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 ZnWO 4 samples do not exceed 0.002 -0.8 mBq/kg (depending on the radionuclide), the total α activity is in the range: 0.2 − 2 mBq/kg. Particular radioactivity, β active 65 Zn and α active 180 W, has been detected. The effect of the re-crystallization on the radiopurity of the ZnWO 4 crystal has been studied. The radioactive contamination of samples of the ceramic details of the set-ups used in the crystals growth has been checked by low background γ spectrometry. A project scheme on further improvement of the radiopurity level of the ZnWO 4 crystal scintillators is briefly addressed.

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Double beta processes in 64 Zn, 70 Zn, 180 W, and 186 W have been searched for with the help of large volume (0.1 − 0.7 kg) low background ZnWO 4 crystal scintillators at the Gran Sasso National Laboratories of the INFN. Total time of measurements exceeds 10 thousands hours. New improved half-life limits on double electron capture and electron capture with positron emission in 64 Zn have been set, in particular (all the limits are at 90% C.L.): T 0ν2ε 1/2 ≥ 1.1 × 10 20 yr, T 2νεβ +

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Energy resolution, light yield, non-proportionality in the scintillation response, α/β ratio, pulse shape for γ rays and α particles were studied with CdWO 4 crystal scintillators. Some indication for a difference in the emission spectra for γ rays and α particles was observed. No dependence of CdWO 4 pulse shape on emission spectrum wavelengths under laser, α particles and γ ray excitation was observed. Dependence of scintillation pulse shape for γ quanta and α particles and pulse-shape discrimination ability on temperature was measured in the range of 0 − 24 • C.