J. Jakubek

FITPix — fast interface for Timepix pixel detectors

The semiconductor pixel detector Timepix contains an array of 256 × 256 square pixels with pitch 55 μm. In addition to high spatial granularity the single quantum counting detector Timepix can provide also energy or time information in each pixel. This device is a powerful tool for radiation and particle detection, imaging and tracking. A new readout interface for silicon pixel detectors of the Medipix family has been developed in our group in order to provide a higher frame rate and enhanced flexibility of operation. The interface consists of a field programmable gate array, a USB 2.0 interface chip, DAC, ADC and a circuit which generates bias voltage for the sensor. The main control system is placed in the FPGA circuit which fully controls the Timepix device. This approach offers an easy way how to include new functionality and extended operation. The interface for Timepix supports all operation modes of the detector (counting, TOT, timing). The FITPix is a successor of the USB 1.22 Interface and the electronic readout is built with the latest available components, which allows achieving up to 90 frames per second with a single detector. The frame rate is about 20 times faster compared to the previous system while it maintains all same capabilities supported. In addition FITPix newly enables an adjustable clock frequency and hardware triggering which is a useful tool when there is the need for synchronized operation of multiple devices. Three modes of hardware trigger have been implemented: hardware trigger which starts the measurement, hardware trigger which terminates the measurement and hardware trigger which controls measurement fully. The entire system is fully powered through the USB bus. FITPix supports also readout from several detectors in chain in which case just an external power source is required. FITPix is a fully flexible device and the user needs no other equipment. FITPix combines high performance and mobility and it opens new fields of applications. The current version of the FITPix interface has dimension 45 mm × 60 mm.

3D sensitive voxel detector of ionizing radiation based on Timepix device

Position sensitive detectors are evolving towards higher segmentation geometries from 0D (single pad) over 1D (strip) to 2D (pixel) detectors. Each step has brought up substantial expansion in the field of applications. The next logical step in this evolution is to design a 3D, i.e. voxel detector. The voxel detector can be constructed from 2D volume element detectors arranged in layers forming a 3D matrix of sensitive elements — voxels. Such detectors can effectively record tracks of energetic particles. By proper analysis of these tracks it is possible to determine the type, direction and energy of the primary particle. One of the prominent applications of such device is in the localization and identification of gamma and neutron sources in the environment. It can be also used for emission and transmission radiography in many fields where standard imagers are currently utilized. The qualitative properties of current imagers such as: spatial resolution, efficiency, directional sensitivity, energy sensitivity and selectivity (background suppression) can be improved. The first prototype of a voxel detector was built using a number of Timepix devices. Timepix is hybrid semiconductor detector consisting of a segmented semiconductor sensor bump-bonded to a readout chip. Each sensor contains 256x256 square pixels of 55 μm size. The voxel detector prototype was successfully tested to prove the concept functionality. The detector has a modular architecture with a daisy chain connection of the individual detector layers. This permits easy rearrangement due to its modularity, while keeping a single readout system for a variable number of detector layers. A limitation of this approach is the relatively large inter-layer distance (4 mm) compared to the pixel thickness (0.3 mm). Therefore the next step in the design is to decrease the space between the 2D detectors.

First tests of a Medipix-1 pixel detector for X-ray dynamic defectoscopy

Abstract Recent theoretical damage material models describe the dynamic development of voids and microcracks in materials under plastic deformation. For these models, experimental verification is needed. We propose direct and non-destructive observation of the propagation of material damage by measuring changes in transmission of X-rays penetrating a stressed material, using a photon-counting X-ray imager. The present contribution aims to demonstrate the applicability of silicon and gallium-arsenide devices for X-ray transmission measurements with a specimen of high-ductile aluminium alloy under study. The first experiments to determine the resolution and the sensitivity of the proposed method with the Medipix-1 pixel detector are presented.

Spectrometric properties of TimePix pixel detector for X-ray color and phase sensitive radiography

The semiconductor pixel detector TimePix is a newly developed successor of the Medipix2 device. Each TimePix pixel is provided with preamplifier, discriminator and counter. Discriminators allow full suppression of the noise and selection of energy range of interest. Each counter can be configured to work in one of three principal operation modes: 1. counting of detected particles; 2. measurement of particle energy; 3. measurement of time of interaction. Possibility of per pixel energy measurement presents a substantial advantage for X-ray radiography with polychromatic X-ray sources (tubes). This feature allows to utilize normally not desirable beam-hardening phenomenon for material determination. If the radiographic system is equipped with a microfocus X-ray tube enabling phase sensitive imaging, the spectrometric properties of TimePix bring further advantages as the phase effects are energy dependent. This contribution presents a compact X-ray microradiographic phase sensitive system based on nanofocus X-ray tube and position sensitive single photon counting pixel detector TimePix (256 times 256 square pixels, pitch of 55 mum) with 300 mum thick silicon sensor. The spectral sensitivity of the detector together with the polychromatic nature of the beam allows material determination (color imaging). Moreover, in phase sensitive configuration it is possible to distinguish a transmission (attenuation) image from a phase (refractive) image. Spatial resolution of the system is on the submicrometer level and measuring times in order of seconds.

Selective detection of secondary particles and neutrons produced in ion beam therapy with 3D sensitive voxel detector

Ion beam therapy is a rapidly developing method for treatment of certain types of cancer. A main advantage of ions is that they deposit most of the energy at the end of their range according to the Bragg curve. Unfortunately, the ion beam often generates a substantial amount of energetic secondary particles with not negligible range. Thus, a fraction of the dose is deposited by other than the primary ions outside of the planned volume. It is, therefore, important to estimate and experimentally verify the distributions of these secondary particles. It is particularly difficult to identify fast neutrons generated by ions in tissue. Fast neutrons are usually detected via their interaction (scattering) with hydrogen nuclei (proton). The proton recoiled by the scattered neutron is subsequently detected by a suitable sensor. The problem is that certain fraction of secondary particles consists of protons as well. Therefore, it is necessary to distinguish protons recoiled by neutrons from protons naturally present in the sample. In this work we present the experimental technique enabling the separation of fast neutrons from protons. The technique uses a 3D sensitive voxel detector composed of several layers of Timepix pixel detectors. These layers are interlaced with a hydrogen rich material (plastic) serving as a convertor of neutrons to recoiled protons. The device records the traces of all interacting radiation providing the time stamp and/or deposited energy for each single particle. A proton passing through the detector creates a trace in all layers, whereas a protons recoiled by neutron originates in the convertor inside of the structure creating a trace in the inner layers only. This way it is possible to distinguish the protons from neutrons with very high selectivity. The technique can be easily extended for detection of slow neutrons. An initial experimental study to register the outcoming neutron radiation was performed at the Heidelberg Ion Beam Therapy Center (HIT) in Germany using medical proton and carbon ion beams. This work is carried out in frame of the Medipix Collaboration.

Detection and track visualization of primary and secondary radiation in hadron therapy beams with the pixel detector Timepix

The interaction of proton and hadron beams of relativistic energy in matter is accompanied by the production of penetrating highly energetic secondary particles and nuclear reaction products which can affect the desired highly localized deposition of energy of the primary high LET particles for radiotherapy purposes. The observation of all ionizing particles arising from and accompanying the primary radiation as well as the direct measurement of particle energy loss, trajectory, local energy deposition and lateral straggling can be directly provided by the semiconductor pixel detector Timepix. This device operates as an active nuclear emulsion providing on-line visualization of particle traces. Results of a pilot experiment are presented with proton and carbon ion beams in the energy range 48–220 MeV/u and 88–430 MeV, respectively produced at the Heavy Ion Therapy HIT facility in Heidelberg.

Compact System for High Resolution X-ray Transmission Radiography, In-line Phase Enhanced Imaging and Micro CT of Biological Samples

Phase imaging visualizes phase shift of photons which passed the sample. Although phase sensitive X-ray imaging offers many advantages it is not routinely used in biological research due to demands of high intensity and highly coherent X-ray beam which is accessible mainly at synchrotron facilities. Phase sensitive imaging can be also carried out with microfocus X-ray tubes. However, the low beam intensity of such systems prolongs the exposure time to such an extent that common digital imaging detectors (CCD, Flat panels) are insufficient due to low efficiency, dark current and noise. This contribution presents a compact phase contrast enhanced imaging system based on a microfocus X-ray tube and the single photon counting pixel detector Medipix2. The spectral sensitivity of the detector together with the polychromatic nature of the beam allows distinguishing an absorption image from a phase image. Spatial resolution of the system can be on the sub micrometer level and measuring times less than a minute. Applications of the system for biological samples are presented. The simplicity of the system allows for routine laboratory work including dynamic in-vivo studies.

Measurement of secondary radiation during ion beam therapy with the pixel detector Timepix

In ion beam therapy the finite range of the ion beams in tissue and the presence of the Bragg-peak are exploited. Unpredictable changes in the patient`s condition can alter the range of the ion beam in the body. Therefore it is desired to verify the actual ion range during the treatment, preferably in a non-invasive way. Positron emission tomography (PET) has been used successfully to monitor the applied dose distributions. This method however suffers from limited applicability and low detection efficiency. In order to increase the detection efficiency and to decrease the uncertainties, in this study we investigate the possibility to measure secondary charged particles emerging from the patient during irradiation. An initial experimental study to register the particle radiation coming out of a patient phantom during the therapy was performed at the Heidelberg Ion Beam Therapy Center (HIT) in Germany. A static narrowly-focused beam of carbon ions was directed into a head phantom. The emerging secondary radiation was measured with the position-sensitive Timepix detector outside of the phantom. The detector, developed by the Medipix Collaboration, consists of a silicon sensor bump bonded to a pixelated readout chip (256 × 256 pixels with 55 μm pitch). Together with the USB-based readout interface, Timepix can operate as an active nuclear emulsion registering single particles online with 2D-track visualization. In this contribution we measured the signal behind the head phantom and investigated its dependence on the beam energy (corresponding to beam range in water 2–30 cm). Furthermore, the response was measured at four angles between 0 and 90 degrees. At all investigated energies some signal was registered. Its pattern corresponds to ions. Differences in the total amount of signal for different beam energies were observed. The time-structure of the signal is correlated with that of the incoming beam, showing that we register products of prompt processes. Such measurements are less likely to be influenced by biological washout processes than the signal registered by the PET technique, coming from decays of beam-induced radioactive nuclei. This work demonstrates that the Timepix detector is able to register ions emerging from the patient during the treatment by carbon ion beams. In future work it will be investigated which information about the incoming beam can be gained from the analysis of the measured data.

Position sensitive detection of neutrons in high radiation background field

We present the development of a high-resolution position sensitive device for detection of slow neutrons in the environment of extremely high γ and e(-) radiation background. We make use of a planar silicon pixelated (pixel size: 55 × 55 μm(2)) spectroscopic Timepix detector adapted for neutron detection utilizing very thin (10)B converter placed onto detector surface. We demonstrate that electromagnetic radiation background can be discriminated from the neutron signal utilizing the fact that each particle type produces characteristic ionization tracks in the pixelated detector. Particular tracks can be distinguished by their 2D shape (in the detector plane) and spectroscopic response using single event analysis. A Cd sheet served as thermal neutron stopper as well as intensive source of gamma rays and energetic electrons. Highly efficient discrimination was successful even at very low neutron to electromagnetic background ratio about 10(-4).

Imaging with secondary radiation in hadron therapy beams with the 3D sensitive voxel detector

In this work we present the technique enabling visualization of the field of scattered and secondary ions generated by primary beam. The technique uses a small 3D sensitive voxel detector composed of several layers of Timepix pixel detectors. The device is placed close to the irradiated object (outside of the primary ion beam) recording the traces of all radiation coming from the sample. Detector provides the timestamp and/or deposited energy for each single particle. The shapes of traces are very typical for different particles which allows for separation of ions from other background such as electrons and gamma photons. The 3D information form the voxel detector allows for reconstruction of direction of incoming radiation. Therefore it is possible to distinguish whether particle came with the beam or it was generated or scattered later. The back projection of reconstructed directions for all registered ions can be in suitable geometry used for generation of image of distribution of scattering or fragmentation in the volume of the irradiated object. The initial experimental study to register the out coming ion radiation from testing phantom was performed at the Heidelberg Ion Beam Therapy Center (HIT) in Germany using medical carbon ion beam.