Influence of Annealing on the Optical and Scintillation Properties of CaWO$_4$ Single Crystals

TL;DR

This study examines how oxygen annealing improves the optical clarity and scintillation performance of CaWO$_4$ crystals used in dark matter detection by reducing absorption and scattering, with notable enhancements in light yield and energy resolution.

Contribution

It demonstrates that oxygen annealing significantly reduces absorption and scattering in CaWO$_4$ crystals without affecting intrinsic light yield, improving their suitability for dark matter experiments.

Findings

Absorption coefficient at 430 nm reduced by a factor of 6

Measured light yield increased by approximately 40% after annealing

Energy resolution at 59.5 keV improved by about 12%

Abstract

We investigate the influence of oxygen annealing on the room temperature optical and scintillation properties of CaWO$_4$ 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 ($\sim430$ nm) by a factor of $\sim6$ and leads to an even larger reduction of the scattering coefficient. Furthermore, the annealing has no significant influence on the \emph{intrinsic} light yield. An additional absorption occurring at $\sim400$ nm suggests the formation of O$^-$ hole centers. Light-yield measurements at room temperature where one crystal surface was mechanically roughened showed an increase of the \emph{measured} light yield by $\sim40 %$ and an improvement of the energy resolution at 59.5 keV by $\sim12 %$ 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.