PurposeActive matrix flat panel imagers (AMFPIs) with thin-film transistor arrays experience image quality degradation by electronic noise in low-dose radiography and fluoroscopy. One potential solution is to overcome electronic noise using avalanche gain in an amorphous selenium (a-Se) (HARP) photoconductor in indirect AMFPI. In this work, we aim to improve temporal performance of HARP using a novel composite hole blocking layer (HBL) structure and increase optical quantum efficiency (OQE) to CsI:Tl scintillators by tellurium (Te) doping.ApproachTwo different HARP structures were fabricated: Composite HBL samples and Te-doped samples. Dark current and optical sensitivity measurements were performed on the composite HBL samples to evaluate avalanche gain and temporal performance. The OQE and temporal performance of the Te-doped samples were characterized by optical sensitivity measurements. A charge transport model was used to investigate the hole mobility and lifetime of the Te-doped samples in combination with time-of-flight measurements.ResultsThe composite HBL has excellent temporal performance, with ghosting below 3% at 10 mR equivalent exposure. Furthermore, the composite HBL samples have dark current <10−10 A/cm2 and achieved an avalanche gain of 16. Te-doped samples increased OQE from 0.018 to 0.43 for 532 nm light. The addition of Te resulted in 2.1% first-frame lag, attributed to hole trapping within the layer.ConclusionsThe composite HBL and Te-doping can be utilized to improve upon the limitations of previously developed indirect HARP imagers, showing excellent temporal performance and increased OQE, respectively.
Active matrix flat panel imagers (AMFPIs) with thin-film transistor (TFT) arrays have become the dominant technology for digital x-ray imaging. However, their performance is degraded by electronic noise in low dose imaging applications. One potential solution is to overcome electronic noise using avalanche gain in an amorphous selenium (a-Se) photoconductor in indirect AMFPI, known as the scintillating high-gain avalanche rushing photoconductor AMFPI (SHARP-AMFPI). We previously developed two SHARP-AMFPI prototypes, however both have several areas of desired improvement. In this work, we fabricate and characterize HARP samples with a composite hole blocking layer (HBL) structure to reliably maintain avalanche fields while reducing temporal effects, as well as samples with tellurium (Te) alloyed a-Se to increase the optical quantum efficiency (OQE) to thallium doped cesium iodide (CsI:Tl) columnar scintillators. Our measurements show that the composite HBL has improved temporal performance over the original prototype, with ghosting below 3% at 10 mR equivalent exposure and no noticeable lag observed. We also show that the layer has comparable dark current to the previously used organic HBL and can reach an avalanche gain of 16. We aim to further reduce the dark current by improving the formulation of the n-type metal oxide layer using different deposition methods. Introducing Te-alloying to HARP samples shows an increase in OQE from 0.018 to 0.43 for 532 nm light. The addition of Te resulted in increased lag, attributed to charge trapping within the layer. Future work will investigate arsenic and chlorine co-doping to restore charge transport in this layer.
Active matrix flat panel imagers (AMFPIs) with thin film transistor (TFT) arrays are becoming the standard for digital x-ray imaging due to their high image quality and real time readout capabilities. However, in low dose applications their performance is degraded by electronic noise. A promising solution to this limitation is the Scintillator High-Gain Avalanche Rushing Photoconductor AMFPI (SHARP-AMFPI), an indirect detector that utilizes avalanche amorphous selenium (a-Se) to amplify optical signal from the scintillator prior to readout. We previously demonstrated the feasibility of a large area SHARP-AMFPI, however there are several areas of desired improvement. In this work, we present a newly fabricated SHARP-AMFPI prototype detector with the following developments: metal oxide hole blocking layer (HBL) with improved electron transport, transparent bias electrode for increased optical coupling, and detector assembly allowing for a back-irradiation (BI) geometry to improve detective quantum efficiency of scintillators. Our measurements showed that the new prototype has improved temporal performance, with lag and ghosting below 1%. We also show an improvement in optical coupling from 25% to 90% for cesium iodide (CsI) scintillator emissions. The remaining challenge of the SHARP-AMFPI is to reduce the dark current to prevent dielectric breakdown under high bias and further increase optical quantum efficiency (OQE) to CsI scintillators. We are proposing to use a newly developed quantum dot (QD) oxide layer, which shows to reduce the dark current by an order of magnitude, and tellurium doping, which could increase OQE to 85% to CsI at avalanche fields, in future prototype detectors.
Direct active matrix flat panel imagers (AMFPIs) using amorphous selenium (a-Se) offer high intrinsic spatial resolution but have limited x-ray quantum efficiency at general radiographic energies due to selenium’s low atomic number. Conversely, indirect AMFPIs using inorganic scintillators typically have superior x-ray quantum efficiency at these energies, but inferior spatial resolution and increased noise due to optical effects in the scintillator. These inherent limitations motivate alternative AMFPI designs to further improve detector xray sensitivity and signal-to-noise performance. Towards this goal, this work constructs and experimentally investigates the x-ray imaging performance of a novel direct-indirect prototype imager referred to as Hybrid AMFPI. The imager comprises a direct conversion a-Se layer that may be coupled to an interchangeable scintillator screen through a transparent blocking layer and bias electrode. In this direct-indirect “hybrid” configuration, a-Se serves as both an x-ray and optical sensor. Readout is performed by a thin-film transistor array with 85 μm pixel pitch. The prototype imager’s x-ray sensitivity, modulation transfer function (MTF), noise power spectrum (NPS) and detective quantum efficiency (DQE) are measured in a direct AMFPI configuration (i.e. a-Se alone) and in a Hybrid configuration under identical x-ray exposure conditions and the results are compared. Contrast-detail and spatial resolution phantoms are also imaged using direct, Hybrid and indirect AMFPI configurations under identical exposure conditions to evaluate differences in their imaging performance.
Amorphous selenium is a unique wide-bandgap disordered material, that shows a deterministic single-carrier hole impact ionization process which results in a very low excess noise factor. A key feature of the avalanche phenomenon in amorphous selenium is that transport at high electric fields shifts to non-activated extended states and this necessitates the need to obtain microscopic access into the relaxation dynamics of non-equilibrium 'hot' holes in extended states. Another interesting aspect of elemental selenium is the similarity in short range order that exists across all allotropic forms. Thus, we employ an in-house ensemble Monte Carlo algorithm, in which we take into consideration scattering from acoustic and non-polar optical phonons to describe the general details of the extended-state hole-phonon interaction. The delocalized extended state transport in the amorphous phase is modeled using the band-transport lattice theory of its crystalline counterpart, trigonal selenium. The energy and phonon band structure along with the density of states and acoustic/optical deformation potentials for the crystalline phase was calculated using density functional theory and a parabolic approximation to the density of states function was used in the simulation. We validate our calculated drift mobility with experimental results in the perpendicular and parallel directions to the c-axis, in the unit cell for trigonal selenium. Moreover, in the direction perpendicular to the c-axis we show that acoustic and non-polar optical phonons are able to maintain a stable hole-energy distribution as long as the electric field is lower than the critical value of 650 kV/cm. Beyond a certain critical electric field, holes in selenium can get 'hot' and gain energy at a faster rate than they loose to the lattice.
Photon counting detectors (PCDs) have the potential to improve x-ray imaging, however current crystalline materials are still hindered by high production cost and performance limitations. We are developing a novel direct conversion amorphous Selenium (a-Se) based field-Shaping multi-Well Avalanche Detector (SWAD) for photon counting breast imaging applications. SWAD’s multi-well Frisch grid design creates separate absorption and sensing regions capable of depth independent avalanche gain. The improved temporal response from unipolar time-differential (UTD) charge sensing combined with tunable avalanche gain within the well region attains the fast timing and energy resolution necessary for successful photon counting under clinical settings. The avalanche gain in a-Se sensors varies rapidly as a function of electric field, which may affect the overall energy resolution of the detector. The goal of this work is to investigate the uniformity of avalanche gain as a function carrier position within the a-Se bulk region for different SWAD design parameters. Our simulation results show that for the geometries modeled, the variation in avalanche gain along different field lines across the multi-well region can be kept below 4.2% by using multi-well pillars with a high aspect ratio. Additionally, the variation in avalanche gain was evaluated for charge clouds generated by incident x-rays with energies of 20, 40 and 60 keV. In all cases the variation in avalanche gain was found to decrease with increasing charge cloud size. For an optimized SWAD geometry, the variation in gain was negligible for each incident x-ray energy simulated.
KEYWORDS: Modulation transfer functions, Sensors, X-rays, X-ray detectors, Signal detection, Systems modeling, Iodine, X-ray imaging, Selenium, Signal to noise ratio, Image quality, Photon counting
Contrast-enhanced digital mammography using spectroscopic x-ray detectors may improve image quality relative to existing contrast-enhanced breast imaging approaches. We present a framework for theoretical modelling of signal and noise in contrast-enhanced spectral mammography (CESM) and apply our framework to systems that use a spectroscopic amorphous selenium (a-Se) field-Shaping multi-Well Avalanche Detector (SWAD) which uses avalanche gain to overcome the low conversion gain of a-Se. We modelled an approach that uses an a-Se SWAD with 100x100 μm2 detector elements, a converter thickness of 300 μm, an avalanche gain of ten, a 10-keV electronic noise floor and two energy bins. We modelled the influence of quantum efficiency, conversion gain, avalanche gain, characteristic emission, electronic noise, energy thresholding and image subtraction on the modulation transfer function (MTF), noise power spectrum (NPS) and iodine contrast. We investigated the choice of energy thresholds for the task of visualizing iodine signals. Our analysis demonstrates that reabsorption of characteristic photons yields energy-bin-dependent MTFs. As a result, spectral subtraction of low-energy and high-energy images enhances high spatial frequencies relative to low spatial frequencies. This effect, combined with better noise performance when using the lowest possible threshold to separate low-energy photons from electronic noise, results in better imaging performance than when reabsorption is suppressed through thresholding. Our theoretical framework enables quantifying trade offs between contrast, spatial resolution and noise for analysis of novel approaches for CESM, and provides a theoretical platform for comparison of CESM with existing approaches.
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.
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
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 241Am 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.
KEYWORDS: Photon counting, Sensors, Monte Carlo methods, Selenium, Crystals, Semiconductors, X-ray imaging, Detector development, X-rays, Signal detection, Breast imaging, Electrodes, Signal attenuation, Prototyping, Luminescence
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.
Active matrix flat panel imagers (AMFPI) have become the dominant detector technology for digital radiography and fluoroscopy. For low dose imaging, electronic noise from the amorphous silicon thin film transistor (TFT) array degrades imaging performance. We have fabricated the first prototype solid-state AMFPI using a uniform layer of avalanche amorphous selenium (a-Se) photoconductor to amplify the signal to eliminate the effect of electronic noise. We have previously developed a large area solid-state avalanche a-Se sensor structure referred to as High Gain Avalanche Rushing Photoconductor (HARP) capable of achieving gains of 75. In this work we successfully deposited this HARP structure onto a 24 x 30 cm2 TFT array with a pixel pitch of 85 μm. An electric field (ESe) up to 105 Vμm-1 was applied across the a-Se layer without breakdown. Using the HARP layer as a direct detector, an X-ray avalanche gain of 15 ± 3 was achieved at ESe = 105 Vμm-1. In indirect mode with a 150 μm thick structured CsI scintillator, an optical gain of 76 ± 5 was measured at ESe = 105 Vμm-1. Image quality at low dose increases with the avalanche gain until the electronic noise is overcome at a constant exposure level of 0.76 mR. We demonstrate the success of a solid-state HARP X-ray imager as well as the largest active area HARP sensor to date.
Active Matrix Flat Panel Imagers (AMFPI) based on an array of thin film transistors (TFT) have become the dominant technology for digital x-ray imaging. In low dose applications, the performance of both direct and indirect conversion detectors are limited by the electronic noise associated with the TFT array. New concepts of direct and indirect detectors have been proposed using avalanche amorphous selenium (a-Se), referred to as high gain avalanche rushing photoconductor (HARP). The indirect detector utilizes a planar layer of HARP to detect light from an x-ray scintillator and amplify the photogenerated charge. The direct detector utilizes separate interaction (non-avalanche) and amplification (avalanche) regions within the a-Se to achieve depth-independent signal gain. Both detectors require the development of large area, solid state HARP. We have previously reported the first avalanche gain in a-Se with deposition techniques scalable to large area detectors. The goal of the present work is to demonstrate the feasibility of large area HARP fabrication in an a-Se deposition facility established for commercial large area AMFPI. We also examine the effect of alternative pixel electrode materials on avalanche gain. The results show that avalanche gain > 50 is achievable in the HARP layers developed in large area coaters, which is sufficient to achieve x-ray quantum noise limited performance down to a single x-ray photon per pixel. Both chromium (Cr) and indium tin oxide (ITO) have been successfully tested as pixel electrodes.
An indirect digital x-ray detector is designed, fabricated, and tested. The detector integrates a high speed, low noise
CMOS substrate with two types of amorphous semiconductors on the circuit surface. Using a laterally oriented layout
a-Si:H or a-Se can be used to coat the CMOS circuit and provide high speed photoresponse to complement the high speed
circuits possible on CMOS technology. The circuit also aims to reduce the effect of slow carriers by integrated a Frisch
style grid on the photoconductive layer to screen for the slow carriers. Simulations show a uniform photoresponse for
photons absorbed on the top layer and an enhanced response when using a Frisch grid. EQE and noise results are
presented. Finally, possible applications and improvements to the area of indirect x-ray imaging that are capable of easily
being implemented on the substrate are suggested.
Digital imaging systems for medical applications use amorphous silicon thin-film transistor (TFT) technology due to its
ability to be manufactured over large areas. However, TFT technology is far inferior to crystalline silicon CMOS
technology in terms of the speed, stability, noise susceptibility, and feature size. This work investigates the feasibility of
integrating an imaging array fabricated in CMOS technology with an a-Se detector. The design of a CMOS passive pixel
sensor (PPS) array is presented, in addition to how an 8×8 PPS array is integrated with the 75 micron thick stabilized
amorphous selenium detector. A non-linear increase in the dark current of 200 pA, 500 pA and 2 nA is observed with
0.27, 0.67 and 1.33 V/micron electric field respectively, which shows a successful integration of selenium layer with the
CMOS array. Results also show that the integrated Selenium-CMOS PPS array has good responsivity to optical light and
X-rays, leaving the door open for further research on implementing CMOS imaging architectures going forward.
Demonstrating that the PPS chips using CMOS technology can use a-Se as a detector is thus the first step in a promising
path of research, which should yield substantial and exciting results for the field. Though area may still prove
challenging, larger CMOS wafers can be manufactured and tiled to allow for a large enough size for certain diagnostic
imaging applications and potentially even large area applications like digital mammography.
In indirect digital x-ray detectors, photodetectors such as hydrogenated amorphous silicon (a-Si:H) p-i-n photodetectors
are used to convert the optical photons generated by the scintillating material to collectible electron-hole
pairs. A problem that arises during the collection of the charges is that the mobility and lifetime of both types
of carriers (electrons and holes) differ. In a-Si:H, the mobility of holes is much lower than that of electrons which
leads to depth-dependent signal variations and causes the charge collection time to be extensive. It has been
shown that the use of a Frisch grid can reduce the effect of the slower carriers in direct x-ray detectors. The
Frisch grid is essentially a conducting grid that shields carriers from the collecting electrode until they are in close
proximity. When the pixel electrodes are properly biased, the grid prevents the slow moving carriers (traveling
away from the collecting electrode) from being collected and puts more weight on the fast moving carriers, thus
allowing the total charge to be collected in less time.
In this paper we investigate the use of a Frisch grid in a-Si:H p-i-n photodetectors for indirect x-ray detectors.
Through simulations and theoretical analysis we determine the grid line sizes and positioning that will be most
effective for practical p-i-n photodetector designs. In addition we compare the results of photodetectors with
and without the grid to characterize the improvement achievable.
KEYWORDS: Photodetectors, Electrodes, Selenium, Sensors, Gamma ray imaging, Photomultipliers, Crystals, X-ray imaging, X-ray detectors, Signal to noise ratio
We propose a new indirect x-ray and gamma-ray detector which is comprised of a scintillating crystal coupled with an
amorphous selenium (a-Se) metal-semiconductor-metal (MSM) photodetector. A lateral Frisch grid is embedded
between the anode and the cathode to provide (1) unipolar charge sensing and (2) avalanche multiplication gain during
hole transport inside the detection region. Unipolar charge sensing operation reduces the persistent photocurrent lag and
increases the speed of the photodetector because most of the pixel charge is induced during carrier transport inside the detection region. Also, with proper biasing of the electrodes, we can create a high-field region between the lateral Frisch grid and the cathode for avalanche multiplication gain. Thus, we can convert the photodetector into a photomultiplier for higher signal-to-noise ratio and single photon-counting gamma-ray imaging. We present for the first time, a fabricated amorphous selenium lateral Frisch photodetector and present preliminary results of the measured photocurrents in response to a blue light emitting diode.
Incomplete charge collection due to poor electron mobility in amorphous selenium (a-Se) results in depth-dependent
signal variations. The slow signal rise-time for the portion of the induced charge due to electron-movement towards the
anode and significant electron trapping cause ballistic deficit. In this paper, we investigate Frisch-grid detector design to
reduce the depth dependent noise, increase the photon count-rate, and improve the spectral performance of positively
biased amorphous selenium radiation detectors. In addition, we analyze the impact of using the Frisch grid detector
design on x-ray sensitivity, detective quantum efficiency (DQE), modulation transfer function (MTF), and image lag of
integrating-mode a-Se radiation detectors. Preliminary results based on theory are presented for emerging digital medical
imaging modalities such as mammography tomosynthesis and fluoroscopy.
A new pixel readout architecture is presented for amorphous selenium (a-Se) direct conversion radiation detectors.
Photon-counting operation provides excellent sensitivity to low radiation doses but saturates the system at medium to
high doses due to the poor charge transport properties of a-Se. Thus, we present an a-Se readout circuit design that is
also capable of dynamically operating in integrating mode to enable the detection of high-dose radiation. This extends
the resolvable dynamic range of the imaging system from very low gamma-ray count rates to very high flux x-ray
radiations. The readout circuit is very promising for applications such as mammography tomosynthesis, and its benefits
can also be extended to other radiation detectors. Finally, we present preliminary spectroscopy results with a-Se.
KEYWORDS: Sensors, Electrodes, Selenium, Near field, Signal detection, Absorption, Electronics, Electric field sensors, Electron transport, X-ray detectors
We investigate amorphous Selenium Frisch-grid detector design to improve the spectral performance, reliability of
single photon detection, and image lag for radiation imaging and detection. Incomplete charge collection due to the low
electron mobility in amorphous Selenium results in depth-dependent signal variations. The slow signal rise-time for the
portion of the induced charge due to electron-movement towards the anode and significant electron trapping cause
ballistic deficit. This phenomenon can be observed from spectrum tailing (also called "electron tailing" for a-Se) and the
wide Gaussian spectrum at low photon energies. The implications of this analysis for the design of new Selenium-based
photoconductors are discussed, and some preliminary simulation results of the theory are presented.
Permanent breast seed implant (PBSI) brachytherapy technique was recently introduced as an alternative to high dose
rate (HDR) brachytherapy and involves the permanent implantation of radioactive 103Palladium seeds into the surgical
cavity of the breast for cancer treatment. To enable accurate seed implantation, this research introduces a gamma camera
based on a hybrid amorphous selenium detector and CMOS readout pixel architecture for real-time imaging of
103Palladium seeds during the PBSI procedure. A prototype chip was designed and fabricated in 0.18-μm n-well CMOS
process. We present the experimental results obtained from this integrated photon counting readout pixel.
Photon counting is emerging as an alternative detection technique to conventional photon integration. In photon counting systems, the value of each image pixel is equal to the number of photons that are absorbed by the radiation detector. The proposed pixel architecture provides a method for energy windowing and serial readout for low-dose gamma-ray imaging. Each pixel is comprised of a radiation detector and integrated analog and digital circuitry. A prototype was developed on a printed circuit board (PCB) using discrete electronic components. In this research, we present the experimental results for the operation of the photon counting pixel with energy windowing and investigate the compromise between pixel noise level and photon count rate.
Photon counting is an emerging detection technique that is promising for mammography tomosynthesis imagers. In photon counting systems, the value of each image pixel is equal to the number of photons that interact with the detector. In this research, we introduce the design and implementation of a low noise, novel selective photon counting pixel for digital mammography tomosynthesis in crystalline silicon CMOS (complementary metal oxide semiconductor) 0.18 micron technology. The design comprises of a low noise charge amplifier (CA), two low offset voltage comparators, a decision-making unit (DMU), a mode selector, and a pseudo-random counter. Theoretical calculations and simulation results of linearity, gain, and noise of the photon counting pixel are presented.
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