Characterization of Neutron-Induced Defects in Isotopically Enriched Lithium Tetraborate

摘要

Lithium Tetraborate (LTB) is assessed for use as a material for neutron detection. LTB isotopically enriched in either Li-6 or B-10 provides a medium that efficiently captures and transmutes neutrons into more readily detected forms of material or energy. Neutron detection is desirable to detect elicit movement of special nuclear material or nuclear weapons. Single crystals of LTB, Li-6 or B-10 enriched, were neutron irradiated, and electron paramagnetic resonance was used to detect point defects induced by the neutrons. Multiple defects were noted in the irradiated crystals. Two particular defects, perturbed lithium vacancies and lithium clusters are proposed as induced by thermal neutrons. Parallel experiments on silver doped LTB indicated that fast neutrons induce interstitial defects including interstitial oxygen, lithium, and silver atoms in silver doped crystals. Electron paramagnetic resonance, electron-nuclear double resonance, pulsed anneal, and thermo luminescence studies prior to neutron irradiation concluded that silver doped LTB crystals contain silver point defects that trap both electrons and holes. Pulsed anneal and thermo luminescence studies of all crystal types prior to neutron irradiation suggest neutron induced defects are significantly more stable that as grown defects. Thermo luminescence may be a viable technique to detect neutron interactions in the crystals. Lastly, 511 keV gamma radiation observed after neutron irradiation of all crystals is attributed to positron annihilation in the crystals. The Li-6(Li-7,2n)C-11 reaction is suggested as the responsible mechanism for positron production. The resulting C-11 nucleus radio actively decays via emission of a positron with a half life of 20.39 minutes.