G. Prekas

Electric field distributions in CdZnTe due to reduced temperature and x-ray irradiation

Real-time Pockels imaging is performed on semi-insulating CdZnTe to measure the electric field profile in the material bulk. In steady-state room temperature conditions the measured electric field profile is uniform, consistent with a low space charge concentration. At temperatures <270 K a significant nonuniform electric field profile is observed, which we explain in terms of temperature-induced band bending at the metal-semiconductor interface, causing the formation of positive space charge in the bulk. Similar electric field distortion effects are observed when room temperature CdZnTe is irradiated by x-rays, causing a high rate of photoinduced charge injection.

Investigation of the internal electric field distribution under in situ x-ray irradiation and under low temperature conditions by the means of the Pockels effect

The internal electric field distribution in cadmium zinc telluride (CdZnTe) x-ray and γ-ray detectors strongly affects their performance in terms of charge transport and charge collection properties. In CdZnTe detectors the electric field distribution is sensitively dependent on not only the nature of the metal contacts but also on the working conditions of the devices such as the temperature and the rate of external irradiation. Here we present direct measurements of the electric field profiles in CdZnTe detectors obtained using the Pockels electo-optic effect whilst under in situ x-ray irradiation. These data are also compared with alpha particle induced current pulses obtained by the transient current technique, and we discuss the influence of both low temperature and x-ray irradiation on the electric field evolution. Results from these studies reveal strong distortion of the electric field consistent with the build-up of space charge at temperatures below 250 K, even in the absence of external irradiation. Also, in the presence of x-ray irradiation levels a significant distortion in the electric field is observed even at room temperature which matches well the predicted theoretical model.

Influence of Contacts on Electric Field in an Au/(CdZn)Te/Au Detector: A Simulation

We report our simulations on the influence of contacts on charge collection in semi-insulating (CdZn)Te with Au contacts under radiation flux, employing simultaneous solutions of the drift-diffusion and Poisson equations. The type of the space charge and the distribution of the electric field in the Au/(CdZn)Te/Au structure at high fluxes reflect the combined influence of charge generated by band bending at the electrodes, and from photogenerated carriers trapped at deep levels. We show that the space charge originating from the latter approaches dominance at high fluxes while the influence of the contacts becomes negligible. The ratio of trapping and collection times at low fluxes strongly depends on band bending, due mainly to a change in the occupation of deep levels by injection or depletion from the contacts. Such dependence is weak at high fluxes; in this case, the space charge due to trapped carriers prevails over that formed due to band bending. These phenomena can cause the formation an electric-field minimum within the device (the pinch point), the position of which is influenced by the nature of the contacts. The field minimum can completely disappear or develop into a dead layer as band bending changes.

High-Performance and Cost-Effective Detector Using Microcolumnar CsI:Tl and SiPM

We are developing a technique to fabricate high spatial resolution and cost-effective photon counting detectors using silicon photomultipliers (SiPMs) and microcolumnar structured scintillator. Photon counting detectors using SiPMs are of much interest to the gamma- and X-ray detector community, but they have limitations at low energy due to their dark noise. In this paper, we report on vapor deposition of CsI:Tl directly onto a SiPM, a technique that improves optical coupling and allows for detection of low energy gamma- and X-rays. It simultaneously addresses related issues of light loss and light spread in the scintillator, thereby improving the performance of the detector. Devices made by this technique may be used for both photon counting and gamma- and X-ray imaging.

Direct and indirect detectors for X-ray photon counting systems

Most currently available X-ray or gamma ray imaging detectors are based on energy integration over a certain period of time. We have been developing X-ray and gamma ray detectors based on the photon counting (with energy determination) concept using both direct and indirect radiation conversion, together with associated application-specific integrated circuits (ASICs). As an alternative to our ASIC design approach, we are also exploiting the potential of state-of-the-art silicon photomultipliers (SiPMs) and discrete electronics. In this paper we discuss the advantages and disadvantages of these two approaches and we report our latest results on our ASIC design efforts and our achievements on SiPM/CsI:Tl detector configurations. We will also discuss the potential uses and advantages that each offers to applications in medicine, imaging, homeland security and industry.

Direct Deposition of Microcolumnar Scintillator on CMOS SSPM Array: Toward a Photon Counting Detector for X‐Ray/Gamma Ray Imaging

We are developing a modular, low‐cost, photon‐counting detector based on a scintillator coupled to a solid‐state photodetector. A working prototype was successfully developed by depositing CsI:Tl directly onto a CMOS SSPM array designed by RMD and custom‐fabricated by a commercial foundry. The device comprised a 6×6 array of 1.5×1.5 mm2 macro‐pixels, each containing a 36×36 array of resistively coupled micro‐pixels, that was subjected to vapor deposition of columnar CsI:Tl. Direct deposition eliminates the gap between the scintillator and SSPM and creates a better optical bond than does index‐matching grease. This paper compares the performance of SSPMs with directly deposited CsI:Tl, in terms of signal‐to‐noise ratio and light spread, against devices using monolithic single crystals or pixelated single crystals coupled to the SSPM. Due to the reduction in light scattering and optical losses in the interface, the directly deposited CsI:Tl demonstrated significantly better position sensitivity, with at least a factor of 2 increase in SNR compared to a single crystal. These data indicate that a photodetector with substantially smaller macro‐pixel dimensions than used here could be used to implement a low‐energy X‐ray/gamma‐ray imaging and spectroscopy detector, particularly for applications where high resolution is of prime importance.We are developing a modular, low‐cost, photon‐counting detector based on a scintillator coupled to a solid‐state photodetector. A working prototype was successfully developed by depositing CsI:Tl directly onto a CMOS SSPM array designed by RMD and custom‐fabricated by a commercial foundry. The device comprised a 6×6 array of 1.5×1.5 mm2 macro‐pixels, each containing a 36×36 array of resistively coupled micro‐pixels, that was subjected to vapor deposition of columnar CsI:Tl. Direct deposition eliminates the gap between the scintillator and SSPM and creates a better optical bond than does index‐matching grease. This paper compares the performance of SSPMs with directly deposited CsI:Tl, in terms of signal‐to‐noise ratio and light spread, against devices using monolithic single crystals or pixelated single crystals coupled to the SSPM. Due to the reduction in light scattering and optical losses in the interface, the directly deposited CsI:Tl demonstrated significantly better position sensitivity, with at lea...

A modular high-resolution photon-counting x-ray detector

For over a century, x-ray imaging detectors (whether film or digital) have formed images by integrating x-ray interactions over a finite acquisition time without regard to event energy or number of events. While the benefits of these ‘energy-integrating’ detectors are well established, their performance—especially in terms of high contrast ratios— is suboptimal. Consequently, the next generation of x-ray detectors for computed tomography (CT) and digital radiography must be capable of counting individual photons and characterizing (and even recording) their energies. In the case of medical imaging, increased sensitivity at low energy will allow reduced radiation dosage to patients and provide contrast ratios equivalent to those obtained by other imaging methods, while improving spatial resolution and reducing noise for the same quantum efficiency. The advanced energy capability will also enable reconstructions with fewer beam-hardening and minimal ghosting artifacts due to lag, and carry the potential for single-exposure, multiple-energy imaging.1 In response to these needs and opportunities, we are developing a family of modular, highly configurable photon-counting, energy-discriminating, high-resolution imaging devices based on a novel combination of cadmium zinc telluride (CdZnTe) semiconductor radiation sensors and high-resolution custom application-specific integrated circuits. Our goal for our Advanced Photon Counting DetectorTM (APCD) module is to combine these two high-performance components into small (e.g., 1 1cm2 and 2 2cm2/ modules that tile seamlessly, forming detectors of arbitrary size and 2or 3D shape (see Figure 1). APCD components and APCD-based detectors are being designed for critical, highly demanding applications such as medical x-ray CT and munitions inspection, as well as for medical and nonmedical x-ray CT and radiography in general. We intend APCD devices to be used to read out other solid-state Figure 1. (a) Cadmium zinc telluride (CdZnTe) or CdTe sensors will be bump-bonded to Advanced Photon Counting Detector (APCD) application-specific integrated circuit (ASIC) CMOS chips to form APCD detector modules (e.g., a 2 2cm2 module of 80 80 pixels). (b) Modules may then be tiled to form APCD detectors, continuous pixel arrays of arbitrary size (e.g., 4 4 modules, 8 8cm2, 320 320 pixels). To do so, modules will be mechanically attached individually to printed circuit motherboards supplying power and containing control and readout electronics. Modules may be individually removed for service.

Real-time Imaging of the Electric field Distribution in CdZnTe at low temperature

Real time imaging of the electric field distribution in CZT at low temperature has been carried out using the Pockels electro-optical effect. CZT detectors have been observed to show degraded spectroscopic resolution at low temperature due to so-called ‘polarization’ phenomena. By mounting a CZT device in a custom optical cryostat, we have used Pockels imaging to observe the distortion of the electric field distribution in the temperature range 240K - 300K. At 240K the electric field has a severely non-uniform depth distribution, with a high field region occupying ∼10% of the depth of the device under the cathode electrode and a low field in the remainder of the device. Using an alpha particle source positioned inside the vacuum chamber we have performed simultaneous alpha particle transient current (TCT) measurements. At low temperatures the alpha particle current pulses become significantly shorter, consistent with the reduced electron drift time due to a non-uniform electric field. These data provide useful insights into the mechanisms which limit the spectroscopic performance of CZT devices at reduced temperature.