Direct Conversion X-ray Detectors Research Articles

Evaluating the Effects of Breaking Vacuum During the Fabrication of Amorphous Selenium Detectors.

Direct conversion X-ray detectors offer high spatial resolution and improved sensitivity over indirect conversion detectors. As interest increases in their applications, including utilization as the top layer in dual-layer detectors for polyenergetic X-ray detection, additional studies on fabrication techniques are required. Amorphous selenium is a well-studied high-Z semiconductor capable of high-resolution and high-sensitivity imaging in direct architectures. It is already commercially available for mammography, and much work has gone into developing it for higher-energy applications (>20 keV.) To fully attenuate energies required for the low energy (<35 keV) top layer in dual-layer detectors, a-Se thicknesses greater than 200 μm must be fabricated. However, evaporation crucibles have limited capacity and require reloading of crucible material to achieve higher thicknesses, which is performed by opening the chamber and breaking vacuum mid-fabrication. We investigate the effects of splitting the fabrication into two depositions - exposing the sample to air in between - on device performance. We find that there is no significant effect on the transport properties, and a small range of performance parameters can be found - suggesting small fluctuations between devices of ± 5%, independent of fabrication technique and thickness. This implies that as we increase our thicknesses to those required for the low-energy layer of the dual-layer detector, we can expect performance to be maintained.

A detachable interface for stable low-voltage stretchable transistor arrays and high-resolution X-ray imaging

Challenges associated with stretchable optoelectronic devices, such as pixel size, power consumption and stability, severely brock their realization in high-resolution digital imaging. Herein, we develop a universal detachable interface technique that allows uniform, damage-free and reproducible integration of micropatterned stretchable electrodes for pixel-dense intrinsically stretchable organic transistor arrays. Benefiting from the ideal heterocontact and short channel length (2 μm) in our transistors, switching current ratio exceeding 106, device density of 41,000 transistors/cm2, operational voltage down to 5 V and excellent stability are simultaneously achieved. The resultant stretchable transistor-based image sensors exhibit ultrasensitive X-ray detection and high-resolution imaging capability. A megapixel image is demonstrated, which is unprecedented for stretchable direct-conversion X-ray detectors. These results forge a bright future for the stretchable photonic integration toward next-generation visualization equipment.

The Effect of Air Ionization in Testing Perovskite-Based Direct Conversion X-ray Detectors

The Effect of Air Ionization in Testing Perovskite-Based Direct Conversion X-ray Detectors

Response characteristics of MAPbBr3 direct conversion X-ray detectors based on measurements and Monte Carlo simulation

In recent years, higher requirements have been placed on the response characteristics of X-ray detectors in the field of medical diagnostic imaging. Due to this, high sensitivity, high attenuation coefficient and low cost detection materials need to be developed. In this paper, the geometric model of a detector was established by Geant4 code. The absorption efficiency and mass attenuation coefficient of MAPbBr3 crystals were calculated in the energy range of 30 keV to 100 keV. Compared with the mass attenuation coefficient of the NIST database, the deviation was within 1.39%. The signal charge number and detection sensitivity of the X-ray interaction with the MAPbBr3 crystal ware calculated. Compared with the CdTe crystal and α-Se, the MAPbBr3 crystal still had a larger detection sensitivity under a smaller applied electric field, which was approximately 9 times higher than that of α-Se. This result indicated that the detection sensitivity could be greatly improved by using a high atomic number and high charge mobility-lifetime product. Based on the simulation results, the 2 mm thick MAPbBr3 crystal exhibited the highest detection sensitivity at 60 keV X-rays, which was in agreement with the experimental results.

High spatial resolution direct conversion amorphous selenium X-ray detectors with monolithically integrated CMOS readout

Recent progress in the field of micron-scale spatial resolution direct conversion X-ray detectors for high-energy synchrotron light sources serve applications ranging from nondestructive and noninvasive microscopy techniques which provide insight into the structure and morphology of crystals, to medical diagnostic measurement devices. Amorphous selenium (a-Se) as a wide-bandgap thermally evaporated photoconductor exhibits ultra-low thermal generation rates for dark carriers and has been extensively used in X-ray medical imaging. Being an amorphous material, it can further be deposited over large areas at room temperatures and at substantially lower costs as compared to crystalline semiconductors. To address the demands for a high-energy and high spatial resolution X-ray detector for synchrotron light source applications, we have thermally evaporated a-Se on a Mixed-Mode Pixel Array Detector (MM-PAD) Application Specific Integrated Circuit (ASIC). The ASIC format consists of 128 × 128 square pixels each 150 μm on a side. A 200 μm a-Se layer was directly deposited on the ASIC followed by a metal top electrode. The completed detector assembly was tested with 45 kV Ag and 23 kV Cu X-ray tube sources. The detector fabrication, performances, Modulation Transfer Function (MTF) measurements, and simulations are reported.

Facile preparation of fully amorphous bulk PbO-Ga2O3 system for direct photon-charge conversion X-ray detector with low dark current and high photocurrent

An amorphous state makes the material free of interface and ensures the overall uniformity of the material and device, to the benefit of large-area electronic devices, such as amorphous silicon and indium gallium zinc oxide display devices and amorphous selenium flat panel detectors. Elements with high atomic number endow the PbO-Ga2O3 system materials with high X-ray absorption coefficients, and wide band gap leading to a low dark current and high signal-to-noise ratio. These advantages are in line with the criteria for a practical X-ray detector development. Herein, the direct photon-charge conversion X-ray detectors based on fully amorphous bulk PbO-Ga2O3 system [(PbO)x(Ga2O3)1-x (x = 0.698, 0.747, 0.761, 0.789)] was fabricated by using high-temperature melt quenching method. Under a fixed electric field of 100 V/mm and X-ray irradiation dose rate of 0.57 μGy ms−1, the dark current as low as 1.1 pA mm−2, photocurrent up to 1600 pA mm−2, signal-to-noise ratio up to 800, comparable sensitivity as 2.02 μC Gy−1 cm−2 of the as fabricated X-ray detector were obtained. All the results suggest that amorphous bulk PbO-Ga2O3 system materials are potential candidates for developing of direct photon-charge conversion X-ray detectors.

Nanocrystalline CaWO4 and ZnWO4 Tungstates for Hybrid Organic-Inorganic X-ray Detectors.

Hybrid materials combining an organic matrix and high-Z nanomaterials show potential for applications in radiation detection, allowing unprecedented device architectures and functionality. Herein, novel hybrid organic-inorganic systems were produced using a mixture of tungstate (CaWO4 or ZnWO4) nanoparticles with a P3HT:PCBM blend. The nano-tungstates with a crystallite size of 43 nm for CaWO4 and 30 nm for ZnWO4 were synthesized by the hydrothermal method. Their structure and morphology were characterized by X-ray diffraction and scanning electron microscopy. The hybrid systems were used to fabricate direct conversion X-ray detectors able to operate with zero bias voltage. The detector performance was tested in a wide energy range using monochromatic synchrotron radiation. The addition of nanoparticles with high-Z elements improved the detector response to X-ray radiation compared with that of a pure organic P3HT:PCBM bulk heterojunction cell. The high dynamic range of our detector allows for recording X-ray absorption spectra, including the fine X-ray absorption structure located beyond the absorption edge. The obtained results suggest that nanocrystalline tungstates are promising candidates for application in direct organic-inorganic X-ray detectors.

Direct Conversion X-ray Detector with Micron-Scale Pixel Pitch for Edge-Illumination and Propagation-Based X-ray Phase-Contrast Imaging.

In this work, we investigate the potential of employing a direct conversion integration mode X-ray detector with micron-scale pixels in two different X-ray phase-contrast imaging (XPCi) configurations, propagation-based (PB) and edge illumination (EI). Both PB-XPCi and EI-XPCi implementations are evaluated through a wave optics model—numerically simulated in MATLAB—and are compared based on their contrast, edge-enhancement, visibility, and dose efficiency characteristics. The EI-XPCi configuration, in general, demonstrates higher performance compared to PB-XPCi, considering a setup with the same X-ray source and detector. However, absorption masks quality (thickness of X-ray absorption material) and environmental vibration effect are two potential challenges for EI-XPCi employing a detector with micron-scale pixels. Simulation results confirm that the behavior of an EI-XPCi system employing a high-resolution detector is susceptible to its absorption masks thickness and misalignment. This work demonstrates the potential and feasibility of employing a high-resolution direct conversion detector for phase-contrast imaging applications where higher dose efficiency, higher contrast images, and a more compact imaging system are of interest.

Dark Current Modeling for a Polyimide-Amorphous Lead Oxide-Based Direct Conversion X-ray Detector.

The reduction of the dark current (DC) to a tolerable level in amorphous selenium (a-Se) X-ray photoconductors was one of the key factors that led to the successful commercialization of a-Se-based direct conversion flat panel X-ray imagers (FPXIs) and their widespread clinical use. Here, we discuss the origin of DC in another X-ray photoconductive structure that utilizes amorphous lead oxide (a-PbO) as an X-ray-to-charge transducer and polyimide (PI) as a blocking layer. The transient DC in a PI/a-PbO detector is measured at different applied electric fields (5–20 V/μm). The experimental results are used to develop a theoretical model describing the electric field-dependent transient behavior of DC. The results of the DC kinetics modeling show that the DC, shortly after the bias application, is primarily controlled by the injection of holes from the positively biased electrode and gradually decays with time to a steady-state value. DC decays by the overarching mechanism of an electric field redistribution, caused by the accumulation of trapped holes in deep localized states within the bulk of PI. Thermal generation and subsequent multiple-trapping (MT) controlled transport of holes within the a-PbO layer governs the steady-state value at all the applied fields investigated here, except for the largest applied field of 20 V/μm. This suggests that a thicker layer of PI would be more optimal to suppress DC in the PI/a-PbO detector presented here. The model can be used to find an approximate optimal thickness of PI for future iterations of PI/a-PbO detectors without the need for time and labor-intensive experimental trial and error. In addition, we show that accounting for the field-induced charge carrier release from traps, enhanced by charge hopping transitions between the traps, yields an excellent fit between the experimental and simulated results, thus, clarifying the dynamic process of reaching a steady-state occupancy level of the deep localized states in the PI. Practically, the electric field redistribution causes the internal field to increase in magnitude in the a-PbO layer, thus improving charge collection efficiency and temporal performance over time, as confirmed by experimental results. The electric field redistribution can be implemented as a warm-up time for a-PbO-based detectors.

Nucleation Engineering in Sprayed MA3Bi2I9 Films for Direct-Conversion X-ray Detectors.

Metal halide perovskite and its derivatives show great promise in X-ray detection. However, large-scale fabrication of high-quality thick perovskite films is still full of challenges due to the complicated crystal nucleation process that always introduces lots of cracks or pinholes in the final perovskite film. Here, a MA3Bi2I9 film was fabricated by the cost-effective, scalable spraying process, and MACl was used as an additive to effectively tune the crystallization process. As a result, a dense MA3Bi2I9 film constituted by large grains was obtained, which has a high carrier mobility of ∼1 cm2 V-1 s-1 and a large activation energy (Ea) for ion migration of 0.91 eV. Thanks to the outstanding optoelectronic characteristics, X-ray detectors with a configuration of ITO/MA3Bi2I9/Au show a sensitivity of 35 μC Gyair-1 cm-2 and a limit of detection (LoD) of 0.14 μGyairs-1, which is outstanding compared with commercial α-Se detectors.

Direct conversion X-ray detectors with 70 pA cm−2 dark currents coated from an alcohol-based perovskite ink

A poly(methyl methacrylate) blocking layer was found to reduce the dark current density in CH3NH3PbI3-based direct-conversion X-ray detectors to as low as 70 pA cm−2.

Direct Conversion X-Ray Detectors with High Sensitivity at Low Dose Rate Based on All-Inorganic Lead-Free Perovskite Wafers

Direct Conversion X-Ray Detectors with High Sensitivity at Low Dose Rate Based on All-Inorganic Lead-Free Perovskite Wafers

Photon-Counting Detector CT: Key Points Radiologists Should Know.

Photon-counting detector (PCD) CT is a new CT technology utilizing a direct conversion X-ray detector, where incident X-ray photon energies are directly recorded as electronical signals. The design of the photon-counting detector itself facilitates improvements in spatial resolution (via smaller detector pixel design) and iodine signal (via count weighting) while still permitting multi-energy imaging. PCD-CT can eliminate electronic noise and reduce artifacts due to the use of energy thresholds. Improved dose efficiency is important for low dose CT and pediatric imaging. The ultra-high spatial resolution of PCD-CT design permits lower dose scanning for all body regions and is particularly helpful in identifying important imaging findings in thoracic and musculoskeletal CT. Improved iodine signal may be helpful for low contrast tasks in abdominal imaging. Virtual monoenergetic images and material classification will assist with numerous diagnostic tasks in abdominal, musculoskeletal, and cardiovascular imaging. Dual-source PCD-CT permits multi-energy CT images of the heart and coronary arteries at high temporal resolution. In this special review article, we review the clinical benefits of this technology across a wide variety of radiological subspecialties.

Sensitive direct-conversion X-ray detectors formed by ZnO nanowire field emitters and β-Ga2O3 photoconductor targets with an electron bombardment induced photoconductivity mechanism

Sensitive X-ray detection is needed in diverse areas motivated by a common desire to reduce radiation dose. Cold cathode X-ray detectors operating with a photoelectron multiplication mechanism called electron bombardment induced photoconductivity (EBIPC) have emerged as promising candidates for low-dose X-ray detection. Herein, the cold cathode detectors formed by ZnO nanowire field emitters and β - Ga 2 O 3 photoconductor targets were proposed for sensitive direct-conversion X-ray detection. The charge carrier transport mechanism of EBIPC effect in X-ray detectors was investigated to achieve a high internal gain ( 2.9 × 10 2 ) and high detection sensitivity ( 3.0 × 10 3 μCGy air − 1 cm − 2 ) for a 6 keV X-ray at the electric field of 22.5 V μm − 1 . Furthermore, the proposed X-ray detectors showed the features of fast response time (40 ms), long-term stability (0.6% for 1 h), and low detection limit ( 0.28 mGy air s − 1 ), suggesting that the direct-conversion cold cathode X-ray detectors are ideal candidates for low-energy X-ray detecting and imaging applications.

Halide-modulated self-assembly of metal-free perovskite single crystals for bio-friendly X-ray detection

Halide-modulated self-assembly of metal-free perovskite single crystals for bio-friendly X-ray detection

An X-ray detector based on a Pt-MgZnO-Pt structure

Direct-conversion X-ray detectors made of wide-bandgap semiconductors have many advantages and important applications. In this study, a direct-conversion X-ray detector based on Pt-MgZnO-Pt structure was fabricated by radio frequency magnetron sputtering. The X-ray detection characteristics of the device were studied. The optimized device with a structure of Pt-MgZnO-Pt exhibited a responsivity of 72.6 nC Gyair –1 cm–2, response time of ∼0.1 s (rise)/∼0.4 s (fall), and a high SNR of 206 at a bias of 30 V under a dose rate of 100 mGyair/s. The experimental results demonstrate the good application prospects of the wide-bandgap MgZnO-based X-ray detectors.

An aerosol-liquid-solid process for the general synthesis of halide perovskite thick films for direct-conversion X-ray detectors

Summary Medical X-ray computed tomography requires imaging at a low dose rate and in a large area. Halide perovskites have shown high potential for X-ray detection, but a pressing challenge is the current lack of low-cost methods for large-scale fabrication of high-quality thick perovskite films. Here we demonstrate and elucidate an aerosol-liquid-solid process to enable continuous growth of uniform halide perovskite films at low temperature over a large area of 100 cm2, which are vertically monolithic, and thus beneficial to carrier transport. Direct-conversion X-ray detectors based on the representative CsPbI2Br films in conjunction with a deliberated, interface-engineered C-electrode have realized an unprecedented sensitivity (≥1.48 × 105 μC Gyair−1 cm−2) and a low limit of detection (280 nGyair s−1). We have further demonstrated the high-resolution radiographic imaging capability of these films. These results lay the groundwork for large-scale application of halide perovskites in radiation detectors.

An Aerosol-Liquid-Solid Process for the General Synthesis of Halide Perovskite Thick Films for Direct Conversion X-Ray Detectors

Medical X-ray computed tomography requires fast imaging at a low dose rate and in a large area. Halide perovskites have shown high potential for X-ray detection, but a pressing challenge is the current lack of low-cost methods for large-scale fabrication of high-quality thick perovskite films. Here we demonstrate and elucidate an aerosol-liquid-solid process to enable continuous growth of uniform halide perovskites films at low temperature over a large area of 100 cm2, which are vertically monolithic and monocrystalline, and thus beneficial to carrier transport. Direct conversion X-ray detectors based on the representative CsPbI2Br films in conjunction with a deliberated, interface-engineered C-electrode have realized an unprecedented sensitivity (≥1.48×105 μC Gyair−1cm−2). We have further demonstrated the high-resolution radiographic imaging capability of these films. These results lay the groundwork for large scale application of halide perovskites in radiation detectors.

Bilayer lead oxide X-ray photoconductor for lag-free operation

Polycrystalline Lead Oxide (poly-PbO) was considered one of the most promising photoconductors for the direct conversion X-ray medical imaging detectors due to its previous success in optical imaging, i.e., as an optical target in so-called Plumbicon video pick-up tubes. However, a signal lag which accompanies X-ray excitation, makes poly-PbO inapplicable as an X-ray-to-charge transducer in real-time X-ray imaging. In contrast, the recently synthesized Amorphous Lead Oxide (a-PbO) photoconductor is essentially lag-free. Here, we report on our approach to a PbO detector where a thin layer of a-PbO is combined with a thick layer of poly-PbO for lag-free operation. In the presented a-PbO/poly-PbO bilayer structure, the poly-PbO layer serves as an X-ray-to-charge transducer while the a-PbO acts as a lag prevention layer. The hole mobility in the a-PbO/poly-PbO bilayer structure was measured by photo-Charge Extraction by Linearly Increasing Voltage technique at different temperatures and electric fields to investigate charge transport properties. It was found that the hole mobility is similar to that in a-Se—currently the only commercially viable photoconductor for the direct conversion X-ray detectors. Evaluation of the X-ray temporal performance demonstrated complete suppression of signal lag, allowing operation of the a-PbO/poly-PbO detector in real-time imaging.

Flexible Inkjet-Printed Triple Cation Perovskite X-ray Detectors.

Flexible direct conversion X-ray detectors enable a variety of novel applications in medicine, industry, and science. Hybrid organic-inorganic perovskite semiconductors containing elements of high atomic number combine an efficient X-ray absorption with excellent charge transport properties. Due to their additional cost-effective and low-temperature processability, perovskite semiconductors represent promising candidates to be used as active materials in flexible X-ray detectors. Inspired by the promising results recently reported on X-ray detectors that are based on either triple cation perovskites or inkjet-printed perovskite quantum dots, we here investigate flexible inkjet-printed triple cation perovskite X-ray detectors. The performance of the detectors is evaluated by the X-ray sensitivity, the dark current, and the X-ray stability. Exposed to 70 kVp X-ray radiation, reproducible and highly competitive X-ray sensitivities of up to 59.9 μC/(Gyaircm2) at low operating voltages of 0.1 V are achieved. Furthermore, a significant dark current reduction is demonstrated in our detectors by replacing spin-coated poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) with sputtered NiOx hole transport layers. Finally, stable operation of a flexible X-ray detector for a cumulative X-ray exposure of 4 Gyair is presented, and the applicability of our devices as X-ray imaging detectors is shown. The results of this study represent a proof of concept toward flexible direct conversion X-ray detectors realized by cost-effective and high-throughput digital inkjet printing.