An Investigation of Radiation Damage Induced by Hydroxyl and Oxygen Impurities In BaF 2 Crystal
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ABSTRACT The radiation effect in hydrolyzed BaF 2 was investigated through tile changes in their optical absorption and EPR spectra before and after -y-irradiation. The resulti demonstrated that hydroxyl and oxygen can be easily introduced into BaF 2 by means of a hydrolysis, the most likely modes are 011- ions substituting for fluorine and 02- ions substituting for fluorine associated with charge-compensating fluorine vacancies O,- - F+. Combining with the Hartree-Fock-Slater local-density discrete variational (HFS-X,,-DV) cluster calculation on some possible defects related to hydrogen and oxygen impurities, we propose that the radiation damage observed in hydrolyzed BaF 2 can be explained in terms of 01- and O,2- - F+ dissociation through a radiolysis. 1. INTRODUCTION Since the discovery of a fast emission component at 220 nm with a very short decay time of 0.6 ns[l], BaF 2 has been widely used in nuclear physics and high energy physics due to its outstanding time resolution and high detection efficiency for "/-ray. However, when Bae 2 detector subjects to long term exposures of radiation, the scintillation light output will decrease. Therefore the radiation damage in BaF 2 crystal becomes an extremely important problem for its applications. Recently, much research has been done in understanding the mechanism of radiation damage in BaF 2 . The results appeared that the intrinsic radiation hardness of pure BaF 2 may be quite good, and the radiation damage observed so far is due to either impurities or defects[23]. By using the Atomic Absorption Spectrometry (AAS), Inductively Coupled Plasma (ICP) and optical absorption spectrophotometry, Li et.al.[3] have measured the correlations between
radiation damage and impurities in BaF2 . Their conclusions showed that the contents of cation impurities in their crystals may have no substantial influence on the radiation damage. But they indicated that, among the anion impurities, hydroxyl and oxygen ions can be easily introduced into BaF2 crystal. Radiation effect of hydroxyl-and-oxygen-containing crystals has been widely investigated[416]. It has found that the impurities modify the radiation damage in these crystals. However,
only a little work on radiation damage in such impurities doped BaF 2 has been reported. Pena et.al.[17] have studied the radiation effect in hydrolyzed BaF 2 and provided much valuable information, but the measurement on the radiation-induced change in optical absorption was not extended to the region of the fast emissions. By using the optical absorption and Electron Paramagnetic Resonance (EPR), we emphasized the research on the above change as it would directly reflect the radiation effect on the scintillation property of the crystal. In the meantime, the electronic structures of some possible defects related to hydroxyl and oxygen impurities were calculated with Ilartree-Pock-Slater local-density discrete variational method (IIFS-Xe-DVM) 447 Mat. Res. Soc. Symp. Proc. Vol. 348. 01994 Materials Research Society
and some modes for the defects
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