Michal Platkevic

Large area pixel detector WIDEPIX with full area sensitivity composed of 100 Timepix assemblies with edgeless sensors

The superior properties of the single particle counting semiconductor pixel detectors in radiation imaging are well known. They are namely: very high dynamic range due to digital counting, absence of integration and read-out noise, high spatial resolution and energy sensitivity. The major disadvantage of current pixel devices preventing their broad exploitation has been their relatively small sensitive area of few cm2. This disadvantage is often solved using tiling method placing many detector units side by side forming a large matrix. The current tiling techniques require rather large gaps of few millimeters between tiles. These gaps stand as areas insensitive to radiation which is acceptable only in some applications such as diffraction imaging. However standard transmission radiography requires fully continuous area sensitivity. In this article we present the new large area device WIDEPIX composed of a matrix of 10 × 10 tiles of silicon pixel detectors Timepix (each of 256 × 256 pixels with pitch of 55 μm) having fully sensitive area of 14.3 × 14.3 cm2 without any gaps between the tiles. The device contains a total of 6.5 mega pixels. This achievement was reached thanks to new technology of edgeless semiconductor sensors together with precise alignment technique and multilevel architecture of readout electronics. The mechanical construction of the device is fully modular and scalable. This concept allows replacing any single detector tile which significantly improves production yield. The first results in the field of X-ray radiography and material sensitive X-ray radiography are presented in this article.

Miniaturized X-ray telescope for VZLUSAT-1 nanosatellite with Timepix detector

We present the application of a Timepix detector on the VZLUSAT-1 nanosatellite. Timepix is a compact pixel detector (256×256 square pixels, 55×55 μm each) sensitive to hard X-ray radiation. It is suitable for detecting extraterrestrial X-rays due to its low noise characteristics, which enables measuring without special cooling. This project aims to verify the practicality of the detector in conjunction with 1-D Lobster-Eye optics to observe celestial sources between 5 and 20 keV. A modified USB interface (developed by IEAP at CTU in Prague) is used for low-level control of the Timepix. An additional 8-bit Atmel microcontroller is dedicated for commanding the detector and to process the data onboard the satellite. We present software methods for onboard post-processing of captured images, which are suitable for implementation under the constraints of the low-powered embedded hardware. Several measuring modes are prepared for different scenarios including single picture exposure, solar UV-light triggered exposure, and long-term all-sky monitoring. The work has been done within Medipix2 collaboration. The satellite is planned for launch in April 2017 as a part of the QB50 project with an end of life expectancy in 2019.

Spatially correlated and coincidence detection of fission fragments with the pixel detector Timepix

Charged-particle coincidence correlated measurements such as angular correlations between rare and main fission fragments measured with conventional detectors provide only partial and limited information (energy cutoff, narrow range of studied ion Z numbers). Many of these drawbacks arise from the standard solid state detectors used so far which can be solved simultaneously by usage of highly segmented single-quantum counting pixel detectors. The Timepix pixel device, which is equipped with energy and time sensitivity capability per pixel, provides high granularity, wide dynamic range and per pixel threshold. This detector operated with integrated USB-readout interfaces such as the USB 1.0 and FITPix devices and the data acquisition software tool Pixelman, both developed for the pixel detectors of the Medipix-family, enables a variety of instrumental configurations, visualization, real-time event-by-event selection as well as vacuum and portability of operation for flexible measurements on different targets and setups. These features combined with event track analysis provide enhanced signal to noise ratio with a high suppression of background and unwanted events. The detector provides multi-parameter information (position, energy and time) for basically all types of ionizing particles in a wide dynamic range of energy (pixel energy threshold ≈ 4 keV), interaction/arrival time (timepix clock step ≥ 100 ns) and position (pixel size = 55 μm). High selectivity is achieved by spatial and time correlation in the same sensor. In addition, several detectors can be run in coincidence. The open and close exposition (shutter) time as well as the readout DAQ can be fully synchronized. For this purpose, we have assembled a modular multi-parameter, tunable and extendable coincidence detector array system based on two and more Timepix devices which can be coupled with supplementary detectors (solid state ΔE detectors and/or ionization chambers) for enhanced ion selectivity. We describe the individual configurations and techniques together with the experiments carried out at several neutron beam/source facilities. We summarize the results and capabilities of application.

Evaluation of the ATLAS-MPX devices for neutron field spectral composition measurement in the ATLAS experiment

A network of 15 Medipix2-based devices (ATLAS-MPX devices) has been installed at various positions in the ATLAS detector within the framework of the ATLAS-MPX collaboration. The aim of the network is to perform real-time measurement of spectral characteristics and composition of the main radiation types in the experiment including slow and fast neutrons, especially during the initial low luminosity LHC operation. This contribution describes the network structure and focuses on the neutron efficiency calibration process of the ATLAS-MPX devices and its simulation in order to predict the behavior of the device in complex neutron fields.

Image Accumulation in Pixel Detector Gated by Late External Trigger Signal and its Application in Imaging Activation Analysis

Single quantum counting pixel detectors of Medipix type are starting to be used in various radiographic applications. Compared to standard devices for digital imaging (such as CCDs or CMOS sensors) they present significant advantages: direct conversion of radiation to electric signal, energy sensitivity, noiseless image integration, unlimited dynamic range, absolute linearity. In this article we describe usage of the pixel device TimePix for image accumulation gated by late trigger signal. Demonstration of the technique is given on imaging coincidence instrumental neutron activation analysis (Imaging CINAA). This method allows one to determine concentration and distribution of certain preselected element in an inspected sample.

Energy (TOF) and position sensitive detection of ultra cold neutrons with micrometric resolution using the TimePix pixel detector

Ultra-cold neutrons (UCN) are neutrons with very small kinetic energies (below 300 neV). The energy is so low that they are reflected from the surface of many materials under any angle of incidence. These neutrons can be thus trapped by an effective Fermi potential and stored inside material “bottles”. Recently bound quantum states of neutrons in the earth’s gravitational field have been observed with UCN. A detector with spatial resolution of around 1 μm is required to be able to resolve such individual states. Being uncharged and having very low energy, ultra-cold neutrons cannot be detected in the silicon pixel detector directly but can be converted into charged particles in a thin layer deposited onto the sensor surface.

Precision Luminosity of LHC Proton–Proton Collisions at 13 TeV Using Hit Counting With TPX Pixel Devices

A network of Timepix (TPX) devices installed in the ATLAS cavern measures the LHC luminosity as a function of time as a stand-alone system. The data were recorded from 13-TeV proton–proton collisions in 2015. Using two TPX devices, the number of hits created by particles passing the pixel matrices was counted. A van der Meer scan of the LHC beams was analyzed using bunch-integrated luminosity averages over the different bunch profiles for an approximate absolute luminosity normalization. It is demonstrated that the TPX network has the capability to measure the reduction of LHC luminosity with precision. Comparative studies were performed among four sensors (two sensors in each TPX device) and the relative short-term precision of the luminosity measurement was determined to be 0.1% for 10-s time intervals. The internal long-term time stability of the measurements was below 0.5% for the data-taking period.

Evaluation of local radiation damage in silicon sensor via charge collection mapping with the Timepix read-out chip

Studies of radiation hardness of silicon sensors are standardly performed with single-pad detectors evaluating their global electrical properties. In this work we introduce a technique to visualize and determine the spatial distribution of radiation damage across the area of a semiconductor sensor. The sensor properties such as charge collection efficiency and charge diffusion were evaluated locally at many points of the sensor creating 2D maps. For this purpose we used a silicon sensor bump bonded to the pixelated Timepix read-out chip. This device, operated in Time-over-threshold (TOT) mode, allows for the direct energy measurement in each pixel. Selected regions of the sensor were intentionally damaged by defined doses (up to 1012 particles/cm2) of energetic protons (of 2.5 and 4 MeV). The extent of the damage was measured in terms of the detector response to the same ions. This procedure was performed either on-line during irradiation or off-line after it. The response of the detector to each single particle was analyzed determining the charge collection efficiency and lateral charge diffusion. We evaluated the changes of these parameters as a function of radiation dose. These features are related to the local properties such as the spatial homogeneity of the sensor. The effect of radiation damage was also independently investigated measuring local changes of signal response to γ, and X rays and alpha particles.

Analogue signal from common electrode of pixelated detector for triggering and spectroscopy

A significant advantage of high granularity and highly sensitive semiconductor pixel detectors is the possibility to directly observe single tracks of charged particles. In many cases, however, these particles are accompanied with unwanted background radiation overlapping traces of interest. The detection selectivity can be increased using a triggering approach usually provided by an external trigger from other detecting devices such as ionization chambers, scintillating or semiconductor detectors. A self-trigger from the same sensor would be highly desirable. Unfortunately the Medipix/Timepix devices are not equipped with such self-trigger feature. A solution which is presented in this contribution makes use of the analog signal taken from the common electrode of the pixelated sensor. This signal, called back-side-pulse, is amplified by a custom made charge sensitive preamplifier which, after shaping, can be used as fast trigger and as independent spectroscopic signal. The stability and energy resolution of this analog signal is, however, strongly affected by electromagnetic noise interference from the digital read-out chip and its interface. In this article we present the solution based on the construction of a hardware galvanic shielded extender which isolates and effectively suppresses such noise interference. The result enables selective self-triggering according to the deposited energy of the detected particle. The technique and operation of the prototype are demonstrated on measurements with heavy charged particles from radioactive α-sources 241Am and 239Pu.

Detection and Real Time Spectroscopy of Charged Particles with the TimePix Pixel Detector

We tested the position—, spectral— and time—resolution capability of the TimePix semiconductor detector together with the USB readout interface and Pixelman control and DAQ software tool for detection and visualization of particles. Event—by—event spectroscopy can be achieved by real time analysis of the characteristic tracks and specific response of different radiation in the pixel detector.