Abstract

Crystals of tungstates and molybdates are widely used to create scintillation detectors of ionizing radiation and are of interest for the search and study of neutrinoless double beta decay. Such experiments require high precision and impose strict requirements on the optical quality and purity of scintillator crystals.

Single crystals Li₂WO₄ and Li₂W_1-xMo_xO₄ doped with molybdenum (x = 0.0125, 0.05) were grown by the Chokhralsky method with a low temperature gradient, which allows obtaining the required crystals.

Method

The Czochralski method with a low temperature gradient was developed for growing large oxide crystals for use in scintillators (LTG Cz).

During melting of the charge, the temperature in the growth furnace was elevated at a rate of 100 degrees/hour to a value of 760 °C and was kept at this temperature for 5 hours until the melt was completely homogenized. Before seeding, the temperature was decreased to 733 °C. The first growth experiments were carried out on a platinum seed holder, and subsequently on monocrystalline seeds cut from the obtained Li₂WO₄ crystals.

Results

The crystal, in which 1.25% of tungsten was replaced by molybdenum, visually completely repeated the appearance of pure Li₂WO₄: it was transparent, optically homogeneous, without visible defects and was characterized by a convex shape of the crystallization front.

A single crystal with a molybdenum content of 5 mol.% reached 65 mm in length and 40 mm in diameter. Its upper part remained transparent, however, at a distance of 25 mm from the seed, macroscopic structural defects began to form, the crystal acquired a yellowish tint, and a general deterioration in its optical quality was recorded.

Conclusion

The LTG Cz method has successfully grown a series of visually defect-free single crystals of Li₂WO₄, both unalloyed and with partial replacement of [WO₄]^2- by [MoO₄]^2- (1.25 mol.% and 5 mol.%, respectively).