Development and Characterization of Lithium Indium Diselenide for Neutron Detection and Imaging Applications

Lithium indium diselenide [LISe] is under development as a single crystal semiconductor detector for neutron detection applications. Enriched in lithium-6, a neutron sensitive isotope, this wide-band gap semiconductor possesses the inherent neutron-gamma discrimination afforded by the thermal neutron capture reaction energy while providing distinct efficiency advantages over lithiated conversion layer detectors. The overarching theme of this work is to characterize the fundamental properties of this material to optimize its performance in neutron detection applications. The work presented here includes the identification of a suitable metallurgical contact for advanced detector fabrication, fundamental electronic property characterization, and proof-of-principle fast neutron imaging performance. Candidate contact materials were deposited through radio frequency magnetron sputtering. The primary metrics used to identify a robust contact were adhesion to the LISe surface and current voltage characteristics. Among the numerous contacts investigated, indium demonstrated the best adhesion properties. Its viability was demonstrated through the fabrication of a pixelated thermal neutron imaging detector (LTNI). Charge generation, transport, and trapping properties were investigated with emphasis on the stability of these properties post-operation in high thermal neutron flux fields. Neutron and alpha spectroscopy, photoinduced current transient spectroscopy, Raman spectroscopy, trap-filled limited voltage, and photoconductivity measurements were used to probe the charge transport and trapping mechanisms. Moderate transport properties were identified with respect to comparable technologies. Defect studies demonstrated that the type and density of defects strongly influenced performance of the detector. Encouraged by the performance of LTNI, an imaging detector was fabricated by coupling a LISe crystal to a 256 x 256 channel Timepix Application Specific Integrated Circuit to maximize spatial resolution. The fast neutron spatial resolution for 9MeV [electron-Volts] neutrons was investigated via a knife edge experiment. The measured efficiency was in agreement with the Evaluated Nuclear Data File cross-section database. The ultimate spatial resolution of the system was determined as 1.55 millimeters via the 10-90% decrease in contrast of the one-dimensional edge spread function. In conclusion, this material has been shown to exhibit suitable properties warranting further development for high efficiency slow neutron applications guided by the results of this work