Ballistic Deficit Pulse Processing in Cadmium-Zinc-Telluride Pixel Detectors for High-Flux X-ray Measurements.

High-flux X-ray measurements with high-energy resolution and high throughput require the mitigation of pile-up and dead time effects. The reduction of the time width of the shaped pulses is a key approach, taking into account the distortions from the ballistic deficit, non-linearity, and time instabilities. In this work, we will present the performance of cadmium−zinc−telluride (CdZnTe or CZT) pixel detectors equipped with digital shapers faster than the preamplifier peaking times (ballistic deficit pulse processing). The effects on energy resolution, throughput, energy-linearity, time stability, charge sharing, and pile-up are shown. The results highlight the absence of time instabilities and high-energy resolution (<4% FWHM at 122 keV) when ballistic deficit pulse processing (dead time of 90 ns) was used in CZT pixel detectors. These activities are in the framework of an international collaboration on the development of spectroscopic imagers for medical applications (mammography, computed tomography) and non-destructive testing in the food industry.

Incomplete Charge Collection at Inter-Pixel Gap in Low- and High-Flux Cadmium Zinc Telluride Pixel Detectors.

The success of cadmium zinc telluride (CZT) detectors in room-temperature spectroscopic X-ray imaging is now widely accepted. The most common CZT detectors are characterized by enhanced-charge transport properties of electrons, with mobility-lifetime products µeτe > 10-2 cm2/V and µhτh > 10-5 cm2/V. These materials, typically termed low-flux LF-CZT, are successfully used for thick electron-sensing detectors and in low-flux conditions. Recently, new CZT materials with hole mobility-lifetime product enhancements (µhτh > 10-4 cm2/V and µeτe > 10-3 cm2/V) have been fabricated for high-flux measurements (high-flux HF-CZT detectors). In this work, we will present the performance and charge-sharing properties of sub-millimeter CZT pixel detectors based on LF-CZT and HF-CZT crystals. Experimental results from the measurement of energy spectra after charge-sharing addition (CSA) and from 2D X-ray mapping highlight the better charge-collection properties of HF-CZT detectors near the inter-pixel gaps. The successful mitigation of the effects of incomplete charge collection after CSA was also performed through original charge-sharing correction techniques. These activities exist in the framework of international collaboration on the development of energy-resolved X-ray scanners for medical applications and non-destructive testing in the food industry.

Charge Sharing and Charge Loss in High-Flux Capable Pixelated CdZnTe Detectors.

Cadmium zinc telluride (CdZnTe) detectors are known to suffer from polarization effects under high photon flux due to poor hole transport in the crystal material. This has led to the development of a high-flux capable CdZnTe material (HF-CdZnTe). Detectors with the HF-CdZnTe material have shown promising results at mitigating the onset of the polarization phenomenon, likely linked to improved crystal quality and hole carrier transport. Better hole transport will have an impact on charge collection, particularly in pixelated detector designs and thick sensors (>1 mm). In this paper, the presence of charge sharing and the magnitude of charge loss were calculated for a 2 mm thick pixelated HF-CdZnTe detector with 250 µm pixel pitch and 25 µm pixel gaps, bonded to the STFC HEXITEC ASIC. Results are compared with a CdTe detector as a reference point and supported with simulations from a Monte-Carlo detector model. Charge sharing events showed minimal charge loss in the HF-CdZnTe, resulting in a spectral resolution of 1.63 ± 0.08 keV Full Width at Half Maximum (FWHM) for bipixel charge sharing events at 59.5 keV. Depth of interaction effects were shown to influence charge loss in shared events. The performance is discussed in relation to the improved hole transport of HF-CdZnTe and comparison with simulated results provided evidence of a uniform electric field.

Energy Recovery of Multiple Charge Sharing Events in Room Temperature Semiconductor Pixel Detectors.

Multiple coincidence events from charge-sharing and fluorescent cross-talk are typical drawbacks in room-temperature semiconductor pixel detectors. The mitigation of these distortions in the measured energy spectra, using charge-sharing discrimination (CSD) and charge-sharing addition (CSA) techniques, is always a trade-off between counting efficiency and energy resolution. The energy recovery of multiple coincidence events is still challenging due to the presence of charge losses after CSA. In this work, we will present original techniques able to correct charge losses after CSA even when multiple pixels are involved. Sub-millimeter cadmium-zinc-telluride (CdZnTe or CZT) pixel detectors were investigated with both uncollimated radiation sources and collimated synchrotron X rays, at energies below and above the K-shell absorption energy of the CZT material. These activities are in the framework of an international collaboration on the development of energy-resolved photon counting (ERPC) systems for spectroscopic X-ray imaging up to 150 keV.

Room-temperature performance of 3 mm-thick cadmium-zinc-telluride pixel detectors with sub-millimetre pixelization.

Cadmium-zinc-telluride (CZT) pixel detectors represent a consolidated choice for the development of room-temperature spectroscopic X-ray imagers, finding important applications in medical imaging, often as detection modules of a variety of new SPECT and CT systems. Detectors with 3-5 mm thicknesses are able to efficiently detect X-rays up to 140 keV giving reasonable room-temperature energy resolution. In this work, the room-temperature performance of 3 mm-thick CZT pixel detectors, recently developed at IMEM/CNR of Parma (Italy), is presented. Sub-millimetre detector arrays with pixel pitch less than 500 µm were fabricated. The detectors are characterized by good room-temperature performance even at high bias voltage operation (6000 V cm-1), with energy resolutions (FWHM) of 3% (1.8 keV) and 1.6% (2 keV) at 59.5 keV and 122.1 keV, respectively. Charge-sharing investigations were performed with both uncollimated and collimated synchrotron X-ray beams with particular attention to recovering the charge losses at the inter-pixel gap region. High rate measurements demonstrated the absence of high-flux radiation-induced polarization phenomena up to 25 × 106 photons mm-2 s-1.

Room-temperature X-ray response of cadmium-zinc-telluride pixel detectors grown by the vertical Bridgman technique.

In this work, the spectroscopic performances of new cadmium-zinc-telluride (CZT) pixel detectors recently developed at IMEM-CNR of Parma (Italy) are presented. Sub-millimetre arrays with pixel pitch less than 500 µm, based on boron oxide encapsulated vertical Bridgman grown CZT crystals, were fabricated. Excellent room-temperature performance characterizes the detectors even at high-bias-voltage operation (9000 V cm-1), with energy resolutions (FWHM) of 4% (0.9 keV), 1.7% (1 keV) and 1.3% (1.6 keV) at 22.1, 59.5 and 122.1 keV, respectively. Charge-sharing investigations were performed with both uncollimated and collimated synchrotron X-ray beams with particular attention to the mitigation of the charge losses at the inter-pixel gap region. High-rate measurements demonstrated the absence of high-flux radiation-induced polarization phenomena up to 2 × 106 photons mm-2 s-1. These activities are in the framework of an international collaboration on the development of energy-resolved photon-counting systems for high-flux energy-resolved X-ray imaging.

Digital fast pulse shape and height analysis on cadmium-zinc-telluride arrays for high-flux energy-resolved X-ray imaging.

Cadmium-zinc-telluride (CZT) arrays with photon-counting and energy-resolving capabilities are widely proposed for next-generation X-ray imaging systems. This work presents the performance of a 2 mm-thick CZT pixel detector, with pixel pitches of 500 and 250 µm, dc coupled to a fast and low-noise ASIC (PIXIE ASIC), characterized only by the preamplifier stage. A custom 16-channel digital readout electronics was used, able to digitize and process continuously the signals from each output ASIC channel. The digital system performs on-line fast pulse shape and height analysis, with a low dead-time and reasonable energy resolution at both low and high fluxes. The spectroscopic response of the system to photon energies below (109Cd source) and above (241Am source) the K-shell absorption energy of the CZT material was investigated, with particular attention to the mitigation of charge sharing and pile-up. The detector allows high bias voltage operation (>5000 V cm-1) and good energy resolution at moderate cooling (3.5% and 5% FWHM at 59.5 keV for the 500 and 250 µm arrays, respectively) by using fast pulse shaping with a low dead-time (300 ns). Charge-sharing investigations were performed using a fine time coincidence analysis (TCA), with very short coincidence time windows up to 10 ns. For the 500 µm pitch array (250 µm pitch array), sharing percentages of 36% (52%) and 60% (82%) at 22.1 and 59.5 keV, respectively, were measured. The potential of the pulse shape analysis technique for charge-sharing detection for corner/border pixels and at high rate conditions (250 kcps pixel-1), where the TCA fails, is also shown. Measurements demonstrated that significant amounts of charge are lost for interactions occurring in the volume of the inter-pixel gap. This charge loss must be accounted for in the correction of shared events. These activities are within the framework of an international collaboration on the development of energy-resolved photon-counting systems for high-flux energy-resolved X-ray imaging (1-140 keV).

Dual-polarity pulse processing and analysis for charge-loss correction in cadmium-zinc-telluride pixel detectors.

Charge losses at the inter-pixel gap are typical drawbacks in cadmium-zinc-telluride (CZT) pixel detectors. In this work, an original technique able to correct charge losses occurring after the application of charge-sharing addition (CSA) is presented. The method, exploiting the strong relation between the energy after CSA and the beam position at the inter-pixel gap, allows the recovery of charge losses and improvements in energy resolution. Sub-millimetre CZT pixel detectors were investigated with both uncollimated radiation sources and collimated synchrotron X-rays, at energies below and above the K-shell absorption energy of the CZT material. The detectors are DC coupled to fast and low-noise charge-sensitive preamplifiers (PIXIE ASIC) and followed by a 16-channel digital readout electronics, performing multi-parameter analysis (event arrival time, pulse shape, pulse height). Induced-charge pulses with negative polarity were also observed in the waveforms from the charge-sensitive preamplifiers (CSPs) at energies >60 keV. The shape and the height of these pulses were analysed, and their role in the mitigation of charge losses in CZT pixel detectors. These activities are in the framework of an international collaboration on the development of energy-resolved photon-counting systems for spectroscopic X-ray imaging (5-140 keV).

A laboratory system for element specific hyperspectral X-ray imaging.

X-ray tomography is a ubiquitous tool used, for example, in medical diagnosis, explosives detection or to check structural integrity of complex engineered components. Conventional tomographic images are formed by measuring many transmitted X-rays and later mathematically reconstructing the object, however the structural and chemical information carried by scattered X-rays of different wavelengths is not utilised in any way. We show how a very simple; laboratory-based; high energy X-ray system can capture these scattered X-rays to deliver 3D images with structural or chemical information in each voxel. This type of imaging can be used to separate and identify chemical species in bulk objects with no special sample preparation. We demonstrate the capability of hyperspectral imaging by examining an electronic device where we can clearly distinguish the atomic composition of the circuit board components in both fluorescence and transmission geometries. We are not only able to obtain attenuation contrast but also to image chemical variations in the object, potentially opening up a very wide range of applications from security to medical diagnostics.

3D Correlative Imaging of Lithium Ion Concentration in a Vertically Oriented Electrode Microstructure with a Density Gradient.

The performance of Li+ ion batteries (LIBs) is hindered by steep Li+ ion concentration gradients in the electrodes. Although thick electrodes (≥300 µm) have the potential for reducing the proportion of inactive components inside LIBs and increasing battery energy density, the Li+ ion concentration gradient problem is exacerbated. Most understanding of Li+ ion diffusion in the electrodes is based on computational modeling because of the low atomic number (Z) of Li. There are few experimental methods to visualize Li+ ion concentration distribution of the electrode within a battery of typical configurations, for example, coin cells with stainless steel casing. Here, for the first time, an interrupted in situ correlative imaging technique is developed, combining novel, full-field X-ray Compton scattering imaging with X-ray computed tomography that allows 3D pixel-by-pixel mapping of both Li+ stoichiometry and electrode microstructure of a LiNi0.8 Mn0.1 Co0.1 O2 cathode to correlate the chemical and physical properties of the electrode inside a working coin cell battery. An electrode microstructure containing vertically oriented pore arrays and a density gradient is fabricated. It is shown how the designed electrode microstructure improves Li+ ion diffusivity, homogenizes Li+ ion concentration through the ultra-thick electrode (1 mm), and improves utilization of electrode active materials.

Quantifying the performance of a hybrid pixel detector with GaAs:Cr sensor for transmission electron microscopy.

Hybrid pixel detectors (HPDs) have been shown to be highly effective for diffraction-based and time-resolved studies in transmission electron microscopy, but their performance is limited by the fact that high-energy electrons scatter over long distances in their thick Si sensors. An advantage of HPDs compared to monolithic active pixel sensors is that their sensors do not need to be fabricated from Si. We have compared the performance of the Medipix3 HPD with a Si sensor and a GaAs:Cr sensor using primary electrons in the energy range of 60-300 keV. We describe the measurement and calculation of the detectors' modulation transfer function (MTF) and detective quantum efficiency (DQE), which show that the performance of the GaAs:Cr device is markedly superior to that of the Si device for high-energy electrons.

Development of a multi-detector readout circuitry for ultrahigh energy resolution single-photon imaging applications.

In this paper, we present the design and preliminary performance evaluation of a novel external multi-channel readout circuitry for small-pixel room-temperature semiconductor detectors, namely CdZnTe (CZT) and CdTe, that provide an excellent intrinsic spatial (250 and 500 µm pixel size) and an ultrahigh energy resolution (~1% at 122 keV) for X-ray and gamma-ray imaging applications. An analog front-end printed circuit board (PCB) was designed and developed for data digitization, data transfer and ASIC control of pixelated CZT or CdTe detectors. Each detector unit is 2 cm × 2 cm in size and 1 or 2 mm in thickness, being bump-bonded onto a HEXITEC ASIC, and wire-bonded to a readout detector module PCB. The detectors' front-end is then connected, through flexible cables of up to 10 m in length, to a remote data acquisition system that interfaces with a PC through USB3.0 connection. We present the design and performance of a prototype multi-channel readout system that can read out up to 24 detector modules synchronously. Our experimental results demonstrated that the readout circuitry offers an ultrahigh spectral resolution (0.8 keV at 60 keV and 1.05 keV at 122 keV) with the Cd(Zn)Te/HEXITEC ASIC modules tested. This architecture was designed to allow easy expansion to accommodate a larger number of detector modules, and the flexibility of arranging the detector modules in a large and deformable detector array without degrading the excellent energy resolution.

An operando spatially resolved study of alkaline battery discharge using a novel hyperspectral detector and X-ray tomography.

An experimental technique is described for the collection of time-resolved X-ray diffraction information from a complete commercial battery cell during discharging or charging cycles. The technique uses an 80 × 80 pixel 2D energy-discriminating detector in a pinhole camera geometry which can be used with a polychromatic X-ray source. The concept was proved in a synchrotron X-ray study of commercial alkaline Zn-MnO2 AA size cells. Importantly, no modification of the cell was required. The technique enabled spatial and temporal changes to be observed with a time resolution of 20 min (5 min of data collection with a 15 min wait between scans). Chemical changes in the cell determined from diffraction information were correlated with complementary X-ray tomography scans performed on similar cells from the same batch. The clearest results were for the spatial and temporal changes in the Zn anode. Spatially, there was a sequential transformation of Zn to ZnO in the direction from the separator towards the current collector. Temporally, it was possible to track the transformation of Zn to ZnO during the discharge and follow the corresponding changes in the cathode.

High Energy Resolution Hyperspectral X-Ray Imaging for Low-Dose Contrast-Enhanced Digital Mammography.

Contrast-enhanced digital mammography (CEDM) is an alternative to conventional X-ray mammography for imaging dense breasts. However, conventional approaches to CEDM require a double exposure of the patient, implying higher dose, and risk of incorrect image registration due to motion artifacts. A novel approach is presented, based on hyperspectral imaging, where a detector combining positional and high-resolution spectral information (in this case based on Cadmium Telluride) is used. This allows simultaneous acquisition of the two images required for CEDM. The approach was tested on a custom breast-equivalent phantom containing iodinated contrast agent (Niopam 150®). Two algorithms were used to obtain images of the contrast agent distribution: K-edge subtraction (KES), providing images of the distribution of the contrast agent with the background structures removed, and a dual-energy (DE) algorithm, providing an iodine-equivalent image and a water-equivalent image. The high energy resolution of the detector allowed the selection of two close-by energies, maximising the signal in KES images, and enhancing the visibility of details with the low surface concentration of contrast agent. DE performed consistently better than KES in terms of contrast-to-noise ratio of the details; moreover, it allowed a correct reconstruction of the surface concentration of the contrast agent in the iodine image. Comparison with CEDM with a conventional detector proved the superior performance of hyperspectral CEDM in terms of the image quality/dose tradeoff.

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.