We are developing a technique to fabricate high spatial resolution and cost-effective photon counting detectors using silicon photomultipliers (SiPMs) and microcolumnar structured scintillator. Photon counting detectors using SiPMs are of much interest to the gamma-and X-ray detector community, but they have limitations at low energy due to their dark noise. In this paper, we report on vapor deposition of CsI:Tl directly onto a SiPM, a technique that improves optical coupling and allows for detection of low energy gamma-and X-rays. It simultaneously addresses related issues of light loss and light spread in the scintillator, thereby improving the performance of the detector. Devices made by this technique may be used for both photon counting and gamma-and X-ray imaging.The SiPM used in this study comprises a 4 4 array of macropixels, each of which is 3.0 mm 3.0 mm, with 3.36 mm pitch. This SiPM was placed inside a physical vapor deposition chamber and a 0.75 mm thick layer of microcolumnar CsI:Tl was grown on its surface without any damage. Scanning Electron micrographs (SEM) show highly oriented microcolumnar CsI:Tl structure orthogonal to the SiPM surface. These microcolumnar structures are excellent for channeling scintillation light to the SiPM and provide sub-macro-pixel resolution, which is now limited to the size of the macro-pixels. In this study, we report the performance characteristics of the resultant detector in terms of position sensitivity, energy discrimination, optical crosstalk, and signal-to-noise ratio. The performance of the detector is evaluated against that of other CsI:Tl/SiPM combinations, such as mechanically coupled monolithic and laser-pixelated CsI:Tl scintillators. Success of the technique may be gauged by the fact that the photopeak can be realized for a wide range of energies, including those of (60 keV) and(122 keV).
Technological innovations in grazing incidence X-ray optics have been crucial to the advancement of the field of X-ray astronomy. Improvements in X-ray focusing optics translate to higher sensitivity for X-ray telescopes operating in the energy range above 10 keV. Full characterization of the X-ray optics involves measurement of the point spread function, scattering, and reflectivity properties of substrate coatings. This requires a very high spatial resolution, high sensitivity, photon counting and energy discriminating large area detector. In this paper we describe the construction of a detector that is well suited to meet these requirements. A prototype version of this camera was used to calibrate the X-ray focusing optics for the Nuclear Spectroscopic Telescope Array (NuSTAR) mission. Analysis of the data obtained during the ground calibration of the NuSTAR telescopes demonstrated the advantages of such a high resolution 2D detector for hard X-rays (30+ keV); however it showed some limitations for medium energy X-rays (8-30 keV). We present here, alternative methods under investigation to improve performance of the detector for medium energy X-rays such as changing the morphology of the CsI:Tl scintillator, improving light transport from scintillator to EMCCD and using a novel bright scintillator, Ba 2 CsI 5 :Eu.
Li-based semiconductor materials represent a promising alternative to 3-He and scintillation materials for thermal neutron detection and imaging instruments. Semiconductor crystals of LiInSe2, LiInP2Se6, and LiGaInSe2 (LiGa0.5In0.5Se2) were grown using natural and enriched lithium ( 6 Li). The materials were characterized for electronic and optical properties including optical transmission, current-voltage (I-V) characteristic for resistivity, and bandgap. Thermal neutron detectors were fabricated and characterized for neutron and gamma-ray response. Pulse height spectra were collected from a moderated custom-designed 241 AmBe neutron source and a 60 Co gamma-ray source. The LiInSe2 samples exhibited a 2.8 eV cutoff in the optical spectrum and a resistivity of ~8×10 11 Ω•cm. LiInSe2 devices exhibit a noise floor of <30 keV which operated at a field of 630 V/mm, for the 0.8-mm thick device. The Vertical Gradient Freeze (VGF) grown LiInP2Se6 samples exhibited a 2.2 eV cutoff in the optical spectrum and resistivity of ~4×10 12 Ω•cm. The Chemical Vapor Transport (CVT) grown LiInP2Se6 devices exhibit a noise floor of <60 keV which operated at a field of 8,000 V/mm, for the 0.05mm thick device. Furthermore, the long-term stability of LiInSe2 devices during multiple weeks under continuous bias was investigated.
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