Effect of recrystallisation on the radioactive contamination of CaWO4 crystal scintillators

Minimising intrinsic radioactivity of crystal scintillators is of particular importance for experiments searching for rare events. We studied the impact of the crystal production process (recrystallisation) on the level of radioactive contamination of CaWO4 crystal scintillators. Several samples of single crystal scintillators were produced using the recrystallisation procedure. It is shown that this has a significant effect on the radioactive contamination of the crystals. Depending on the stage of recrystallisation the activity due to 210Po (product of 210Pb decay) varies in the range 0.03–1.32 Bq kg−1 while the activity of 238U varies from 0.04 to 0.33 Bq kg−1. We found that uranium is rejected by the crystal with a segregation coefficient ≈0.3. The improvement in radiopurity of CaWO4 by one order of magnitude due to recrystallisation has been demonstrated. The additional benefit of this process is the improvement in the energy resolution. A programme to develop radiopure CaWO4 crystal scintillators is discussed briefly.