Daniel Vavrik

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.

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.

X-ray dynamic observation of the evolution of the fracture process zone in a quasi-brittle specimen

The aim of this work is the evaluation of the fracture process zone while loading a quasi-brittle concrete compound. The regularly used optical observation of the specimen surface does not provide accurate information regarding the fracture zone shape, particularly when this zone is tunnelled inside of the specimen body. Therefore, X-ray dynamic defectoscopy and computed tomography were employed as tools for an extended investigation of process zone evolution. A notched specimen manufactured from silicate composite was subjected to the three-point bending test in a special table-top loading device. On-line radiographic observation of the process zone during the loading experiment serves for overall evaluation, while a tomographic measurement - which is conducted during temporal loading interruption - provides information about the spatial distribution of the newly developed cracks.

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).

Modular pixelated detector system with the spectroscopic capability and fast parallel read-out

A modular pixelated detector system was developed for imaging applications, where spectroscopic analysis of detected particles is advantageous e.g. for energy sensitive X-ray radiography, fluorescent and high resolution neutron imaging etc. The presented system consists of an arbitrary number of independent versatile modules. Each module is equipped with pixelated edgeless detector with spectroscopic ability and has its own fast read-out electronics. Design of the modules allows assembly of various planar and stacked detector configurations, to enlarge active area or/and to improve detection efficiency, while each detector is read-out separately. Consequently read-out speed is almost the same as that for a single module (up to 850 fps). The system performance and application examples are presented.

Metal grain structure resolved with table-top micro-tomographic system

X-ray transmission radiograms of Aluminum alloy contain relatively low contrast features induced by variations of the material chemical composition. Generally, these variations are strongly connected with grains of the metal, where these differ from each to other. Although grains of the studied material have typically dimensions of tens of micrometers only, the material variations can enable the observation of geometry and orientations of grains in the specimen volume employing advanced X-ray micro tomographic method. Analysis of such tiny structures requires high dynamic range of acquired radiograms with high signal to noise ratio and appropriate geometrical magnification. These requirements can be fully satisfied by using the pixelated single photon counting device Medipix, a precise micro-tomographic setup and appropriate data processing. Results will be demonstrated with an Aluminum alloy bar specimen.

Real-time X-ray 2-D and 3-D micro-imaging of living animals with Medipix2 single photon counting detector

In this work we present the study of applicability of a desktop size radiographic/tomographic X-ray system for real-time microscopy and micro-tomography in the fields of biology, entomology, botanic and medical imaging. The apparatus is made up of the single photon counting pixel silicon detector, Medipix2 (matrix of 256×256 square pixels of 55 μm pitch) and a microfocus X-ray tube with a minimum spot size of 5 μm and a tungsten anode. The system has been used for observations of time-dependent processes inside living and still biological and organic samples. Excellent contrast and spatial resolution (micrometer scale) were obtained as a combination of a) low photon energy (40 kVp X-ray tube voltage), b) single photon counting operation, witch avoids integration of dark current c) energy discrimination in each pixel, allowing noise rejection and providing high SNR, d) high effective dynamic range for long exposures, which allows for high signal with high SNR, e) implementation of an original procedure for the energy response calibration of each pixel of the detector matrix, f) high speed read-out hardware and software, which opens the possibility to perform real-time studies of biological processes permitting, e.g., observation of morphological changes, mutations or metamorphosis of living animals and plants. Static and dynamic images of a parasite life cycle from the larva stage to pupa stage are presented here, as well as an in vivo computed tomography of the parasite living inside its host.

Material analysis using characteristic transmission spectra

Conventional X-ray radiography evaluates the attenuation of X-rays to provide information about the examined object. In this approach it remains difficult to distinguish whether the object features are induced by the object geometry or by the material composition. Measuring the X-ray photon flux together with evaluation of the transmission spectra can solve this problem as will be presented in this paper.

Properties of neutron pixel detector based on Medipix-2 device

Neutron transmission radiography can serve as a complementary diagnostic method to X-ray radiography. It can produce contrast images of materials, which are indistinguishable in X-ray images (typically materials containing hydrogen). Good performance of a neutron detecting device is essential for the acquisition of high quality neutron images. The properties of the neutron pixel detector based on the single X-ray photon pixel detector device Medipix-1 (64 /spl times/ 64 square pixels with pitch of 170 /spl mu/m) were already demonstrated and published. Slow neutrons are captured and converted in a surface layer containing Li-6 to tritons and alpha particles which are subsequently detected by a silicon pixel detector. A Medipix-2 detector (256 /spl times/ 256 square pixels with pitch of 55 /spl mu/m) was adapted in a similar way. The detector performance was tested at the NEUTRA station of the Paul Scherrer Institute using a beam of slow neutrons from the spallation source SINQ with an intensity of 3/spl middot/10/sup 6/ neutrons/cm/sup 2/s. The spatial resolution, detection efficiency and other detector properties have been determined and are compared with several types of contemporary neutron imaging systems (Medipix-1, CCD camera, imaging plates). The results demonstrate the superiority of the Medipix-2 based neutron imager in terms of spatial resolution, linearity and dynamic range. The system is very promising in applications for micro-neutron tomography, especially when large detector areas will become available.

Neutron imaging with micrometric spatial resolution

The detection of neutrons with high spatial resolution by a pixelated silicon semiconductor silicon detector requires the use of a thin converter. A 6Li based converter was used for this purpose in our work. We demonstrate that unwanted gamma rays and electrons accompanying neutron beam can be effectively suppressed using pattern recognition of the analyzed particle tracks, where spectroscopic ability of the detector was advantageously used. Neutron detection with micrometric spatial resolution is reached by appropriate fitting of identified heavy charged particles by the Gaussian distribution.