high-Z Sensor Research Articles

Single-photon counting detectors for diffraction-limited light sources

When first introduced, single-photon counting detectors reshaped crystallography at synchrotrons. Their fast readout speed enabled, for example, shutter-less data collection and fine slicing of the rotation angle and boosted the development of new experimental techniques like ptychography. Under optimal conditions, single-photon counting detectors provide an unlimited dynamic range with image noise only limited by the Poisson statistics of the incoming photons. Counting the pulses from individual photons, essentially what made the detectors so successful, also causes the main drawback, which is the loss of efficiency at high photon fluxes due to pulse pileup in the analog front end. To fully take advantage of diffraction-limited light sources, the next-generation single-photon counters need to improve their count rate capabilities in the same order of magnitude as the increased flux. Moreover, fast frame rates (a few kHz) are required to cope with the shorter dwell time achievable, thanks to the higher flux. Detector architecture with multiple comparators and counters can open new possibilities for energy-resolved imaging, while interpixel communication can overcome the issues arising from charge sharing and reduce the loss of efficiency at the pixel corners. Coupling single-photon counting detectors to high-Z sensors for hard X-ray detection (>20 keV) and to low-gain avalanche diodes (LGADs) for soft X-rays is also necessary to make use of the increased coherence of the new light sources over the full radiation spectrum. In this paper, we present possible strategies to improve the performance of single-photon counting detectors at the fourth-generation synchrotron sources and compare them to charge integrating detectors.

Balancing gain and dynamic range in a 25 µm pitch hybrid pixel detector

MÖNCH is a hybrid pixel detector featuring 25 µm pixel pitch and analogue readout for X-ray imaging at synchrotron radiation (SR) facilities. Sub-pixel spatial resolution has been demonstrated using charge sharing and interpolation algorithms. The current prototype version, MÖNCH0.4, features 19 different pixel architectures to assess the design choices and components for an optimised architecture to be used at SR facilities, and to explore the potential use of dynamic gain switching in fine pitch pixels for applications at X-ray free electron lasers (XFELs).Previous characterisation results of the pixel architectures without dynamic gain switching have shown noise levels as low as 21.7 e- r.m.s., which have now been pushed to sub-20 e- r.m.s at room temperature using standard 300 µm-thick silicon sensors. Achieving low noise values however requires high conversion gain and necessitates design choices such as the simplification of the pixel architecture (e.g. by limiting the available choice of in-pixel gains). These compromises ultimately restrain the available dynamic range and prevent the use of MÖNCH with low-gain avalanche diodes (LGADs) or high-Z sensors because of the large signals (internal amplification and high photon energies, respectively) and of large leakage currents.In this paper, we will introduce the MÖNCH project followed by a description of the current prototype along with characterisation results of the pixel architectures without dynamic gain switching for synchrotron applications with an emphasis on noise and dynamic range. These experimental results will be used to fine-tune the design of MÖNCH0.5 to validate the final pixel design. This small prototype should also include additional features from the continuous developments of the PSD detector group towards a full-scale 2 × 3 cm2 MÖNCH1.0.

Concepts for the data flow control on the XIDer readout ASIC

XIDer is a 2D pixelated hybrid X-ray detector that is supposed to act as a multi-purpose platform for diffraction experiments at fourth-generation high-energy synchrotron light sources like the Extremely Brilliant Source of the European Synchrotron Radiation Facility (ESRF-EBS). As such it needs to tackle a wide variety of requirements. These include x-ray energies of 30keV to 100keV imposing the utilisation of high-Z sensors, a wide dynamic range from single photons to fluxes of 1 × 109photonspixel−1s−1 and expected raw data rates up to the order of TBs−1 in the final detector design.As a result, XIDer’s readout ASIC has to provide smart and flexible ways for on-chip data pre-processing to reduce the output data rate to manageable sizes. This article focusses on introducing the ASIC’s novel and unique data-handling concepts like the custom-designed telegram protocol followed by the presentation of exemplary configurations and use cases.

KITE: High frame rate, high count rate pixelated electron counting ASIC for 4D STEM applications featuring high-Z sensor

KITE is a novel mixed-mode CMOS electron counting ASIC developed for the readout of pixelated semiconductor radiation detectors targeted to 4D STEM applications. The chip features a pixel pitch of 100 μm arranged in a 192 × 192 matrix and can be bump-bonded to both Si and high-Z sensor materials. The design of an extremely fast front-end electronics and the use of the instant retrigger mechanism allow for a non-paralyzable counting mode up to above 108 cts/s/pix, which corresponds to a 300 keV electron beam current of 14 pA/pix. The massive parallel readout in combination with two on-chip data compression techniques – 2 × 2 pixel digital binning and floating-point counter encoding – allows for frame rates higher than 100 kfps (this work), in principle extendable to almost 500 kfps with improved readout boards. We successfully characterized a first detector prototype featuring a 1500 μm-thick CdZnTe sensor with fluorescence X-rays from elemental targets and with 200 keV and 300 keV electrons in terms of energy response, threshold trimming accuracy, single event multiplicity distribution, spatial uniformity and temporal stability, count rate capability and imaging performances (MTF and DQE). Custom Monte Carlo simulations complemented when possible the experimental data for cross-validation purposes, generally with a very good matching. Finally, a proof-of-concept 4D STEM acquisition is presented and discussed.

XIDER: a novel X-ray detector for the next generation of high-energy synchrotron radiation sources

Next-generation sources of synchrotron radiation pose significant challenges for 2D pixelated X-ray detectors, such as at the ESRF Extremely Brilliant Source (EBS), the first fourth-generation high-energy synchrotron facility. In particular, scattering and diffraction experiments require fast detectors with a high dynamic range, from single photon sensitivity to pile-up conditions under very high photon fluxes. Furthermore, in the case of high-energy applications, the high-Z sensor materials needed for efficient photon detection introduce other difficulties. Leakage current, bias- and flux-induced polarisation, and afterglow all must be carefully managed for the detector system to reach the required specifications. The XIDER project aims to fulfil the needs of the above-mentioned applications by implementing a novel incremental digital integration readout scheme. XIDER detectors seek to operate efficiently under the high-flux EBS beam of up to 100 keV photons, with a time resolution that can cope with near-continuous and pulsed beams. Simultaneously, non-constant leakage current contributions can be removed for noise-free single photon detection, resulting in a very high dynamic range. This contribution presents the recent developments of the XIDER project, including the first characterisation measurements with cadmium telluride sensors.

Directional-Sensitive X-ray/Gamma-ray Imager on Board the VZLUSAT-2 CubeSat for Wide Field-of-View Observation of GRBs in Low Earth Orbit

We present a miniaturized and wide field-of-view X-ray and Gamma-ray imager consisting of a segmented 2D optics-collimator coupled to the high-sensitivity semiconductor pixel detector Timepix equipped with a high-Z sensor (CdTe 2000 μm thick). The compact payload has been deployed in low-Earth orbit (LEO) onboard the 3U Cubesat VZLUSAT-2 which was launched on 13 January 2022. The instrument is designed to verify small spacecraft borne observation in open space of hard X-ray and Gamma-ray sources both of celestial and atmospheric origin. High-resolution spectral-sensitive X-ray and Gamma-ray images are provided with enhanced event discrimination and wide field-of-view up to 60°. Description of the instrument together with response evaluation and tests in ground with well-defined sources are presented. The intended observational plan for in-orbit measurements is outlined along with astrophysical goals and issues.

Spectral and DQE performance of 300 [formula omitted]m and 500 [formula omitted]m thick GaAs:Cr X-ray photon counting detectors for imaging applications

X-ray photon counting detectors featuring high-Z sensor materials such as CdTe, CdZnTe and GaAs have the possibility to greatly improve the field of medical imaging. In this work, we investigated the energy resolving capabilities and imaging performance of two novel proof-of-concept hybrid photon counting detectors based on the IBEX ASIC and on Chromium-compensated GaAs as direct-detection sensor material targeted for X-ray imaging applications. The two sensors feature a pixel size of 75 μm and two different thicknesses — 300 and 500 μm. Spectral properties were assessed by analyzing energy spectra from fluorescence targets (Ge, Mo, Ba) and direct X-ray tube beam qualities relevant for mammographic (28 kVp, 35 kVp), pediatric/animals/μCT (50 kVp) and extremities/μCT (70 kVp) imaging applications. Imaging properties in terms of MTF and DQE were assessed at the aforementioned direct X-ray beam conditions and as a function of the threshold energy. The energy resolution lies in the range 1.6-2.2 keV FWHM, increasing with the sensor thickness and fluorescence energy of the target. Charge sharing effects are as expected less prominent for thinner sensors. The behavior of the MTF and DQE is not monotonic with the threshold energy and we extracted their optimal values. In this conditions, the MTF equals the one of an ideal pixel for all studied thicknesses and beam quality and the same does the DQE at mammographic conditions. At higher beam energies the DQE decreases the more for thinner sensors due to lower X-ray absorption efficiency.

X-ray imaging with high-Z sensors for the ESRF-EBS Upgrade

The Extremely Brilliant Source (EBS) upgrade of the European Synchrotron Radiation Facility (ESRF) sets new requirements for the detection schemes used in various beamlines. In order to facilitate direct x-ray detection at moderate and high energies in the range of 30–100 keV we investigate the performance of pixelated photon counting detectors using high-Z semiconductor sensors as the sensitive medium. In the context of this work, several prototype modules based on CdTe, CZT and GaAs:Cr pixelated sensors have been developed and tested at ESRF using laboratory X-ray sources and monochromatic synchrotron beams. Each module, consisting of a pixelated sensor bonded to a single Timepix chip, is coupled to the MAXIPIX readout system. A common characterisation procedure is applied to each detector module, focusing on the sensor quality, the spatial characteristics and the imaging performance. The results presented in this work along with further tests will provide the means to determine the best compromise between sensor material and performance for future detector systems.

A wide energy range calibration algorithm for X-ray photon counting pixel detectors using high-Z sensor material

We developed a novel, accurate and robust energy calibration algorithm for single X-ray photon counting detectors suitable for the use of high-Z sensor materials. The calibratable energy range was demonstrated to extend over a wide interval from 12 keV up to 150 keV and the same method can also be applied to lower or higher X-ray energies. Fluorescence lines of several elemental targets are used as reference up to 75 keV. For higher energies, the reference is given by direct X-ray tube spectra end-points — extracted with a semi-empirical model independently of the broadening by electronic noise. The pixel-wise threshold trimming relies on a target curve fitting iterative procedure which is able to cope with offset, gain and flat-field variations. The algorithm is highly parallelizable and can profit from modern multi-core machines. The proposed method is able to deal with the spectral complexity introduced by atomic fluorescence effects arising in high-Z sensor materials such as CdTe, CdZnTe and GaAs for energies above the corresponding K-edges and enhanced by a decreasing pixel size. We report the qualification results for the case study of a 1 mm-thick CdTe sensor with 150 μm pixel size, bonded to an IBEX ASIC and calibrated in the range 12–150 keV. The validity of the method was assessed by analyzing calibrated pulse height spectra recorded with Sn and W fluorescence samples and with a 99mTc radioactive source. The global deviation was found to be smaller than 0.5% at 140.5 keV. The residual threshold dispersion was 0.54 keV rms at the Sn Kα peak and 1.71 keV rms at the 99mTc γ-decay peak, still with a limited impact on the overall energy resolution of the system.

Investigation of low temperature bonding process using indium bumps

Hybrid pixel detector modules are state of the art components of X-ray cameras in synchrotron radiation experiments. Temperature-sensitive, high-Z sensor materials like Germanium, Cadmium-Telluride, Cadmium-Zinc-Telluride and MCT-sensors require an especially low temperature bonding process. The investigation presented in this paper focuses on a low temperature compression bonding process using a ductile bump material, like indium, and a dedicated UBM surface that is able to form intermetallic compounds with this bump metal. The test samples are based on a test chip design with the size of the MEDIPIX3 readout chip and an identical number of bumps, arranged in a 256 × 256 bump matrix with 55 μm pitch in X- and Y-directions. In addition to the indium bumped test chip, two typical UBM-pad configurations were investigated on the corresponding interconnect side: a pad-defined electroplated Ni-Au-UBM and a passivation opening-defined, PVD-deposited NiV-Au pad. Daisy chain structures, 4-Point-probe resistance measurement structures and isolation test structures were implemented to characterize the influence of bonding pressure, bonding temperature and dwell time of bonding pressure on the electrical interconnection resistance. Good bonding results and a corresponding minimum electrical resistance of daisy chains and individual interconnects were achieved at temperatures of 80̂C and 100̂C. A minimum bonding pressure of 3.2 MPa was necessary to achieve reliable bonding results. Daisy chain failures occurred especially in the corner of the chips. After electrical characterization, pull tests were performed to investigate the corresponding bonding area. Pull test results showed a larger bonding area for the pad-defined, electroplated UBM pad configuration in comparison to the passivation opening defined bonding pad.

Tests of UFXC32k chip with CdTe pixel detector

The paper presents the performance of the UFXC32K—a hybrid pixel detector readout chip working with CdTe detectors. The UFXC32K has a pixel pitch of 75 μm and can cope with both input signal polarities. This functionality allows operating with widely used silicon sensors collecting holes and CdTe sensors collecting electrons. This article describes the chip focusing on solving the issues connected to high-Z sensor material, namely high leakage currents, slow charge collection time and thick material resulting in increased charge-sharring effects. The measurements were conducted with higher X-ray energies including 17.4 keV from molybdenum. Conclusions drawn inside the paper show the UFXC32K's usability for CdTe sensors in high X-ray energy applications.

Photon counting microstrip X-ray detectors with GaAs sensors

High-Z sensors are increasingly used to overcome the poor efficiency of Si sensors above 15 keV, and further extend the energy range of synchrotron and FEL experiments. Detector-grade GaAs sensors of 500 μm thickness offer 98% absorption efficiency at 30 keV and 50% at 50 keV . In this work we assess the usability of GaAs sensors in combination with the MYTHEN photon-counting microstrip readout chip developed at PSI. Different strip length and pitch are compared, and the detector performance is evaluated in regard of the sensor material properties. Despite increased leakage current and noise, photon-counting strips mounted with GaAs sensors can be used with photons of energy as low as 5 keV, and exhibit excellent linearity with energy. The charge sharing is doubled as compared to silicon strips, due to the high diffusion coefficient of electrons in GaAs.

LAMBDA 2M GaAs—A multi-megapixel hard X-ray detector for synchrotrons

Synchrotrons can provide very intense and focused X-ray beams, which can be used to study the structure of matter down to the atomic scale. In many experiments, the quality of the results depends strongly on detector performance; in particular, experiments studying dynamics of samples require fast, sensitive X-ray detectors. “LAMBDA” is a photon-counting hybrid pixel detector system for experiments at synchrotrons, based on the Medipix3 readout chip. Its main features are a combination of comparatively small pixel size (55 μm), high readout speed at up to 2000 frames per second with no time gap between images, a large tileable module design, and compatibility with high-Z sensors for efficient detection of higher X-ray energies. A large LAMBDA system for hard X-ray detection has been built using Cr-compensated GaAs as a sensor material. The system is composed of 6 GaAs tiles, each of 768 by 512 pixels, giving a system with approximately 2 megapixels and an area of 8.5 by 8.5 cm2. While the sensor uniformity of GaAs is not as high as that of silicon, its behaviour is stable over time, and it is possible to correct nonuniformities effectively by postprocessing of images. By using multiple 10 Gigabit Ethernet data links, the system can be read out at the full speed of 2000 frames per second. The system has been used in hard X-ray diffraction experiments studying the structure of samples under extreme pressure in diamond anvil cells. These experiments can provide insight into geological processes. Thanks to the combination of high speed readout, large area and high sensitivity to hard X-rays, it is possible to obtain previously unattainable information in these experiments about atomic-scale structure on a millisecond timescale during rapid changes of pressure or temperature.

Basis material decomposition in spectral CT using a semi-empirical, polychromatic adaption of the Beer–Lambert model

Following the development of energy-sensitive photon-counting detectors using high-Z sensor materials, application of spectral x-ray imaging methods to clinical practice comes into reach. However, these detectors require extensive calibration efforts in order to perform spectral imaging tasks like basis material decomposition. In this paper, we report a novel approach to basis material decomposition that utilizes a semi-empirical estimator for the number of photons registered in distinct energy bins in the presence of beam-hardening effects which can be termed as a polychromatic Beer–Lambert model. A maximum-likelihood estimator is applied to the model in order to obtain estimates of the underlying sample composition. Using a Monte-Carlo simulation of a typical clinical CT acquisition, the performance of the proposed estimator was evaluated. The estimator is shown to be unbiased and efficient according to the Cramér–Rao lower bound. In particular, the estimator is capable of operating with a minimum number of calibration measurements. Good results were obtained after calibration using less than 10 samples of known composition in a two-material attenuation basis. This opens up the possibility for fast re-calibration in the clinical routine which is considered an advantage of the proposed method over other implementations reported in the literature.

Feasibility study of a ``4H'' X-ray camera based on GaAs:Cr sensor

A multilayer stacked X-ray camera concept is described. This type of technology is called `4H' X-ray cameras, where 4H stands for high-Z (Z>30) sensor, high-resolution (less than 300 micron pixel pitch), high-speed (above 100 MHz), and high-energy (above 30 keV in photon energy). The components of the technology, similar to the popular two-dimensional (2D) hybrid pixelated array detectors, consists of GaAs:Cr sensors bonded to high-speed ASICs. 4H cameras based on GaAs also use integration mode of X-ray detection. The number of layers, on the order of ten, is smaller than an earlier configuration for single-photon-counting (SPC) mode of detection [1]. High-speed ASIC based on modification to the ePix family of ASIC is discussed. Applications in X-ray free electron lasers (XFELs), synchrotrons, medicine and non-destructive testing are possible.

CdTe Timepix detectors for single-photon spectroscopy and linear polarimetry of high-flux hard x-ray radiation.

Single-photon spectroscopy of pulsed, high-intensity sources of hard X-rays - such as laser-generated plasmas - is often hampered by the pileup of several photons absorbed by the unsegmented, large-volume sensors routinely used for the detection of high-energy radiation. Detectors based on the Timepix chip, with a segmentation pitch of 55 μm and the possibility to be equipped with high-Z sensor chips, constitute an attractive alternative to commonly used passive solutions such as image plates. In this report, we present energy calibration and characterization measurements of such devices. The achievable energy resolution is comparable to that of scintillators for γ spectroscopy. Moreover, we also introduce a simple two-detector Compton polarimeter setup with a polarimeter quality of (98 ± 1)%. Finally, a proof-of-principle polarimetry experiment is discussed, where we studied the linear polarization of bremsstrahlung emitted by a laser-driven plasma and found an indication of the X-ray polarization direction depending on the polarization state of the incident laser pulse.

Review of hybrid pixel detector readout ASICs for spectroscopic X-ray imaging

Semiconductor detector readout chips with pulse processing electronics have made possible spectroscopic X-ray imaging, bringing an improvement in the overall image quality and, in the case of medical imaging, a reduction in the X-ray dose delivered to the patient. In this contribution we review the state of the art in semiconductor-detector readout ASICs for spectroscopic X-ray imaging with emphasis on hybrid pixel detector technology. We discuss how some of the key challenges of the technology (such as dealing with high fluxes, maintaining spectral fidelity, power consumption density) are addressed by the various ASICs. In order to understand the fundamental limits of the technology, the physics of the interaction of radiation with the semiconductor detector and the process of signal induction in the input electrodes of the readout circuit are described. Simulations of the process of signal induction are presented that reveal the importance of making use of the small pixel effect to minimize the impact of the slow motion of holes and hole trapping in the induced signal in high-Z sensor materials. This can contribute to preserve fidelity in the measured spectrum with relatively short values of the shaper peaking time. Simulations also show, on the other hand, the distortion in the energy spectrum due to charge sharing and fluorescence photons when the pixel pitch is decreased. However, using recent measurements from the Medipix3 ASIC, we demonstrate that the spectroscopic information contained in the incoming photon beam can be recovered by the implementation in hardware of an algorithm whereby the signal from a single photon is reconstructed and allocated to the pixel with the largest deposition.

High-speed readout of high-Z pixel detectors with the LAMBDA detector

High-frame-rate X-ray pixel detectors make it possible to perform time-resolved experiments at synchrotron beamlines, and to make better use of these sources by shortening experiment times. LAMBDA is a photon-counting hybrid pixel detector based on the Medipix3 chip, designed to combine a small pixel size of 55 μm, a large tileable module design, high speed, and compatibility with ``high-Z'' sensors for hard X-ray detection. This technical paper focuses on LAMBDA's high-speed-readout functionality, which allows a frame rate of 2000 frames per second with no deadtime between successive images. This takes advantage of the Medipix3 chip's ``continuous read-write'' function and highly parallelised readout. The readout electronics serialise this data and send it back to a server PC over two 10 Gigabit Ethernet links. The server PC controls the detector and receives, processes and stores the data using software designed for the Tango control system. As a demonstration of high-speed readout of a high-Z sensor, a GaAs LAMBDA detector was used to make a high-speed X-ray video of a computer fan.

A germanium hybrid pixel detector with 55μm pixel size and 65,000 channels

Hybrid pixel semiconductor detectors provide high performance through a combination of direct detection, a relatively small pixel size, fast readout and sophisticated signal processing circuitry in each pixel. For X-ray detection above 20 keV, high-Z sensor layers rather than silicon are needed to achieve high quantum efficiency, but many high-Z materials such as GaAs and CdTe often suffer from poor material properties or nonuniformities. Germanium is available in large wafers of extremely high quality, making it an appealing option for high-performance hybrid pixel X-ray detectors, but suitable technologies for finely pixelating and bump-bonding germanium have not previously been available.A finely-pixelated germanium photodiode sensor with a 256 by 256 array of 55μm pixels has been produced. The sensor has an n-on-p structure, with 700μm thickness. Using a low-temperature indium bump process, this sensor has been bonded to the Medipix3RX photoncounting readout chip. Tests with the LAMBDA readout system have shown that the detector works successfully, with a high bond yield and higher image uniformity than comparable high-Z systems. During cooling, the system is functional around -80°C (with warmer temperatures resulting in excessive leakage current), with -100°C sufficient for good performance.

The LAMBDA photon-counting pixel detector and high-Z sensor development

Many X-ray experiments at third-generation synchrotrons benefit from using single-photon-counting detectors, due to their high signal-to-noise ratio and potential for high-speed measurements. LAMBDA (Large Area Medipix3-Based Detector Array) is a pixel detector system based on the Medipix3 readout chip. It combines the features of Medipix3, such as a small pixel size of 55 μm and flexible functionality, with a large tileable module design consisting of 12 chips (1536 × 512 pixels) and a high-speed readout system capable of running at 2000 frames per second.To enable high-speed experiments with hard X-rays, the LAMBDA systemhas been combined with different high-Z sensormaterials. Room-temperature systems using GaAs and CdTe systems havebeen produced and tested with X-ray tubes and at synchrotronbeamlines. Both detector materials show nonuniformities in their rawimage response, but the pixel yield is high and the uniformity can beimproved by flat-field correction, particularly in the case ofGaAs. High-frame-rate experiments show that useful information can begained on millisecond timescales in synchrotron experiments with thesesensors.