STUDY OF THE DEFECTS IN BaFCl AFTER X-RAY IRRADIATION
Wang Zhenjia Xiong Guangnan(Tianjin Institute of Technology, Institute of Material Physics, Tianjin 300191)
BaFX(Cl,Br) possesses the tetragonal structure of PbFCI. Its space group is P4/nmm. BaFX(Cl,Br) activated with divalent Eu2+ are used as X-ray phosphors for medical imaging. The photostimulated luminescence (PSL) process of BaFX (Cl,Br): Eu2+ irradiated by X-ray connects with the F-H center pair. The defects play an important part in PSL.Under 800℃, undoped BaFCl phosphors were prepared in N2 gas atmosphere. After the samples of BaFCl were irradiated by X-ray at 80K, we observed 310nm and 380nm emission peaks under 580nm photostimu-lation in the temperature range of 80K to 273K. The intensities of two peaks depend on the temperature as shown in Fig.1.At low temperature, electrons and self-trapped holes are created in BaFCl under X-ray exposure. Their recombination produces self-trapped excitons, and the excitons annihilate nonradiatively to create F-H center pairs. The process is shown in the Fig.2. Fncenter is regarded as an electron trapped in a vacancy of the halogen ion while H center is regarded as an X2- molecule occupying an X-ion site ( X- is the halogen ion ). When the sample is pholostimulated by 580nm light (F center optical absorption band) the electron of F center runs away and is trapped by the H center. Then it produces the self-trapped exciton with the STE (self-trapped exciton) recombinative luminescence. Fig.3 shows the above process.Yute et al. reported the absorption peaks of F center and H ( Cl2-) center from the absorptive spectrum of BaFCl single crystal at 77K. Stimulating light with the wavelength of F center absorption band irradiates the sample of BaFCl after X-ray exposure. We have measured the self-trapped exciton's luminescence of the F-H (Cl2-) center pair at 380nm. In analogy to the intrinsic luminescence of BaF2 crystal, the 310nm peak is from the self-trapped exciton's luminescence of the F-H (F2-) center pair.The intensity of the two peaks deperds OD the temperature. 310nm emission intensity changes significantly between 80K and 170K. The self-trapped exciton produced by F-H (F2-) center pair is not oriented with its fluorine-fluorine axis in the a-b plane, in fact, may be oriented with the fluorine-fluorine axis at an angle of less than 90° with respect to C axis. This reorientation may require an activation energy which is not available at lower temperature and therefore the 310rm emission would be reduced in intensity below 170K.As stated above, 380nm peak is from the F-H (Cl2-) center pair and 310nm peak is from the F-H (F2-) center pair.