Production of high-purity CaWO₄ crystals for direct dark matter search with CRESST

The direct dark matter search experiment CRESST (Cryogenic Rare Event Search with Superconducting Thermometers) uses scintillating CaWO₄ single crystals as targets to search for possible recoils of dark matter particles. Since several years these CaWO₄ crystals have been produced directly at the Technische Universitaet Muenchen (TUM) including the CaWO₄ powder production from the raw materials CaCO₃ and WO₃, the CaWO₄ crystal growth via the Czochralski method as well as the after-growth treatment of the crystals. In CRESST-II Phase 2 (2013-2015), 4 TUM-grown crystals were operated in the experiment for the first time, showing the best radiopurities of all crystals installed. Therefore, in CRESST-III Phase 1 (started in summer 2016) mainly TUM-grown crystals have been included. The goal for the upcoming CRESST-III Phase 2 is a further improvement of the radiopurity. A method to chemically purify the raw materials was developed at TUM and is presented together with first promising results.