The Development of Lithium Tetraborate Compounds for Thermal Neutron Detection

Due to the shortage of ³He [helium-3], the goal of this project was to develop replacement materials for slow neutron detection in mixed radiation fields. The U.S. Department of Homeland Security (DHS) indicated that replacement materials should have an absolute neutron efficiency of 2.5 cps/ng of ²⁵²Cf (Californium-252) and a neutron/gamma discrimination intrinsic efficiency of 1 x 10^-6 [one gamma-ray response in a million counts]. In this work, the use of amorphous lithium tetraborate (Li₂B₄O₇:Ce) is analyzed as a thermal neutron detector. Also discussed is the synthesis of the lithium tetraborate, using ⁶Li [lithium-6], to form a crystalline powder that is heated simultaneously with cerium oxide (CeO₂) and excess boric acid to produce an optically clear glass. In this study, the structure of glass was probed, the abnormal reduction of Ce⁴⁺ [tetravalent cerium] to Ce³⁺ [trivalent cerium] observed in the fluorescence peak at 360 nm, and irradiation studies with alpha particles, beta particles, gamma-rays, and neutrons were performed. The resulting material was shown to have a light yield of 550 photons/neutron, which is 8% the light output of GS-20, a lithiated glass.