Scintillation Crystal Materials -- Cadmium Tungstate

Cadmium tungstate (CdWO₄) single crystal is a scintillation detector with excellent scintillation properties.

Its X ray absorption coefficient is large, the luminous efficiency is high, the afterglow time is short, and the radiation damage resistance is strong. It has extremely important application value in the field of nuclear medicine, X-ray, CT and container detection.

Cadmium tungstate (CdWO₄) single crystal belongs to the tungstenite structure (monoclinic), its melting point is 1289 °C, with excellent performance, high luminous efficiency and energy resolution. In the process of growth, the crystal is easy to crack because of the cleavage plane. In 1948, the optical properties of cadmium tungstate crystals were reported for the first time. In 1950, some scholars made the crystals by flame. They were yellow green, but they were of poor quality.

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With the development of the preparation technology, it is possible to prepare large size cadmium tungstate single crystals by the Czochralski process. However, the growth of high-quality single crystal cadmium tungstate is still difficult. The main problem is that the optical quality of each CdWO₄ crystal is not uniform enough, the reproducibility of single crystal growth in different batches is poor. There are many kinds of crystal defects such as cracking, inclusion and color center, so there are few units for stable growth of cadmium tungstate single crystal at home and abroad. At present, large size cadmium tungstate crystals are mainly grown by Czochralski method and crucible method.

CdWO₄ single crystals were grown by Czochralski method, and transparent and nearly colorless CdWO₄ single crystals were successfully grown by optimizing the growth process. Through the analysis of SEM, light transmission and fluorescence properties, the results show that the crystal grown has good crystallinity, no macroscopic crystal defect, complete structure and transmittance of 70% in the visible region, the wavelength of the emission peak is in the blue region, but the main problems are as follows: melt volatilization, crystal cleavage along (010) cleavage plane, inclusion formation, crystal coloring, etc..

The crucible reduction method is based on high purity CdO and WO₃, and CdWO₄ polycrystalline material is synthesized by high temperature solid state reaction. The CdWO₄ single crystal was grown by vertical crucible falling method. When the single crystal was grown, the furnace temperature was 1350 °C ~1400 °C, the solid-liquid interface temperature gradient was 30~40 °C /cm, and the crucible descending rate was 0.5 ~ 1.5 mm/h. Compared with the Czochralski method, the crucible method avoids the volatilization of the melt components during the crystal growth process and keeps the homogeneity of the melt composition, the stable temperature field is beneficial to reduce the crystal cracking. Therefore, the method of crucible growth with cadmium tungstate crystal is a process with application and development value.

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