Liviu Matei

Single crystal of LiInSe2 semiconductor for neutron detector

Single crystals of semiconductor-grade lithium indium selenide (LiInSe2) were grown using the vertical Bridgman method. The orthorhombic structure of the materials was verified using powder x-ray diffraction. The room temperature band gap of the crystal was found to be 2.85 eV using optical absorption measurements. Resistivity of LiInSe2, obtained using current-voltage measurements, has semiconducting nature (decreases with increasing temperature) and is in order of 1010 Ω·cm. Photoconductivity measurement showed the photocurrent peak at 445 nm. Nuclear radiation devices were fabricated, and alpha particle detection was observed, suggesting that this material could be a candidate for neutron detection applications.

Comparative gamma spectroscopy with SrI 2 (Eu), GYGAG(Ce) and Bi-loaded plastic scintillators

We are developing new scintillator materials that offer potential for high resolution gamma ray spectroscopy at low cost. Single crystal SrI<inf>2</inf>(Eu) offers ∼3% resolution at 662 keV, in sizes of ∼1 in³. We have developed ceramics processing technology allowing us to achieve cubic inch scale transparent ceramic scintillators offering gamma spectroscopy performance superior to NaI(Tl). Our bismuth-loaded plastic scintillator demonstrates energy resolution of ∼8% at 662 keV, for samples of ∼0.5 cm³.

Large Area

Cd_0.9Zn_0.1Te (CZT) based pixelated radiation detectors have been fabricated and characterized for gamma ray detection. Large area CZT single crystals has been grown using a tellurium solvent method. A 10 ×10 guarded pixelated detector has been fabricated on a ~ 19.5 ×19.5 ×5 mm³ crystal cut out from the grown ingot. The pixel dimensions were 1.3 ×1.3 mm² and were pitched at 1.8 mm. A guard grid was used to reduce interpixel/inter-electrode leakage. The crystal was characterized in planar configuration using electrical, optical and optoelectronic methods prior to the fabrication of pixelated geometry. Current-voltage (I-V) measurements revealed a leakage current of 27 nA at an operating bias voltage of 1000 V and a resistivity of ~ 3.1 ×10¹⁰ Ω-cm. Infrared transmission imaging revealed an average tellurium inclusion/precipitate size less than 8 μm. Pockels measurement has revealed a near-uniform depth-wise distribution of the internal electric field. The mobility-lifetime product in this crystal was calculated to be 6.2 ×10 ^{- 3} cm²/V using alpha ray spectroscopic method. Gamma spectroscopy using a ¹³⁷Cs source on the pixelated structure showed fully resolved 662 keV gamma peaks for all the pixels, with percentage resolution (FWHM) as high as 1.8%.

{\rm Cd}_{0.9}{\rm Zn}_{0.1}{\rm Te}

Detector grade Cd0.9Zn0.1Te single crystals were grown using a tellurium solvent method. Single crystal blocks of volume ~1 cm3 were prepared for detector fabrication and characterization. The grown crystals were characterized using infra-red transmission imaging and Pockels effect measurements. Two detectors in single-polarity charge sensing configurations viz., small pixel, and virtual Frisch grid were fabricated on two crystals obtained from the same section of the ingot. Current-voltage measurements performed in planar configuration exhibited a very low leakage current of ~5 nA at 1000 V and resistivities of the order of 8.5×1010 Ω·cm. Electron drift mobilities of the order of 840 cm2/V.s and electron mobility-lifetime products of the order of 2.7×10-3 cm2/V were calculated from alpha spectroscopy using detectors in planar configuration. The small pixel and the virtual Frisch grid detector showed similar energy resolution of 3.7% for 662 keV gamma rays however, the virtual Frisch grid configuration revealed a better overall performance with a peak-to-Compton ratio of 2.8. A digital spectrometer and related software has been developed using a digitizer card and used to employ offline correction schemes to compensate for the charge loss effects, resulting in significant improvement of the 662 keV peak resolution (1.8% as compared to 3.7% without correction) obtained in the case of small pixel detector.

Pixelated Detector: Fabrication and Characterization

The detection of thermal neutrons has traditionally been accomplished with 3He-tubes, but with the recent shortage of 3He, much research has gone into finding suitable replacements. Both relatively inefficient 10B- and 6LiF-coated silicon diodes and HgI2 have been known for many years, and engineered structures in Si that have been filled with 10B and 6LiF have shown promise. These devices are intended to realize an optimal juxtaposition of neutron-sensitive material and semiconductor and thereby simulate a semiconductor containing B or Li. Such material has been realized for the first time in the form of 6LiInSe2 in which collectable charge from the 6Li(n,t) reaction indicates a neutron event. In this paper we report neutron and gamma responses of 6LiInSe2, we show pulse height spectra from pure gamma sources and from a thermal neutron source, and we derive the μτ product from the position of spectral features as a function of bias voltage. In addition, we demonstrate the observation of the beta decay of 116mIn in samples exposed to thermal neutrons. This feature of the response serves as an additional confirmation of exposure to neutrons.

Cd

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.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 wa...

_{0.9}

Chalcopyrite crystals of 6LiInSe2 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 6LiInSe2 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.

Zn

We present a preliminary design for a novel neutron detection system that is compact, lightweight, and low power consuming, utilizing the CubeSat platform making it suitable for space-based applications. This is made possible using the scintillating crystal lithium indium diselenide (6LiInSe2), the first crystal to include 6Li in the crystalline structure, and a silicon avalanche photodiode (Si-APD). The schematics of this instrument are presented as well as the response of the instrument to initial testing under alpha, gamma and neutron radiation. A principal aim of this work is to demonstrate the feasibility of such a neutron detection system within a CubeSat platform. The entire end-to-end system presented here is 10 cm x 10 cm x 15 cm, weighs 670 grams and requires 5 V direct current at 3 Watts.

_{0.1}

Abstract. We present a preliminary design for a neutron detection system that is compact, lightweight, and low power consuming, utilizing the CubeSat platform making it suitable for space-based applications. This is made possible using the scintillating crystal lithium indium diselenide (LiInSe26), the first crystal to include Li6 in the crystalline structure, and a silicon avalanche photodiode. The schematics of this instrument are presented as well as the response of the instrument to initial testing under alpha radiation. A principal aim of this work is to demonstrate the feasibility of such a neutron detection system within a CubeSat platform. The entire end-to-end system presented here is 10×10×15  cm3, weighs 670 g, and requires 5 V direct current at 3 W.

Te Crystal Growth and Fabrication of Large Volume Single-Polarity Charge Sensing Gamma Detectors

The change in bulk resistivity of CdZnTe (CZT) crystals was measured during infrared (IR) light between 950 and 1000 nm. The crystals are grown using one of the state-of-the-art methods either the traveling heating method or the modified Bridgman method. The change resistivity was evaluated using the steady-state current with and without light. Additionally, the change in current with both IR sources were correlated to the influence of secondary phases (SP) in each crystal using IR transmission microscopy to determine whether the number and size of the impurities has a drastic effect based on the current-voltage (IV) characteristics. SP at various depths within CZT are connected to the existence of variable depth, IR-excitable traps that lie within the bandgap. The release of these traps will significantly affect the overall current in the system. However, the current increase may not match the overall energy of the light utilized are more dependent on the size and quantity for each energy range.