Dual-layer detectors offer the potential for energy separation, allowing for lesion differentiation and material decomposition without the effects of motion blur that occur in dual-energy detection. We have proposed a direct/indirect dual-layer amorphous selenium (a-Se) detector, in which the direct conversion top layer absorbs low energy x-rays and higher energy x-rays pass through to be absorbed by the indirect conversion bottom layer. First studies of the indirect layer, consisting of a thin-film transistor (TFT) flat panel detector (FPD) with an a-Se photoconductive layer, show promising results, but the MTF was limited by the performance of the gadolinium oxysulfide scintillator used. To improve spatial resolution, a CsI:Tl scintillator should be employed. Unfortunately, the emission peak of CsI:Tl scintillators falls outside optimal wavelengths for a-Se photoconduction. By alloying the a-Se with Te and operating at high fields, we improve absorption and signal production in the FPD. In this work, this is we fabricate single pixel a-Se-Te detectors with a parylene blocking layer and give results for Te concentrations of 0%, 10%, 15%, and 20%. While leakage currents and lag increase with Te content, conversion efficiency is improved by over 30%, showing promise for implementation into an FPD with a CsI:Tl scintillator.
We demonstrate measured radiation detection results from a 50 × 50 μm2 pixel size direct-conversion amorphous selenium (a-Se)/CMOS hybrid detector. We are interested in investigating a monolithic hybrid CMOS detector composed of an a-Se photoconductor thin film integrated on a CMOS active pixel readout array. In this work, we attempt to verify the fabrication process of a-Se as a photoconductive layer for integration with an existing CMOS ASIC. For this initial investigation, the RD53B-ATLAS was used to validate the deposition of a-Se for integration with CMOS technology.
In medical imaging, material decomposition and lesion differentiation are essential for the early detection of many severe and deadly diseases. While standard flat panel imagers are capable of providing high-resolution images, they lack the ability to differentiate between X-ray energies and, therefore, soft and hard tissues. Detection of multiple X-ray energies by dual-shot, photon counting, or dual-layer detectors provides a route for differentiation of these tissues based on the spectrum observed; multiple images may be generated based on the energies, allowing for subtraction and a more detailed image of tissues and calcifications. To improve the resolution, dose level, and motion artifacts of these imagers, we propose a new dual-layer detector consisting of a direct conversion amorphous selenium top layer, followed by an indirect conversion scintillator/amorphous selenium bottom layer. In this work, we present the first steps towards building this detector by characterizing the performance of the bottom indirect flat panel. We show that the blocking layer chosen, though unoptimized, performs adequately up to 50 V/um and have fabricated the detector, which will be evaluated for detective quantum efficiency and modulation transfer function.
Amorphous selenium (a-Se) is a high gain, low dark current, large area compatible photoconductor that has received significant attention towards the development of UV and X-ray detectors for medical imaging. Indirect detectors utilizing a-Se often feature blue emitting scintillators due to the high attenuation coefficient of a-Se in that region. However, emission tails from the scintillators often fall out of the conversion range of a-Se, and scintillators with emission peaks outside the absorption of Se cannot be utilized. In order to improve the sensitivity and gain in a-Se indirect detectors, we propose doping a-Se with tellurium as a function of depth, where tail emission will be absorbed by the lower bandgap aSe/Te after primary absorption in the initial Se layer. In addition, we employ a lateral device structure to avoid any absorption at short wavelengths from a transparent electrode or blocking layer. In this work, we present the first steps towards fabricating these devices. Studies of charge transport in doped a-Se/Te devices are performed using the transient photocurrent time-of-flight technique. We report hole and electron mobilities for a-Se1-xTex (x = 0, 0.01, 0.05, 0.10) as a function of applied voltage, along with band gaps and comparisons to previous studies. Fabrication of lateral devices, with and without optical slits, is demonstrated and discussed.
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