The Timepix2 ASIC (application-specific integrated circuit) is the upgraded successor to the Timepix  hybrid pixel detector readout chip. Like the original, Timepix2 contains a matrix of 65k square pixels of 55 μm pitch that can be coupled to a similarly segmented semiconductor sensor, or integrated in an ionising gas detector. The pixels are programmable, with several operation modes and selectable counter depths (up to 18 bits for time-ofarrival, ToA, and up to 14 bits for time-over-threshold, ToT). In ToT and ToA mode, each pixel records the arrival time and energy deposited by particles interacting with the corresponding sensor segment, with an optional separation of timing resolution for ToT and ToA: down to 10 ns each. The gain of the frontend circuit can be programmed to adapt to the quantity of energy deposited in the sensor, yielding a large dynamic range of 0.38 ke to 950 ke . The frontend noise in adaptive gain mode is 380 e rms. The design also introduces some power optimisation features to the Timepix portfolio, such as power masking on selectable parts of the pixel matrix. With all pixels powered on, using 100 MHz for both ToT and ToA clock frequencies, and assuming a sparse particle interaction with the pixels, the matrix is estimated to consume less than 900 mW based on simulation.
The first Medipix chip which aimed at permitting single photon counting on a sizable matrix of pixels was developed in the mid-1990’s. In the following 20 years two families of chips have evolved from that initial effort. The Medipix photon counting family of chips comprises Medipix, Medipix2 and Medipix3. A 4th generation chip, Medipix4, is under development. The Timepix chips were initially more aimed at single particle detection and that family comprises Timepix, the most recent Timepix2 chip (introduced in this Special Issue) and Timepix3. The 4th generation Timepix4 is also under development and a first version will be produced in 2019. This paper seeks to provide a brief introduction to the various members of the Medipix family and provide references to more detailed descriptions already available in the literature.
Medipix3 is a 256×256 channel hybrid pixel detector readout chip working in a single photon counting mode with a new inter-pixel architecture, which aims to improve the energy resolution in pixelated detectors by mitigating the effects of charge sharing between channels. Charges are summed in all 2×2 pixel clusters on the chip and a given hit is allocated locally to the pixel summing circuit with the biggest total charge on an event-by-event basis. Each pixel contains also two 12-bit binary counters with programmable depth and overflow control. The chip is configurable such that either the dimensions of each detector pixel match those of one readout pixel or detector pixels are four times greater in area than the readout pixels. In the latter case, event-by-event summing is still possible between the larger pixels. Each pixel has around 1600 transistors and the analog static power consumption is below 15 μW in the charge summing mode and 9 μW in the single pixel mode. The chip has been built in an 8-metal 0.13 μm CMOS technology. This paper describes the chip from the pixel to the periphery and first electrical results are summarized.
Timepix3 is a hybrid pixel detector readout chip. It features a data driven readout mode where the chip sends out a data packet containing pixel coordinate, time over threshold and time of arrival immediately after the hit is processed by the pixel. The maximum hit rate is 40 Mhits/cm2/s with a minimum time step in the arrival time measurement of 1.56 ns. The pixel matrix consist of 256 × 256 square pixels at a 55 μm pitch and the pixel front end noise is 61 e− RMS. In this paper we present the first radiation measurements with Timepix3 bump bonded to a 300 μm thick silicon sensor. The chip is calibrated per pixel, using internal test pulses and the calibration is verified using X-ray fluorescence. The energy resolution, threshold dispersion and gain dispersion is measured. The energy resolution in time over threshold mode under normal operation conditions is 4.07 keV FWHM at 59.5 keV. At 10.5 keV an energy resolution of 0.72 keV FWHM was achieved in photon counting mode and in time over threshold mode, by optimizing the energy response, we achieved a 1.38 keV FWHM. We also investigate the time walk and present first results on using the time information for track reconstruction.
Funded by the European Space Agency, a miniaturized radiation monitor (MIRAM) is being developed in collaboration of the Institute for Experimental and Applied Physics, Czech Technical University in Prague and ADVACAM s.r.o. in Prague. Within a small, low power consumption and inexpensive unit, this tool provides measurement of the deposited dose and flux estimation for electrons and protons separately to the spacecraft it is attached to. The planned device will integrate a direct-converting pixel detector of the Timepix family (300-1000 μm thick sensor, 256 x 256 pixels, pixel pitch 55 μm), combined with four diodes, providing low power mode and coincidence measurements. Presented are the strategy for the particle-type identification and results from simulations of the detector response for electrons and protons. The strategy and design are based on the experience gained from the investigation of the data received from the Space Application of the Timepix Radiation Monitor (SATRAM) within the last five years. The proficiency of both is analysed using data from MC simulations in Geant4.
Photon-counting pixel detectors are now routinely used on synchrotron beamlines. Many applications benefit from their noiseless mode of operation, single-pixel point spread function and high frame rates. One of their drawbacks is a discontinuous detection area due to the space-consuming wirebonded connections of the readout chips. Moreover, charge sharing limits their efficiency and their energy discrimination capabilities. In order to overcome these issues the ESRF is developing SMARTPIX,a scalable and versatile pixel detector system with minimized dead areas and with energy resolving capabilities based on the MEDIPIX3RX readout chip. SMARTPIX exploits the through-silicon via technology implemented on MEDIPIX3RX, the active edge sensor processing developed in particular at ADVACAM, and the on-chip analog charge summing feature of MEDIPIX3RX. This article reports on system architecture, unit module structure, data acquisition electronics, target characteristics and applications.
Semiconductor single-particle-counting pixel detectors offer many advantages for radiation imaging: high detection efficiency, energy discrimination, noiseless digital integration (counting), high frame rate and virtually unlimited dynamic range. All these properties allow to achieve high quality images. Examples of transmission images and 3D tomographic reconstruction using X-rays and slow neutrons are presented demonstrating effects that can affect the quality of images. A number of obstacles can limit detector performance if not handled. The pixel detector is in fact an array of individual detectors (pixels), each of them has its own efficiency, energy calibration and also noise. The common effort is to make all these parameters uniform for all pixels. However, an ideal uniformity can be never reached. Moreover, it is often seen that the signal in one pixel affects neighboring pixels due to various reasons (charge sharing, crosstalk, etc.). All such effects have to be taken into account during data processing to avoid false data interpretation. The main intention of this contribution is to summarize techniques of data processing and image correction to eliminate residual drawbacks of pixel detectors. It is shown how to extend these methods to handle further physical effects such as hardening of the beam and edge enhancement by deflection. Besides, more advanced methods of data processing such as tomographic 3D reconstruction are discussed. All methods are demonstrated on real experiments from biology and material science performed mostly with the Medipix2 pixel device. A brief view to the future of pixel detectors and their applications also including spectroscopy and particle tracking is given too.
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 detectors based on the Medipix chips have proven themselves as a good tool for spectral X-ray imaging. An important advantage of such detectors is the possibility to use various sensor materials (Si, GaAs:Cr, CdTe etc). Each of these materials has own advantages and disadvantages. The higher Z materials provide the higher photon registration efficiency, but also the greater energy of the fluorescent photons distorting the detector’s energy response. The thickness of the sensor affects the same way: the thicker sensor means the higher photon registration efficiency, but the energy resolution gets worse. At the same time, photons that have passed through the detector without interacting in it can be registered by the next detector. The combination of several detectors with various sensor materials allows increasing the overall photon registration efficiency. In this case, each detector will operate in the optimal energy range. In this work, we consider a system based on three Timepix detectors with sensors made from Si, GaAs:Cr and CdTe. The Monte Carlo simulation was used for the sensors thickness optimization. The possibility and advantages of using such imaging system for spectral CT are demonstrated.
Research using hybrid pixel detectors in medical physics is on the rise. Timepix detectors have arrays of 256 × 256 pixels with a resolution of 55 μm. Here, and by using Timepix counts instead of Hounsfield units, we present a calibration curve of a Timepix detector analog to those used for CT calibration. Experimentation consisted of the characterization of electron density in 10 different kinds of tissue equivalent samples from a CIRS 062M phantom (lung, 3 kinds of bones, fat, breast, muscle, water and air). Radiation of the detector was performed using an orthodontic X-ray machine at 70 KeV and .06 second of tube current with a purpose-built aluminum collimator. Data acquisition was performed at 1 frame per second and taking 3 frames per phantom. We were able to find a curve whose behavior was similar to others already published. This will lead to the verification of the usage of Timepix for identification of different tissues in an organ.