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Schematics of a parallelepiped-shaped virtual Frisch-grid CZT detector (bar). The shielding electrode is decoupled from the cathode cap and extends to the anode side of the device.
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Virtual Frisch-grid CdZnTe detectors potentially can provide energy resolution close to the statistical limit. However, in real detectors, the quality of the crystals used to fabricate the devices primarily determines energy resolution. In this paper, we report our findings on the spectral response of devices and their relation to material-characte...
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... schematic of a bar detector is shown in Fig. 1. The geometry of its electrodes [7] includes the features of two designs previously proposed for parallelepiped-shaped CZT detectors: the CAPture device [9,10], and the Frisch-ring (or capacitive Frisch-grid) detector [11,12]. As in the CAPture device, the cathode electrode extends ~2 mm on the crystal's side surfaces to form a cap. ...
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... detectors. The edge effects are explained [14] by local variations in the electric field causing the clouds to approach the side surfaces where they become trapped. The second problem concerns twins and subgrain boundaries, overlooked during the processes of crystal selection that can directly trap electrons. Both effects are illustrated in Figs. 10 (a-d) that plot the pulse-height spectra and X-ray maps measured for four representative detectors. The first detector (a) has a good spectrum with a narrow peak and high peak-to-Compton ratio. This good spectral response correlates well with the uniform lateral response map measured with a collimated X-ray beam (small dots in the map ...
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... defects: incoherent twins, stacking faults, and subgrain boundaries. Thus, the IR microscope reveals many extended defects as such clusters of Te inclusions [22]. We note that dislocations with their dangling bonds affect electron transport in semiconductors, and their roles in changing a device's performances have been long recognized [29]. Fig. 11 (a) shows a photograph of the Te inclusions clustered along the plane located near the long side surface of a bar. This cluster was visible from one direction only, and for particular orientation of the crystal; we would not have been able to find it before taking the X-ray diffraction topography measurements for this sample. However, in ...
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... of the Te inclusions clustered along the plane located near the long side surface of a bar. This cluster was visible from one direction only, and for particular orientation of the crystal; we would not have been able to find it before taking the X-ray diffraction topography measurements for this sample. However, in the topographic image of Fig. 11 (b), the presence of a twin boundary along the long side of the bar is obvious. The high geometrical contrast in the diffraction pattern results from differences in relative orientation of adjacent domains. The consequence of the presence of the extended defects on the device's performance is illustrated in Figs. 11 (c), (d) and (e) that ...
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... in the topographic image of Fig. 11 (b), the presence of a twin boundary along the long side of the bar is obvious. The high geometrical contrast in the diffraction pattern results from differences in relative orientation of adjacent domains. The consequence of the presence of the extended defects on the device's performance is illustrated in Figs. 11 (c), (d) and (e) that show the responses measured for the same detector with gamma-rays and the collimated X-ray beam. Figure 12 has several more examples of the extended defects observed with synchrotron diffraction topography, and their correlations with the device performances. The topograph of the detector 1 shows two slightly disoriented ...
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... (c), (d) and (e) that show the responses measured for the same detector with gamma-rays and the collimated X-ray beam. Figure 12 has several more examples of the extended defects observed with synchrotron diffraction topography, and their correlations with the device performances. The topograph of the detector 1 shows two slightly disoriented domains (circled) located near both sides of the bar. ...
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... indicating the presence of significant strains within it; no other defects (accept for Te inclusions) were identified with IR. The device fabricated from this bar had an atypical performance. Thus, while the X-ray response was very uniform over device's area except for the one edge where the boundary is located, the peak in the spectrum is broad. Fig. 10. Pulse-height spectra and X-ray mappings measured illustrating two kinds of defects in parallelepiped- shaped detector caused extended defects (see explanations in the text). For detector 3, we located several Te-inclusion clusters (boundaries) similar to those Soldner et al. [22] identified as twins, which stretched across the crystal ...
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... walls of dislocations. The formation of this type of dislocation network is likely to occur during either the post-growth cooling process or annealing. The presence of such walls of dislocation in the crystals we studied might explain why some detectors cannot perform satisfactorily despite having good prerequisite transport properties. Fig. 13 (top) shows a photograph of the etched surface of the bar used to fabricate detector 4; it reveals clearly these aligned dislocations patterns. We re-fabricated detector 4 by placing new contacts on the long sides of its surfaces, and carried out an X-ray scan to see the effect of the dislocations walls. The response map, wherein the darker ...
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... clearly these aligned dislocations patterns. We re-fabricated detector 4 by placing new contacts on the long sides of its surfaces, and carried out an X-ray scan to see the effect of the dislocations walls. The response map, wherein the darker areas represent the areas in the device from which smaller signals were measured, also is shown in Fig. 13 (bottom). The map was filtered to enhance the faded features and, as a result, the density of the dark spots corresponding to the Te inclusions is disproportionally increased. The dark lines seen in the X-ray map perfectly correlate with the dislocation lines in the photograph, verifying that polygonized dislocations affect charge transport. ...
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... 13 (bottom). The map was filtered to enhance the faded features and, as a result, the density of the dark spots corresponding to the Te inclusions is disproportionally increased. The dark lines seen in the X-ray map perfectly correlate with the dislocation lines in the photograph, verifying that polygonized dislocations affect charge transport. Fig. 13. A photograph of the etched surface of the crystal used to fabricate Detector 4 (top) and the X-ray map (bottom) measured for the same crystal re-fabricated as planar detector with the electrodes deposited on the long ...
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Experimental and theoretical investigations are reported to comprehend the structural and lattice dynamical properties of high quality Bridgman grown zb Zn1-xMnxTe (x ≤ 0.15) crystals. Our detailed far-infrared reflectivity and Raman scattering measurements ascertained atypical “intermediate-phonon-mode” behavior for the alloys. Besides perceiving ZnTe- and zb MnTe-like LO and TO phonons, a weak Mn alloy-disorder mode near ∼205 cm⁻¹ is observed. A classical “Drude-Lorentz” model is used for evaluating the dielectric tensors of ternary alloys to simulate composition dependent reflectivity spectra – providing results in good agreement with the experimental data at near normal incidence. Lattice relaxation effects around MnZn and ZnMn from the synchrotron radiation extended x-ray absorption experiments are meticulously incorporated to help construct the perturbation models for simulating impurity modes using a realistic rigid-ion-model in the frame work of an average t-matrix Greens function theory – reconfirming on much firmer ground – the observed intermediate phonon mode behavior.
... CZT detectors operate as single-type carrier (electrons), meaning that the devices' geometrical parameters and contacts configurations should be chosen so to minimize the contribution of uncollected holes, whose effect on output signals depends on the location of interaction points. In this work, we investigated the performance of 6x6x15 mm 3 virtual Frisch-grid detectors [2][3][4][5][6][7][8] assembled in 4x4 detector arrays with a common cathode-readout for correcting pulse-height and rejecting incomplete charge-collection events. This type of detector provides excellent energy resolution, <1.5% FWHM at 662 keV, and large effective areas. ...
... The special resolution provided by the array (determined by the cross-sectional area of the bars) is suitable for using such arrays as detection planes in coded-aperture telescopes. As we demonstrated previously [6][7][8], an energy resolution between 0.8-and 1.5% -FWHM at 662 keV can be achieved with regular spectroscopic-grade CZT material; this is about twice the resolution obtained for 3D devices, 0.6%, made from the pre-selected crystals. This resolution is adequate for resolving most gamma-ray spectra. ...
We present our new results from testing 15-mm-long virtual Frisch-grid CdZnTe detectors with a common-cathode readout for correcting pulse-height distortions. The array employs parallelepiped-shaped CdZnTe (CZT) detectors of a large geometrical aspect ratio, with two planar contacts on the top and bottom surfaces (anode and cathode) and an additional shielding electrode on the crystal's sides to create the virtual Frisch-grid effect. We optimized the geometry of the device and improved its spectral response. We found that reducing to 5 mm the length of the shielding electrode placed next to the anode had no adverse effects on the device's performance. At the same time, this allowed corrections for electron loss by reading the cathode signals to obtain depth information.
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We detail our new results from testing an array of 15-mm long virtual Frisch-grid CdZnTe detectors with a cathode signal readout-scheme intended to improve spectral response by correcting for electron trapping. We designed a novel electrode configuration for these long-drift detectors that ensures an energy resolution close to the statistical limit, and high detection efficiency. However, in reality, the quality of the crystals limits the performance of this type of device. Here, we describe the characterization of the array, show our preliminary results obtained with gamma-ray sources, and expound on their relation to our material-characterization data.
... Fig. 1 is a schematic drawing of a virtual Frisch-grid CZT detector. The geometry of its electrodes [7] includes the features of two designs previously proposed for parallelepiped-shaped CZT detectors: the CAPture device [8][9], and the Frisch-ring (or capacitive Frisch-grid) detector [6][11]. The configuration of the cathode's electrode cap is similar to that in the CAPture device, but extends only 2 mm on the side surface. ...
Cadmium Zinc Telluride (CdZnTe or CZT) is a very attractive material for using as room-temperature semiconductor detectors, because it has a wide bandgap and a high atomic number. However, due to the material's poor hole mobility, several special techniques were developed to ensure its suitability for radiation detection. Among them, the virtual Frisch-grid CZT detector is an attractive option, having a simple configuration, yet delivering an outstanding spectral performance. The goal of our group in Brookhaven National Laboratory (BNL) is to improve the performance of Frisch-ring CZT detectors; most recently, that effort focused on the non-contacting Frisch-ring detector, allowing us to build an inexpensive, large-volume detector array with high energy-resolution and a large effective area. In this paper, the principles of virtual Frisch-grid detectors are described, especially BNL's innovative improvements. The potential applications of virtual Frisch-grid detectors are discussed, and as an example, a hand-held gamma-ray spectrometer using a CZT virtual Frisch-grid detector array is introduced, which is a self-contained device with a radiation detector, readout circuit, communication circuit, and high-voltage supply. It has good energy resolution of 1.4% (FWHM of 662-keV peak) with a total detection volume of ~20 cm3. Such a portable inexpensive device can be used widely in nonproliferation applications, non-destructive detection, radiation imaging, and for homeland security. Extended systems based on the same technology have potential applications in industrial- and nuclear-medical-imaging.
We discussed the design implementation and results from testing 2 × 2-, 3 × 3-, and 2 × 4-arrays of 6×6 ×15 mm3 CdZnTe virtual Frisch-grid detectors. In these measurements we employed a data acquisition system based on the H3D ASIC developed by BNL's Instrumentation Division in collaboration with the University of Michigan for 3D position-sensitive detectors. We used CZT crystals with a range of performance attributes to evaluate practical array configurations and detector-assembling procedures. The detector ratings were assigned based on the pulse-height spectra and correlated with data from X-ray diffraction topography measurements and X-ray response mapping obtained at BNL's National Synchrotron Light Source. The results helped us to better understand the performance limits of these detectors, and to identify future improvements in the array's design and requirements for the new readout ASIC.
Large-volume CdZnTe (CZT) single crystals with electron lifetimes exceeding 10 recently became available commercially for making large effective area gamma-ray arrays. However, significant variations were observed in the performance of detectors made from such arrays. We evaluated the spectroscopic performance of such a large-volume, 20 2015 mm , coplanar-grid (CPG) CdZnTe detector, intended for use in a handheld radioisotope-identifier, which unexpectedly showed a poor spectral performance with an energy resolution of 3.5–4% FWHM measured with 662-keV gamma-rays. To understand the factors affecting its performance, we applied several characterization techniques, viz., white X-ray diffraction topography measurements, IR microscopy, and micron-spatial resolution X-ray mapping. They allowed us to identify major crystal defects in the device that limit its energy resolution and detection efficiency.
Large-volume CdZnTe (CZT) single crystals with electron lifetime exceeding 10 mus have recently become commercially available. This opened the opportunity for making room temperature CZT gamma-ray detectors with extended thicknesses and larger effective areas. However, the extended defects that are present even in the highest-quality material remain a major drawback which affects the availability and cost of large CZT detectors. In contrast to the point defects that control electron lifetime and whose effects on the charge collection can be electronically corrected, the extended defects introduce significant fluctuations in the collected charge, which increase with a crystal's thickness. The extended defects limit the uniformity in the electrons' drift distance in CZT crystals, above which electron trapping cannot effectively be corrected. In this paper, we illustrate the roles of the extended defects in CZT detectors with different geometries. We emphasize that the crystallinity of commercial CZT materials remains a major obstacle on the path to developing thick, large-volume CZT detectors for gamma-ray imaging and spectroscopy.
