 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 ZnWO 4 crystal scintillators are briefly discussed.