Lithium indium diselenide, 6 LiInSe2 or LISe, is a newly developed neutron detection material that shows both semiconducting and scintillating properties. This paper reports on the performance of scintillating LISe crystals for its potential use as a converter screen for cold neutron imaging. The spatial resolution of LISe, determined using a 10% threshold of the Modulation Transfer Function (MTF), was found to not scale linearly with thickness. Crystals having a thickness of 450 µm or larger resulted in an average spatial resolution of 67 µm, and the thinner crystals exhibited an increase in spatial resolution down to the Nyquist frequency of the CCD. The highest measured spatial resolution of 198 µm thick LISe (27 µm) outperforms a commercial 50 µm thick ZnS(Cu): 6 LiF scintillation screen by more than a factor of three. For the LISe dimensions considered in this study, it was found that the light yield of LISe did not scale with its thickness. However, absorption measurements indicate that the 6 Li concentration is uniform and the neutron absorption efficiency of LISe as a function of thickness follows general nuclear theory. This suggests that the differences in apparent brightness observed for the LISe samples investigated may be due to a combination of secondary charged particle escape, scintillation light transport in the bulk and across the LISe-air interface, and variations in the activation of the scintillation mechanism. Finally, it was found that the presence of 115 In and its long-lived 116 In activation product did not result in ghosting (memory of past neutron exposure), demonstrating potential of LISe for imaging transient systems.

Abstract:The material lithium indium diselenide, a single crystal neutron sensitive semiconductor, has demonstrated its capabilities as a high resolution imaging device. The sensor was prepared with a 55 µm pitch array of gold contacts, designed to couple with the Timepix imaging ASIC. The resulting device was tested at the High Flux Isotope Reactor, demonstrating a response to cold neutrons when enriched in 95% 6 Li. The imaging system performed a series of experiments resulting in a <200 µm resolution limit with the Paul Scherrer Institute (PSI) Siemens star mask and a feature resolution of 34 µm with a knife-edge test. Furthermore, the system was able to resolve the University of Tennessee logo inscribed into a 3D printed 1 cm 3 plastic block. This technology marks the application of high resolution neutron imaging using a direct readout semiconductor.

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This paper reports on the charge carrier properties of several lithium indium diselenide (LISe) semiconductors. It was found that the charge collection efficiency of LISe was improved after high flux thermal neutron irradiation including the presence of a typically unobservable alpha peak from hole-only collection. Charge carrier trap energies of the irradiated sample were measured using photo-induced current transient spectroscopy. Compared to previous studies of this material, no significant differences in trap energies were observed. Through trap-filled limited voltage measurements, neutron irradiation was found to increase the density of trap states within the bulk of the semiconductor, which created a polarization effect under alpha exposure but not neutron exposure. Further, the charge collection efficiency of the irradiated sample was higher (14–15 fC) than that of alpha particles (3–5 fC), indicating that an increase in hole signal contribution resulted from the neutron irradiation. Finally, it was observed that significant charge loss takes place near the point of generation, producing a significant scintillation response and artificially inflating the W-value of all semiconducting LISe crystals.