Semiconductor Pixel Research Articles

Characterizing devices for validation of dose, dose rate, and LET in ultra high dose rate proton irradiations.

Ultra high dose rate (UHDR) radiotherapy using ridge filter is a new treatment modality known as conformal FLASH that, when optimized for dose, dose rate (DR), and linear energy transfer (LET), has the potential to reduce damage to healthy tissue without sacrificing tumor killing efficacy via the FLASH effect. Clinical implementation of conformal FLASH proton therapy has been limited by quality assurance (QA) challenges, which include direct measurement of UHDR and LET. Voxel DR distributions and LET spectra at planning target margins are paramount to the DR/LET-related sparing of organs at risk. We hereby present a methodology to achieve experimental validation of these parameters. Dose, DR, and LET were measured for a conformal FLASH treatment plan involving a 250-MeV proton beam and a 3D-printed ridge filter designed to uniformly irradiate a spherical target. We measured dose and DR simultaneously using a 4D multi-layer strip ionization chamber (MLSIC) under UHDR conditions. Additionally, we developed an "under-sample and recover (USRe)" technique for a high-resolution pixelated semiconductor detector, Timepix3, to avoid event pile-up and to correct measured LET at high-proton-flux locations without undesirable beam modifications. Confirmation of these measurements was done using a MatriXX PT detector and by Monte Carlo (MC) simulations. MC conformal FLASH computed doses had gamma passing rates of>95% (3mm/3% criteria) when compared to MatriXX PT and MLSIC data. At the lateral margin, DR showed average agreement values within 0.3% of simulation at 100Gy/s and fluctuations ∼10% at 15Gy/s. LET spectra in the proximal, lateral, and distal margins had Bhattacharyya distances of<1.3%. Our measurements with the MLSIC and Timepix3 detectors shown that the DR distributions for UHDR scenarios and LET spectra using USRe are in agreement with simulations. These results demonstrate that the methodology presented here can be used effectively for the experimental validation and QA of FLASH treatment plans.

Time structures of proton pencil beam scanning delivery on a microsecond scale measured with a pixelated semiconductor detector Timepix3.

The time structures of proton spot delivery in proton pencil beam scanning (PBS) radiation therapy are essential in many clinical applications. This study aims to characterize the time structures of proton PBS delivered by both synchrotron and synchrocyclotron accelerators using a non-invasive technique based on scattered particle tracking. A pixelated semiconductor detector, AdvaPIX-Timepix3, with a temporal resolution of 1.56ns, was employed to measure time of arrival of secondary particles generated by a proton beam. The detector was placed laterally to the high-flux area of the beam in order to allow for single particle detection and not interfere with the treatment. The detector recorded counts of radiation events, their deposited energy and the timestamp associated with the single events. Individual recorded events and their temporal characteristics were used to analyze beam time structures, including energy layer switch time, magnet switch time, spot switch time, and the scanning speeds in the x and y directions. All the measurements were repeated 30 times on three dates, reducing statistical uncertainty. The uncertainty of the measured energy layer switch times, magnet switch time, and the spot switch time were all within 1% of average values. The scanning speeds uncertainties were within 1.5% and are more precise than previously reported results. The measurements also revealed continuous sub-milliseconds proton spills at a low dose rate for the synchrotron accelerator and radiofrequency pulses at 7µs and 1ms repetition time for the synchrocyclotron accelerator. The AdvaPIX-Timepix3 detector can be used to directly measure and monitor time structures on microseconds scale of the PBS proton beam delivery. This method yielded results with high precision and is completely independent of the machine log files.

Particle tracking, recognition and LET evaluation of out-of-field proton therapy delivered to a phantom with implants

Objective. This study aims to assess the composition of scattered particles generated in proton therapy for tumors situated proximal to some titanium (Ti) dental implants. The investigation involves decomposing the mixed field and recording Linear Energy Transfer (LET) spectra to quantify the influence of metallic dental inserts located behind the tumor. Approach. A therapeutic conformal proton beam was used to deliver the treatment plan to an anthropomorphic head phantom with two types of implants inserted in the target volume (made of Ti and plastic, respectively). The scattered radiation resulted during the irradiation was detected by a hybrid semiconductor pixel detector MiniPIX Timepix3 that was placed distal to the Spread-out Bragg peak. Visualization and field decomposition of stray radiation were generated using algorithms trained in particle recognition based on artificial intelligence neural networks (AI NN). Spectral sensitive aspects of the scattered radiation were collected using two angular positions of the detector relative to the beam direction: 0° and 60°. Results. Using AI NN, 3 classes of particles were identified: protons, electrons & photons, and ions & fast neutrons. Placing a Ti implant in the beam’s path resulted in predominantly electrons and photons, contributing 52.2% of the total number of detected particles, whereas for plastic implants, the contribution was 65.4%. Scattered protons comprised 45.5% and 31.9% with and without metal inserts, respectively. The LET spectra were derived for each group of particles identified, with values ranging from 0.01 to 7.5 keV μm−1 for Ti implants/plastic implants. The low-LET component was primarily composed of electrons and photons, while the high-LET component corresponded to protons and ions. Significance. This method, complemented by directional maps, holds the potential for evaluating and validating treatment plans involving stray radiation near organs at risk, offering precise discrimination of the mixed field, and enhancing in this way the LET calculation.

Assessment of the axial resolution of a compact gamma camera with coded aperture collimator

PurposeHandheld gamma cameras with coded aperture collimators are under investigation for intraoperative imaging in nuclear medicine. Coded apertures are a promising collimation technique for applications such as lymph node localization due to their high sensitivity and the possibility of 3D imaging. We evaluated the axial resolution and computational performance of two reconstruction methods.MethodsAn experimental gamma camera was set up consisting of the pixelated semiconductor detector Timepix3 and MURA mask of rank 31 with round holes of 0.08 mm in diameter in a 0.11 mm thick Tungsten sheet. A set of measurements was taken where a point-like gamma source was placed centrally at 21 different positions within the range of 12–100 mm. For each source position, the detector image was reconstructed in 0.5 mm steps around the true source position, resulting in an image stack. The axial resolution was assessed by the full width at half maximum (FWHM) of the contrast-to-noise ratio (CNR) profile along the z-axis of the stack. Two reconstruction methods were compared: MURA Decoding and a 3D maximum likelihood expectation maximization algorithm (3D-MLEM).ResultsWhile taking 4400 times longer in computation, 3D-MLEM yielded a smaller axial FWHM and a higher CNR. The axial resolution degraded from 5.3 mm and 1.8 mm at 12 mm to 42.2 mm and 13.5 mm at 100 mm for MURA Decoding and 3D-MLEM respectively.ConclusionOur results show that the coded aperture enables the depth estimation of single point-like sources in the near field. Here, 3D-MLEM offered a better axial resolution but was computationally much slower than MURA Decoding, whose reconstruction time is compatible with real-time imaging.

Characterisation of a customised 4-chip Timepix3 module for charged-particle tracking

Ion-beam radiotherapy is a growing cancer treatment modality because it offers a superior dose distribution in the patient compared with conventional radiotherapy using X-rays. Thanks to their versatility, application-specific integrated circuits (ASIC) increasingly gain interest for research into ion imaging and ion-beam characterisation. Timepix3 is a hybrid semiconductor pixel detector, which offers nanosecond time binning as well as dead-time-free and noise-free data-driven readout at a pixel pitch of 55µm × 55µm. In this work, a novel 4-chip Timepix3 mini-tracker (quad module) was characterised in a therapeutic proton beam. The quad module has two detection layers equipped with two Timepix3 chips each, which are stacked like a particle telescope at a distance of 20.3mm. In a detection layer, two Timepix3 chips share the same sensitive silicon sensor. The surface area of the silicon sensor is approximately 28mm × 14mm. Apart from the pixels at the chip edges and the masked pixels, the quad module showed a uniform counting response to the mono-energetic proton irradiation without noticeable defects. The measurement accuracy of the energy deposition was found to be better than 1% at 340keV. The synchronisation between the four chips of the quad module showed systematic delays of up to 25ns. When these delays are corrected, the time resolution of the quad module is (1.17 ± 0.03)ns. The time resolution could be further improved with the implementation of a time-walk correction. It is concluded that the quad module fulfils the requirements to be used as a charged-particle tracker in ion-beam radiotherapy. Future testing will focus on the radiation hardness, dose-rate dependence and response to high-LET radiation.

Directional-sensitive wide field-of-view monitoring of high-intensity proton beams by spectral tracking of scattered particles with scattering foil and miniaturized radiation camera

Monitoring and characterization of particle beams in wide-range is often necessary in research and many applications with particle accelerators. The quantitative measurement and evaluation of composition especially of high-intensity beams are limited and can become a challenge with conventional methods especially with simplified instrumentation for ease of deployment. For this purpose, we developed a novel technique based on high-resolution spectral-sensitive tracking of single particles scattered from the beam path by a thin foil. We use a compact radiation camera equipped with the semiconductor pixel detector Timepix3 together with dedicated Monte-Carlo simulations. Particle-event type discrimination and directional information are produced by the detector spectral-tracking response together with particle-type resolving power derived from experimental calibrations. Directional- and spectral-sensitive components can be resolved in wide field-of-view. Quantification of the primary beam intensity is extrapolated by numerical calculations. Demonstration and evaluation of the technique are provided by measurements with 33 MeV protons from a light ion cyclotron accelerator.Scattered particles originating from the thin foil, the accelerator beam nozzle, and the air space along the beam path are detected and evaluated.

Probability distribution maps of deposited energy with sub-pixel resolution for Timepix3 detectors

The Medipix3 Collaboration has developed the Timepix3 chip, which can provide information about individual particles interacting with a hybrid pixelated semiconductor detector. The Timepix3 generates a data packet including time and energy information for each pixel receiving a charge above the discriminator threshold. From a set of such packages generated by one particle the total deposited energy and the sub-pixel center of mass (c.m) position of the interaction can be calculated. However, sub-threshold losses will distort both the measured energy and c.m calculation. This work aims to generate probability maps for the most probable initial interaction position and energy for a given detected cluster. The correction maps were calculated for different energies and sub-pixel c.m positions. It is crucial to realistically simulate the front-end response of the Timepix3 detector to be able to match and understand the experimental data and generate accurate probability maps. The correction maps from the simulations using the simulation tool Allpix 2 are applied to experimental data utilizing an Am-241 source to irradiate a Minipix Timepix3, leading to sub-pixel resolution and improved energy resolution for Timepix3 detectors.

Processing of secondary radiation and cosmic ray data measured at flight altitudes with the pixel detector Timepix

&#x0D; &#x0D; &#x0D; Experimental data of the secondary radiation field and cosmic rays inside passenger aircraft in the atmosphere at airline altitudes (10-12 km) were processed. High-resolution data were measured by the semiconductor pixel detector Timepix operated in a miniaturized radiation camera MiniPIX-Timepix. The detector provides precise characterization with quantum sensitivity in terms of deposited energy and visualization of the charged particle radiation and X-ray field. The data were processed at the pre-processing and processing level with an integrated SW tool (Data Processing Engine-DPE). Results and physics data products consist of particle flux, dose rate, deposited energy, deposited dose, field composition into broad particle classes (protons, electrons, X-rays) as well as detailed visualization of the radiation field with quantum imaging registration of single-particle tracks. In this work, the detailed analysis of two measurements, as well as comparative graphs of selected results between ten flights are presented. Results of the total absorbed dose are compared with values measured also by Timepix detectors on ground and in LEO orbit onboard a satellite.&#x0D; &#x0D; &#x0D;

Quantum-imaging detection of secondary neutrons in proton radiotherapy fields

Secondary radiation fields encountered in proton radiotherapy environments contain different particle species produced in a broad range of energies and directions. Experimental knowledge of the composition and spectral characteristics of such complex fields is valuable for operation and protection of instruments and personnel, design and optimization of irradiations as well as planning and validation of treatment plans. The neutron component, which are produced with non-negligible yield, is in particular challenging to measure and discriminate from other radiations by conventional detectors. In order to measure in such complex fields the neutron component, both fast and thermal, we make use of the semiconductor pixel detector Timepix3 equipped with a silicon sensor and a neutron converter mask. The detector was before calibrated with well-defined neutron fields. In this work, we characterize the secondary radiation field and examine in particular the neutron component behind a large water-equivalent phantom irradiated by a 190 MeV clinical proton beam. The detected neutrons have a predominant fast neutron component. No thermal neutrons are observed in the measured data. The neutron-induced interactions in the detector are resolved in a high background with enhanced discrimination by quantum-imaging visualization, micrometer scale pattern recognition and high-resolution spectral-sensitive tracking of single particles. Detailed results are provided in wide range in terms of composition of the mixed-radiation field, total and partial fluxes and dose rates as well as particle deposited dose and linear-energy-transfer (LET) spectra.

Thermal neutron detection and track recognition method in reference and out-of-field radiotherapy FLASH electron fields using Timepix3 detectors.

Objective.This work presents a method for enhanced detection, imaging, and measurement of the thermal neutron flux.Approach. Measurements were performed in a water tank, while the detector is positioned out-of-field of a 20 MeV ultra-high pulse dose rate electron beam. A semiconductor pixel detector Timepix3 with a silicon sensor partially covered by a6LiF neutron converter was used to measure the flux, spatial, and time characteristics of the neutron field. To provide absolute measurements of thermal neutron flux, the detection efficiency calibration of the detectors was performed in a reference thermal neutron field. Neutron signals are recognized and discriminated against other particles such as gamma rays and x-rays. This is achieved by the resolving power of the pixel detector using machine learning algorithms and high-resolution pattern recognition analysis of the high-energy tracks created by thermal neutron interactions in the converter.Main results. The resulting thermal neutrons equivalent dose was obtained using conversion factor (2.13(10) pSv·cm2) from thermal neutron fluence to thermal neutron equivalent dose obtained by Monte Carlo simulations. The calibrated detectors were used to characterize scattered radiation created by electron beams. The results at 12.0 cm depth in the beam axis inside of the water for a delivered dose per pulse of 1.85 Gy (pulse length of 2.4μs) at the reference depth, showed a contribution of flux of 4.07(8) × 103particles·cm-2·s-1and equivalent dose of 1.73(3) nSv per pulse, which is lower by ∼9 orders of magnitude than the delivered dose.Significance. The presented methodology for in-water measurements and identification of characteristic thermal neutrons tracks serves for the selective quantification of equivalent dose made by thermal neutrons in out-of-field particle therapy.

Measurement of the time structure of FLASH beams using prompt gamma rays and secondary neutrons as surrogates

Objective. The aim of this study was to investigate the feasibility of online monitoring of irradiation time (IRT) and scan time for FLASH proton radiotherapy using a pixelated semiconductor detector. Approach. Measurements of the time structure of FLASH irradiations were performed using fast, pixelated spectral detectors based on the Timepix3 (TPX3) chips with two architectures: AdvaPIX-TPX3 and Minipix-TPX3. The latter has a fraction of its sensor coated with a material to increase sensitivity to neutrons. With little or no dead time and an ability to resolve events that are closely spaced in time (tens of nanoseconds), both detectors can accurately determine IRTs as long as pulse pile-up is avoided. To avoid pulse pile-up, the detectors were placed well beyond the Bragg peak or at a large scattering angle. Prompt gamma rays and secondary neutrons were registered in the detectors’ sensors and IRTs were calculated based on timestamps of the first charge carriers (beam-on) and the last charge carriers (beam-off). In addition, scan times in x, y, and diagonal directions were measured. The experiment was carried out for various setups: (i) a single spot, (ii) a small animal field, (iii) a patient field, and (iv) an experiment using an anthropomorphic phantom to demonstrate in vivo online monitoring of IRT. All measurements were compared to vendor log files. Main results. Differences between measurements and log files for a single spot, a small animal field, and a patient field were within 1%, 0.3% and 1%, respectively. In vivo monitoring of IRTs (95–270 ms) was accurate within 0.1% for AdvaPIX-TPX3 and within 6.1% for Minipix-TPX3. The scan times in x, y, and diagonal directions were 4.0, 3.4, and 4.0 ms, respectively. Significance. Overall, the AdvaPIX-TPX3 can measure FLASH IRTs within 1% accuracy, indicating that prompt gamma rays are a good surrogate for primary protons. The Minipix-TPX3 showed a somewhat higher discrepancy, likely due to the late arrival of thermal neutrons to the detector sensor and lower readout speed. The scan times (3.4 ± 0.05 ms) in the 60 mm distance of y-direction were slightly less than (4.0 ± 0.06 ms) in the 24 mm distance of x-direction, confirming the much faster scanning speed of the Y magnets than that of X. Diagonal scan speed was limited by the slower X magnets.

Dose estimation in X-ray backscatter imaging with Timepix3 and TLD detectors

Nowadays, the detection of backscatter X-rays has become an essential part of road security scanners, where individuals are at risk of an accidental exposure to the pulsed photon radiation fields generated by the devices that implement this technique. Dose to soft-tissue measurements of a pulsed radiation field generated by a commercial cargo X-ray backscattering device are presented using a Timepix3 detector. To accurately estimate the imparted dose, the energy spectrum of a backscattering cargo scanner is measured with a thick CdZnTe hybrid semiconductor pixel detector. The deconvolution of this measured spectrum is performed and the contributions to the dose are calculated by means of the mass energy-absorption coefficients for soft-tissue. This methodology was assessed through a Monte-Carlo simulation, together with verification measurements using thermoluminescent detectors (TLDs) as a standard dosimetry system. Measurements of direct irradiation, for a 30 s scan with the device at 2 m from the detector, yielded a dose of (4.9 ± 0.7) μGy and (3.6 ± 0.3) μGy for the Timepix3 and TLDs, respectively. In a simulated cargo environment, with the detector behind a 1 mm thick steel wall, 30 s scan time, and 2 m source to detector distance, the dose measured by the Timepix3 was (1.7 ± 0.1) μGy, while for the TLDs (1.4 ± 0.6) μGy, showing that the proposed methodology has a similar response as compared to a standard dosimetry system.

Surface figure measurement tool based on a radial shearing interferometer using a geometric phase lens with various spherical wavefronts.

We describe a compact radial shearing interferometer based on a geometric phase lens as a surface figure measurement tool. Based on the polarization and diffraction characteristics of a geometric phase lens, two radially sheared wavefronts are simply generated, and the surface figure of a specimen can be immediately reconstructed from the radial wavefront slope calculated with four phase-shifted interferograms obtained from a polarization pixelated complementary metal-oxide semiconductor camera. In order to increase the field of view, moreover, the incident wavefront is adjusted according to the shape of the target, which allows the reflected wavefront to become planar. By the combination of the incident wavefront formula and the measurement result by the proposed system, the whole surface figure of the target can be instantaneously reconstructed. As the experimental result, the surface figures of various optical components were reconstructed at the extended measurement area with less than 0.78µm deviations, and it was confirmed that the radial shearing ratio was fixed independent of the surface figures.

Out-of-field measurements and simulations of a proton pencil beam in a wide range of dose rates using a Timepix3 detector: Dose rate, flux and LET

Stray radiation produced by ultra-high dose-rates (UHDR) proton pencil beams is characterized using ASIC-chip semiconductor pixel detectors. A proton pencil beam with an energy of 220 MeV was utilized to deliver dose rates (DR) ranging from conventional radiotherapy DRs up to 270 Gy/s. A MiniPIX Timepix3 detector equipped with a silicon sensor and integrated readout electronics was used. The chip-sensor assembly and chipboard on water-equivalent backing were detached and immersed in the water-phantom. The deposited energy, particle flux, DR, and the linear energy transfer (LET(Si)) spectra were measured in the silicon sensor at different positions both laterally, at different depths, and behind the Bragg peak. At low-intensity beams, the detector is operated in the event-by-event data-driven mode for high-resolution spectral tracking of individual particles. This technique provides precise energy loss response and LET(Si) spectra with radiation field composition resolving power. At higher beam intensities a rescaling of LET(Si) can be performed as the distribution of the LET(Si) spectra exhibits the same characteristics regardless of the delivered DR. The integrated deposited energy and the absorbed dose can be thus measured in a wide range. A linear response of measured absorbed dose was obtained by gradually increasing the delivered DR to reach UHDR beams. Particle tracking of scattered radiation in data-driven mode could be performed at DRs up to 0.27 Gy/s. In integrated mode, the saturation limits were not reached at the measured out-of-field locations up to the delivered DR of over 270 Gy/s. A good agreement was found between measured and simulated absorbed doses.

Quantitative comparison of planar coded aperture imaging reconstruction methods

Imaging distributions of radioactive sources plays a substantial role in nuclear medicine as well as in monitoring nuclear waste and its deposit. Coded Aperture Imaging (CAI) has been proposed as an alternative to parallel or pinhole collimators, but requires image reconstruction as an extra step. Multiple reconstruction methods with varying run time and computational complexity have been proposed. Yet, no quantitative comparison between the different reconstruction methods has been carried out so far.This paper focuses on a comparison based on three sets of hot-rod phantom images captured with an experimental γ-camera consisting of a Tungsten-based MURA mask with a 2 mm thick 256 × 256 pixelated CdTe semiconductor detector coupled to a Timepix© readout circuit.Analytical reconstruction methods, MURA Decoding, Wiener Filter and a convolutional Maximum Likelihood Expectation Maximization (MLEM) algorithm were compared to data-driven Convolutional Encoder-Decoder (CED) approaches. The comparison is based on the contrast-to-noise ratio as it has been previously used to assess reconstruction quality. For the given set-up, MURA Decoding, the most commonly used CAI reconstruction method, provides robust reconstructions despite the assumption of a linear model. For single image reconstruction, however, MLEM performed best of all analytical reconstruction methods, but took on average 45 times longer than MURA Decoding. The fastest reconstruction method is the Wiener Filter with a run time 4.3 times faster compared to MURA Decoding and a mediocre quality. The CED with a specifically tailored training set was able to succeed the most commonly used MURA decoding on average by a factor between 1.37 and 2.60 and an equal run time.

Spectral tracking of proton beams by the Timepix3 detector with GaAs, CdTe and Si sensors

Position and directional-sensitive spectrometry of energetic charged particles can be performed with high resolution and wide dynamic range (energy, direction) with the hybrid semiconductor pixel detectors Timepix/Timepix3. The choice of semiconductor sensor material, thickness, and properties such as the reverse bias voltage, greatly determine detector sensitivity and resolving power for spectrometry and particle tracking. We investigated and evaluated the spectral tracking resolving power such as deposited energy and linear-energy-transfer (LET) spectra with the Timepix3 detector with different semiconductor sensors, based on GaAs:Cr, CdTe, and Si, using well-defined radiation sources in terms of radiation type (protons), energy, and incident direction to the detector sensor. Measurements of particle incident direction in a wide range were performed with collimated monoenergetic proton beams of various energies in the range 8–31 MeV at the U120-M cyclotron at the NPI CAS Rez near Prague. All detectors were per-pixel calibrated. This work enables to examine and perform a detailed study of charge sharing and charge collection efficiency in semiconductor sensors. The results serve to optimise the detector chip-sensor assembly configuration for measurements especially with high-LET particles in ion radiotherapy and outer space. The work underway includes evaluation of newly refined semi-insulating GaAs sensors and improved radiation hard semiconductor sensors SiC.

Readout chip with RISC-V microprocessor for hybrid pixel detectors

Hybrid single-photon counting pixel detectors have recently been widely used for X-ray and ionizing particle detection in medicine, high-energy physics, and material science. Many different chips have been developed for the readout of the semiconductor pixel sensor. Typically, developed ASICs have very limited digital logic and do not provide substantial data processing. In this paper, we present the readout chip that integrates the readout channels matrix with a RISC-V-based microprocessor SoC. The designed device has been prototyped in an FPGA and sent to production in a CMOS 40 nm process. Integration of a pixel matrix with the RISC-V-based central processing unit significantly improved the detector functionality. It enabled the device to work independently without external assistive device usage and execute many algorithms, e.g., calibration, threshold scanning, and data filtering, on-chip. Communication between the CPU and the pixel matrix was carried out through the dedicated Pixel Matrix Controller with the CPU standard I/O operations usage. This specialized peripheral consists of a coprocessor responsible for precise matrix control, a data converter for data conversion acceleration, and control and status registers connected to the core data bus. Many algorithms have been developed and tested, one of which is the intelligent real-time filtering of regions of interest.

The 100[formula omitted]PET project: A small-animal PET scanner for ultra-high resolution molecular imaging with monolithic silicon pixel detectors

Recent developments in semiconductor pixel detectors allow a new generation of positron-emission tomography (PET) scanners that, combined with advanced image reconstruction algorithms, will allow for a few hundred microns spatial resolutions. Such novel scanners will pioneer ultra-high resolution molecular imaging, a field that is expected to have an enormous impact in several medical domains, neurology among others.The University of Geneva, the École Polytechnique Fédérale de Lausanne, and the University of Lucerne, have launched the 100μPET project that aims to produce a small-animal PET scanner with ultra-high resolution. The scanner will be composed of 4 ”towers”, each containing a stack of 60 monolithic silicon pixel sensors for the direct measurement of the annihilation photons. The sensors are 270 μm thick, providing unprecedented depth-of-interaction measurement. Monte Carlo simulations were done simulating different scanner conditions, resulting in a spatial resolution down to 0.2 mm FWHM and a sensitivity of 3.2%.

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.

Charge sharing in pixelated semiconductor sensors

The charge sharing between neighboring pixels in pixelated sensors can be used to measure particle or x-ray coordinates with accuracy better than the pixel pitch. The accurate model of the charge distribution shape is essential to achieve ultimate coordinate accuracy. The charge sharing is caused by charge carriers diffusion on the path from the generation point to pixels. This paper is focused on the diffusion of the initially compact charge cloud in the field free region. The diffusion equation solutions are obtained using separation of variable and Fourier synthesis method for different initial conditions and resulting charge distributions are integrated over pixel areas. The look up table containing pre-calculated values for pixel charge fractions is proposed to speed up numerical calculations.