X-ray Pixel Detectors Research Articles

Multi-energy reconstructions, central electron temperature measurements, and early detection of the birth and growth of runaway electrons using a versatile soft x-ray pinhole camera at MST.

A multi-energy soft x-ray pinhole camera has been designed, built, and deployed at the Madison Symmetric Torus to aid the study of particle and thermal transport, as well as MHD stability physics. This novel imaging diagnostic technique employs a pixelated x-ray detector in which the lower energy threshold for photon detection can be adjusted independently on each pixel. The detector of choice is a PILATUS3 100K with a 450 μm thick silicon sensor and nearly 100 000 pixels sensitive to photon energies between 1.6 and 30 keV. An ensemble of cubic spline smoothing functions has been applied to the line-integrated data for each time-frame and energy-range, obtaining a reduced standard-deviation when compared to that dominated by photon-noise. The multi-energy local emissivity profiles are obtained from a 1D matrix-based Abel-inversion procedure. Central values of Te can be obtained by modeling the slope of the continuum radiation from ratios of the inverted radial emissivity profiles over multiple energy ranges with no a priori assumptions of plasma profiles, magnetic field reconstruction constraints, high-density limitations, or need of shot-to-shot reproducibility. In tokamak plasmas, a novel application has recently been tested for early detection, 1D imaging, and study of the birth, exponential growth, and saturation of runaway electrons at energies comparable to 100 × Te,0; thus, early results are also presented.

X-ray rocking curve imaging on large arrays of extremely tall SiGe microcrystals epitaxial on Si

This work investigates layers of densely spaced SiGe microcrystals epitaxially formed on patterned Si and grown up to extreme heights of 40 and 100 µm using the rocking curve imaging technique with standard laboratory equipment and a 2D X-ray pixel detector. As the crystalline tilt varied both within the epitaxial SiGe layers and inside the individual microcrystals, it was possible to obtain real-space 2D maps of the local lattice bending and distortion across the complete SiGe surface. These X-ray maps, showing the variation of crystalline quality along the sample surface, were compared with optical and scanning electron microscopy images. Knowing the distribution of the X-ray diffraction peak intensity, peak position and peak width immediately yields the crystal lattice bending locally present in the samples as a result of the thermal processes arising during the growth. The results found here by a macroscopic-scale imaging technique reveal that the array of large microcrystals, which tend to fuse at a certain height, forms domains limited by cracks during cooling after the growth. The domains are characterized by uniform lattice bending and their boundaries are observed as higher distortion of the crystal structure. The effect of concave thermal lattice bending inside the microcrystal array is in excellent agreement with the results previously presented on a microscopic scale using scanning nanodiffraction.

Strategies for on-chip digital data compression for X-ray pixel detectors

The continued desire for X-ray pixel detectors with higher frame rates will stress the ability of application-specific integrated circuit (ASIC) designers to provide sufficient off-chip bandwidth to reach continuous frame rates in the 1 MHz regime. To move from the current 10 kHz to the 1 MHz frame rate regime, ASIC designers will continue to pack as many power-hungry high-speed transceivers at the periphery of the ASIC as possible. In this paper, however, we present new strategies to make the most efficient use of the off-chip bandwidth by utilizing data compression schemes for X-ray photon-counting and charge-integrating pixel detectors. In particular, we describe a novel in-pixel compression scheme that converts from analog to digital converter units to encoded photon counts near the photon Poisson noise level and achieves a compression ratio of > 1.5× independent of the dataset. In addition, we describe a simple yet efficient zero-suppression compression scheme called “zeromask” (ZM) located at the ASIC's edge before streaming data off the ASIC chip. ZM achieves average compression ratios of > 4×, > 7×, and > 8× for high-energy X-ray diffraction, ptychography, and X-ray photon correlation spectroscopy datasets, respectively. We present the conceptual designs, register-transfer level block diagrams, and the physical ASIC implementation of these compression schemes in 65 nm CMOS. When combined, these two digital compression schemes could increase the effective off-chip bandwidth by a factor of 6–12×.

Co-axial silicon/perovskite heterojunction arrays for high-performance direct-conversion pixelated X-ray detectors

Creating monolithic silicon/perovskite structures is a promising approach to engage emerging perovskite materials with silicon circuitry, which is essential to achieve industry-scale applications such as high performance X-ray detection. In particular, to achieve pixelated perovskite is a key step to address the issue of electrical crosstalk and low spatial resolution in imaging caused by inefficient migration and collection of carriers in the direction of signal collection. However, with the existing top-down methods, the thickness of patterned perovskite is limited to be in submicron level, which is far from highly efficient absorption of X-ray energy, further compromising the detection sensitivity. Here, we successfully demonstrate 3D hybrid perovskite crystal arrays with the thickness of ~300 μm on pixelated silicon substrate (forming co-axial silicon/perovskite heterojunction arrays) by melting PbBr2 and in situ chemical vapor conversion. Both morphological and optical properties of the resulting heterojunction arrays are systematically investigated. Furthermore, we demonstrate a high performance direct-conversion flat panel X-ray detector which exhibits high sensitivity of 242 μC Gyair−1 cm−2 that is much higher than the commercially available α-Se, and fast response speed (rising and falling time are 0.5 ms and 1.3 ms, respectively). The proposed strategy addresses the trade-off problem between the high sensitivity (requiring ~ mm-thickness perovskite crystal) and high spatial resolution (patterning of perovskite crystal) to achieve high-performance X-ray detection. This work not only offers a new pathway to fabricate pixelated μm-thick perovskite-based X-ray detectors, but also impacts on the application and functionalization of perovskite materials in silicon circuitry.

Medical Applications of Tissue-Equivalent, Organic-Based Flexible Direct X-Ray Detectors

The aim of this study is to assess direct X-ray detectors based on organic thin films, fabricated onto flexible plastic substrates and operating at ultra-low bias (<1 V), for different medical applications. With this purpose, flexible fully organic pixelated X-ray detectors have been tested at the imaging beamline SYRMEP (SYnchrotron Radiation for MEdical Physics) at the Italian synchrotron Elettra, Trieste. The detectors’ performance has been assessed for potential employment both as reliable wearable personal dosimeters for patients and as flexible X-ray medical imaging systems. A spatial resolution of 1.4 lp mm-1 with a contrast of 0.37 has been evaluated. Finally, we validate the detector using X-ray doses and energies typically employed for actual medical radiography, and using X-ray beam pulses provided by a commercial dental radiography system, recording a sensitivity of 1.6×105 µC Gy-1 cm-3 with a linear response with increasing of the dose rates and a reliable signal to 100 ms X-rays pulses.

Digital integration: a novel readout concept for XIDER, an X-ray detector for the next generation of synchrotron radiation sources

This paper introduces the incremental digital integration concept, a novel readout scheme for the front-end of 2D pixelated X-ray detectors. The readout scheme includes features found in both current photon-counting and charge-integrating devices and is particularly suitable for detectors that need to operate with very high photon flux, under strong pileup conditions, and have to provide high sensitivity with noise-free effective operation. The core concept of this scheme is to slice the total exposure time required to acquire a given image in very short time intervals of the order of one microsecond or less that we call subframes. The detector signal is integrated for each subframe in the analog domain and digitised individually at the pixel level. This scheme allows to identify and discard those values for which the measured signal is only built from dark current or noise contributions. The final pixel value is then obtained in the digital domain by summing up the values of the non-discarded subframes. In the proposed scheme all this signal processing happens at pixel level in the detector front end. Reaching high dynamic range is not the only advantage of this method, digital integration offers the possibility of continuous cancellation of the dark current contributions even if they are not stable or fluctuate in time. This opens the possibility of building integrating detectors able to operate at high duty cycles, including continuous beam, with high-Z compound semiconductor sensors, a major challenge for the upcoming synchrotron radiation sources such as the ESRF Extremely Brilliant Source (EBS) currently under advanced construction that will become the first fourth-generation high-energy synchrotron facility worldwide. The XIDER detector, the first implementation of the incremental digital integration scheme, is planned to be a very fast and versatile high dynamic range detector optimised for high energy scattering and diffraction applications at ESRF-EBS and future similar facilities.

Microsecond time-resolved X-ray diffraction for the investigation of fatigue behavior during ultrasonic fatigue loading.

A new method based on time-resolved X-ray diffraction is proposed in order to measure the elastic strain and stress during ultrasonic fatigue loading experiments. Pure Cu was chosen as an example material for the experiments using a 20 kHz ultrasonic fatigue machine mounted on the six-circle diffractometer available at the DiffAbs beamline on the SOLEIL synchrotron facility in France. A two-dimensional hybrid pixel X-ray detector (XPAD3.2) was triggered by the strain gage signal in a synchronous data acquisition scheme (pump-probe-like). The method enables studying loading cycles with a period of 50 µs, achieving a temporal resolution of 1 µs. This allows a precise reconstruction of the diffraction patterns during the loading cycles. From the diffraction patterns, the position of the peaks, their shifts and their respective broadening can be deduced. The diffraction peak shift allows the elastic lattice strain to be estimated with a resolution of ∼10-5. Stress is calculated by the self-consistent scale-transition model through which the elastic response of the material is estimated. The amplitudes of the cyclic stresses range from 40 to 120 MPa and vary linearly with respect to the displacement applied by the ultrasonic machine. Moreover, the experimental results highlight an increase of the diffraction peak broadening with the number of applied cycles.

Characterization of a system that combines energy dispersive X-ray diffraction with X-ray fluorescence and its potential applications in archeometry

We report a proof of concept constructing an experimental setup to explore the imaging capability of a single X-ray detector, simulating a pixelated X-ray detector, to measure EDXRF (Energy Dispersive X-ray Fluorescence) and EDXRD (Energy Dispersive X-ray Diffraction) image spectra simultaneously. Combining a conventional rotating anode Cu X-ray source and assembling a single X-ray detector (Si-PIN Amptek, 139 eV at 5.98 keV) on a computer controlled XY stage, a proxy of an imaging X-ray detector was constructed for the measurement of simultaneous EDXRD and EDXRF of the same spot of a sample. The main advantages of this setup are the virtual need for sample preparation and the simultaneous XRF-XRD measurements of the same spot, enabling a combined and consistent analysis of a sample. The time consumed in an analysis using this setup is largely due to the need for acquiring multiple (several hundred) single spectra, which can be compensated, by using a variable acquisition time, depending on the count rate, exploring the high dynamic range of the X-ray emission during data acquisition. A Python code was written for offline data filtering and analysis. Using a simple geometrical model, d-spacings were calculated, and the model predictions were superimposed to the ED (Energy Dispersive) surface plots with good agreement. The instrument proved to work according to expectations and helped set the main experimental parameters for a more compact and portable setup under development.

Double-side pixelated X-ray detector based on metamorphic InGaAs/InAlAs quantum well

Due to the high atomic number, low band gap and high electron mobility of III-IV semiconductors, the use of metamorphic InGaAs/InAlAs quantum well-based devices was proposed for fast pixelated photon detectors. In this work, we are presenting a double side-segmented quantum well (QW) device, discussing its spatial resolution and analysing the crosstalk between pixels. The fabricated devices were tested with needle-shaped beams of synchrotron radiation with different energies and spot sizes. The position of the synchrotron radiation was estimated with a 1.3-μm precision. The charge spread in the material and related crosstalk function between pixels were extracted from the position estimation measurements of the photon beams. The results show that the cross-talk between pixels is actually responsible for the different resolutions obtained, regardless of the experimental conditions, pointing out the importance of the geometry of the fabricated devices. Furthermore, it has been observed that the QW pixelation is mandatory for hybrid detector technology.

A computational tool for simulation and design of tangential multi-energy soft x-ray pin-hole cameras for tokamak plasmas.

A new tool has been developed to calculate the spectral, spatial, and temporal responses of multi-energy soft x-ray (ME-SXR) pinhole cameras for arbitrary plasma densities (n e,D), temperature (T e), and impurity densities (n Z). ME-SXR imaging provides a unique opportunity for obtaining important plasma properties (e.g., T e, n Z, and Z eff) by measuring both continuum and line emission in multiple energy ranges. This technique employs a pixelated x-ray detector in which the lower energy threshold for photon detection can be adjusted independently. Simulations assuming a tangential geometry and DIII-D-like plasmas (e.g., n e,0 ≈ 8 × 1019 m-3 and T e,0 ≈ 2.8 keV) for various impurity (e.g., C, O, Ar, Ni, and Mo) density profiles have been performed. The computed brightnesses range from few 102 counts pixel-1 ms-1 depending on the cut-off energy thresholds, while the maximum allowable count rate is 104 counts pixel-1 ms-1. The typical spatial resolution in the mid-plane is ≈0.5 cm with a photon-energy resolution of 500 eV at a 500 Hz frame rate.

Evaluation of the UFXC32k photon-counting detector for pump–probe experiments using synchrotron radiation

This paper presents the performance of a single-photon-counting hybrid pixel X-ray detector with synchrotron radiation. The camera was evaluated with respect to time-resolved experiments, namely pump-probe-probe experiments held at SOLEIL. The UFXC camera shows very good energy resolution of around 1.5 keV and allows the minimum threshold setting to be as low as 3 keV keeping the high-count-rate capabilities. Measurements of a synchrotron characteristic filling mode prove the proper separation of an isolated bunch of photons and the usability of the detector in time-resolved experiments.

A Bulk Control Circuit for Open-Loop Front-Ends for X-Ray Pixel Detectors

In this paper, we present a bulk control circuit to correct the chip-to-chip process variations of an open-loop nonlinear front-end (FE) for X-ray pixel detectors. Our study was carried out in the framework of the Depfet sensor with signal compression detector development for the European X-ray free electron laser. The presented circuit is capable to stabilize the FE response in presence of threshold voltage variations, acting on the bulk voltages of the FE’s transistors and exploiting the body effect. The control circuit does not affect the noise performances of the FE. The working principle of the proposed control circuit and the first experimental results obtained with a first prototype realized in the 130-nm IBM technology are presented in this work.

Design and optimization of collection efficiency and conversion gain of buried p-well SOI pixel X-ray detector

Buried P-Well (BPW) technology was used in silicon-on-insulator pixels (SOIPIX) to suppress the back-gate effect, the major challenge in SOIPIX. In this work, we have designed and optimized two novel pixel structures, which are based on different BPW design layouts, to study the carrier collection efficiency and conversion gain of the pixel unit used in SOIPIX X-ray detectors. The first structure has an extended BPW region connected with a P+ node. In the second structure, a separated BPW ring region is formed surrounding the P+ node. Two X-ray sources with different photon energies have been applied in the simulation of excess carrier generation. The results indicated that the first structure had higher collection efficiency while the second structure had a slightly better conversion gain. As a result, the total photoelectric voltage of the first structure is about two times that of the second structure, where low doping concentration (&lt;1 × 1016 cm‒3) in the BPW region is preferred. Such a study of design and optimization of BPW technology is very important for applications in SOIPIX detectors

The formation of amplitude spectra in X-ray pixel detectors made of gallium arsenide.

This study aims to analyse energy spectra formation in semiconductor X-ray pixel detectors using a simple experimental method. The calculations were performed for the pixel detectors made of high-resistivity gallium arsenide compensated by chromium GaAs (Cr). A peculiar feature of these detectors is an extremely short lifetime of the holes. When using ordinary detectors with planar electrodes the spectra with high energy resolution could not be observed. In this study, the shape of amplitude spectra of gamma rays were calculated with energy W0 = 60 and 17 keV. The calculations were performed for the pixel detector of GaAs (Cr) with the thickness of d = 500μm and pixel pitch of 50μm. The mobility of electrons and holes were assumed to be μn = 3000 cm2/Vs, μp = 300 cm2/Vs, and the lifetimes were τn = 20 ns and τp = 1 ns, respectively. It was demonstrated that in the pixel detector, where there was practically no collection of holes and the amplitude spectra occurred with the energy resolution of 3.5 keV. The calculations show that energy spectra of the pixel detectors has a high energy resolution at an appropriate polarity applied bias voltage. The calculation results were conformed by the experimental data.

On reachable energy resolution of SiPM based scintillation counters for X-ray detection

Presently, silicon photomultipliers (SiPMs) are very attractive devices to replace photomultipliers for light detection in many different fields. For example, they could be used in detectors of photons with energies of 20–150 keV for medical and nondestructive testing applications. The small size and high electron gain of SiPMs make them very attractive candidates for pixelated X-ray detectors operating in a photon counting mode. In this research we evaluated the detector performance that can be reached with up-to-date Lu-based scintillators. Application of LYSO:Ce and LFS-3 scintillators provides high count rate capability because of a short scintillator decay time of ∼ 40 ns but enables reaching just a moderate energy resolution. Meanwhile, with a LuYAG:Pr scintillator of quite low non-proportionality one can attain much better energy resolution at a reduced rate.

Energy calibration and gain correction of pixelated spectroscopic x-ray detectors using correlation optimised warping

We describe the implementation of a reliable, robust and flexible gain correction and energy calibration algorithm for pixelated spectroscopic x-ray detectors. This algorithm uses a data processing method known as correlation optimised warping which aligns shifted datasets by means of a segmental linear stretching and compression of the spectral data in order to best correlate with a reference spectrum. We found the algorithm to be very robust against low-count spectroscopy, and was reliable in a range of different spectroscopic applications. Analysis of the integrated spectrum over all pixels for a Cerium K-alpha x-ray emission (at 34.72 keV) yielded a peak width of 2.45 keV before alignment and 1.11 keV after alignment. This compares favourably with the best in class pixel peak width of 0.76 keV and the mean peak width for all pixels of 1.00 keV. We also found the algorithm to be more user friendly than other peak-search algorithms because there is less external input. A key advantage of this algorithm is that it requires no prior knowledge of the input spectral characteristics, shape or quality of the data. This therefore lends itself to being useful for in-line processing and potentially removes the need for a separate calibration standard (e.g. a radioactive source). This algorithm can be used for any system that simultaneously collects large numbers of spectral data—including multi-element detectors.

Direct X-ray photoconversion in flexible organic thin film devices operated below 1 V.

The application of organic electronic materials for the detection of ionizing radiations is very appealing thanks to their mechanical flexibility, low-cost and simple processing in comparison to their inorganic counterpart. In this work we investigate the direct X-ray photoconversion process in organic thin film photoconductors. The devices are realized by drop casting solution-processed bis-(triisopropylsilylethynyl)pentacene (TIPS-pentacene) onto flexible plastic substrates patterned with metal electrodes; they exhibit a strong sensitivity to X-rays despite the low X-ray photon absorption typical of low-Z organic materials. We propose a model, based on the accumulation of photogenerated charges and photoconductive gain, able to describe the magnitude as well as the dynamics of the X-ray-induced photocurrent. This finding allows us to fabricate and test a flexible 2 × 2 pixelated X-ray detector operating at 0.2 V, with gain and sensitivity up to 4.7 × 104 and 77,000 nC mGy−1 cm−3, respectively.

At home accurate XRD measurements with an hybrid pixel X-ray detector: a comparison with CCD, APS-CMOS commercial detectors and theoretical model

At home accurate XRD measurements with an hybrid pixel X-ray detector: a comparison with CCD, APS-CMOS commercial detectors and theoretical model

Front end ASIC for AGIPD, a high dynamic range fast detector for the European XFEL

The Adaptive Gain Integrating Pixel Detector (AGIPD) is a hybrid pixel X-ray detector for the European-XFEL. One of the detector's important parts is the radiation tolerant front end ASIC fulfilling the European-XFEL requirements: high dynamic range—from sensitivity to single 12.5keV-photons up to 104 photons. It is implemented using the dynamic gain switching technique with three possible gains of the charge sensitive preamplifier. Each pixel can store up to 352 images in memory operated in random-access mode at ⩾4.5 MHz frame rate. An external vetoing may be applied to overwrite unwanted frames.

Wafer-scale pixelated scintillator and specially designed data acquisition system for fiber optic taper array-coupled digital x-ray detector

A digital x-ray detector scheme based on a pixelated scintillator coupled with a fiber optic (FOT) array is suitable for many high-resolution x-ray imaging applications. However, certain challenges need to be addressed for fabrication of wafer-scale uniform pixelated x-ray scintillators. In addition, difficulties associated with implementation of the data acquisition system for acquiring output image data from the multiple image sensors used in the detector also need to be addressed. In this paper, a 2×2 FOT array-coupled digital x-ray detector scheme using a 5-in. pixelated scintillator is proposed. A novel fabrication setup along with the corresponding processes for fabricating the wafer-scale pixelated scintillator and implementation of a specially designed embedded data acquisition system based on a single embedded micro-processer (ARM) and four field-programmable gate array (FPGA) chips are discussed in detail. Preliminary experiments demonstrate that this pixelated scintillator-based digital x-ray detector scheme with an active imaging area of about 100mm×100mm shows considerable potential for use in high-resolution x-ray imaging.