Polycrystalline HgI2 Research Articles

A new generation of direct X-ray detectors for medical and synchrotron imaging applications

Large-area X-ray imaging is one of the most widely used imaging modalities that spans several scientific and technological fields. Currently, the direct X-ray conversion materials that are being commercially used for large-area (> 8 cm × 4 cm without tiling) flat panel applications, such as amorphous selenium (a-Se), have usable sensitivities of up to only 30 keV. Although there have been many promising candidates (such as polycrystalline HgI2 and CdTe), none of the semiconductors were able to assuage the requirement for high energy (> 40 keV) large-area X-ray imaging applications due to inadequate cost, manufacturability, and long-term performance metrics. In this study, we successfully demonstrate the potential of the hybrid Methylammonium lead iodide (MAPbI3) perovskite-based semiconductor detectors in satisfying all the requirements for its successful commercialization in synchrotron and medical imaging. This new generation of hybrid detectors demonstrates low dark current under electric fields needed for high sensitivity X-ray imaging applications. The detectors have a linear response to X-ray energy and applied bias, no polarization effects at a moderate bias, and signal stability over long usage durations. Also, these detectors have demonstrated a stable detection response under BNL’s National Synchrotron Light Source II (NSLS-II) 70 keV monochromatic synchrotron beamline.

Sensitive and Fast Direct Conversion X‐Ray Detectors Based on Single‐Crystalline HgI2 Photoconductor and ZnO Nanowire Vacuum Diode

AbstractPhotoconductive cold cathode X‐ray detectors are highly promising for use in medical diagnosis, industrial inspection, and scientific research applications. However, development of highly sensitive, fast, and stable direct conversion X‐ray detectors remains very challenging because of structural and material limitations. This article presents a direct conversion X‐ray detector based on a HgI2 crystal and a ZnO nanowire vacuum diode with sensitivity of 6.8 × 103 µCGyair−1 cm−2, which is nearly two orders of magnitude higher than that of a commercial a‐Se X‐ray detector. The X‐ray detector, which uses a high‐crystallization HgI2 photoconductor, shows a response time of 0.24 ms, thus significantly outperforming the polycrystalline HgI2 X‐ray detector. In addition, the current limitation between the HgI2 photoconductor and the ZnO nanowire cold cathodes guaranteed device reliability under high applied electric fields for practical applications. The findings presented in this work demonstrate that the proposed X‐ray detector based on the HgI2 photoconductor and the ZnO nanowire vacuum diode is highly efficient and stable.

Surface morphology and electrical properties of polycrystalline mercuric iodide films based on coplanar grid electrode

Polycrystalline mercuric iodide (HgI2) films have been grown on ITO-coated glass substrates using hot-wall physical vapor deposition (HWPVD) with the different source temperatures. The influence of source temperature on the structure and morphology of the polycrystalline HgI2 films is investigated. It is found that the source temperature plays an important role in the improvement of the structure and morphology. The uniformly (001)-oriented polycrystalline HgI2 films can be obtained at the source temperature of 85 °C. The electrical properties of high quality polycrystalline HgI2 films based on coplanar-grid electrode are investigated at room temperature under irradiation conditions by 5.9 keV X-ray irradiation from 55Fe source. For (001)-oriented polycrystalline HgI2 films, the dark current is in the order of 10−8 A and photo current is in the order of 10−7 A with the applied voltage of 40 V. However, a stable signal of capacity in the order of 10−12 F is also obtained under low and high frequency respectively.

Textured α-HgI2 ceramics for sensitive X-ray detection

In the prospect of manufacturing ionizing radiation detectors, dense ceramics of α-HgI2 with a strong preferred orientation were elaborated. X-ray diffraction showed that most crystallites exhibited their crystal c-axis perpendicular to the ceramic plane, which should be favorable for optimum electrical transport properties. Dark current densities about 2nAmm−2 (under 0.2Vμm−1 bias) were measured and were shown to arise mostly from the release of carriers from trap defects (Poole–Frenkel emission). The X-ray sensitivity ≈1600nCmGy−1cm−2 and the μτ value ≈3×10−5cm2V−1 were comparable to those of other forms of polycrystalline HgI2. The HgI2 ceramics also exhibited a perfectly linear response as a function of the X-ray dose.

Simulation and characterization of polycrystalline mercuric iodide film detector based on coplanar grid electrode

Polycrystalline mercuric iodide films are being developed as a new detector technology for X-ray imaging at room temperature. Polycrystalline mercuric iodide (HgI2) films have been grown on ITO-coated glass substrates using hot-wall physical vapor deposition (HWPVD) with the different source temperatures. The electrical properties of high quality polycrystalline HgI2 detector based on coplanar-grid electrode are investigated at room temperature. The HgI2 device of high quality polycrystalline films has low leakage current before X-ray radiation and stable capacity under low frequency. Different coplanar-grid electrode patterns are designed by varying the widths of grid and gap. The finite elements analysis is used to simulate the potential distribution of the coplanar-grid HgI2 device. The electrode design of pattern A (the width of grid is 150 μm and the width of gap is 500 μm) is the best. The electrical characteristic of coplanar-grid device using mask A verifies the feasible simulation of electrode design for pattern A.

Improvement in pixel signal uniformity of polycrystalline mercuric iodide films for digital X-ray imaging

We investigated polycrystalline mercuric iodide (HgI2) that exhibits uniform pixel signals for its use in digital X-ray imaging. To fabricate thin polycrystalline HgI2 films, the particle-in-binder (PIB) method is used because it enables the fabrication of X-ray conversion films at a low temperature and a normal pressure. Moreover, it has a large-scale deposition capacity at a low cost. Although the thin layers fabricated by the PIB method have such advantages, they are chemically unstable and show poor reproducibility and nonuniform X-ray response. To solve these problems, in this study, additional physical and chemical treatments were performed along with the PIB method after taking the size confinement effect of photoconductive particles into consideration. Morphological and electrical properties were measured to investigate the effects of the physical and chemical treatments.

Development of active matrix flat panel imagers incorporating thin layers of polycrystalline HgI2 for mammographic x-ray imaging

Active matrix flat-panel imagers (AMFPIs) offer many advantages and have become ubiquitous across a wide variety of medical x-ray imaging applications. However, for mammography, the imaging performance of conventional AMFPIs incorporating CsI:Tl scintillators or a-Se photoconductors is limited by their relatively modest signal-to-noise ratio (SNR), particularly at low x-ray exposures or high spatial resolution. One strategy for overcoming this limitation involves the use of a high gain photoconductor such as mercuric iodide (HgI2) which has the potential to improve the SNR by virtue of its low effective work function (WEFF). In this study, the performance of direct-detection AMFPI prototypes employing relatively thin layers of polycrystalline HgI2 operated under mammographic irradiation conditions over a range of 0.5 to 16.0 mR is presented. High x-ray sensitivity (corresponding to WEFF values of ∼19 eV), low dark current (<0.1 pA mm−2) and good spatial resolution, largely limited by the size of the pixel pitch, were observed. For one prototype, a detective quantum efficiency of ∼70% was observed at an x-ray exposure of ∼0.5 mR at 26 kVp.

Electrical properties of polycrystalline mercury iodide film

Abstract The outstanding properties of mercury iodide make it an ideal material as room temperature X-ray and gamma-ray detectors. For this purpose, electrical and irradiation properties of high quality polycrystalline HgI 2 detector are investigated at room temperature under irradiation conditions by 5.9 keV X-ray irradiation from a 55 Fe source. For (0 0 1)-oriented polycrystalline HgI 2 film, the dark current is in the order of 10 −10 A and photocurrent is in the order of 10 −9 A with the applied voltage of 40 V. However, a good signal to noise ratio of 4.44 and an energy resolution of 4.1% for X-ray detecting is also obtained.

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.

Properties of Polycrystalline HgI2 Films Fabricated by Using a Particle in Binder Method

Properties of Polycrystalline HgI2 Films Fabricated by Using a Particle in Binder Method

Investigation of the signal behavior at diagnostic energies of prototype, direct detection, active matrix, flat-panel imagers incorporating polycrystalline HgI2

Active matrix, flat-panel x-ray imagers based on a-Si:H thin-film transistors offer many advantages and are widely utilized in medical imaging applications. Unfortunately, the detective quantum efficiency (DQE) of conventional flat-panel imagers incorporating scintillators or a-Se photoconductors is significantly limited by their relatively modest signal-to-noise ratio, particularly in applications involving low x-ray exposures or high spatial resolution. For this reason, polycrystalline HgI2 is of considerable interest by virtue of its low effective work function, high atomic number and the possibility of large-area deposition. In this study, a detailed investigation of the properties of prototype, flat-panel arrays coated with two forms of this high-gain photoconductor are reported. Encouragingly, high x-ray sensitivity, low dark current and spatial resolution close to the theoretical limits were observed from a number of prototypes. In addition, input-quantum-limited DQE performance was measured from one of the prototypes at relatively low exposures. However, high levels of charge trapping, lag and polarization, as well as pixel-to-pixel variations in x-ray sensitivity are of concern. While the results of the current study are promising, further development will be required to realize prototypes exhibiting the characteristics necessary to allow practical implementation of this approach. Publisher's note: The title was changed from: 'Signal behavior of polycrystalline HgI2 at diagnostic energies of prototype, direct detection, active matrix, flat-panel imagers' to 'Investigation of the signal behavior at diagnostic energies of prototype, direct detection, active matrix, flat-panel imagers incorporating polycrystalline HgI2' 24 hours after initial publication to correct a mistake.

Systematic investigation of the signal properties of polycrystalline HgI2 detectors under mammographic, radiographic, fluoroscopic and radiotherapy irradiation conditions

The signal properties of polycrystalline mercuric iodide (HgI2) film detectors, under irradiation conditions relevant to mammographic, radiographic, fluoroscopic and radiotherapy x-ray imaging, are reported. Each film detector consists of an ∼230 to ∼460 µm thick layer of HgI2 (fabricated through physical vapour deposition or a screen-print process) and a thin barrier layer, sandwiched between a pair of opposing electrode plates. The high atomic number, high density and low effective ionization energy, WEFF, of HgI2 make it an attractive candidate for significantly improving the performance of active matrix, flat-panel imagers (AMFPIs) for several x-ray imaging applications. The temporal behaviour of current from the film detectors in the presence and in the absence of radiation was used to examine dark current levels, the lag and reciprocity of the signal response, x-ray sensitivity and WEFF. The results are discussed in the context of present AMFPI performance. This study provides performance data for a wide range of potential medical x-ray imaging applications from a single set of detectors and represents the first investigation of the signal properties of polycrystalline mercuric iodide for the radiotherapy application.

TH-C-I-611-09: Development of Direct Detection Active Matrix Flat-Panel Imagers Employing Mercuric Iodide for Diagnostic Imaging

Purpose: To characterize the performance of prototype direct‐detection active matrix flat‐panel imagers (AMFPIs) employing recently improved forms of polycrystalline mercuric iodide photoconductors as x‐ray converters for fluoroscopic imaging.Method and Materials: While direct and indirect detection flat‐panel imagers offer good performance for many applications, their modest signal per interacting x‐ray limits the DQE performance under imaging conditions of low exposure such as in fluoroscopy. One possible pathway to overcome this limitation involves the use of a high gain photoconductive x‐ray converter such as mercuric iodide (HgI2). Accordingly, performance results from a number of AMFPI prototypes employing two types of polycrystalline HgI2 (a form created through physical vapor deposition,PVD, and a particle‐in‐binder form involving screen printing, PIB) are reported. Each prototype incorporates an array with a pixel format of 768×768 and a pixel pitch of 127 μm, and was operated under fluoroscopic conditions. The measured performance was compared to theoretical calculations based on a cascaded systems model. Results: Performance of the prototypes is reported in terms of pixel properties as well as MTF, NPS and DQE. Recent significant improvements in the properties of HgI2photoconductors, such as chemical stability, low dark current (< 10 pA/mm2), low lag, and high x‐ray sensitivity (corresponding to an effective ionization energy approaching the theoretical limit of ∼5 eV) resulted in a high DQE performance at low exposures. Pixel‐to‐pixel non‐uniformities, which tend to reduce the dynamic range, remain high (∼10% to 30%) and thus require further optimization. Conclusions: While the development of polycrystalline HgI2 as an x‐ray converter for AMFPIs has achieved important milestones related to various material properties, various challenges remain. Nevertheless, the high x‐ray sensitivity and DQE observed at low exposures demonstrate the potential for input‐quantum‐limited fluoroscopic operation. This work was supported by NIH grant R01‐EB000558.

Synthesis and properties of HgI 2 intercalation of Bi 2Sr 2CaCu 2O y superconductor

Polycrystalline or single crystals of HgI 2 intercalated Bi 2Sr 2CaCu 2O y (Bi2212) have been synthesized and their properties have been studied together with those of HgBr 2 intercalates. Single crystals of HgI 2 intercalated Bi2212 were synthesized by a stepwise intercalation method using iodine intercalate as a secondary host material. These could not be obtained through direct intercalation methods by which polycrystalline sample were obtained. From ac susceptibility and resistivity measurements, superconducting onset temperature ( T con) for polycrystalline HgI 2 intercalate was found to be 82 K which was higher than that of the host (75 K). On the other hand, T con for single crystal HgI 2 intercalate was shown to be 79–80 K which is lower than that of host (95–97 K). The effect of HgI 2 or HgBr 2 intercalation on T c tried discussion from the view point of hole doping and interblock coupling effect.

Direct conversion X-ray sensors: sensitivity, DQE and MTF

Imaging characteristics of photoconductive flat panel X-ray image detectors, such as X-ray sensitivity, detective quantum efficiency (DQE), and resolution in terms of the modulation transfer function (MTF) are examined with applications to amorphous selenium (a-Se), polycrystalline HgI2 and CdZnTe detectors. A theoretical model has been developed for the calculation of X-ray sensitivity of a pixellated detector by using the Shockley–Ramo theorem, the weighting potential of the individual pixel and the final trapped charge distribution across the photoconductor. The X-ray sensitivity of pixellated X-ray detectors mostly depends on the mobility and lifetime product of charges that move towards the pixel electrodes, and the extent of dependence increases with decreasing pixel per unit detector thickness. A cascaded linear system model that includes incomplete charge collection and interaction depth dependent conversion gain and charge collection stages is considered for the calculation of the zero spatial frequency detective quantum efficiency DQE(0) of a direct conversion X-ray image detector. The DQE(0) performance of a-Se, HgI2 and CdZnTe detectors is examined for fluoroscopic applications. It is shown that, in addition to high quantum efficiency, both high conversion gain and high charge collection efficiency are required to improve the DQE performance of an X-ray image detector. The factors affecting the MTF of a-Se flat-panel detectors for digital mammography are investigated. Both theoretical and experimental methods are examined. A theoretical model has been developed based on cascaded linear system analysis with parallel processes to take into account the effect of K-fluorescence. This model has been used to understand the performance of a small-area prototype detector with 85 μm pixel size. The presampling MTF of the prototype has been measured and compared to a theoretical calculation based on the model. The calculation shows that K-fluorescence accounts for a 15% reduction in the MTF at the Nyquist frequency of the prototype detector. The measurement of presampling MTF of the prototype detector reveals an additional source of blurring, which is probably due to charge carrier trapping in the blocking layer at the interface between a-Se and the active matrix. This introduces a drop in presampling MTF at high spatial frequencies.

Ｂｉ２Ｓｒ２ＣａＣｕ２Ｏｙ超伝導体へのＨｇＩ２インターカレーションの研究

Polycrystalline or single crystal HgI2 intercalated Bi2Sr2CaCu2Oy, (Bi2212) superconductors have been synthesized and their structure and superconductivity have been studied. According to X-ray diffraction, the a and c axis-lengths were estimated to be 0.54 nm and 2.26 um, respectively. The expansion of c axis length was 0.72 nm, compared to that of host Bi2212 (3.09nm/2=1.54nm), while a axis length did not changed. From ac susceptibility and resistivity measurements, superconducting transition onset temperature (Tc) for polycrystalline HgI2 intercalate was found to be 82K, which is higher than that of host 75K. On the other hand, resistivity measurements for single crystal HgI2 intercalate was showed to be 75K, which is lower than that of host 95K. The effect of HgI2 intercalation on the superconductivity is discussed from the viewpoint of hole concentration change.

Charge collection and absorption-limited sensitivity of x-ray photoconductors: Applications to a-Se and HgI2

A generalized expression for charge carrier transport and absorption-limited sensitivity of x-ray photoconductors is derived by analytically solving the continuity equation for both holes and electrons considering the drift of electrons and holes in the presence of deep traps. The normalized sensitivity equation is applied to stabilized a-Se and HgI2. In the latter case, the sensitivity model is fitted to published data to determine the electron and hole ranges (6.4×10−6 cm2/V and 7×10−8 cm2/V, respectively) in screen-printed polycrystalline HgI2.

Thick films of X-ray polycrystalline mercuric iodide detectors

Polycrystalline HgI2 thick film detectors are among the leading semiconductor materials to be used as direct converters in X-ray digital radiography. Their properties along with a survey of the properties of alternative materials, such as PbI2 or A-Se, will be given. The preparation of HgI2 detector plates, both by direct evaporation (Physical vapor deposition, (PVD)) and by binding the individual crystallites with polymeric glue, forming screen-printed (SP) detector plates, will be described. The microstructure of the PVD thick films showing a columnar morphology, as determined by SEM measurements, will be shown. The X-ray response to radiological X-ray generator of 85kVp using the current integration mode will be reported for both PVD and SP films. Finally, some X-ray images taken at Xerox–Parc using HgI2 polycrystalline detectors will be shown.

Theoretical and experimental sensitivity to X-rays of single and polycrystalline HgI 2 compared with different single-crystal detectors

The response for medical X-ray generator of single and polycrystalline HgI 2 to 65 kVp X-rays has been compared to the response of single-crystal CdZnTe and PbI 2 detectors using the current integration mode. The dark current density expressed in pA/mm 2 and sensitivity expressed in μC/R×cm 2 are given for these detectors for several thicknesses. The polycrystalline HgI 2 thick film detectors were fabricated using the hot wall physical vapor-phase deposition (PVD) method.