Salim Reza

Detector Developments at DESY

With the increased brilliance of state-of-the-art synchrotron radiation sources and the advent of free-electron lasers (FELs) enabling revolutionary science with EUV to X-ray photons comes an urgent need for suitable photon imaging detectors. Requirements include high frame rates, very large dynamic range, single-photon sensitivity with low probability of false positives and (multi)-megapixels. At DESY, one ongoing development project - in collaboration with RAL/STFC, Elettra Sincrotrone Trieste, Diamond, and Pohang Accelerator Laboratory - is the CMOS-based soft X-ray imager PERCIVAL. PERCIVAL is a monolithic active-pixel sensor back-thinned to access its primary energy range of 250 eV to 1 keV with target efficiencies above 90%. According to preliminary specifications, the roughly 10 cm × 10 cm, 3.5k × 3.7k monolithic sensor will operate at frame rates up to 120 Hz (commensurate with most FELs) and use multiple gains within 27 µm pixels to measure 1 to ∼100000 (500 eV) simultaneously arriving photons. DESY is also leading the development of the AGIPD, a high-speed detector based on hybrid pixel technology intended for use at the European XFEL. This system is being developed in collaboration with PSI, University of Hamburg, and University of Bonn. The AGIPD allows single-pulse imaging at 4.5 MHz frame rate into a 352-frame buffer, with a dynamic range allowing single-photon detection and detection of more than 10000 photons at 12.4 keV in the same image. Modules of 65k pixels each are configured to make up (multi)megapixel cameras. This review describes the AGIPD and the PERCIVAL concepts and systems, including some recent results and a summary of their current status. It also gives a short overview over other FEL-relevant developments where the Photon Science Detector Group at DESY is involved.

Investigation on the directional dark-field signals from paperboards using a grating interferometer

Recent advancements in the grating interferometer based Phase Contrast X-ray Imaging (PCXI) technique enables high quality dark-field images to be obtained using conventional X-ray tubes. The dark-field images map the scattering inhomogeneities inside objects. Since, the dark-field image is constructed by considering only those photons which are scattered while passing through the objects, it can reveal useful information about the object inner structures, such as, the fibre structures inside paperboards. The end-use performance of paperboards, such as the printing quality and the stiffness depends on the uniformity in the thickness and the structures of the coating layer of the paperboards. The uniformity in the coating layer is determined by the coating techniques, the coating materials and the topography of the base sheet. In this article, the dark-field signals from four paperboard samples with different quality indices are analysed. The isotropic and the anisotropic scattering coefficients for all of the samples have been calculated. Based on the correlation between the isotropic coefficients and the quality indices of the paperboards, a new method for paperboard quality measurement has been suggested.

Spectral resolution in pixel detectors with single photon processing

Pixel detectors based on photon counting or single photon processing readout are becoming popular for spectral X-ray imaging. The detector is based on deep submicron electronics with functions to determine the energy of each individual photon in every pixel. The system is virtually noiseless when it comes to the number of the detected photons. However noise and variations in system parameters affect the determination of the photon energy. Several factors affect the energy resolution in the system. In the readout electronics the most important factors are the threshold dispersion, the gain variation and the electronic noise. In the sensor contributions come from charge sharing, variations in the charge collection efficiency, leakage current and the statistical nature of the charge generation, as described by the Fano factor. The MEDIPIX technology offers a powerful tool for investigating these effects since energy spectra can be captured in each pixel. In addition the TIMEPIX chip, when operated in Time over Threshold mode, offers an opportunity to analyze individual photon interactions, thus addressing charge sharing and fluorescence. Effects of charge sharing and the properties of charge summing can be investigated using MEDIPIX3RX. Experiments are performed using both Si and CdTe detectors. In this paper we discuss the various contributions to the spectral noise and how they affect detector response. The statements are supported with experimental data from MEDIPIX-type detectors.

Non-Destructive Method to Resolve the Core and the Coating on Paperboard by Spectroscopic X-ray Imaging

Quality control is an important issue in the paperboard industry. A typical sheet of paperboard contains a core of cellulose fibers [C6H10O5], coated on one or both sides with layers of calcium car ...

Measurement of the sensitive profile in a solid state silicon detector, irradiated by X-rays

A newly constructed solid state silicon dose profile detector is characterized concerning its sensitive profile. The use of the MEDIPIX2 sensor system displays an excellent method to align an image of an X-ray slit to a sample under test. The scanning from front to reverse side of the detector, show a decrease in sensitivity of 20%, which indicates a minority charge carrier lifetime of 0.18 ms and a diffusion length of 460 μm. The influence of diced edges results in a volumetric efficiency of 59%, an active volume of 1.2 mm2 of total 2.1 mm2.

On the Charge Collection Efficiency of the PERCIVAL Detector

The PERCIVAL soft X-ray imager is being developed by DESY, RAL, Elettra, DLS, and PAL to address the challenges at high brilliance Light Sources such as new-generation Synchrotrons and Free Electro ...

Report on recent results of the PERCIVAL soft X-ray imager

The PERCIVAL (Pixelated Energy Resolving CMOS Imager, Versatile And Large) soft X-ray 2D imaging detector is based on stitched, wafer-scale sensors possessing a thick epi-layer, which together with back-thinning and back-side illumination yields elevated quantum efficiency in the photon energy range of 125–1000 eV. Main application fields of PERCIVAL are foreseen in photon science with FELs and synchrotron radiation. This requires high dynamic range up to 105 ph @ 250 eV paired with single photon sensitivity with high confidence at moderate frame rates in the range of 10–120 Hz. These figures imply the availability of dynamic gain switching on a pixel-by-pixel basis and a highly parallel, low noise analog and digital readout, which has been realized in the PERCIVAL sensor layout. Different aspects of the detector performance have been assessed using prototype sensors with different pixel and ADC types. This work will report on the recent test results performed on the newest chip prototypes with the improved pixel and ADC architecture. For the target frame rates in the 10–120 Hz range an average noise floor of 14e− has been determined, indicating the ability of detecting single photons with energies above 250 eV. Owing to the successfully implemented adaptive 3-stage multiple-gain switching, the integrated charge level exceeds 4 10^6 e− or 57000 X-ray photons at 250 eV per frame at 120 Hz. For all gains the noise level remains below the Poisson limit also in high-flux conditions. Additionally, a short overview over the updates on an oncoming 2 Mpixel (P2M) detector system (expected at the end of 2016) will be reported.

Detecting Cr contamination in water using X-ray fluorescence

With the rapid growth in population and the overwhelming demand of industrial consumer products around the world, the amount of generated wastes is also increasing. Therefore, the optimal utilization of wastes and the waste management policies are very important in order to protect the environment [1]. The most common way of waste management is to dispose them into city dumps and landfills. These disposal sites may produce toxic and green house gases and also a substantial amount of leachate, which can affect the environment [2]. Leachate is liquid, which, while percolating through wastes in a landfill, extracts soluble and suspended solids. Leachate contains toxic and harmful substances, such as Chromium (Cr), Arsenic, Lead, Mercury, Benzene, Chloroform and Methylene Chloride, and can contaminate surface water and aquifers.

Readout cross-talk for alpha-particle measurements in a pixelated sensor system

Simulations in Medici are performed to quantify crosstalk and charge sharing in a hybrid pixelated silicon detector. Crosstalk and charge sharing degrades the spatial and spectral resolution of single photon processing X-ray imaging systems. For typical medical X-ray imaging applications, the process is dominated by charge sharing between the pixels in the sensor. For heavier particles each impact generates a large amount of charge and the simulation seems to over predict the charge collection efficiency. This indicates that some type of non modelled degradation of the charge transport efficiency exists, like the plasma effect where the plasma might shield the generated charges from the electric field and hence distorts the charge transport process. Based on the simulations it can be reasoned that saturation of the amplifiers in the Timepix system might generate crosstalk that increases the charge spread measured from ion impact on the sensor.

Suitable Post Processing Algorithms for X-Ray Imaging using Oversampled Displaced Multiple Images

X-ray imaging systems such as photon counting pixel detectors have a limited spatial resolution of the pixels, based on the complexity and processing technology of the readout electronics. For X-ray imaging situations where the features of interest are smaller than the imaging system pixel size, and the pixel size cannot be made smaller in the hardware, alternative means of resolution enhancement require to be considered. Oversampling with the usage of multiple displaced images, where the pixels of all images are mapped to a final resolution enhanced image, has proven a viable method of reaching a sub-pixel resolution exceeding the original resolution. The effectiveness of the oversampling method declines with the number of images taken, the sub-pixel resolution increases, but relative to a real reduction of imaging pixel sizes yielding a full resolution image, the perceived resolution from the sub-pixel oversampled image is lower. This is because the oversampling method introduces blurring noise into the mapped final images, and the blurring relative to full resolution images increases with the oversampling factor. One way of increasing the performance of the oversampling method is by sharpening the images in post processing. This paper focus on characterizing the performance increase of the oversampling method after the use of some suitable post processing filters, for digital X-ray images specifically. The results show that spatial domain filters and frequency domain filters of the same type yield indistinguishable results, which is to be expected. The results also show that the effectiveness of applying sharpening filters to oversampled multiple images increase with the number of images used (oversampling factor), leaving 60-80% of the original blurring noise after filtering a 6 x 6 mapped image (36 images taken), where the percentage is depending on the type of filter. This means that the effectiveness of the oversampling itself increase by using sharpening filters, and more images taken can be considered worth the effort.