A. Zwerger

The Influence of Pixel Pitch and Electrode Pad Size on the Spectroscopic Performance of a Photon Counting Pixel Detector With CdTe Sensor

The quality and availability of room temperature CdTe sensor material for X-ray detection has improved significantly in the last years. A CdTe sensor with different pixel pitches and electrode pad sizes was bump-bonded to a Medipix2-MXR ASIC. With this photon counting detector we were able to investigate the influence of pixel pitch and electrode pad size on the energy response functions. The accurate knowledge of the energy response is crucial for energy resolving X-ray imaging. Therefore we exposed the detector to gamma rays of 241Am and 57Co. In the following analysis of the energy response spectra we determined the number of events in the photo peak, the energy resolution and the threshold energy where the photo peak is found (photo peak position) caused by the absorption of the 59.5 keV photons of 241Am. For the energy calibration we used the 59.5 keV photons of 241Am and the 122 keV photons of 57Co. Concerning energy resolution, energy threshold calibration and photo peak position our measurements show good agreement with the expected behaviour. The number of events in the photo peak strongly depends on the pixel pitch and the electrode pad configuration.

Energy resolution and transport properties of CdTe-Timepix-Assemblies

CdTe is a promising material for the detection of γ- and X-rays as 1 mm thick CdTe sensors offer an absorption efficiency that is higher than 50 {%} for photon energies up to 120 keV. Therefore 1mm thick CdTe from Acrorad has been flip-chipped as the sensor material on a Timepix readout-electronics ASIC at the Freiburg Materials Research Center (FMF). The transport properties of the CdTe material are investigated by determining the μτ-product of the charge carriers by illuminating the non-collecting electrode with alpha particles. The method commonly used to determine the μτ-product for non-pixelated devices was adapted for the Medipix pixelated readout-electronics. The position of the alpha particles on the pixel matrix provides spatial information, which was used to create a mapping of the μτ-product. The energy resolutions and the positions of the noise edge of two CdTe-Timepix assemblies, one with a pixel pitch of 110 x 110 μm2 and another one with a pixel pitch of 55 x 55 μm2 were investigated by performing a threshold scan of a 241Am source. A comparison of the energy spectra obtained with the two assemblies with different pixel pitches confirms the strong dependence of charge sharing on the energy spectra, as expected.

Characterization of photon counting pixel detectors based on semi-insulating GaAs sensor material

Hybrid semiconductor pixel detectors are considered of high interest for synchrotron applications like diffraction and imaging experiments. However, at photon energies above 30 keV, high-Z sensor materials have to be used due to the weak absorption of the most commonly used sensor material, for instance silicon wafers with a thickness of a few hundred μm. Besides materials like CdTe and Ge, semi-insulating, chromium compensated SI-GaAs(Cr) proves to be a promising sensor material for applications with X-rays in the mid-energy range up to ~60 keV. In this work, material characterisation of SI-GaAs(Cr) wafers by electrical measurements and synchrotron white beam topography as well as the characterization and application of pixel detector assemblies based on Medipix readout chips bump-bonded to 500 μm thick SI-GaAs(Cr) sensors are presented. The results show a very homogeneous material with high resistivity and good electrical properties of the electrons as well as a very promising imaging performance of the detector assemblies.

Crystal growth and characterization of detector grade (Cd,Zn) Te crystals

(Cd,Zn)Te crystals were grown from the melt by the Bridgman method with diameters of 25 and 75 mm. The material was doped with indium. The structural quality of the crystals was constantly improved to increase the single crystalline areas. These improvements reduced the number of grains in the 25 mm and 75 mm diameter, respectively. The crystals were characterized by electrical and optical methods to evaluate the relation between structural quality and detector performance. A reduction of the etch pitch density down to 2times103 cm-2 was achieved by the optimization of the crystal growth The resistivity of the crystals was in the range of 2times109 up to 5times1010 Omega cm. The detector performance was tested with different radiation sources. The product of mobility-lifetime of charge carriers was 3times10-3 cm2/V. The energy resolution for different radiation energies were measured for detector thickness of 1 mm and 10 mm.

Optimization of Medipix-2 Threshold Masks for Spectroscopic X-Ray Imaging

Spectroscopic X-ray imaging enhances image contrast and provides advanced object information due to energy resolution. The Medipix-2 chip is a photon counting semiconductor detector and features two energy thresholds for energy selective imaging. The aim of this study is to present the development of optimized threshold adjustment masks with small energy windows of about 3 keV width using a monochromatic X-ray source for equalization and to demonstrate the benefits of spectroscopic X-ray imaging using an integrated circuit as object.

Digital spectroscopic system based on large volume stacked coplanar grid (Cd,Zn)Te detectors

Gamma-ray spectroscopic systems with high efficiency and energy resolution have become important in many fields. Both aspects are provided in the system we present: an improved multi channel analyzer with digital signal processing and large volume stacked detectors with a Peltier cooling device. The paper shows the performance of the system in the laboratory and outdoor

Investigation of crystallographic and detection properties of CdTe at the ANKA synchrotron light source

The crystallographic properties of a semiconducting CdTe pixelated sensor were investigated at the ANKA synchrotron facility (KIT, Karlsruhe) by means of back reflection white beam topography and high resolution X-ray diffraction. From the results, several orientation contrast features were identified that could be assigned to small angle grain boundaries of 0.01°. Those structures are disseminated in the whole area of the investigated crystal and form a mosaic structure network of tiled and twisted blocks. The topographic mapping of the sensor was correlated with its X-ray response map. The comparison demonstrates the presence of similar features, proving that the structural quality of the sensor material influences the charge carrier transport and consequently the detector performances.

A comparison of various strategies to equalize the lower energy thresholds of a CdTe Medipix2 hexa detector for X-ray imaging applications

Technological advances have made possible the development of pixelized photon counting semiconductor detectors, many of which are used in X-ray imaging to resolve the spectral composition of the incident photons. Here, in a so-called Hexa detector, we employ a 3 × 2 array of Medipix2 MXR readout chips, bump bonded to a cadmium telluride sensor of 1 mm thickness with a pixel pitch of 165 μm. Each pixel in this assembly offers two variable energy thresholds, which enables counting of only those photons within an energy range of interest. As manufacturing tolerances cause deviations in each of the pixels responses, the two thresholds can be calibrated for every pixel to render their response to radiation more homogeneous. In this work, we compare various methods that we chose to equalize the lower thresholds: a) the noise edge of the detector electronics, the characteristic X-rays from b) silver and c) tantalum foils as well as flat fields obtained at d) 40 and e) 120 kVp. It will be shown that the energy dependence in the resulting adjustment bit maps are only small, whereas the question as to which strategy to choose (peak position vs. image homogeneity) will have a greater influence on the resulting corrections. Additionally, we observed a decrease in the mean adjustment values with increasing distance from the central axis of the Hexa detector under study.

Flatfield Correction Optimization for Energy Selective X-Ray Imaging With Medipix3

Pixelated photon counting semiconductor X-ray detectors like the Medipix feature adjustable energy thresholds allowing selective counting of photons of a specified energy. This development permits for energy selective X-ray imaging with advanced material information. Furthermore the photon counting function principle of these detectors allows X-ray imaging with reduced noise, providing contrast improvement in low contrast objects. The aim of this study is to analyze the behavior of the new Medipix3 detector, especially regarding flatfield correction for X-ray imaging applications. First high resolution low contrast X-ray images and energy selective X-ray images acquired with the Medipix3 detector are presented as well in this paper.

Saturation effects of CdTe photon counting detectors under high photon fluxes

Pixelated photon counting semiconductor detectors enable to resolve the spectral composition of the incident photons in X-ray imaging, provided that a detector is operated in the absence of pulse pile-up and sensor polarisation. The purpose of this study was to examine the imaging properties of Medipix2 MXR arrays under high photon fluxes, which do not meet this requirement. The detectors studied feature a 1 mm thick cadmium telluride sensor and pixel pitches of 110 μm and 165 μm. We show that the critical photon flux, at which the detectors linear response breaks down, can be shifted towards higher values by increasing the IKrum current in the charge sensitive preamplifier, which corresponds to reducing the pulse shaping time. The negative impact of this procedure on the energy response function, which was measured using an 241Am source, seems only moderate. Furthermore, we describe various kinds of image artefacts due to sensor defects. While bulks of non-counting pixels always appear at the same position and shrink with increased IKrum values, non-counting single pixels randomly show up at different positions during image acquisition and their number rises with an increased photon flux. Additionally, we demonstrate that the critical flux varies among the pixels, leading to large-scale image inhomogeneities at high photon fluxes.