The technological advances introduced by additive manufacturing techniques have significantly improved the ability to generate functional composites with a wide variety of mechanical and optical properties. Progress in the additive manufacturing of scintillating particle composites could enable new capabilities that span applications in nuclear nonproliferation, nuclear energy and basic science. The present work focuses on developing capabilities for additively manufacturing scintillating particle composites where successful implementation could enable cost-effective highperformance detectors for a wide range of applications. The results demonstrate the optical and response characteristics of arranged scintillating glass particle composites that are optically transparent, mechanically robust and respond to incident fast neutrons.
Neutron detectors are used for illicit material detection, neutron radiography, stellar investigations of chemical content including biological compounds in planetary terrain and to monitor nuclear power plant fuel products and radioactive waste. Li-containing chalcogenide materials are promising alternative thermal neutron detection materials due to the incorporation of the <sup>6</sup>Li isotope at high density. <sup>6</sup>LiInSe<sub>2</sub> is limited in its effective thermal neutron efficiency by <sup>115</sup>In neutron capture which results in gamma decay rather than charge creation. This study includes investigations of mixed crystalline material <sup>6</sup>LiIn<sub>1-x</sub>Ga<sub>x</sub>Se<sub>2</sub> where the indium concentration is reduced by Ga substitution. The optical properties have been tuned by gallium substitution and radiation response has been observed.
The development of a thermal neutron imaging sensor constructed with semiconducting lithium indium diselenide is presented. Both a computational and experimental investigation were conducted. In the computational investigation, it is shown that the imaging potential of Lithium Indium Diselenid (LISe) is excellent, even when using a large pixel pitch through the use of super sampling. In the experimental investigation, it was found that a single pixel LISe detector using detector super sampling shows a spatial variation in the count rate, which is a clear sign of imaging capability. However, a good image was not obtained in the first experiment and may be caused by a variety of experimental conditions. Finally, a search is still underway to find a suitable contact metal with good mechanical adhesion for wedge bonding.
Chalcopyrite crystals of <sup>6</sup>LiInSe<sub>2</sub> have recently been shown to respond to gamma and thermal neutron radiation. Thus far, large crystals have been prepared although the charge collection efficiency has not been sufficient for high energy resolution. In an effort to improve energy resolution needed for gamma spectroscopy as well as pulse shape discrimination for mixed gamma neutron fluxes, the precipitate concentration within the <sup>6</sup>LiInSe<sub>2</sub> crystal have been studied. The precipitate volume greatly affects the energy resolution in the pulse height spectrum. Further, the charge mobility varies greatly with holes being preferentially trapped by these precipitates or some other defect site within the crystal.
Impurity analysis and compositional distribution studies have been conducted on a crystal of
LiInSe<sub>2</sub>, a compound semiconductor which recently has been shown to respond to ionizing radiation.
IR microscopy and laser induced breakdown spectroscopy (LIBS) revealed the presence of
inclusions within the crystal lattice. These precipitates were revealed to be alkali and alkaline earth
elemental impurities with non-uniform spatial distribution in the crystal. LIBS compositional maps
correlate the presence of these impurities with visual color differences in the crystal as well as a
significant shift of the band gap. Further, LIBS revealed variation in the ratio of I-III-VI2 elemental
constituents throughout the crystal. Analysis of compositional variation and impurities will aid in
discerning optimal synthesis and crystal growth parameters to maximize the mobility-lifetime
product and charge collection efficiency in the LiInSe<sub>2</sub> crystal. Preliminary charge trapping
calculations have also been conducted with the Monte Carlo N-particle eXtended (MCNPx) package
indicating preferential trapping of holes during irradiation with thermal neutrons.