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Saint-Laurent, Canada

Goldan A.H.,State University of New York at Stony Brook | Rowlands J.A.,Thunder Bay Regional Research Institute | Tousignant O.,Anrad Corporation | Karim K.S.,University of Waterloo
Journal of Applied Physics

The use of high resistivity amorphous solids as photodetectors, especially amorphous selenium, is currently of great interest because they are readily produced over large area at substantially lower cost compared to grown crystalline solids. However, amorphous solids have been ruled out as viable radiation detection media for high frame-rate applications, such as single-photon-counting imaging, because of low carrier mobilities, transit-time-limited photoresponse, and consequently, poor time resolution. To circumvent the problem of poor charge transport in amorphous solids, we propose unipolar time-differential charge sensing by establishing a strong near-field effect using an electrostatic shield within the material. For the first time, we have fabricated a true Frisch grid inside a solid-state detector by evaporating amorphous selenium over photolithographically prepared multi-well substrates. The fabricated devices are characterized with optical, x-ray, and gamma-ray impulse-like excitations. Results prove the proposed unipolar time-differential property and show that time resolution in non-dispersive amorphous solids can be improved substantially to reach the theoretical limit set by spatial spreading of the collected Gaussian carrier cloud. © 2013 AIP Publishing LLC. Source

Tousignant O.,Anrad Corporation | Karim K.S.,University of Waterloo | Rowlands J.A.,Thunder Bay Regional Research Institute
Applied Physics Letters

We demonstrate a high granularity multi-well solid-state detector with the unipolar time-differential property. Results show an improvement in the temporal pulse response by more than two orders-of-magnitude using amorphous selenium as the photoconductive film. The significance of the results presented here is the ability to reach the intrinsic physical limit for detector pulse speed by transitioning from the slow transit-time-limited response which depends on the bulk carrier transport mechanism, to the ultrafast dispersion-limited response which depends on the spatial spreading of the collected carrier packet. © 2012 American Institute of Physics. Source

Kabir M.Z.,Concordia University at Montreal | Chowdhury L.,University of Toronto | DeCrescenzo G.,University of Toronto | Tousignant O.,Anrad Corporation | And 3 more authors.
Medical Physics

Purpose: A numerical model and the experimental methods to study the x-ray exposure dependent change in the modulation transfer function (MTF) of amorphous selenium (a-Se) based active matrix flat panel imagers (AMFPIs) are described. The physical mechanisms responsible for the x-ray exposure dependent change in MTF are also investigated. Methods: A numerical model for describing the x-ray exposure dependent MTF of a-Se based AMFPIs has been developed. The x-ray sensitivity and MTF of an a-Se AMFPI have been measured as a function of exposure. The instantaneous electric field and free and trapped carrier distributions in the photoconductor layer are obtained by numerically solving the Poisson's equation, continuity equations, and trapping rate equations using the backward Euler finite difference method. From the trapped carrier distributions, a method for calculating the MTF due to incomplete charge collection is proposed. Results: The model developed in this work and the experimental data show a reasonably good agreement. The model is able to simultaneously predict the dependence of the sensitivity and MTF on accumulated exposure at different applied fields and bias polarities, with the same charge transport parameters that are typical of the particular a-Se photoconductive layer that is used in these AMFPIs. Under negative bias, the MTF actually improves with the accumulated x-ray exposure while the sensitivity decreases. The MTF enhancement with exposure decreases with increasing applied field. Conclusions: The most prevalent processes that control the MTF under negative bias are the recombination of drifting holes with previously trapped electrons (electrons remain in deep traps due to their long release times compared with the time scale of the experiments) and the deep trapping of drifting holes and electrons. © 2010 American Association of Physicists in Medicine. Source

Allec N.,University of Waterloo | Abbaszadeh S.,University of Waterloo | Fleck A.,Grand River Regional Cancer Center | Tousignant O.,Anrad Corporation | Karim K.S.,University of Waterloo
IEEE Transactions on Nuclear Science

Dual-layer, or stacked, detectors reduce motion artifacts in combined X-ray images, such as k-edge images, by acquiring low-and high-energy signals simultaneously. In this work we constructed a prototype single pixel dual-layer detector using amorphous selenium (a-Se) as the conversion material based on the same technology used for commercial large area flat panel imagers. A cascaded detector model was used to model the detector and for comparison with the experimental measurements. The detector was demonstrated to obtain contrast-enhanced mammography signals using an iodinated contrast agent. The experimentally obtained contrast was compared with the model and good agreement was found demonstrating the feasibility of the dual-layer technology. For comparison purposes, a single-layer single pixel detector capable of k-edge imaging but prone to motion artifacts (acquiring low-and high-energy signals sequentially) was also studied. © 2012 IEEE. Source

Kasap S.,University of Saskatchewan | Frey J.B.,University of Saskatchewan | Belev G.,University of Saskatchewan | Tousignant O.,Anrad Corporation | And 10 more authors.

In the last ten to fifteen years there has been much research in using amorphous and polycrystalline semiconductors as x-ray photoconductors in various x-ray image sensor applications, most notably in flat panel x-ray imagers (FPXIs). We first outline the essential requirements for an ideal large area photoconductor for use in a FPXI, and discuss how some of the current amorphous and polycrystalline semiconductors fulfill these requirements. At present, only stabilized amorphous selenium (doped and alloyed a-Se) has been commercialized, and FPXIs based on a-Se are particularly suitable for mammography, operating at the ideal limit of high detective quantum efficiency (DQE). Further, these FPXIs can also be used in real-time, and have already been used in such applications as tomosynthesis. We discuss some of the important attributes of amorphous and polycrystalline x-ray photoconductors such as their large area deposition ability, charge collection efficiency, x-ray sensitivity, DQE, modulation transfer function (MTF) and the importance of the dark current. We show the importance of charge trapping in limiting not only the sensitivity but also the resolution of these detectors. Limitations on the maximum acceptable dark current and the corresponding charge collection efficiency jointly impose a practical constraint that many photoconductors fail to satisfy. We discuss the case of a-Se in which the dark current was brought down by three orders of magnitude by the use of special blocking layers to satisfy the dark current constraint. There are also a number of polycrystalline photoconductors, HgI2 and PbO being good examples, that show potential for commercialization in the same way that multilayer stabilized a-Se x-ray photoconductors were developed for commercial applications. We highlight the unique nature of avalanche multiplication in a-Se and how it has led to the development of the commercial HARP video-tube. An all solid state version of the HARP has been recently demonstrated with excellent avalanche gains; the latter is expected to lead to a number of novel imaging device applications that would be quantum noise limited. While passive pixel sensors use one TFT (thin film transistor) as a switch at the pixel, active pixel sensors (APSs) have two or more transistors and provide gain at the pixel level. The advantages of APS based x-ray imagers are also discussed with examples. © 2011 by the authors; licensee MDPI, Basel, Switzerland. Source

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