Photoemission and Cathodoluminescence of Doped Lithium Tetraborate Crystals Being Developed for Neutron Detection
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Photoemission and Cathodoluminescence of Doped Lithium Tetraborate Crystals Being Developed for Neutron Detection Christina L. Dugan1, Robert L. Hengehold1, Stephen R. McHale1, Yaroslav Losovyj2,3, John W. McClory1, and James C. Petrosky1 1 Engineering Physics, Air Force Institute of Technology, Wright Patterson AFB, Ohio 2 Physics, University of Nebraska, Lincoln, Nebraska 3 Center for Advanced Microstructures and Devices, Louisiana State University, Baton Rouge, Louisiana ABSTRACT Photoemission spectroscopy using synchrotron radiation was used to determine the energy level structure of Mn doped Li2 B4O7 crystals. Photoemission studies provided evidence of Mn in the bulk crystal at 47.2 eV. Valence band analysis provided the presence of surface states but no acceptor sites. Cathodoluminescence studies were also made on undoped and Mn doped Li2 B4O7 using various beam energies from 5 to 10 KeV at room temperature. Self trapped exciton emission states are evident in the undoped and Mn doped Li2B4O7 sample ranging in energies from 3.1 to 4.1 eV. INTRODUCTION In order to better protect the United States against further acts of terrorism within its borders, ports of embarkation, and abroad, smaller, more reliable, sensitive and faster special nuclear material detection capabilities must be explored. Neutron detection has the advantages of low natural background count, few neutron sources in normal commerce operations, and different shielding characteristics compared to gamma rays. Current neutron detection devices are, however, not sensitive enough, bulky, and too expensive for wide use applications.[1] A primary advantage of using solid state media such as semiconductors for detectors is that more dense material yields smaller devices. Solid state neutron device design must utilize a material with a high neutron absorption cross section. Lithium tetraborate, Li2B4O7, is a crystalline material containing high densities of 6Li and 10B, both materials with high neutron absorption cross sections. The 10B neutron capture cross section for thermal neutrons is 3850 barns and 6Li has a cross section of 940 barns. [2] The primary objective of this research is to determine the material characteristics of doped Li2B4O7 in order to produce more efficient neutron detection devices. Since Li2B4O7 is an insulator, it is necessary to dope it with impurities such as manganese, copper, or silver so it may become p type, and thus increase the number of positive charge carriers, or n type to increase the number of electron charge carriers. This would allow for the use of Li2B4O7 in heterojunction or homojunction semiconductor devices. An accurate bandmap and electronic configuration of undoped Li2B4O7 exists. [3] Through a combination of cathodoluminescence and photoemission spectroscopy the doped Li2B4O7 electronic band structure and other surface and bulk characteristics can be determined. EXPERIMENT A cathodoluminescence system and two separate photoemission systems were used to study undoped and Mn doped Li2B4O7. The cathodoluminescen
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