Radiation Damage Mechanisms In Scintillator Materials: Applications to BaF 2 and CeF 3
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L. E. Halliburton and G. J. Edwards Physics Department, West Virginia University, Morgantown, WV 26506 ABSTRACT
Results from recent radiation damage studies in high quality BaF 2 and CeF 3 crystals are presented. Optical absorption and electron paramagnetic resonance (EPR) techniques are used to identify specific radiation damage mechanisms. Specific attention is given to the role of oxygen and hydrogen in the room temperature damage of BaF 2. Also, Mn 2+ ions are shown to change valence state in BaF 2 during room temperature irradiation. Numerous optical absorption bands are created in CeF 3 during irradiations at low temperature. These bands are associated with electron traps (either F centers or Ce 2 + ions) and they thermal anneal below room temperature. An EPR spectrum, assigned to F centers, is observed in low-temperature irradiated CeF 3. I. INTRODUCTION Nearly all scintillator materials exhibit radiation damage when exposed to sufficiently high radiation fields. There are, in general, two categories of damage mechanisms. In typical halide materials, the radiolysis process produces widely separated vacancies and interstitials (e.g., F centers and H centers) by an exciton relaxation mechanism. This process is present even in "perfect" crystals. A more pervasive, damage process involves the trapping of
radiation-induced holes and electrons at native defects and impurities introduced into the
crystal during growth. Transition-metal ions and rare-earth ions are found in most scintillator materials at the part-per-million level. This is also true for hydrogen and vacancies. Ionizing radiation redistributes the electrons among these impurities and native defects, and the resulting valence changes result in unwanted optical absorption bands. Spectroscopic techniques, such as optical absorption, luminescence, and magnetic resonance (EPR and ENDOR) provide sensitive, high resolution approaches to identify and characterize the various defects formed by radiation in scintillator materials. It is important to determine the model of each major defect and how it is produced and annihilated. Thermal stability is a key parameter since many defects are stable only at low temperature, but they still can cause significant transient absorption or luminescence at room temperature. In the present paper, we describe a series of experimental results obtained from highquality BaF 2 and CeF 3 crystal irradiated either at room temperature or below. A mechanism is proposed whereby OH- ions in BaF 2 are dissociated by radiation to form an absorption band in the 200-nm region. The effects of low-temperature irradiation of CeF 3 also is reported.
423 Mat. Res. Soc. Symp. Proc. Vol. 348. 01994 Materials Research Society
H. RESULTS FROM BaF 2 CRYSTALS
A. Low-Temperature Irradiation Optical absorption and EPR results were obtained from a typical high-quality BaF 2 crystal irradiated near liquid-nitrogen temperature. In these low-temperature experiments, the sample size was 2 x 3 x 5 mm3 and the source of radiation was an x-ray tube op
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