GaN-Based Neutron Scintillators with a 6 LiF Conversion Layer
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GaN-Based Neutron Scintillators with a 6LiF Conversion Layer Andrew G. Melton1, Eric Burgett2, Nolan Hertel3, and Ian T. Ferguson1 1 Department of Electrical and Computer Engineering, University of North Carolina at Charlotte, Charlotte, NC, 28223, USA 2 Nuclear Engineering Program, Idaho State University, Pocatello, Idaho 83209, USA 3 Nuclear Engineering and Radiological Engineering Program, George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA ABSTRACT Thin film gallium nitride (GaN) scintillators have been produced by MOCVD and made neutron-sensitive by applying an enriched lithium-6 fluoride (6LiF) conversion layer. The 6 Li(n,α) reaction produces both alpha and triton particles, which have very penetration depths in GaN. The range and energy deposition characteristics of these particles in GaN have been simulated. Alpha-induced scintillation was measured in silicon-doped GaN using an americium241 (241Am) source. The thermal neutron responses of the 6LiF-coated GaN scintillator were tested using two thermal neutron sources, an 241Am-Be source inside a graphite pile and a reactor source. The scintillator was found to have a linear response to thermal neutron flux level over a range of more than three orders of magnitude. INTRODUCTION Neutron detectors must incorporate isotopes with high neutron reaction cross-sections to produce secondary ionizing radiation, such as heavy charged particles. The state of the art for decades has been the helium-3 (3He) gas-filled proportional counter. However, 3He is extremely rare (0.0001% of naturally occurring He), and is thus primarily manufactured through tritium decay. Tritium production has slowed dramatically in recent years, and demand for 3He has increased, resulting in a global shortage. This has motivated development of alternative detection materials. Solid-state devices have several advantages over gas-based detectors, including mechanical robustness and higher density of neutron-sensitive nuclei. Gallium nitride (GaN) is a mature optoelectronic semiconductor and has been shown to have superior radiation hardness compared to silicon and gallium arsenide [1], making GaN a viable solid-state neutron detection material. In this work, 10 µm thick GaN scintillators were grown by metalorganic chemical vapor deposition (MOCVD) on 2 inch sapphire substrates. Thermal neutron sensitivity was achieved by through the use of a lithium fluoride conversion layer, which was enriched to 95% 6Li. This conversion layer reacts with incident thermal neutrons via the 6Li(n,α), producing 2.05 MeV alpha particles and 2.73 MeV tritons which are emitted in opposite directions. These secondary ionizing radiation particles produce electron-hole pairs in the GaN. These electron-hole pairs recombine to produce scintillation photons. The macroscopic thermal neutron (En = 0.0253 eV) cross-section, Σ, for enriched LiF is 57.51 cm-1. Both undoped and Si-doped GaN films were investigated.
EXPERIMENT The GaN scintillators investigated in this
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