Hybrid Pixel Research Articles

Investigation of thick GaAs:Cr pixel sensors for X-ray imaging applications

The new generation of synchrotron radiation sources sets advanced requirements in the performance of hybrid pixel detectors. A critical point in the design of such detectors is the choice of the sensor material. Chromium-compensated Gallium Arsenide (GaAs:Cr) is a promising candidate for use in pixel detectors, targeting imaging applications in the intermediate energy range 20–50 keV. In this work, thick GaAs:Cr sensors grown recently using the Liquid Encapsulated Czochralski (LEC) method were bonded to Timepix chips. The performance of the sensors was characterised by using various X-ray sources in view of imaging applications. The uniform illumination of the sensors reveals inhomogeneities due to the presence of crystal defects in the microscopic level. Using a micro-focused X-ray beam we extract sensitivity maps and the mobility-lifetime product at the pixel level. The results obtained confirm that the non-uniformities observed at the sensor level are directly linked with the microscopic defects. Overall, the efficient use of such sensors in X-ray imaging applications relies on their stability over irradiation-time.

Statistically correcting dynamical electron scattering improves the refinement of protein nanocrystals, including charge refinement of coordinated metals.

Electron diffraction allows protein structure determination when only nanosized crystals are available. Nevertheless, multiple elastic (or dynamical) scattering, which is prominent in electron diffraction, is a concern. Current methods for modeling dynamical scattering by multi-slice or Bloch wave approaches are not suitable for protein crystals because they are not designed to cope with large molecules. Here, dynamical scattering of nanocrystals of insulin, thermolysin and thaumatin was limited by collecting data from thin crystals. To accurately measure the weak diffraction signal from the few unit cells in the thin crystals, alow-noise hybrid pixel Timepix electron-counting detector was used. The remaining dynamical component was further reduced in refinement using a likelihood-based correction, which was introduced previously for analyzing electron diffraction data of small-molecule nanocrystals and was adapted here for protein crystals. The procedure is shown to notably improve the structural refinement, in one case allowing the location of solvent molecules. It also allowed refinement of the charge states of bound metal atoms, an important element in protein function, through B-factor analysis of the metal atoms and their ligands. These results clearly increase the value of macromolecular electron crystallography as a complementary structural biology technique.

Fast digital lossy compression for X-ray ptychographic data.

Increases in X-ray brightness from synchrotron light sources lead to a requirement for higher frame rates from hybrid pixel array detectors (HPADs), while also favoring charge integration over photon counting. However, transfer of the full uncompressed data will begin to constrain detector design, as well as limit the achievable continuous frame rate. Here a data compression scheme that is easy to implement in a HPAD's application-specific integrated circuit (ASIC) is described, and how different degrees of compression affect image quality in ptychography, a commonly employed coherent imaging method, is examined. Using adaptive encoding quantization, it is shown in simulations that one can digitize signals up to 16383 photons per pixel (corresponding to 14 bits of information) using only 8 or 9 bits for data transfer, with negligible effect on the reconstructed image.

Precision measurement of pixel sensor capacitance

The capacitance of the charge collection node of a sensor system is an important parameter for the design of the analog front-end electronics. The analog front-end of high-granularity sensors like for example hybrid pixel detectors need to be optimized for timing resolution, power consumption, and electronics noise—parameters which all depend on the pixel capacitance. Current pixel detector developments for the HL-LHC upgrade typically use silicon sensors with a pixel size in the order of 50 × 50 μm2 which have a pixel capacitance of several tens of fF, depending on the sensor geometry. We have developed a dedicated integrated circuit to be bump-bonded to a pixel sensor, which enables a precise pixel capacitance measurement by using the charge-based capacitance measurement method. In this paper, we will describe the measurement method and the implementation of the capacitance measurement chip (Pixcap65) and show measurement results of a planar pixel sensor whose pixel capacitance is influenced by variations of the implant geometry.

A Projector-Camera System Using Hybrid Pixels with Projection and Capturing Capabilities

A Projector-Camera System Using Hybrid Pixels with Projection and Capturing Capabilities

Study of charge sharing effect in a GaAs:Cr-based Timepix3 detector

Hybrid pixel semiconductor detectors find more and more applications in modern experimental physical setups. In particular, pixelated detectors based on GaAs:Cr sensor and Timepix3 chip are used in R&D for a state-of-the-art Transition Radiation Detector prototype at CERN. Motivation and usage aspects for GaAs:Cr-Timepix3 device in the experiment are covered in the talk. Authors” contribution to the work of the research group is a study of charge collection and transportation processes in the sensor including fluorescence and so-called charge sharing effect. These are able to significantly affect both spatial and energy resolution of the system. Estimates for the effects are obtained via numerical modelling and then used to analyze the impact on the detector performance.

Full-field X-ray fluorescence imaging with a straight polycapillary X-ray collimator

Due to the availability of X-ray imaging detectors, full-field X-ray fluorescence (FF-XRF) imaging technique has become achievable, which provides an alternative to scanning X-ray fluorescence imaging with a micro-focus X-ray beamline. In this paper, we present a setup based on straight capillary optics and an energy-dispersive hybrid pixel detector, which can perform simultaneous mapping of several chemical elements. The photon transmission efficiency and spatial resolution are compared between two X-ray collimation setups: one using pinhole optics and one using straight polycapillary optics. There is a tradeoff between the spatial resolution and transmission efficiency when considering X-ray optics. When optimizing the spatial resolution, using straight capillary optics achieved a higher intensity gain when comparing with the pinhole setup. Characterization of the polycapillary imaging setup is performed through analyzing various samples in order to investigate the spatial frequency response and the energy sensitivity. This developed setup is capable of FF-XRF imaging in characteristic energies below 20 keV, while for higher energies the spatial resolution is affected by photon transmission through the collimator. This work shows the potential of the FF-XRF instrument in the monitoring of toxic metal distributions in environmental mapping measurements.

Characterization of an architecture for front-end pixel binning in an integrating pixel array detector

Optimization of an area detector involves compromises between various parameters like frame rate, read noise, dynamic range and pixel size. We have implemented and tested a novel front-end binning design in a photon-integrating hybrid pixel array detector using the MM-PAD-2.0 pixel architecture. In this architecture, the pixels can be optionally binned in a 2×2 pixel configuration using a network of switches to selectively direct the output of 4 sensor pixels to a single amplifier input. Doing this allows a trade-off between frame rate and spatial resolution. Tests show that the binned pixels perform well, but with some degradation on performance as compared to an un-binned pixel. The increased parasitic input capacitance does reduce the signal collected per x-ray as well as increases the noise of the pixel. The increase in noise is, however, less than the factor of 2 increase one would observe for binning in post-processing. Spatial scans across the binned pixels show that no measured signal intensity is lost at the inner binning unit boundaries. In the high flux regime, at a 2×2 pixel wide beam spot (FWHM) size, binned mode responds linearly up to a photon flux of 107 x-rays/s, and performs comparably with un-binned mode up to a photon flux of 108 x-rays/s. While this study demonstrates a proof of concept for front-end binning in integrating detectors, we also identify changes to this early-stage prototype which can further improve the performance of binning pixel structures.

Characterisation and mapping of scattered radiation fields in interventional radiology theatres

We used the Timepix3 hybrid pixel detector technology in order to determine the exposure of medical personnel to ionizing radiation in an interventional radiology room. We measured the energy spectra of the scattered radiation generated by the patient during X-ray image-guided interventional procedures. We performed measurements at different positions and heights within the theatre. We first observed a difference in fluence for each staff member. As expected, we found that the person closest to the X-ray tube is the most exposed while the least exposed staff member is positioned at the patient’s feet. Additionally, we observed a shift in energy from head to toe for practitioners, clearly indicating a non-homogenous energy exposure. The photon counting Timepix3 detector provides a new tool for radiation field characterisation that is easier-to-use and more compact than conventional X-ray spectrometers. The spectral information is particularly valuable for optimising the use of radiation protection gear and improving dosimetry surveillance programs. We also found the device very useful for training purposes to provide awareness and understanding about radiation protection principles among interventional radiology staff.

Radiation hard monolithic CMOS sensors with small electrodes for High Luminosity LHC

The upgrade of the tracking detectors for the High Luminosity-LHC (HL-LHC) requires the development of novel radiation hard silicon sensors. The development of Depleted Monolithic Active Pixel Sensors targets the replacement of hybrid pixel detectors with radiation hard monolithic CMOS sensors. We designed, manufactured and tested radiation hard monolithic CMOS sensors in the TowerJazz 180 nm CMOS imaging technology with small electrodes pixel designs. These designs can achieve pixel pitches well below current hybrid pixel sensors (typically 50 × 50μm) for improved spatial resolution. Monolithic sensors in our design allow to reduce multiple scattering by thinning to a total silicon thickness of only 50μm. Furthermore monolithic CMOS sensors can substantially reduce detector costs. These well-known advantages of CMOS sensor for performance and costs can only be exploited in pp-collisions at HL-LHC if the DMAPS sensors are designed to be radiation hard, capable of high hit rates and have a fast signal response to satisfy the 25 ns bunch crossing structure of LHC. Through the development of the MALTA and Mini-MALTA sensors we show the necessary steps to achieve radiation hardness at 1015 neq/cm2 for DMAPS with small electrode designs. The sensors combine high granularity (pitch 36.4x36.4μm2), low detector capacitance (<5fF/pixel) of the charge collection electrode (3μm), low noise (ENC≈10 e−) and low power operation (1μW/pixel) with a fast signal response (25 ns bunch crossing). The sensors feature arrays of 512 × 512 (MALTA) and 16 × 64 (Mini-MALTA) pixels. To cope with high hit rates expected at HL-LHC (>200 MHz/cm2) we have implemented a novel high-speed asynchronous readout architecture. The paper summarises the optimisation of the pixel design to achieve radiation hard pixel designs with full efficiency after irradiation at >98% after 1015 neq/cm2).

BDAQ53, a versatile pixel detector readout and test system for the ATLAS and CMS HL-LHC upgrades

BDAQ53 is a readout system and verification framework for hybrid pixel detector readout chips of the RD53 family. These chips are designed for the upgrade of the inner tracking detectors of the ATLAS and CMS experiments. BDAQ53 is used in applications where versatility and rapid customization are required, such as in laboratory testing environments, test beam campaigns, and permanent setups for quality control measurements. It consists of custom and commercial hardware, a Python-based software framework, and FPGA firmware. BDAQ53 is developed as open source software with both software and firmware being hosted in a public repository.

A deep learning approach to multi-track location and orientation in gaseous drift chambers

Accurate measuring the location and orientation of individual particles in a beam monitoring system is of particular interest to researchers in multiple disciplines. Among feasible methods, gaseous drift chambers with hybrid pixel sensors have the great potential to realize long-term stable measurement with considerable precision. In this paper, we introduce deep learning to analyze patterns in the beam projection image to facilitate three-dimensional reconstruction of particle tracks. We propose an end-to-end neural network based on segmentation and fitting for feature extraction and regression. Two segmentation branches, named binary segmentation and semantic segmentation, perform initial track determination and pixel-track association. Then pixels are assigned to multiple tracks, and a weighted least squares fitting is implemented with full back-propagation. Besides, we introduce a center-angle measure to judge the precision of location and orientation by combining two separate factors. The initial position resolution achieves 8.8 μm for the single track and 11.4 μm (15.2 μm) for the 1–3 tracks (1–5 tracks), and the angle resolution achieves 0.15°and 0.21°(0.29°) respectively. These results show a significant improvement in accuracy and multi-track compatibility compared to traditional methods.

Development of the Projection-Based Material Decomposition Algorithm for Multienergy CT

Current advances in the development of hybrid pixel detectors allow analyzing an energy spectrum of the incoming X-ray radiation with a keV resolution. This spectral information can be utilized to determine the material composition of the studied object based on the known material attenuation dependency on energy. The algorithm discussed and implemented in this article solves an optimization problem with a cost function based on Beer's Law. A successful application of this procedure to the real data requires an accurate model of the detector response. A Monte-Carlo simulation of the registration process in the Timepix3-based detector is performed for the generation of the detector signal corresponding to a monochromatic beam depending on the detector properties. The material decomposition algorithm is applied to simulated data with a response function similar to the real detector. The obtained results are in accordance with the expected values of material concentrations, and the variance mostly depends on a statistical error of the simulation and properties of the detector. An application of the implemented algorithm to experimental data is attempted but the results contain qualitative and quantitative errors. Shortcomings of the current implementation and possible improvements of the detector model and decomposition procedure are discussed.

Sub-pixel electron detection using a convolutional neural network

Modern direct electron detectors (DEDs) provided a giant leap in the use of cryogenic electron microscopy (cryo-EM) to study the structures of macromolecules and complexes thereof. However, the currently available commercial DEDs, all based on the monolithic active pixel sensor, still require relative long exposure times and their best results have only been obtained at 300 keV. There is a need for pixelated electron counting detectors that can be operated at a broader range of energies, at higher throughput and higher dynamic range. Hybrid Pixel Detectors (HPDs) of the Medipix family were reported to be unsuitable for cryo-EM at energies above 80 keV as those electrons would affect too many pixels. Here we show that the Timepix3, part of the Medipix family, can be used for cryo-EM applications at higher energies. We tested Timepix3 detectors on a 200 keV FEI Tecnai Arctica microscope and a 300 keV FEI Tecnai G2 Polara microscope. A correction method was developed to correct for per-pixel differences in output. Timepix3 data were simulated for individual electron events using the package Geant4Medipix. Global statistical characteristics of the simulated detector response were in good agreement with experimental results. A convolutional neural network (CNN) was trained using the simulated data to predict the incident position of the electron within a pixel cluster. After training, the CNN predicted, on average, 0.50 pixel and 0.68 pixel from the incident electron position for 200 keV and 300 keV electrons respectively. The CNN improved the MTF of experimental data at half Nyquist from 0.39 to 0.70 at 200 keV, and from 0.06 to 0.65 at 300 keV respectively. We illustrate that the useful dose-lifetime of a protein can be measured within a 1 second exposure using Timepix3.

BioMAX - the first macromolecular crystallography beamline at MAX IV Laboratory.

BioMAX is the first macromolecular crystallography beamline at the MAX IV Laboratory 3 GeV storage ring, which is the first operational multi-bend achromat storage ring. Due to the low-emittance storage ring, BioMAX has a parallel, high-intensity X-ray beam, even when focused down to 20 µm × 5 µm using the bendable focusing mirrors. The beam is tunable in the energy range 5-25 keV using the in-vacuum undulator and the horizontally deflecting double-crystal monochromator. BioMAX is equipped with an MD3 diffractometer, anISARA high-capacity sample changer and an EIGER 16M hybrid pixel detector. Data collection at BioMAX is controlled using the newly developed MXCuBE3 graphical user interface, and sample tracking is handled by ISPyB. The computing infrastructure includes data storage and processing both at MAX IV and the Lund University supercomputing center LUNARC. With state-of-the-art instrumentation, a high degree of automation, a user-friendly control system interface and remote operation, BioMAX provides an excellent facility for most macromolecular crystallography experiments. Serial crystallography using either a high-viscosity extruder injector or the MD3 as a fixed-target scanner is already implemented. The serial crystallography activities at MAX IV Laboratory will be further developed at the microfocus beamline MicroMAX, when it comes into operation in 2022. MicroMAX will have a 1 µm × 1 µm beam focus and a flux up to 1015 photons s-1 with main applications inserial crystallography, room-temperature structure determinations and time-resolved experiments.

Characterisation of the impacts of the environmental variables on Timepix3 Si sensor hybrid pixel detector performance

The study was conducted to calibrate and characterise the response of the Timepix3 photon-counting hybrid pixel detector. The study was conducted to calibrate and characterise the response of the Timepix3 photon-counting hybrid pixel detector. The goal was also to determine the impact of the angular variation of the detector to the source, of temperature change, and of ambient or strobe light on the on the detector response (measured fluence and energy spectrum). The impacts were studied using X-ray fluorescence lines, as well as Am-241 and Fe-55 radioactive sources. Angular variation measurements indicated angular dependence. This dependency increased with the angle and increased with lower energies. A decrease in fluence of up to 98.4% was recorded for Fe-55 (5.89 keV) and 43.1% for Am-241 (59.56 keV) at an angle of 90°. Temperature measurements showed a 4% decrease of photon count when increasing the temperature from 10 °C to 36 °C. Energy spectra were shifted to lower energies when the temperature increased. Measurements with variable light intensities showed no variations in terms of fluence or energy spectra. However, if it was a strobe light, the fluence was overestimated by 10% and the spectral shape presented an additional artefactual peak around 3 keV. To restrict the variability of the detector response and avoid a time-consuming calculation of the error factor, due to the detector’s temperature variation, we showed that it is necessary to keep the measurement temperature as close as possible to the temperature at which the calibration was performed. We also discovered the necessity of focusing on other relevant parameters such as the effect of the ambient light level or the angle of incidence of the X-ray beam impinging the detector on the detector response. This enabled us to propose a set of correction factors that can be used for other applications.

Hybrid Pixel EELS Detector: Low Noise, High Speed, and Large Dynamic Range

Journal Article Hybrid Pixel EELS Detector: Low Noise, High Speed, and Large Dynamic Range Get access Benjamin Plotkin-Swing, Benjamin Plotkin-Swing Nion Co., Kirkland, Washington, United States Search for other works by this author on: Oxford Academic Google Scholar Tracy Lovejoy, Tracy Lovejoy Nion Co., Kirkland, Washington, United States Search for other works by this author on: Oxford Academic Google Scholar Niklas Dellby, Niklas Dellby Nion Co., Kirkland, Washington, United States Search for other works by this author on: Oxford Academic Google Scholar George Corbin, George Corbin Nion Co., Kirkland, Washington, United States Search for other works by this author on: Oxford Academic Google Scholar Matthew Hoffman, Matthew Hoffman Nion Co., Kirkland, Washington, United States Search for other works by this author on: Oxford Academic Google Scholar Sacha De Carlo, Sacha De Carlo Dectris, Baden-Daettwil, Aargau, Switzerland Search for other works by this author on: Oxford Academic Google Scholar Luca Piazza, Luca Piazza Dectris, Baden-Daettwil, Aargau, Switzerland Search for other works by this author on: Oxford Academic Google Scholar Chris Meyer, Chris Meyer Nion Co., Kirkland, Washington, United States Search for other works by this author on: Oxford Academic Google Scholar Andreas Mittelberger, Andreas Mittelberger Nion Co., Kirkland, Washington, United States Search for other works by this author on: Oxford Academic Google Scholar Ondrej Krivanek Ondrej Krivanek Nion Co., Kirkland, Washington, United States Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 26, Issue S2, 1 August 2020, Pages 1928–1930, https://doi.org/10.1017/S1431927620019856 Published: 01 August 2020

The Phase 2 Upgrade of the CMS Inner Tracker

A new silicon tracker will be built for the Phase 2 Upgrade of the CMS experiment to fully exploit the increased luminosity delivered by the HL-LHC. The innermost part, called the Inner Tracker, will be exposed to extreme conditions such as unprecedented radiation levels of 1.2Grad and 2.3×1016neq/cm2 and hit rate of 3.2GHz/cm2. The new Inner Tracker relies on many novel solutions and technologies that allow for a design of a light and radiation-hard pixel detector of high performance. The hybrid pixel modules will be composed of pixel sensors with pixel size of 2500μm2 and a new ASIC, designed in 65 nm CMOS technology, developed by the RD53 collaboration. A novel scheme of serial powering will be deployed to power the pixel modules and new technologies will be used for a high bandwidth readout system. The mechanics will be lightweight, based on carbon-fibre material and two-phase CO2 cooling. In this contribution, the design of the CMS Inner Tracker system will be presented along with the prospective design choices.

The TSV process in the hybrid pixel detector for the High Energy Photon Source

HEPS-BPIX3 is the third prototype of a single-photon counting pixel detector with 1.4 million pixels developed for applications using synchrotron light sources. It follows the previous prototypes with a pixel size of 150μm×150μm and frame rate up to 1.2 kHz at 20-bit dynamic range. We started to upgrade it with the through silicon via (TSV) process to reduce the insensitive gap between modules. The TSV process drills vias at the pads of the readout chip, and makes connections between the front and back sides. The redistribution layer (RDL) and bump bonding are performed at the bottom of readout chip to connect the control, readout signals and power supplies to the PCB. By using this so-called three-dimensional (3D) integration techniques, the insensitive area of the module is reduced from 26.3% to 11.8%. The assembled modules have been tested with X-ray and synchrotron light source, and the results show that the readout chips work well following the TSV process without any measurable performance degradation.

Particle tracking and radiation field characterization with Timepix3 in ATLAS

Four hybrid pixel detectors of Timepix3 technology, installed in the ATLAS experiment, were continuously taking data from April 2018 until the end of the Run-2 data taking period (December 2019). These detectors are synchronized with each other and the LHC orbit clock. They are capable of resolving the bunch structure of the LHC beams due to their time resolution of ∼2ns. Thus, they allow the characterization of the radiation field inside and outside bunch-crossing periods. This is shown for Timepix3 detectors at the extended barrel (x=-3.58 m, y=0.97 m, z=2.83 m). We apply pattern recognition methods to decompose the radiation field and determine the directionality of the minimum ionizing particles (MIP) component of the radiation field.