Growth Of CdWO₄ Scintillation Crystals By Vertical Bridgman Process

CdWO₄ (CWO) single crystal is a well-known scintillator with excellent properties. The crystal has attractive performances such as high light yield, short radiation length, high density, non-deliquescence and the radiation stability. The unique properties make the crystal to be very valuable for radiation detection with high resolution, especially for medical imaging and security checking. In the previous works, most investigations were focused on the Czochralski process. However, it is difficulties for Czochralski technique to grow large-size CWO crystals with high quality. Trying to overcome the difficulties in Czochralski growth, the vertical Bridgman process was used to grow CWO crystals in our laboratory. In this thesis, we present the first report on the vertical Bridgman growth of CWO crystals. This work proves that the vertical Bridgman process is a promising technique for the mass growth of large-size CWO crystals with high quality.Using the initial agents CdO and WO3 with high purity, the polycrystalline material CdWO₄ was prepared by solid state reaction at elevated temperature. CWO crystals were grown in a resistively heated vertical Bridgman furnace, which was controlled at 1350～1400oC. Growth process was driven by lowering the crucible at a rate of 0.5～1.5 mm/h. The temperature gradient across solid-liquid interface was controlled as 30～40℃/cm. By means of the process described above, the pale yellow-brown and transparent CWO crystal as large asΦ40×70 mm has been grown successfully. The crystal was characterized by DTA/TG, XRD, optical transmission, photoluminescence and X-ray stimulated luminescence spectrum. X-ray diffraction rocking curve indicates the desirable crystalline quality of the grown crystal. The optical measurements show a high optical transmittance in the visible wavelength range and a strong emission with central wavelength at 470 nm under X-ray excitation.In order to explore the effective method to improve the optical quality of the crystals, some technical problems such as the growth parameters , component ratio of feed material, method of avoiding the harmful volatilization of the melt and annealing conditions were investigated. The typical defects in the crystals were investigated. The optical transmittance spectra and photoluminescence spectra of the crystals grown with different conditions was measured, the influence of the growth process on the optical homogeneity of the crystals was also discussed. Based on the experimental results, the following conclusions could be confirmed: (1) The polycrystalline material with accurate stoichiometric composition was suitable for growing CWO crystals with high quality by the vertical Bridgman process; (2) By sealing the platinum crucibles charged with feed materials, the harmful CdO volatilization could be avoided effectively in the crystal growth. This process is helpful to grow CWO crystals with high quality. (3) To decrease the cracking in the crystal growth, the crystal should be grown along the preferred growth direction [001]. (4)By means of annealing treatment in oxygen atmosphere, the oxygen vacancy in the crystals could be decreased and the optical transmittance could be increased. The annealing treatment is also helpful to avoid the cracking of the crystals in the fabrication process.