Influence of annealing on the optical and scintillation properties of CaWO4CaWO4 single crystals

We investigate the influence of oxygen annealing on the room temperature optical and scintillation properties of CaWO4CaWO4 single crystals that are being produced for direct Dark Matter search experiments. The applied annealing procedure reduces the absorption coefficient at the peak position of the scintillation spectrum (∼430 nm) by a factor of ∼6 and leads to an even larger reduction of the scattering coefficient. Furthermore, the annealing has no significant influence on the intrinsic light yield. An additional absorption occurring at ∼400 nm suggests the formation of O-O- hole centers. Light-yield measurements at room temperature where one crystal surface was mechanically roughened showed an increase of the measured light yield by ∼40% and an improvement of the energy resolution at 59.5 keV by ∼12% for the annealed crystal. We ascribe this result to the reduction of the absorption coefficient while the surface roughening is needed to compensate for the also observed reduction of the scattering coefficient after annealing.

► We investigate the effects of oxygen annealing of a CaWO4CaWO4 single crystal.
► We observed a reduction of the absorption coefficient by a factor of ∼6∼6.
► We found that a reduction of the scattering coefficient can be compensated by surface roughening.
► An additional absorption at ∼400nm suggests the formation of O-O- hole centers.
► We report an improvement of the measured light yield and energy resolution at 59.5 keV by ∼40%∼40% and ∼12%∼12%, respectively.