Martin Jakubek

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 inspection of composite materials for aircraft structures using detectors of Medipix type

This work presents an overview of promising X-ray imaging techniques employed for non-destructive defectoscopy inspections of composite materials intended for the Aircraft industry. The major emphasis is placed on non-tomographic imaging techniques which do not require demanding spatial and time measurement conditions. Imaging methods for defects visualisation, delamination detection and porosity measurement of various composite materials such as carbon fibre reinforced polymers and honeycomb sendwiches are proposed. We make use of the new large area WidePix X-ray imaging camera assembled from up to 100 edgeless Medipix type detectors which is highly suitable for this type of measurements.

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.

Radiographic Observation of Damage Zone Evolution in High Ductile Specimen

This work reports on new results of the experimental observation of material damage evolution in high ductile flat specimens manufactured from aluminium alloy. Failures in ductile materials precede intensive internal material damage evolution. Not only damage existence but also its quantification has to be determined for a numerical simulations purpose. An experimental method called “X-Ray Dynamic Defectoscopy (XRDD)” was developed from this reason [1]. The test sample is illuminated by X-rays during the loading process. Measured changes in transmission represent alterations of effective thickness of the specimen. The effective thickness changes are understood as weakening of the material due to damage volume fraction and transversal thickness reduction (contraction) resulting from loading stress. Alterations in the sample thickness due to void volume fraction are separated from the total thickness reduction using independent optical measurement of the out-of plane displacement field by the photometric stereo method. We observe time evolution of the damage zone shape and proportional volume fraction of voids (damage intensity) by XRDD. In-plane strain field and consequent plastic strain intensity at a surface is investigated by the optical Method of Interpolated Ellipses [2].

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.

3D imaging of radiation damage in silicon sensor and spatial mapping of charge collection efficiency

Radiation damage in semiconductor sensors alters the response and degrades the performance of many devices ultimately limiting their stability and lifetime. In semiconductor radiation detectors the homogeneity of charge collection becomes distorted while decreasing the overall detection efficiency. Moreover the damage can significantly increase the detector noise and degrade other electrical properties such as leakage current. In this work we present a novel method for 3D mapping of the semiconductor radiation sensor volume allowing displaying the three dimensional distribution of detector properties such as charge collection efficiency and charge diffusion rate. This technique can visualize the spatially localized changes of local detector performance after radiation damage. Sensors used were 300 μm and 1000 μm thick silicon bump-bonded to a Timepix readout chip which serves as an imaging multichannel microprobe (256 × 256 square pixels with pitch of 55 μm, i.e. all together 65 thousand channels). Per pixel energy sensitivity of the Timepix chip allows to evaluate the local charge collection efficiency and also the charge diffusion rate. In this work we implement an X-ray line scanning technique for systematic evaluation of changes in the performance of a silicon sensor intentionally damaged by energetic protons.

Dynamic defectoscopy with flat panel and CdTe Timepix X-ray detectors combined with an optical camera

Damage of gradually loaded ductile materials involves a number of physical processes which are highly nonlinear and have different intensity and extent. Dynamic defectoscopy (i.e. defectoscopy of time changing damage processes) combining an X-ray/optical imaging system is proposed for online visualization and analysis of the complex behaviour of such materials. A large area flat panel detector with rather long read out time is used for overall observation of slow damage processes. On the other hand, a semiconductor CdTe Timepix detector with small active area allows following the rapid damage processes occurring in the final phase of specimen failure. Optical imaging of the specimen surface was utilized for analysing the specimen deformations.

Probe and scanning system for 3D response mapping of pixelated semiconductor detector with X-rays and the timepix device

The development of new radiation detectors of different semiconductor materials (Si, CdTe, GaAs, …) brings the necessity to test and evaluate their response and detection performance such as the spatial homogeneity and local charge collection efficiency. A number of these materials exhibit a certain degree of inhomogeneity, which is needed to be determined in order to eliminate its negative effects. Similarly, such testing is desired as well in order to determine the extent of radiation damage in detectors. We decided to build a size-configurable beam and detector positioning system to probe the collection of charge spatially localized deposited by X-rays on a pixelated detector. The principle of this system is based on the use of a collimated parallel X-ray beam with a line profile, which delivers a defined charge at a specific location in 3D in the sensor. The beam can be sent onto the pixelated sensor at a low angle, which allows determining, for a given angle and detector position, the depth of intera...

X-ray based methods for 3D characterization of charge collection and homogeneity of sensors with the use of Timepix chip

Timepix is a universal readout chip for pixel detectors which can be connected to various semiconductor sensors. The device has a 256×256 matrix of square pixels with a pitch of 55 µm. Every single pixel is able to measure the collected charge. The traditional material used for sensors is mono-crystalline silicon (Si). However, other materials such as gallium arsenide (GaAs) or cadmium telluride (CdTe) are applicable as well. To describe the properties of the sensors it is important to probe and evaluate the charge collection efficiency and its homogeneity across sensor area (or if possible even in its volume).