CaWO4 Crystal Growth

[ Czochralski Method ] [ Crystals Grown at TUM ]


Several years ago the CRESST collaboration started to produce CaWO4 crystals in-house in the crystal laboratory of TUM. In this way the selection of raw materials, the growth environment and the different growth parameters can be influenced and the availability of ultra-pure crystals with optimized light yields can be ensured.


Czochralski Method

At TUM CaWO4 crystals are grown via the Czochralski method (see A. Erb and J.-C. Lanfranchi, CrystEngComm, 2013, 15, 2301). Therefore, CaWO4 powder produced via a solid state reaction from the raw materials CaCO3 and WO3 is melted at ~1600° C in a Rh crucible. A seed crystal is lowered into the melt and drawn up again under rotation. In this way a single crystal with the same crystallographic orientation as the seed crystal forms.

After growth, an after-heater keeps the grown crystal at a temperature of 1200° C for 20 hours and regulates a slow cool-down afterwards to reduce internal stresses and to avoid cleavage.

 


Crystals Grown at TUM

In CRESST-II cylindrical detector crystals with a height and diameter of 4cm were installed. Therefore, raw crystals with a typical diameter of about 45 mm, and a mass of about 800 g were produced. For the smaller crystals of CRESST-III (20x20x10mm3) also the geometry of the raw crystals was adapted to a diameter of about 30 mm. Cutting and polishing brings the raw crystal into the desired detector form.

In CRESST-II Phase 2 TUM-grown crystals were included for the first time. They have an improved radiopurity compared to commercial crystals. Excellent results were achieved especially for the crystal TUM40 (see CRESST-II Phase 2). In CRESST-III Phase 1 80% of the crystals installed were produced at TUM.



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