EPR and ENDOR Studies of Point Defects in Lithium Tetraborate Crystals.

摘要

Lithium tetraborate (Li2B4O7 or LTB) is a promising material for both radiation dosimetry and neutron detection applications. LTB crystals can be grown pure or doped with different impurities including transition-metal and rare-earth ions. Research in this dissertation focuses on undoped LTB crystals and LTB crystals doped with copper and silver. Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) are used to characterize point defects in the lithium tetraborate crystals. Thermoluminescence (TL), photoluminescence (PL), photoluminescence excitation (PLE), and optical absorption (OA) are also used. An intrinsic hole trap associated with lithium vacancies is characterized with EPR and ENDOR and its thermal stability is determined using thermoluminescence. A 'perturbed' hole trap due to Ag2+ ions is characterized in doped crystals using EPR data alone. This method is tested on a previously studied hole center where both EPR and ENDOR were used. New x-ray induced centers are identified in copper-doped crystals. These include two Cu2+ hole centers and two Cu0 electron centers. These centers are characterized with EPR and their thermal stability explains TL peaks in glow curves. Finally, a comprehensive study utilizing EPR, OA, PL, and PLE data provide convincing explanations for the absorption and emission features of silver-doped crystals.