Improvement of radiopurity level of enriched ¹¹⁶CdWO₄ and ZnWO₄ crystal scintillators by recrystallization

As low as possible radioactive contamination of a detector plays a crucial role to improve sensitivity of a double beta decay experiment. The radioactive contamination of a sample of ¹¹⁶CdWO₄ crystal scintillator by thorium was reduced by a factor ≈10, down to the level 0.01 mBq/kg (²²⁸Th), by exploiting the recrystallization procedure. The total alpha activity of uranium and thorium daughters was reduced by a factor ≈3, down to 1.6 mBq/kg. No change in the specific activity (the total α activity and ²²⁸Th) was observed in a sample of ZnWO₄ crystal produced by recrystallization after removing ≈0.4 mm surface layer of the crystal.