SWAD: inherent photon counting performance of amorphous selenium multi-well avalanche detector

Abstract

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 aSe 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.

Cascaded systems analysis of charge sharing in cadmium telluride photon‐counting x‐ray detectors

“…However, the low atomic number of Si results in low quantum efficiency for general radiographic and CT x‐ray spectra. Recently, amorphous selenium ( a ‐Se) has been proposed as a photoconductor for SPC and spectroscopic x‐ray imaging . Amorphous Se has the advantage of established manufacturing processes, but has low conversion gain, which may lead to poor energy resolution and loss of photon counts below electronic noise floors .…”

“…Amorphous Se has the advantage of established manufacturing processes, but has low conversion gain, which may lead to poor energy resolution and loss of photon counts below electronic noise floors . However, Stavro et al proposed using a field‐shaping multiwell avalanche detector (SWAD) structure with a ‐Se, which yields effective conversion gains comparable to CdTe, CZT, and Si.…”

Photon counting performance of amorphous selenium and its dependence on detector structure

“…A large pixel size (1 mm 2 ) was chosen to match the experimental conditions of previously published PHS measurements and compare the inherent energy resolution and count rate linearity of three different detector configurations. For the small pixel sizes needed in mammography (≤100 μm), the PHS may be degraded by spatial charge sharing effects 52 caused by K-fluorescence reabsorption and charge cloud spreading, which were omitted in this work. However, these effects were investigated separately in our previous work for using pixel sizes relevant for mammography (100 and 85 μm).…”

“…However, these effects were investigated separately in our previous work for using pixel sizes relevant for mammography (100 and 85 μm). 52 With regard to charge sharing due to K-fluorescence reabsorption into neighboring pixels, the lower K-fluorescence energy of a-Se leads to less effect compared with higher Z materials. With regard to charge sharing due to charge cloud spreading across pixel boundaries, a-Se detectors have an inherent advantage compared with CdTe/CZT detectors due to their relatively low carrier mobility and high electric field, which result in negligible lateral expansion of the charge cloud during drift.…”

Deriving depth‐dependent light escape efficiency and optical Swank factor from measured pulse height spectra of scintillators

“…This phenomenon is studied using pulse height spectroscopy, which characterizes the probability distribution of the number of quanta output by the converter per x‐ray interaction. Pulse height spectroscopy has been used by investigators to evaluate x‐ray conversion in x‐ray image intensifiers, scintillators used in screen‐film, computed radiography and digital radiography systems, photoconductors used in direct FPIs such as a ‐Se, and photon counting detectors for mammography and computed tomography …”