Timepix Detector Research Articles

Stigmatic imaging of secondary ions in MeV-SIMS spectrometry by linear Time-of-Flight mass spectrometer and the TimePix detector

Secondary ion mass spectrometry (SIMS), based on primary ions within the MeV energy domain, also known as MeV-SIMS, is a subject of increasing scientific interest. The main drive for the interest in the development of MeV-SIMS is the ability to desorb high yields of large non-fragmented organic molecular ions from the sample surface. This makes MeV-SIMS particulary useful in imaging of biological tissues.Imaging methods based on scanning a focused primary ion beam are associated with demanding focusing of the heavy energetic ions. As an alternative, stigmatic imaging mode has been studied here, applying point-to-point imaging characteristics of secondary ions in the linear Time-Of-Flight mass spectrometer. In stigmatic imaging approaches, spatial resolution is independent of the focussed spot size of the ionising primary ion beam, but instead dependant on the ability of the ion optics to project an image of the ion distributions removed from the surface onto a position sensitive ion detector.

Single absorption grating based X-ray phase contrast CT using Timepix detector

Single absorption grating based X-ray phase contrast imaging (XPCI) with a hybrid pixel detector Timepix is a promising technique in many application fields (material sciences, biology, medicine etc.). Conventional coded aperture approach with microfocus source uses two absorption gratings where one of them is moved in steps (so-called phase stepping). In contrast to that, the proposed XPCI approach provides the absorption, phase contrast and dark field images from a single acquisition. This leads to higher mechanical stability of the system and decrease of the delivered dose. Moreover, faster data acquisition increases the usability of the system for CT measurements and dynamic phase radiography. Custom-built laboratory X-ray imaging setup equipped with Timepix detector and nano-focus X-ray tube was utilized for the measurements. The maximal source to detector distance (SDD) available in the setup is roughly 90 cm. Such system can provide long enough phase propagation distance while using the absorption grating with 60 μm strip period, 55 μm large detector pixels and three pixel illumination. The system enables insertion and positioning of both 1D and 2D absorption gratings producing micro-line or micro-pencil beams, respectively. The positioning stages of the used system allow to acquire a series of phase contrast images taken under different angles and thus to carry out phase contrast computed tomography (CT). CT reconstructions were performed by both in-house and commercial software packages. Results of pilot CT measurements of phantoms objects are presented in the contribution.

Multi-spectral X-ray transmission imaging using a BSI CMOS Image Sensor

In this work we study the performance to obtain X-ray images of a Back Side Illuminated CMOS Image Sensor, the Omnivision OV5647, empoying X-rays from tube with a palladium anode and voltages from 7.5 keV to 50 keV. The performance is compared with the Timepix detector operating in the Time Over Threshold mode. False color images are obtained using data from different energies and brightnesses, to fussion different information on the same picture. The different attenuations are analyzed and discussed in terms of the charge detection efficiency of the CMOS sensor, measured using Fluorescence X-rays and gamma rays from calibrated sources.

Stack of Timepix-based detectors with Si, GaAs:Cr and CdTe sensors with optimized thickness for spectral CT

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.

3D reconstruction of particle tracks in a 2 mm thick CdTe hybrid pixel detector

We demonstrate how the latest generation of hybrid pixel detectors of the Timepix family can be used to reconstruct 3 dimensional particle tracks on a microscopic scale, additionally determining the stopping power along the particles’ paths. In an experiment, a Timepix3 detector with a 2 mm thick planar CdTe sensor was irradiated in a 40 GeV/c pion beam and used in a similar way to a time-projection chamber: The coordinates x and y were given by the trajectory projection (pixel pitch: 55,upmu hbox {m}), the z-coordinate was reconstructed from the charge carrier drift time measurement (time binning: 1.5625 ns). The achievable z-resolution was studied at different bias voltages. Systematic inaccuracies due to an imprecise drift time model were determined and separated from the intrinsic uncertainty given by the time resolution. It was shown that a z-resolution of 60,upmu hbox {m} could be achieved by a perfect modeling of the drift time. With the presented z-reconstruction methodology, we studied the charge collection efficiency as a function of interaction depth, which was then used to apply a charge loss correction to the per-pixel energy measurements. 3D event displays of pion, muon and electron tracks are shown.

Improvement of TimePix energy resolution correcting threshold variations

The Timepix detector is an important tool in spectroscopic imaging since it allows the determination of the energy of absorbed photons using its time over threshold capability. In this paper, however, we look at the energy resolution using the single photon counting mode combined with threshold scans. The pixel circuit of the TimePix detector involves a discriminator logic, which compares the collected amount of charge in each pixel with a threshold level (THL). The counter is incremented only if the induced signal surpasses this threshold. The THL is typically calibrated w.r.t. energy in the proximity of the noise edge, however, due to gain mismatch of the individual pixels, the threshold levels of pixels deviate if a DAC value far from the calibration point is set. This energy threshold mismatch far from the calibration point was corrected in this paper based on spectrum matching between individual pixels. The derived method is compared to straightforward calibration using multiple monochromatic peaks. To this end a threshold scan was performed using a TimePix Si-1mm detector (utilizing counting priciples) in presence of a Am-241 gamma source. The peak at energy 59.5 keV was fitted for each pixel to determine the actual THL values for all pixels at this energy. Then the full spectrums for all pixels were matched, using one fixed point. The matching process gave us two matrices of correction factors, the first matrix corrects the THL gain and the second its offset. In a second evaluation we measured and fitted the x-ray fluorescence of zirconium and copper during threshold scan in addition to the Am-241 peak. The gain and offset of each pixel were then determined by linear fit of the peak positions. The two calibration methods were evaluated by measuring the full width of half maximum FWHM of two other fluorescence peaks (23.17 keVand 26.2 keV peaks of cadmium). The evaluation shows that linear fit of multiple peaks results in a comparable FWHM (reduction of 21%) over spectrum matching the (reduction of 20%), while, opposite to spectrum matching, necessitating multiple measurement setps and THL scans.

Study of Power Consumption of Timepix3 Detector

The Timepix3 readout chip—the latest member of the Medipix family of hybrid pixel detectors—brought several new functionalities in comparison with the older Timepix, i.e. a high hit-rate, a time granularity of 1.5625 ns, a data-driven readout scheme (with a per pixel dead time of approximately 475 ns), and the capability of measuring Time-over-Threshold (ToT) and Time-of-Arrival (ToA) in each pixel at the same time. However, the high power consumption of the Timepix3 in the standard setting prevents its use in applications with limited power budget. Moreover, the high power consumption poses the risk of overheating the sensor so that proper cooling is crucial. The presented work investigates the effect of different settings in the analogue and a digital part of the Timepix3 detector on its power consumption. Measurements were performed with the Timepix3 chipboard. The firmware of the Katherine readout was modified so that the user can monitor the power consumptions of analogue and digital part “on-line” (directly in the control software). In standard settings, a power consumption of approximately 1.3 W was found. By changes of internal DACs, the consumption could be reduced to 650 mW. Further reduction was achieved by the change of the clock management in the digital part of the Timepix3. In result, a power consumption of 216 mA could be achieved. In these low power settings, the ToA clock was reduced to 10 MHz and thus the time binning was 100 ns. The energy resolution was not affected significantly. The pixel dead time is also negatively affected when the matrix clock is reduced. In the case of 10 MHz, the minimal per pixel dead time is 1.9 μs.

Study of the radiation fields in LEO with the Space Application of Timepix Radiation Monitor (SATRAM)

We present the analysis of data taken by the Space Application of Timepix Radiation Monitor (SATRAM). It is centred on a Timepix detector (300 μm thick silicon sensor, pixel pitch 55 μm, 256 × 256 pixels). It was flown on Proba-V, an Earth observing satellite of the European Space Agency (ESA) from an altitude of 820 km on a sun-synchronous orbit, launched on May 7, 2013. A Monte Carlo simulation was conducted to determine the detector response to electrons (0.5–7 MeV) and protons (10–400 MeV) in an omnidirectional field taking into account the shielding of the detector housing and the satellite. With the help of the simulation, a strategy was developed to separate electrons, protons and ions in the data. The measured dose rate and stopping power distribution are presented as well as SATRAM’s capability to measure some of the stronger events in Earth’s magnetosphere. The stopping power, the cluster height and the shape of the particle tracks in the sensor were used to separate electrons, protons and ions. The results are presented as well. Finally, the pitch angles for a short period of time were extracted from the data and corrected with the angular response determined by the simulation.

First results from the LUCID-Timepix spacecraft payload onboard the TechDemoSat-1 satellite in Low Earth Orbit

The Langton Ultimate Cosmic ray Intensity Detector (LUCID) is a payload onboard the satellite TechDemoSat-1, used to study the radiation environment in Low Earth Orbit (∼635 km). LUCID operated from 2014 to 2017, collecting over 2.1 million frames of radiation data from its five Timepix detectors on board. LUCID is one of the first uses of the Timepix detector technology in open space, with the data providing useful insight into the performance of this technology in new environments. It provides high-sensitivity imaging measurements of the mixed radiation field, with a wide dynamic range in terms of spectral response, particle type and direction. The data has been analysed using computing resources provided by GridPP, with a new machine learning algorithm that uses the Tensorflow framework. This algorithm provides a new approach to processing Medipix data, using a training set of human labelled tracks, providing greater particle classification accuracy than other algorithms. For managing the LUCID data, we have developed an online platform called Timepix Analysis Platform at School (TAPAS). This provides a swift and simple way for users to analyse data that they collect using Timepix detectors from both LUCID and other experiments. We also present some possible future uses of the LUCID data and Medipix detectors in space.

Timepix in LEO Orbit onboard the VZLUSAT-1 Nanosatellite: 1-year of Space Radiation Dosimetry Measurements

The VZLUSAT-1 satellite, the first Czech CubeSat, was successfully launched on June 23, 2017, to a 510 km Sun-synchronous low-Earth orbit. It carries several scientific payloads including a Timepix detector as focal plane imager for the X-Ray telescope onboard. The Timepix detector contributes significantly to the satellite data collection, with more than 25 000 sampling acquisitions in the first year of deployment. Despite limitations of the satellite attitude control system, necessary for capturing X-Ray images of the Sun, the Timepix detector allows measuring the space radiation environment along the satellite orbit. As of September 2018, we conducted 33 whole-Earth mappings, recording radiation doses around the planet. Further, we show data from scans of the South Atlantic Anomaly and polar radiation horns, where the location and acquisition time were tailored to minimize event pile-up and particle track overlap. Since October 2017, the optics segment of the onboard X-Ray telescope was deployed, which exposed the Timepix detector unshielded to free open space. This change produced entirely new observations namely of low energy charged particles and a significant increase of measured particle flux. We also registered the effects of exposing the sensor to direct intense sunlight. We will summarize on the actual performance of the custom readout interface, which exceeds expectations in the constrained environment of the low-cost and low-powered CubeSat nanosatellite.

Rospix: modular software tool for automated data acquisitions of Timepix detectors on Robot Operating System

We present software for interfacing FITPix and USB Lite compatible readout electronics using Robot Operating System (ROS). ROS is widely adopted middleware for integration of sensors, processing algorithms and logic to autonomous systems such as robots, unmanned helicopters, and robotic payloads. Thanks to ROS, Timepix detectors can be deployed in automated experiments on platforms spanning from traditional desktop computers to small ARM devices such as Raspberry Pi and Odroid. Acquisition and detector settings can be controlled in Linux shell allowing deployment on headless devices. Using ROS networking capabilities, captured frames and detector control can be transmitted via a network, which allows building distributed processing pipelines. The proposed software is a lightweight package, easily connectable to existing visualization, logging, and processing software in ROS. It offers simple bindings to custom Python and C++ programs for real-time control of the acquisition or processing of the captured frames. Rospix was deployed on NASA suborbital rocket, which was successfully launched on April 4, 2018, from Kwajalein Atoll by Pennsylvania State University. Rospix provides reliable solution for mobile robots where the collected data are used in real time to guide the robot through an environment. We release Rospix using the GitHub platform and welcome the community to contribute on the first project connecting the fields of ionizing radiation imaging and mobile robotics.

Timepix3 detector network at ATLAS experiment

We describe the technical design and show results of first measurements of a network based on the Timepix3 and the Katherine readout (with Gigabit Ethernet interface) installed in the ATLAS cavern in the LHC in January 2018. The network consists of four Timepix3 detectors, arranged in two two-layer telescopes. All detectors in the network are synchronized with each other and the LHC orbit clock. The technical solution and the concept of the project are described in details. Since the radiation field inside ATLAS is rather harsh, only the detector units (sensors with power supplies) of the system are placed in the cavern. The detectors are connected to the readout electronics (situated in a rack room with radiation levels comparable to the natural background radiation) by 80 m long cables. The technical design was tested for distances up to 120 m. First measurements are presented, which demonstrate the capabilities of the ATLAS-TPX3 network.

Dynamic range and resolving power of the Timepix detector to heavy charged particles

The spectrometric and particle tracking response of the Timepix detector for charged particle detection was examined for energetic (i.e. penetrating) heavy charged particles in a wide range of energies and directions. The aim of this study was to examine the detector's resolving power including particles approaching the minimum-ionizing regime, in particular energetic protons. The per-pixel energy range, of importance namely for heavy charged particles, was also investigated. This work complements and extends the ongoing task to analyze and describe the response and resolving power of the detector in a wide range of particle types, energy (energy loss) and direction. The methodology of event discrimination in terms of these degrees of freedom was expanded and refined. Resolution and event classification were based on analyses of cluster morphology parameters together with spectrometric, tracking and correlated derived quantities such as linear-energy-transfer (LET) and ratio cluster height to cluster path length.

A synchronization and data acquisition system for silicon detectors

A dedicated synchronization bus has been developed and integrated into the FITPix COMBO device. It can be used as Timepix read-out (involving back-side-pulse acquisition) or as a simple spectrometer device—Spectrig (when an external single pad sensor is connected, e.g. Δ E detector). The synchronization bus permits the implementation of a system of up to 32 distinct devices running in clock locked mode while the absolute value of the timestamp is distributed to all devices in the system. Any combination of Timepix or spectrometer devices (up to 32) can be connected-up to create a final measurement set-up. The synchronization bus has also been designed to control a trigger signal and busy signal to allow an effective filtration of unpaired events when coincidence measurement is performed. The system has been tested with a Δ E− E telescope consisting of a thin detector and Timepix. The thin detector has been used for Δ E and the Timepix detector for E measurements. The Δ E detector has an area of 10×10 mm2 and a thickness of 12 μm with non-uniformity of 8%. The area and thickness of Timepix are 14×14 mm2 and 300 μm, respectively. The detection system can provide simultaneous information about position, energy, time and type of registered particles with high synchronization accuracy. Test measurements have been carried out with an alpha particle source (Ra-226) in a vacuum and the results of these measurements are presented in this paper.

The LUCID-Timepix spacecraft payload and the CERN@school educational programme

The Langton Ultimate Cosmic ray Intensity Detector (LUCID) is a payload onboard the satellite TechDemoSat-1, used to study the radiation environment in Low Earth Orbit (∼635 km). LUCID operated from 2014 to 2017, collecting over 2.1 million frames of radiation data from its five Timepix detectors on board. LUCID is one of the first uses of the Timepix detector technology in open space, with the data providing useful insight into the performance of this technology in new environments. The data has been analysed using computing resources provided by GridPP, with a novel machine learning algorithm. For managing the LUCID data, we have developed an online platform called Timepix Analysis Platform at School (TAPAS). This provides a swift and simple way for users to analyse data that they collect using Timepix detectors from both LUCID and other school based Timepix projects. These projects constitute a secondary school programme 'CERN@school' that give a framework for novel implementations of conventional classroom experiments using Timepix, as well as letting students contribute to large international scientific collaborations and devise their own research projects.

Diamondpix: A CVD Diamond Detector With Timepix3 Chip Interface

A pixelated 2-D detector combining chemical-vapor-deposited diamond and the Timepix3 chip (“Diamondpix”) is presented. Its conceptual design with a brief description of the Timepix3 chip acquisition modes is outlined. The performance has been tested with fluorescence X-rays, fast neutrons, and electron beam. A first energy calibration has been obtained with X-rays and compared with an equivalent silicon Timepix3 detector. Measurements on fast neutrons and other radioactive source demonstrated a good gamma/neutron rejection capability. Moreover, Diamondpix has been exposed to a beam of ultrarelativistic electrons showing that it can act as a very powerful monitor of beam position, measuring simultaneously the charge released inside the detector and the time of arrival (ToA) of the particles by reconstructing the time profile of the beam bunches. Finally, high-intensity measurements show some delayed signals probably related to the trap defects inside the diamond. The first study of their spatial distribution correlated with the measurements of the charge released inside the diamond and ToA is also discussed.

Dosimetry in mixed neutron-gamma fields with a Timepix detector

We used the hybrid semiconductor pixel detector Timepix as an area dosimeter in mixed neutron-gamma fields. The surface of the detector was covered with several converters based on 6Li, polyethylene, Th and enriched U. We used three different neutron sources for calibration of the device – thermal neutrons from graphite pile, 241Am-Be and 252Cf. The desired neutron component of dosimetry and operational quantities can be determined by counting events under the converters. The segmentation of the Timepix detector and its capability to measure energy deposition of each interacting quantum of ionizing radiation allows a separation of neutron events from X-ray/gamma or electron interaction. Thus, the device can also be used for simultaneous X-ray and gamma dosimetry, where it is in some aspects superior to most of the commercial devices, given its detection threshold of 3–5 keV.

Resolving power of pixel detector Timepix for wide-range electron, proton and ion detection

The resolving power of the Timepix detector for wide-range charged particle detection has been examined and evaluated in defined radiation fields. The goal is to broadly characterize mixed-radiation fields consisting namely of X-rays and charged particles in terms of particle-types (species), spectral response (energy loss) and direction in wide field-of-view (essentially 2π) with a single compact tracking detector. Tests and calibration measurements were performed with the same device at electron, proton and ion fields at various energies and incident directions. Event-by-event detection, together with pattern recognition analysis of the single particle tracks, are exploited to analyze events according to three degrees of freedom—the particle type (X-rays, light and heavy charged particles), energy range (low or high energy—depending on their range being smaller or larger than the pixel size of the detector semiconductor sensor) and direction (incident angle to the sensor plane). Characteristic values are determined for the cluster analysis morphology parameters, the particles stopping power or Linear Energy Transfer and derived correlated quantities. Ratios and correlations between selected parameters are analyzed including 2D-scatter plots. A physics-based wide-range classification is proposed for a total of 8 broad event groups—in terms of light charged particles (electrons, muons) of both low and high energy incident perpendicular (type 1, including X-rays) or high energy non-perpendicular (type 5), protons of low energy omnidirectional (type 2) and high energy non-perpendicular (type 6), alpha particles and light ions of low energy omnidirectional (type 3) and high energy non-perpendicular (type 7), and heavy ions of low energy omnidirectional (type 4) and high energy non-perpendicular (type 8).

Abstract 2229: Mammography assessment of microcalcifications using a pixel detector system

Abstract Breast cancer is a major public health problem being the most frequently diagnosed malignancy and the second leading cause of dead from cancer among women in the United States. In 2015, there were expected around 231,840 new cases of invasive breast cancer, as well as 60,290 cases of in situ breast cancer, and 40,290 deaths. Worldwide, it represents 23% of the total cases diagnosed of cancer and 14% of the cancer deaths in female. Diagnosing breast cancer before it has the potential to invade surrounding tissue and metastasize can increase the chance of survival, reduce poor prognosis and the need of aggressive treatment. This is one of the main goals of early diagnosis that advocates mammography, the clinical examination and the breast self-examination as screening modalities for breast cancer. Recently, other technologies have been proposed like ultrasound and magnetic resonance imaging (MRI). Although these screening methods have reduced the relative risk of death, they have some limitations in some population groups. The aim of this work is adapting and testing a semiconductor pixel detector, Timepix in a custom-swine breast phantom to evaluate its technical feasibility to introduce this technology in a clinical setting for mammography assessment based on microcalcification detection. Swine model is commonly used in medical research based on anatomical and histopathological characteristics closely resemble those of humans, and this is an advantage over the wax-based phantoms that contains aluminum oxide (Al2O3) specks, nylon fibers and other inclusions to mimic microcalcifications and histopathological structures. The Timepix detector was used together with a Tungsten microfocus X-ray to detect hydroxyapatite crystals mimicking malignant microcalcifications with sizes ranged between 153-827μm, embedded into the swine phantom. To compare the detection performance in the swine phantom, a commercial phantom has been used (Model 015 CIRS Inc., Norfolk, VA, US). Both phantoms were tested on a clinical mammography system (Selenia Dimensions AWS 5000 ET19) and by using the experimental setup with the Timepix. This approach has potential to be used for mammography screening in different population groups based on its performance in microcalcifications detection. The results are highly relevant for further studies of the feasibility of pixel detectors for risk assessment and early diagnosis of breast cancer. Citation Format: Carlos Ávila Bernal, Gerardo Roque, Roberto Rueda, Jorge Racedo. Mammography assessment of microcalcifications using a pixel detector system [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2229.

Precision Measurements of Induced Radioactivity and Absolute Luminosity Determination With TPX Detectors in LHC Proton–Proton Collisions at 13 TeV

A network of Timepix (TPX) devices installed in the ATLAS cavern measures the Large Hadron Collider (LHC) luminosity as a stand-alone system. The data were recorded from 13-TeV proton–proton collisions in 2016. Using two TPX devices, the number of hits created by particles passing the pixel matrices was counted. Absolute luminosity is determined with the van der Meer scan technique by separating the LHC proton beams and measuring the widths of the beams in low-intensity LHC proton–proton collisions. The exact determination of the activation background contributes to the overall precision of the TPX luminosity measurements. The activation background varies in time due to induced radioactivity at the different positions of the TPX devices in the ATLAS cavern. The activation at a given time depends on the history of the LHC operation. A detailed study of induced radioactivity has been performed to reduce the uncertainty on both the relative and absolute luminosity measurements.