Codoped Lithium Sodium Iodide With Tl+ and Eu2+ Activators for Neutron Detector

Abstract

Investigations of high-efficiency neutron-sensitive scintillators are very important for a wide variety of applications including special nuclear material detection, fundamental research, radiation safety, and industrial monitoring, among others. Responding to this need, we have been studying the Li x Na(1– x )I (conventionally called LNI) crystal system, using enriched 6Li because of its high neutron cross section. We have successfully grown single crystals of LNI with $x$ values in the range of 4%–20%, by the vertical Bridgman technique. Sodium iodide (NaI) and lithium iodide (LiI) have the same rock salt crystal structure capable of forming a monolithic solid solution over the entire concentration range. These LNI crystals are codoped with a few mol percent of Tl+ and Eu2+ activator to achieve the desired scintillation properties. X-ray excited luminescence spectra of LNI crystals show nearly identical main emission peaks around 440 nm independent of Li concentration. Neutron detection measurements using an americium/beryllium (AmBe) source show that increasing Li concentration from 4 to 20 mol% causes the neutron light yield (LY) to drop from 150 000 to 50 000 photons/neutron, while the gamma equivalent energy (GEE) for neutrons rises from 3.65 to 4.8 MeV. When measured at 662 keV (137Cs), the gamma ray LY behaves similarly, decreasing with the increase of Li content. This can be attributed to the lower fraction of Na, which affects both stopping power and scintillation efficiency. Pulse-shape discrimination measurements show well-defined differences between the gamma and neutron scintillator response.