Matthew C. Veale

Pixellated Cd(Zn)Te high-energy X-ray instrument

We have developed a pixellated high energy X-ray detector instrument to be used in a variety of imaging applications. The instrument consists of either a Cadmium Zinc Telluride or Cadmium Telluride (Cd(Zn)Te) detector bump-bonded to a large area ASIC and packaged with a high performance data acquisition system. The 80 by 80 pixels each of 250 μm by 250 μm give better than 1 keV FWHM energy resolution at 59.5 keV and 1.5 keV FWHM at 141 keV, at the same time providing a high speed imaging performance. This system uses a relatively simple wire-bonded interconnection scheme but this is being upgraded to allow multiple modules to be used with very small dead space. The readout system and the novel interconnect technology is described and how the system is performing in several target applications.

An ASIC for the Study of Charge Sharing Effects in Small Pixel CdZnTe X-Ray Detectors

An Application Specific Integrated Circuit (ASIC) has been developed at the Rutherford Appleton Laboratory (RAL) to study the small pixel effect in spectroscopic CdTe and CdZnTe detectors. The PIXIE ASIC consists of four arrays of 3 × 3 channels flip chip bonded directly to the detector pixels. The active circuitry of each channel is a charge sensitive preamplifier and an output buffer which is multiplexed directly off chip. Each of the four detector arrays has a different anode geometry. The HEXITEC series of small pixel detectors developed at RAL have demonstrated energy resolutions of ~1 keV per pixel for both CdTe and CdZnTe, however, charge sharing events account for between 30-40% of the total count rate and can lead to degradation of the spectroscopy if not corrected for. The PIXIE ASIC will be used to study the effect of anode geometry on charge sharing and other aspects of the small pixel effect.

K-edge subtraction imaging using a pixellated energy-resolving detector

This paper presents preliminary work aimed at assessing the feasibility of K-edge subtraction imaging using the spectroscopic information provided by a pixellated energy-resolving Cadmium Zinc Telluride detector, having an active area of 20×20 pixels 250 μm in size. Images of a test object containing different amounts of Iodine-based contrast agent were formed above and below the K-edge of Iodine (33.2 keV) by integrating, pixel by pixel, different windows of the spectrum. The results show that the optimum integration window for details 1-2 mm in diameter is between 2 keV and 5 keV. Concentrations of down to 50 μg Iodine/ml were detected in a 1-mm diameter tube with an entrance dose of 100 μGy.

Dark-field hyperspectral X-ray imaging

In recent times, there has been a drive to develop non-destructive X-ray imaging techniques that provide chemical or physical insight. To date, these methods have generally been limited; either requiring raster scanning of pencil beams, using narrow bandwidth radiation and/or limited to small samples. We have developed a novel full-field radiographic imaging technique that enables the entire physio-chemical state of an object to be imaged in a single snapshot. The method is sensitive to emitted and scattered radiation, using a spectral imaging detector and polychromatic hard X-radiation, making it particularly useful for studying large dense samples for materials science and engineering applications. The method and its extension to three-dimensional imaging is validated with a series of test objects and demonstrated to directly image the crystallographic preferred orientation and formed precipitates across an aluminium alloy friction stir weld section.

A CdTe detector for hyperspectral SPECT imaging

A Cadmium Telluride (CdTe) detector has been developed for multiple-radioisotope SPECT imaging. The 2 × 2 cm detector has 80 × 80 pixels on a 250 μm pitch and a three-side buttable design so that it can be tiled into larger arrays. The detector is termed hyperspectral as it measures the energy of every photon that interacts in the CdTe to give fully spectroscopic information from 5–200 keV in each pixel. The detector has been tested for applications in multiple-radioisotope SPECT imaging using a 1 mm diameter pinhole configuration and standard phantom test objects containing Tc-99m, I-123 and Ga-67. The detector has an average pixel energy resolution (FWHM) of 0.75% at the I-123 photopeak of 159 keV. We demonstrate the systems capability of resolving spatial features of 2 mm, although the spatial resolution of the detector is limited only by the pixel size and pinhole magnification factor. These characteristics are superior to alternative detectors currently in use in clinical SPECT systems. When imaging multiple radioisotopes simultaneously, we show that there is very little cross-talk between adjacent photopeaks, leading to superior image contrast. The detector is also capable of resolving fluorescence x-rays from the radioactive source, which could be used to improve image count statistics or derive information about the attenuation properties of the source. The performance presented here, and the ability to tile the detector modules to create a clinically useful field of view, makes this technology a strong candidate to be used in future solid-state SPECT cameras.

A 10 cm × 10 cm CdTe Spectroscopic Imaging Detector based on the HEXITEC ASIC

The 250 μ m pitch 80x80 pixel HEXITEC detector systems have shown that spectroscopic imaging with an energy resolution of <1 keV FWHM per pixel can be readily achieved in the range of 5–200 keV with Al-pixel CdTe biased to −500 V. This level of spectroscopic imaging has a variety of applications but the ability to produce large area detectors remains a barrier to the adoption of this technology. The limited size of ASICs and defect free CdTe wafers dictates that building large area monolithic detectors is not presently a viable option. A 3-side buttable detector module has been developed to cover large areas with arrays of smaller detectors. The detector modules are 20.35 × 20.45 mm with CdTe bump bonded to the HEXITEC ASIC with coverage up to the edge of the module on three sides. The fourth side has a space of 3 mm to allow I/O wire bonds to be made between the ASIC and the edge of a PCB that routes the signals to a connector underneath the active area of the module. The detector modules have been assembled in rows of five modules with a dead space of 170 μ m between each module. Five rows of modules have been assembled in a staggered height array where the wire bonds of one row of modules are covered by the active detector area of a neighboring row. A data acquisition system has been developed to digitise, store and output the 24 Gbit/s data that is generated by the array. The maximum bias magnitude that could be applied to the CdTe detectors from the common voltage source was limited by the worst performing detector module. In this array of detectors a bias of −400 V was used and the detector modules had 93 % of pixels with better than 1.2 keV FWHM at 59.5 keV. An example of K-edge enhanced imaging for mammography was demonstrated. Subtracting images from the events directly above and below the K-edge of the Iodine contrast agent was able to extract the Iodine information from the image of a breast phantom and improve the contrast of the images. This is just one example where the energy spectrum per pixel can be used to develop new and improve existing X-ray imaging techniques.

Identification of simulants for explosives using pixellated X-ray diffraction

A new method of material identification has been developed utilising pixellated X-ray diffraction (PixD) to probe the molecular structure of hidden items. Since each material has a unique structure, this technique can be used to “fingerprint” items and has significant potential for use in security applications such as airport baggage scanning. The pixellated diffraction technique allows two distinct forms of diffraction, angular-dispersive and energy-dispersive X-ray diffraction, to be combined, exploiting the benefits of both. Thus, fast acquisition times are possible with a small system which contains no moving parts and can be easily implemented. In this work, the capability of the system to identify specific materials within a sample is highlighted. Such an approach would be highly beneficial for detecting explosive materials which are concealed amongst or inside other masking items. The technology could easily be added to existing baggage scanning equipment and would mean that if a suspicious item is seen in a regular X-ray image, the operator of the equipment could analyse the object in detail without opening the bag. The net result would be more accurate analysis of baggage content and faster throughput, as manual searching of suspicious objects would not be required.

X-ray micro-beam characterization of a small pixel spectroscopic CdTe detector

A small pixel, spectroscopic, CdTe detector has been developed at the Rutherford Appleton Laboratory (RAL) for X-ray imaging applications. The detector consists of 80 × 80 pixels on a 250 μm pitch with 50 μm inter-pixel spacing. Measurements with an 241Am γ-source demonstrated that 96% of all pixels have a FWHM of better than 1 keV while the majority of the remaining pixels have FWHM of less than 4 keV. Using the Diamond Light Source synchrotron, a 10 μm collimated beam of monochromatic 20 keV X-rays has been used to map the spatial variation in the detector response and the effects of charge sharing corrections on detector efficiency and resolution. The mapping measurements revealed the presence of inclusions in the detector and quantified their effect on the spectroscopic resolution of pixels.

Materials identification using a small-scale pixellated x-ray diffraction system

A transmission x-ray diffraction system has been developed using a pixellated, energy-resolving detector (HEXITEC) and a small-scale, mains operated x-ray source (Amptek Mini-X). HEXITEC enables diffraction to be measured without the requirement of incident spectrum filtration, or collimation of the scatter from the sample, preserving a large proportion of the useful signal compared with other diffraction techniques. Due to this efficiency, sufficient molecular information for material identification can be obtained within 5 s despite the relatively low x-ray source power. Diffraction data are presented from caffeine, hexamine, paracetamol, plastic explosives and narcotics. The capability to determine molecular information from aspirin tablets inside their packaging is demonstrated. Material selectivity and the potential for a sample classification model is shown with principal component analysis, through which each different material can be clearly resolved.

Investigating the suitability of GaAs:Cr material for high flux X-ray imaging

Semi-insulating wafers of GaAs material with a thickness of 500μm have been compensated with chromium by Tomsk State University. Initial measurements have shown the material to have high resistivity (3 × 109Ωcm) and tests with pixel detectors on a 250 μm pitch produced uniform spectroscopic performance across an 80 × 80 pixel array. At present, there is a lack of detectors that are capable of operating at high X-ray fluxes (> 108 photons s-1 mm-2) in the energy range 5–50 keV. Under these conditions, the poor stopping power of silicon, as well as issues with radiation hardness, severely degrade the performance of traditional detectors. While high-Z materials such as CdTe and CdZnTe may have much greater stopping power, the formation of space charge within these detectors degrades detector performance. Initial measurements made with GaAs:Cr detectors suggest that many of its material properties make it suitable for these challenging conditions. In this paper the radiation hardness of the GaAs:Cr material has been measured on the B16 beam line at the Diamond Light Source synchrotron. Small pixel detectors were bonded to the STFC Hexitec ASIC and were irradiated with 3 × 108 photons s-1 mm-2 monochromatic 12 keV X-rays up to a maximum dose of 0.6 MGy. Measurements of the spectroscopic performance before and after irradiation have been used to assess the extent of the radiation damage.