A. Reznik

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.

Indirect flat-panel detector with avalanche gain: fundamental feasibility investigation for SHARP-AMFPI (scintillator HARP active matrix flat panel imager).

An indirect flat-panel imager (FPI) with avalanche gain is being investigated for low-dose x-ray imaging. It is made by optically coupling a structured x-ray scintillator CsI(Tl) to an amorphous selenium (a-Se) avalanche photoconductor called HARP (high-gain avalanche rushing photoconductor). The final electronic image is read out using an active matrix array of thin film transistors (TFT). We call the proposed detector SHARP-AMFPI (scintillator HARP active matrix flat panel imager). The advantage of the SHARP-AMFPI is its programmable gain, which can be turned on during low dose fluoroscopy to overcome electronic noise, and turned off during high dose radiography to avoid pixel saturation. The purpose of this paper is to investigate the important design considerations for SHARP-AMFPI such as avalanche gain, which depends on both the thickness dSe and the applied electric field ESe of the HARP layer. To determine the optimal design parameter and operational conditions for HARP, we measured the ESe dependence of both avalanche gain and optical quantum efficiency of an 8μm HARP layer. The results were used in a physical model of HARP as well as a linear cascaded model of the FPI to determine the following x-ray imaging properties in both the avalanche and nonavalanche modes as a function of ESe: (1) total gain (which is the product of avalanche gain and optical quantum efficiency); (2) linearity; (3) dynamic range; (4) gain nonuniformity resulting from thickness nonuniformity; and (5) effects of direct x-ray interaction in HARP. Our results showed that a HARP layer thickness of 8μm can provide adequate avalanche gain and sufficient dynamic range for x-ray imaging applications to permit quantum limited operation over the range of exposures needed for radiography and fluoroscopy.

Avalanche multiplication phenomenon in amorphous semiconductors : Amorphous selenium versus hydrogenated amorphous silicon

Although the effect of the impact ionization and the consequent avalanche multiplication in amorphous selenium (a-Se) was established long ago and has led to the development and commercialization of ultrasensitive video tubes, the underlying physics of these phenomena in amorphous semiconductors has not yet been fully understood. In particular, it is puzzling why this effect has been evidenced at practical electric fields only in a-Se among all amorphous materials. For instance, impact ionization seems much more feasible in hydrogenated amorphous silicon (a-Si:H) since the charge carrier mobility in a-Si:H is much higher than that in a-Se and also the amount of energy needed for ionization of secondary carriers in a-Si:H is lower than that in a-Se. Using the description of the avalanche effect based on the lucky-drift model recently developed for amorphous semiconductors we show how this intriguing question can be answered. It is the higher phonon energy in a-Si:H than that in a-Se, which is responsible f...

A solid-state amorphous selenium avalanche technology for low photon flux imaging applications

PURPOSE The feasibility of a practical solid-state technology for low photon flux imaging applications was investigated. The technology is based on an amorphous selenium photoreceptor with a voltage-controlled avalanche multiplication gain. If this photoreceptor can provide sufficient internal gain, it will be useful for an extensive range of diagnostic imaging systems. METHODS The avalanche photoreceptor under investigation is referred to as HARP-DRL. This is a novel concept in which a high-gain avalanche rushing photoconductor (HARP) is integrated with a distributed resistance layer (DRL) and sandwiched between two electrodes. The avalanche gain and leakage current characteristics of this photoreceptor were measured. RESULTS HARP-DRL has been found to sustain very high electric field strengths without electrical breakdown. It has shown avalanche multiplication gains as high as 10(4) and a very low leakage current (< or = 20 pA/mm2). CONCLUSIONS This is the first experimental demonstration of a solid-state amorphous photoreceptor which provides sufficient internal avalanche gain for photon counting and photon starved imaging applications.

Kinetics of the photostructural changes in a-Se films

The kinetics of the photodarkening effect has been studied experimentally for amorphous selenium (a-Se) layers at room temperature and at an elevated temperature (35°C) close to the glass transition. By switching an intense pumping light on and off with a period of 100s, we have studied the kinetics of both the buildup of photodarkening and its relaxation (recovery). It was found that at 35°C, only a reversible component of photodarkening has been observed. This result has been interpreted within the framework of a phenomenological model assuming that photodarkening is caused by light-induced transitions of structural units from their ground states into metastable states. Our estimate for the energy barrier EB between these states obtained for the photodarkening process (EB∼0.8eV) coincides with that obtained from the analysis of the relaxation process. At room temperature, an irreversible component of photodarkening has been observed along with the reversible one. The energy barrier responsible for the r...

Design and feasibility of active matrix flat panel detector using avalanche amorphous selenium for protein crystallography

Protein crystallography is the most important technique for resolving the three-dimensional atomic structure of protein by measuring the intensity of its x-ray diffraction pattern. This work proposes a large area flat panel detector for protein crystallography based on direct conversion x-ray detection technique using avalanche amorphous selenium (a-Se) as the high gain photoconductor, and active matrix readout using amorphous silicon (a-Si:H) thin film transistors. The detector employs avalanche multiplication phenomenon of a-Se to make the detector sensitive to each incident x ray. The advantages of the proposed detector over the existing imaging plate and charge coupled device detectors are large area, high dynamic range coupled to single x-ray detection capability, fast readout, high spatial resolution, and inexpensive manufacturing process. The optimal detector design parameters (such as detector size, pixel size, and thickness of a-Se layer), and operating parameters (such as electric field across the a-Se layer) are determined based on the requirements for protein crystallography application. The performance of the detector is evaluated in terms of readout time (<1 s), dynamic range (approximately 10(5)), and sensitivity (approximately 1 x-ray photon), thus validating the detectors efficacy for protein crystallography.

Enhanced Detection Efficiency of Direct Conversion X-ray Detector Using Polyimide as Hole-Blocking Layer

In this article we demonstrate the performance of a direct conversion amorphous selenium (a-Se) X-ray detector using biphenyldisnhydride/1,4 phenylenediamine (BPDA/PPD) polyimide (PI) as a hole-blocking layer. The use of a PI layer with a-Se allows detector operation at high electric fields (≥10 V/μm) while maintaining low dark current, without deterioration of transient performance. The hole mobility of the PI/a-Se device is measured by the time-of-flight method at different electric fields to investigate the effect of the PI layer on detector performance. It was found that hole mobility as high as 0.75 cm2/Vs is achievable by increasing the electric field in the PI/a-Se device structure. Avalanche multiplication is also shown to be achievable when using PI as a blocking layer. Increasing the electric field within a-Se reduces the X-ray ionization energy, increases hole mobility, and improves the dynamic range and sensitivity of the detector.

Anisotropy of the carrier effective masses in bulk α-PbO

The electronic properties of an individual α-PbO layer and bulk material consisting of stacked interacting layers are studied with the first-principle calculations. It was found that while for an individual layer the top of the valence band is extremely flat, for stacked layers it becomes dispersive due to the interlayer interaction. As a result, an effective mass of holes drops from m ∗ =3 8m0 in a single layer to m ∗ =2 .44m0 in bulk α-PbO. In contrast, the conduction band is almost independent of the layer interaction: for both individual layer and bulk material the bottom of the conduction band is characterized by sharp dispersion. The electron effective mass is practically unchanged and is about m ∗ =0 .4m0. The calculation of the electron density distribution suggests that heavy holes are a result of a strong localization of lone pair p electrons on the oxygen atom which form the top of the valence band. A similar effect of heavy holes is observed for another photoconductor like amorphous Se (Gobrecht H. and Tausend A., in Proceedings of the International Conference on the Physics of Semiconductor, Paris, 1964 (Academic Press Inc., New York) 1965, p. 1189) for which the lone pairs of the valence electrons play an important role in photo-generation. editors choice Copyright c EPLA, 2012

Detectors with dual-ended readout by silicon photomultipliers for high resolution positron emission mammography applications

We used silicon photomultipliers coupled to LYSO scintillator crystals in dual-ended readout configuration to evaluate detector performance for depth of interaction (DOI) positron emission tomography (PET) applications. By experiments and Monte Carlo simulation, we investigated the effect of scintillator crystal geometry and surface finishing on DOI, energy and timing resolution. Measurements indicate almost linear dependency of signal asymmetry on DOI, with sensitivity of 7.1% mm-1 and DOI resolution of 2.1+0.6 mm for saw-cut, and 1.3% mm-1 and 9.0±1.5 mm for polished scintillator crystals. Unlike polished scintillator crystals, the energy resolution in saw-cut crystals depends on DOI. It improves from 19% when DOI is in the center, to 14% with DOI at either end of the crystal, while it remains independent of DOI at 14% for polished scintillators. Similarly, we observed degradation of timing resolution in saw-cut crystals compared to polished ones. We deduce that the trade-off between DOI and energy resolution is mediated by scintillator crystal surface finishing and geometry. However, accurate extraction of DOI can be used to correct for DOI dependency of energy resolution. Based on our experimental results we conclude that saw-cut 2×2×20 mm3 LYSO crystals coupled to silicon photomultipliers (SiPMs) in dual-ended readout configuration are suitable candidates for DOI-PET applications such as positron emission mammography (PEM).

X‐ray imaging with amorphous selenium: Pulse height measurements of avalanche gain fluctuations

Avalanche multiplication in amorphous selenium (a-Se) can provide a large, adjustable gain for active matrix flat panel imagers (AMFPI), enabling quantum noise limited x-ray imaging during both radiography and fluoroscopy. In the case of direct conversion AMFPI, the multiplication factor for each x ray is a function of its depth of interaction, and the resulting variations in gain can reduce the detective quantum efficiency (DQE) of the system. An experimental method was developed to measure gain fluctuations by analyzing images of individual x rays that were obtained using a video camera with an a-Se target operated in avalanche mode. Pulse height spectra (PHS) of the charge produced per x ray were recorded for monoenergetic 30.9, 49.4, and 73.8keV x-ray sources. The rapid initial decay and long tail of each PHS can be explained by a model in which positive charge dominates the initiation of avalanche. The Swank information factor quantifies the effect of gain fluctuation on DQE and was calculated from the PHS. The information factor was found to be 0.5 for a 25μm a-Se layer with a maximum gain of ∼300. Changing the energy of the incident x ray influenced the range of the primary photoelectron and noticeably affected the tail of the experimental PHS, but did not significantly change the avalanche Swank factor.