Growth of Halide Scintillators with a Modified Heat Exchange Method
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Abstract: To test the potential of the heat exchange method for growing scintillator crystals, ingots of mono-, bi- and trivalent halides were prepared. Except for monovalent halides, the removal of oxygen compounds is indispensable. In the case of CaF 2 and BaF2 , the purification by reaction with NH4 F and a superheating of the melt could be included in the crystal growth process. Clear, monocrystalline ingots of these substances were grown in 4f-heated glassy carbon crucibles without seed crystals. Oxygen-free starting material had to be prepared for the growth of inclusion free CeF 3 crystals. Introduction:
The most common crystal growth techniques like the Bridgman, Verneuil and Czochralski method provide cylinder or rod-shaped crystal boules. By cutting or grinding these raw single crystals into the desired shape, a considerable amount of material is lost. Aside from this loss, during the subsequent shaping the crystals may crack and, in the case of hard materials, the processing requires much time. These problems are reduced by growing the crystals directly in the desired net shape, e.g. with a gradient solidification method or a heat exchange method, which in principle means the same. In these techniques the molten material crystallizes in a crucible, as the crystallization heat is drawn off through the bottom of the vessel by defined cooling [1]. The crystallized body has exactly the shape provided by the crucible. The heat exchange method has some further advantages making it interesting for industrial production of crystals: no motion of mechanical parts is necessary, small temperature gradients lower the danger of
cracking due to thermal stress, and no sophisticated and expensive equipment is needed. Additionally, the method has a large potential for upscaling the crystal size at reasonable expense. The method was first described 1925 by St6ber who grew hemispherical crystals of NaNO3 , Bismuth and ice [1]. 1973 Viechnicki and Schmid showed a way to grow large boules and plates of sapphire with HEM [2]. Recently Biderman et al. nicely demonstrated the shape growth potential of the heat exchange method by growing dome-shaped sapphire single crystals [3]. Halides have also been grown with HEM: Lamsen and Holmes prepared LiYF 1974 [4], the growth of Ni doped MgF 2 was reported by Reed et al. 1977 [5], and large CaF 2 crystals were grown by Horowitz et al. 1987 [6]. The possibility of growing large shaped crystals at low expense and with high quality should make the heat exchange method (HEM) ideally suitable for the growth of scintillator crystals, especially for the big ones needed inhigh energy physics. To learn the HEM growth method, we first tried growing KC1 and CaF 2. Then we applied the method to BaF 2 and CeF 3, which are among the most interesting fast scintillators [7].
513 Mat. Res. Soc. Symp. Proc. Vol. 348. 01994 Materials Research Society
E2Werimental: In growing halide crystals with a heat exchange method, we used a small if-heated furnace and round crucibles with diameters up to 70 mm
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