Photon counting detectors (PCD) have the potential to improve x-ray imaging; however, they are still hindered by high costs and performance limitations. By using amorphous selenium (a-Se), the cost of PCDs can be significantly reduced compared with modern crystalline semiconductors, and enable large-area deposition. We are developing a direct conversion field-shaping multiwell avalanche detector (SWAD) to overcome the limitation of low carrier mobility and low charge conversion gain in a-Se. SWAD’s dual-grid design creates separate nonavalanche interaction (bulk) and avalanche sensing (well) regions, achieving depth-independent avalanche gain. Unipolar time differential (UTD) charge sensing, combined with tunable avalanche gain in the well region allows for fast response and high charge gain. We developed a probability-based numerical simulation to investigate the impact of UTD charge sensing and avalanche gain on the photon counting performance of different a-Se detector configurations. Pulse height spectra (PHS) for 59.5 and 30 keV photons were simulated. We observed excellent agreement between our model and previously published PHS measurements for a planar detector. The energy resolution significantly improved from 33 keV for the planar detector to ∼7 keV for SWAD. SWAD was found to have a linear response approaching 200 kcps / pixel.
Photon counting detectors (PCD) have the potential to improve x-ray imaging, however they are still hindered by high production costs and performance limitations. By using amorphous Selenium (a-Se) the cost of PCDs can be significantly reduced compared to currently used crystalline semiconductors and enable large area deposition. To overcome the limitation of low carrier mobility and low charge conversion gain in a-Se, we are developing a novel direct conversion a-Se field-Shaping multi-Well Avalanche Detector (SWAD). SWADs multi-well, dual grid design creates separate non-avalanche interaction (bulk) and avalanche sensing (well) regions, achieving depth-independent avalanche gain. Unipolar time differential (UTD) charge sensing, combined with tunable avalanche gain in the well region allows for fast timing and comparable charge conversion gain to crystalline semiconductors. In the present work we developed a probability based numerical simulation to model the charge generation, transport and signal collection of three different a-Se detector configurations and systematically show the improvements in energy resolution attributed to UTD charge sensing and avalanche gain. Pulse height spectra (PHS) for each detector structure, exposed to a filtered <sup>241</sup>Am source, are simulated and compared against previously published PHS measurements of a conventional a-Se detector. We observed excellent agreement between our simulation of planar a-Se and the measured results. The energy resolution of each generated PHS was estimated by the full-width-at-half-maximum (FWHM) of the primary photo-peak. The energy resolution significantly improved from ~33 keV for the planar a-Se detector to ~7 keV for SWAD utilizing UTD charge sensing and avalanche gain.
Single-photon-counting (SPC) x-ray detectors are expected to play a key role in the next generation of medical x-ray imaging. The spatial resolution of SPC x-ray detectors is an important design criterion, in particular for mammography in which one of the primary aims is to detect and differentiate micro-calcifications. The purpose of this abstract is to extend the cascaded systems approach to investigate the influence of reabsorption of characteristic x rays on SPC spatial resolution. A parallel-cascaded model is used to describe reabsorption of characteristic x rays following photoelectric interactions. We use our model to calculate the large-area gain and modulation transfer function (MTF) of amorphous selenium (a-Se) SPC detectors that use a field-Shaping multi-Well Avalanche Detector (SWAD) structure to overcome the low conversion gain of a-Se. Our model accounts for emission and reabsorption of characteristic x rays, x-ray conversion to electron-hole pairs, avalanche gain and gain variance, integration of secondary quanta in detector elements, electronic noise, and energy threshold. Theoretical predictions are compared with the results of Monte Carlo simulations. Our analysis shows that under mammographic imaging conditions, the a-Se/SWAD structure with an avalanche gain of 10 or greater results in minimal loss of photon counts below the electronic noise floor for electronic noise levels ~500 - 700 e-h pairs. Double counting of characteristic x-rays inflates the large-area gain by ~20% relative to the quantum efficiency, and results in modest MTF degradation relative to energy-integrating systems. Excellent agreement between theoretical and Monte Carlo analyses was observed. This approach provides a theoretical framework for understanding SPC detector performance and for system optimization
Proc. SPIE. 10132, Medical Imaging 2017: Physics of Medical Imaging
KEYWORDS: Semiconductors, Photon counting, Sensors, Signal attenuation, Electrodes, Luminescence, Crystals, X-rays, Monte Carlo methods, Detector development, X-ray imaging, Selenium, Signal detection, Prototyping, Breast imaging
Photon counting detectors (PCD) with energy discrimination capabilities have the potential for improved detector performance over conventional energy integrating detectors. Additionally, PCDs are capable of advanced imaging techniques such as material decomposition with a single exposure, which may have significant impact in breast imaging applications. Our goal is to develop a large area amorphous Selenium (a-Se) photon counting detector. By using our novel direct conversion field-Shaping multi-Well Avalanche Detector (SWAD) structure, the inherent limitations of low charge conversion gain and low carrier mobility of a-Se can be overcome. In this work we developed a spatio-temporal charge transport model to investigate the effects of charge sharing, energy loss and pulse pileup for SWAD. Using a monoenergetic 20 keV source we found that 32% of primary interactions have K-fluorescence emissions that escape the target pixel, 62.5% of which are reabsorbed in neighboring pixels, while 37.5% escape the detector entirely for a 100 μm × 100 μm pixel size. Simulated pulse height spectra for an input count rate of 50,000 counts/s/pixel with a 2 μs dead time was also generated, showing a photopeak FWHM = 2.6 keV with ~10% pulse pileup. Additionally we present the first time-of-flight (TOF) measurements from prototype SWAD samples, showing successful unipolar time differential (UTD) charge sensing. Our simulation and initial experimental results show that SWAD has potential towards making a large area a-Se based PCD for breast imaging applications.
Photon counting detectors (PCDs) have the potential to improve x-ray imaging, however they are still hindered by several
performance limitations and high production cost. By using amorphous Selenium (a-Se) the cost of PCDs can be
significantly reduced compared to crystalline materials and enable large area detector fabrication. To overcome the
problem of low carrier mobility and low charge conversion gain in a-Se, we are developing a novel direct conversion a-
Se field-Shaping multi-Well Avalanche Detector (SWAD). SWAD circumvents the charge transport limitation by using a
Frisch grid built within the readout circuit, reducing charge collection time to ~200 ns. Field shaping permits depth
independent avalanche gain in wells, resulting in total conversion gain that is comparable to Si and CdTe. In the present
work we investigate the effects of charge sharing and energy loss to understand the inherent photon counting performance
for SWAD at x-ray energies used in breast imaging applications (20-50keV). The energy deposition profile for each
interacting x-ray was determined with Monte Carlo simulation. For the energy ranges we are interested in, photoelectric
interaction dominates, with a k-fluorescence yield of approximately 60%. Using a monoenergetic 45 keV beam incident
on a target pixel in 400um of a-Se, our results show that only 20.42 % and 22.4 % of primary interacting photons have kfluorescence
emissions which escape the target pixel for 100um and 85um pixel sizes respectively, demonstrating SWAD’s
potential for high spatial resolution applications.