George Belev

Amorphous and Polycrystalline Photoconductors for Direct Conversion Flat Panel X-Ray Image Sensors

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.

Spatially resolved measurement of high doses in microbeam radiation therapy using samarium doped fluorophosphate glasses

The measurement of spatially resolved high doses in microbeam radiation therapy has always been a challenging task, where a combination of high dose response and high spatial resolution (microns) is required for synchrotron radiation peaked around 50 keV. The x-ray induced Sm3+ → Sm2+ valence conversion in Sm3+ doped fluorophosphates glasses has been tested for use in x-ray dosimetry for microbeam radiation therapy. The conversion efficiency depends almost linearly on the dose of irradiation up to ∼5 Gy and saturates at doses exceeding ∼80 Gy. The conversion shows strong correlation with x-ray induced absorbance of the glass which is related to the formation of phosphorus-oxygen hole centers. When irradiated through a microslit collimator, a good spatial resolution and high “peak-to-valley” contrast have been observed by means of confocal photoluminescence microscopy.

Lateral metal-semiconductor-metal photodetectors based on amorphous selenium

We report a lateral metal-semiconductor-metal (MSM) photodetector (PD) based on an amorphous selenium (a-Se). The PD exhibits a dark current below 200 fA under electric fields ranging from 6 to 12 V/μm, a responsivity of up to 0.45 A/W, a photogain of 1.2 near short wavelengths of 468 nm, and a high-speed photoresponse with a rise time of 50 μs, fall time of 60 μs, and time constant of 30 μs, respectively. The lateral MSM PD based on a-Se has great potential for use in digital x-ray imaging applications.

X-ray induced Sm3+ to Sm2+ conversion in fluorophosphate and fluoroaluminate glasses for the monitoring of high-doses in microbeam radiation therapy

Fluorophosphate and fluoroaluminate glasses doped with trivalent samarium were evaluated as sensors of x-ray radiation for microbeam radiation therapy at the Canadian Light Source using the conversion of trivalent Sm3+ to the divalent form Sm2+. Both types of glasses show similar conversion rates and may be used as a linear sensor up to ∼150 Gy and as a nonlinear sensor up to ∼2400 Gy, where saturation is reached. Experiments with a multi-slit collimator show high spatial resolution of the conversion pattern; the pattern was acquired by a confocal fluorescence microscopy technique. The effects of previous x-ray exposure may be erased by annealing at temperatures exceeding the glass transition temperature Tg while annealing at TA < Tg enhances the Sm conversion. This enhancement is explained by a thermally stimulated relaxation of host glass ionic matrix surrounding x-ray induced Sm2+ ions. In addition, some of the Sm3+-doped glasses were codoped with Eu2+-ions but the results show that there is no marked ...

Dark current in multilayer stabilized amorphous selenium based photoconductive x-ray detectors

We report on experimental results which show that the dark current in n-i-p structured, amorphous selenium films is independent of i-layer thickness in samples with consistently thick blocking layers. We have observed, however, a strong dependence on the n-layer thickness and positive contact metal chosen. These results indicate that the dominant source of the dark current is carrier injection from the contacts and any contribution from carriers thermally generated in the bulk of the photoconductive layer is negligible. This conclusion is supported by a description of the dark current transients at different applied fields by a model which assumes only carrier emission over a Schottky barrier. This model also predicts that while hole injection is initially dominant, some time after the application of the bias, electron injection may become the dominant source of dark current.

Pseudomonas aeruginosa triggers CFTR-mediated airway surface liquid secretion in swine trachea.

Significance Cystic fibrosis (CF) is a genetic disorder caused by mutations in the gene encoding for the anion channel cystic fibrosis transmembrane conductance regulator (CFTR). Several organs are affected in CF, but most of the morbidity and mortality comes from lung disease caused by the failure to clear bacteria. Bacterial clearance depends on a layer of airway surface liquid (ASL) covering the airways, rich in antimicrobial compounds and mucins, that removes bacteria from the airway through mucociliary clearance. This study provides the first demonstration that inhalation of bacteria triggers CFTR-dependent ASL secretion. We suggest that this response to inhaled pathogens is an important but previously unknown part of the innate immune response that would be missing in CF patients, resulting in reduced bacterial killing and facilitating infection. Cystic fibrosis (CF) is an autosomal recessive genetic disorder caused by mutations in the gene encoding for the anion channel cystic fibrosis transmembrane conductance regulator (CFTR). Several organs are affected in CF, but most of the morbidity and mortality comes from lung disease. Recent data show that the initial consequence of CFTR mutation is the failure to eradicate bacteria before the development of inflammation and airway remodeling. Bacterial clearance depends on a layer of airway surface liquid (ASL) consisting of both a mucus layer that traps, kills, and inactivates bacteria and a periciliary liquid layer that keeps the mucus at an optimum distance from the underlying epithelia, to maximize ciliary motility and clearance of bacteria. The airways in CF patients and animal models of CF demonstrate abnormal ASL secretion and reduced antimicrobial properties. Thus, it has been proposed that abnormal ASL secretion in response to bacteria may facilitate the development of the infection and inflammation that characterize CF airway disease. Whether the inhalation of bacteria triggers ASL secretion, and the role of CFTR, have never been tested, however. We developed a synchrotron-based imaging technique to visualize the ASL layer and measure the effect of bacteria on ASL secretion. We show that the introduction of Pseudomonas aeruginosa and other bacteria into the lumen of intact isolated swine tracheas triggers CFTR-dependent ASL secretion by the submucosal glands. This response requires expression of the bacterial protein flagellin. In patients with CF, the inhalation of bacteria would fail to trigger ASL secretion, leading to infection and inflammation.

X-ray induced effects in stabilized a-Se X-ray photoconductors

Abstract We report on X-ray induced changes in the electrical properties of stabilized amorphous selenium typical of the material used in direct conversion X-ray imaging devices. Carrier mobility and deep-trapping lifetime were measured using time-of-flight and interrupted-field time-of-flight (IFTOF) measurements. The hole and electron drift mobility is not affected by up to 1 R of exposure to 50 kV p X-rays. The hole lifetime decreases from 50 to 27 μs after exposure to 0.48 R. The hole lifetime slowly recovers to its initial value after circa 3000 min. The electron lifetime does not change after exposure to 1 R. The results are explained by an accumulation of excess hole traps due to capture of electrons in deep localized states. We also observe small changes in the dark current after exposure to X-rays.

Relaxation of the electrical properties of vacuum-deposited a-Se1−xAsx photoconductive films: Charge-carrier lifetimes and drift mobilities

The authors have examined the relaxation of the electrical properties of vacuum-deposited x-ray photoconductor-type a-Se1−xAsx (x=0%–0.5%) films in terms of the time evaluation of the deep trapping time τ, i.e., carrier lifetime, and the drift mobility μ from the time the samples were brought to 23 °C after they had been annealed at 55 °C (above the glass-transition temperature Tg) for 30 min. The changes in the lifetime and drift mobility arise from structural-relaxation processes and have been modeled using a stretched exponential-relaxation process, i.e., τ=τ∞+(τo−τ∞)exp[−(t/τsr)β], where τ∞ is the lifetime when the sample is fully relaxed (the final “equilibrium” value), τo is the initial lifetime, τsr is the characteristic structural-relaxation time that controls the relaxation of the observed property, and β is the stretching factor. The authors have examined the relaxation of τ and μ as a function of composition. Within experimental errors, the structural-relaxation time associated with electron an...

Direct-Conversion X-Ray Detector Using Lateral Amorphous Selenium Structure

In this paper, we propose to use a lateral metal-semiconductor-metal (MSM) structure with a thick amorphous selenium (a-Se) layer intended for direct-conversion X-ray detection. For the purposes of demonstration, a variety of single-pixel detectors with electrode spacing ranging from 2 to 10 μm were fabricated and characterized. Compared with the vertical structure, the MSM structure avoids the usage of high voltage, therefore eliminating a safety concern. However, the simulation results indicate that the electric field in such a structure is not uniformly distributed and only confined into a region near the bottom electrodes up to a thickness of ~20 μm. The charge collection is therefore undertaken in the bottom layer and the top layer where a majority of energy deposits instead plays a dominant role in charge generation and diffusion. We believe that the lateral MSM detector with thick a-Se will be feasible for direct-conversion X-ray detection.

Active pixel image sensor for large-area medical imaging

The most widely used pixel architecture is a passive pixel sensor (PPS) where the pixel consists of a detector and an a-Si:H thin-film transistor readout switch. While the PPS has the advantage of being compact and amenable towards high-resolution imaging, the data line capacitance, resistance, and the column charge amplifiers add a large noise component to the PPS that reduces the minimum readable sensor input signal. Building upon previous research into active pixel sensor (APS) based amplified pixel readout circuits, this work investiates a current-mediated APS (C-APS) x-ray detection array for diagnostic medical imaging applications. Preliminary tests indicate linear performance, and a programmable circuits gain via choice of supply voltage and sampling time. In addition, the performance of C-APS amplified pixels is measured from both, a-Si TFT metastability and noise performance perspectives. Theory and measurements indicate that the C-APS pixel architecture is promising for diagnostic medical imaging modalities including low noise, real-time fluoroscopy.