Design Considerations and Testing of Virtual Frisch-Grid ... - IEEE Xplore

IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 60, NO. 4, AUGUST 2013

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Design Considerations and Testing of Virtual Frisch-Grid CdZnTe Detector Arrays Using the H3D ASIC Aleksey E. Bolotnikov, Member, IEEE, Jamie Butcher, Giuseppe S. Camarda, Yonggang Cui, Member, IEEE, Gianluigi De Geronimo, Member, IEEE, Jack Fried, P. M. Fochuk, Anwar Hossain, Member, IEEE, Kihyun H. Kim, Member, IEEE, O. V. Kopach, G. Mahler, Matthew Marshall, B. McCall, Matthew Petryk, Emerson Vernon, Ge. Yang, and Ralph B. James, Fellow, IEEE

Abstract—We discussed the design implementation and results from testing 2 2-, 3 3-, and 2 4-arrays of 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 pulseheight 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. Index Terms—CdZnTe, charge-loss correction, crystal defects, virtual Frisch grid detectors.

I. INTRODUCTION

T

HE presence of extended defects in today’s commercial CdZnTe (CZT) material restricts the size and effective area of individual CZT detectors. To overcome this problem we proposed a robust and low-fabrication-cost array of virtual Frisch-grid detectors that can be used in hand-held and portable systems for imaging and spectroscopy of gamma rays [1]–[6]. The overall energy resolution of the proposed array is FWHM at 662 keV (typically 1.3%) which is times worse [7], [8]. Howthan that of the 3D pixelated detectors, ever, the design’s simplicity and possibility of using readily Manuscript received January 02, 2013; revised May 16, 2013; accepted July 14, 2013. Date of publication August 06, 2013; date of current version August 14, 2013. This work was supported by U.S. Department of Energy, Office of Nonproliferation and Verification Research & Development, NA-22, U.S. Defense Threat Reduction Agency (DTRA) and BNL’s Technology Maturation Award. The manuscript has been authored by Brookhaven Science Associates, LLC under Contract DE-AC02-98CH1-886 with the U. S. Department of Energy. A. E. Bolotnikov, G. S. Camarda, Y. Cui, G. De Geronimo, J. Fried, A. Hossain, K. H. Kim, G. Mahler, M. Petryk, E. Vernon, G. Yang, and R. B. James are with Brookhaven National Laboratory, Upton, NY 11793 USA (e-mail: [email protected]). J. Butcher is with Geneseo University, Geneseo, NY 14454 USA. P. M. Fochuk and O. V. Kopach are with Chernivtsi National University, Chernivtsi 58000, Ukraine. M. Marshall is with the University of New Mexico, Albuquerque, NM 87131 USA. B. McCall is with Alabama A&M University, Huntsville, AL 35810 USA. Digital Object Identifier 10.1109/TNS.2013.2274054

available CZT crystals make such arrays very attractive in applications with slightly relaxed requirements on energy and spatial resolution. We note that it is possible to achieve energy resolution by using detectors fabricated from selected crystals. Nevertheless, our goal is to stay within 1.5% and use unselected CZT crystals as much as possible; such crystals (containing some defects) are less expensive and readily available for making practical CZT detectors today [9]. barIn the present array we employed shaped crystals; however, as we demonstrated in the past, longer bars, can also be used. The detectors, such as only restraining factor of using such detectors is the availability of 20-mm long crystals with acceptable performance, which is less than that of 15-mm-long ones. The spatial resolution of the array in the XY plane is deterin this case, which mined by the size of the crystals, is suitable for coded-aperture telescopes. However, with extra readout channels and slightly modified detectors design, we potentially could achieve sub-millimeter spatial resolution. In the Z direction (interaction depth) we expected a spatial resolution by using the drift times of the electron clouds proof vided by the ASIC. The detection sensitivity, , (the minimum detectable source intensity or flux) is the most critical parameter in many practical applications. It scales as follows:

(1) where is the energy resolution, and is the detector’s effective area. Compared to an array of 3D detectors [7], [8], the energy resolution of 1.3% is about twice as large than what is reported for the best 3D pixelated detector array, viz., 0.6%, which gives only a 1.4 factor of sensitivity loss that can be regained by making larger-area detectors. For this energy resolution, effective area is the dominant factor for increasing the device sensitivity. The virtual Frisch-grid detectors used in this array utilize a non-contacting (capacitive) shielding electrode originally proposed by McGregor et al. [10] and Montémont et al. [11]. We employed long CZT crystals to minimize the region between the virtual Frisch grid and the anode (charge induced region); events taking place in this region contribute to the continuous background in the pulse-height spectrum. Another important

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Fig. 1. Schematics of the 2

2 sub-array with a common cathode.

feature is that we grouped detectors to form 2 2-, 3 3-, or 4 4-sub-arrays operating with a common cathode [1]; because of the mutual shielding effect of many anodes, we were able to reduce the width of the shielding electrode down to 4 mm without compromising the anode screening efficiency from the holes. On the other hand, the signals read out from the common cathode, which now is sensitive to the interaction over the entire crystal thickness, allowed us to measure the electron’s drift time and interaction depth; both parameters are vital in correcting for electron trapping and in rejecting incomplete charge collection (ICC) events including those interacting inside the charge-induced region [1], [12]. Previously we evaluated our first 2 2 array prototype with the common cathode [1]. The main purpose of these studies was to evaluate likely practical-array configurations and detector-assembling procedures. Accordingly, we fabricated and tested several detector-array housings that we populated with detectors grouped into 2 2, 3 3, and 2 4 arrays using a readout system based on the fourth generation of H3D ASIC developed by BNL’s Instrumentation Division in collaboration with the University of Michigan [7], [13], [14]. We also demonstrated capability of the ICC events algorithm in the case of the multiple interaction point/detector events. Previously, we demonstrated it in the case of the single-detector events [1].

carries the small decoupling capacitor and resistor. Narrow contacting strips at the bottom of the holder connect the shielding electrodes of all the detectors to ground. Each CZT detector is encapsulated inside an ultra-thin polyester shell enveloping the spring contacts on the detector’s top- and bottom-surfaces. The spring contacts cover the entire surfaces of the detector’s cathodes and anodes to prevent damaging of the crystals’ surfaces when the springs are compressed. This configuration assures the facile replacement of individual detectors, whilst conferring resistance to any shocks or external impacts. The spring contacts preclude the need to use conductive epoxy to connect the detectors. The backside of the substrate is used for mounting the ASIC chips and other electronic components. If necessary, the bottom surface of the fanout with ASIC chips also can accommodate a Peltier cooling-plate mounted directly on the substrate. Thin Teflon-insulated wires connect the common cathodes to the sockets on the fanout substrate. Each wire passes through the tiny gap between the corners of the four adjacent detectors, and is soldered directly to the decoupling capacitor. One of the goals of these measurements was to verify that potential cross-talks between the cathode wires and the anodes have no effect on the array performance. We tested several prototypes of the array’s housing made from different plastic materials and with slightly different designs. These tests helped us to find the optimal detector configurations within the array. In addition, these tests helped us to answer the following questions: (1) Are there any interferences (cross-talk) between the cathode wires and the detector’s anodes and between the adjacent common cathodes; (2) How many detectors can be arranged with a single cathode before exceeding the maximum allowable leakage current; (3) How reliable are the spring contacts; and, (4) What is the optimal bias applied to the common cathodes. B. Detectors

II. EXPERIMENTAL A. Array Design There are several requirements to making the robust and inexpensive array: (1) The array must be assembled with minimum spaces between the detectors (ideally, the detectors should be mounted side-by-side with no gaps in between them); (2) All readout electronics (for the cathodes and anodes) should be placed behind the detectors to avoid attenuation of incident gamma rays; (3) The design of the array should offer an easy way to replace any individual detector, without breaking its structural integrity (avoid using permanent epoxy); and, (4) The readout electronics, with a front-end ASIC, should be mounted on the opposite side of the fanout substrate used for mounting detectors. Fig. 1 illustrates a schematic of our current design. A thinwall honeycomb plastic holder is attached firmly to the fan-out substrate. The detectors, each furnished with two flat spring contacts, are slid inside the holder’s cells where they fit tightly, and then gently pressed to the substrate using a thin conductive plate (or board) placed on the top of them. This plate also connects together the cathodes of the detectors within a sub-array, and

As stated before, the main purpose of these measurements was to test the design of the array and assembling procedure; we were less concerned about achieving its best performance which primarily depends on the quality of the crystals used to fabricate the array. Thus, we used 9 detectors remaining from the batch of the detectors originally supplied by Endicott Interconnects for previous projects. The bulk resistivity and the electron product of the material used to fabricate these detectors, as specified by the vendor and independently verified by us, were and , respectively. Despite such outstanding material properties, not all of the detectors showed good spectral performance. Some fraction of the detectors could not be used because of excessively high total leakage currents attributed either to leaky side surfaces or to conductive subgrain boundaries propagating across a detector between the cathode and anode contacts. We note that by polishing and etching side surfaces of such detectors often reduces the leakage current. The main reason of poor spectral responses is a presence of randomly distributed subgrain boundaries (or their networks) which neither can be controlled during the growth nor revealed during the detector fabrication by

BOLOTNIKOV et al.: DESIGN CONSIDERATIONS AND TESTING OF VIRTUAL FRISCH-GRID CdZnTe DETECTOR ARRAYS

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TABLE I CRITERIA OF CRYSTAL RATING

Fig. 3. A low-grade detector illustrating the same parameters as those in Fig. 2. The response map was taken with a 50-mm step size.

Fig. 2. Example of a top-rated detector: (a) uncorrected pulse-height spectrum, (b) white X-ray beam diffraction topograph, (c) and (d) X-ray response maps. In (d), the XY axis represents the step numbers (30-mm step size in this case), while Z represents the amplitude of the signal in channels.

vendors [8]. Normally, we employ white X-ray beam diffraction topography to reveal subgrain boundaries [15] but chemical etching of CZT surfaces can also be used [16]. It is practically impossible to predict accurately the extent of the subgrain boundaries effects on detector performances. Our studies indicate a two-fold role of subgrain boundaries. Dislocations that form subgrain boundaries accumulate impurities and secondary phases (primarily Te precipitates) leading to very inhomogeneous distribution of trapping centers for carriers which, in turn causes fluctuations in the collected charge signal. These fluctuations depend on the spatial distribution and orientation of subgrain boundaries and concentrations of trapping centers inside them (access of trapping centers above their average level in CZT bulk). The second effect of subgrain boundaries is variations of the local electric field; this effect is particularly important in detectors with large geometrical aspect ratios such as the virtual Frisch-grid detectors in which a nonparallel electric field can cause strong edge effects (charge trapping near side surfaces). We rated the quality of these detectors as 3 and 4 on a scale of 1 to 5, except for one detector with a very poor performance and thus, a rating of 2. Table I summarizes our typical criteria

Fig. 4. An acceptable detector (rating 4) illustrating the same features as those in Fig. 2. The response map was taken with a 50-mm step size.

for rating the CZT crystals. As an example, Figs. 2–4 show the pulse-height spectra, white X-ray beam diffraction topographs, and X-ray response maps measured for three detectors with ratings of 5, 2, and 4, respectively. Nine detectors were sufficient to assemble and test three arrays: 2 2 and 3 3 with single common cathodes, and 2 4 with two common cathodes. The total number of detectors in a sub-array is limited by the maximum leakage current tolerated by the cathode channel of the ASIC, viz., up to 100 nA in this case. Table II shows a map of the measured leakage currents of nine detectors from the 3 3 array at a 2500-V applied bias and at a temperature of . These data demonstrate that even a 4 4 array also can function satisfactorily with a single common cathode.

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TABLE II DETECTOR LEAKAGE CURRENT(NA) AT-2500 V CATHODE BIAS

Fig. 5. Assembling steps of the 3 3 array: (a) crystal encapsulated in the ultra-thin polyester shell; (b) encapsulated detector furnished with two spring contacts and a 4-mm-wide shielding electrode (aluminum tape with a thin conducting adhesive layer); (c) assembled detectors inside the cells of the plastic housing mounted on the fanout substrate. Table II contains the leakage currents (nA) measured for the 9 detectors from the 3 3 array at and at room temperature, .

C. Fabricating Detectors and Assembling Arrays We encapsulated the CZT bar crystals inside an ultra-thin polyester shell with two flat CuBe spring contacts1 placed on their top and bottom surfaces in the ultra-thin polyester shells (Fig. 5(a))). The shell evenly envelopes the edges of the spring contacts and slightly presses them to the platinum electrodes of the crystals (Fig. 5(b))). A 4-mm wide shielding electrode (aluminum tape with a thin conducting adhesive layer) was wrapped around the detector’s sidewalls near the anode. The assembled detectors were slid inside the cells of the plastic housing mounted on the fanout substrate and then gently pressed in from the common cathode side (Fig. 5(c))). Table II in Fig. 5 shows the values of the total (bulk and surface) leakage currents (nA) measured at room temperature, , for each detector from the 3 3 array at bias applied on the common cathode. D. Readout System Each array under test was plugged into the detector board and enclosed inside the test box containing a readout system based on the H3D ASIC. In these measurements, we chose the same peaking time to shape the output signals read out from the cathode and anodes. During the measurements, the test box with electronics and detectors was placed inside an environmental chamber that maintained the temperature of the detectors at . We note that without cooling, a single ASIC generated 1FOTOFAB,

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enough heat to raise the temperature of the CZT detectors inside the tightly closed up box to . The array was irradiated with uncollimated , , and -sources placed above the cathodes. The cathode’s bias was set at 2500 V. We used the full readout mode of the ASIC meaning that any of the detectors from the array could initiate the readout sequence of the measured signals. For each interaction event, the ASIC generates a stream of data containing the amplitudes and drift times of each electron cloud generated inside the detectors. The data stream was saved in a file in the computer’s memory and then analyzed off-line. The fourth generation of H3D ASIC used in these measurements demonstrated excellent performance in amplitude and time measurement, low trigger threshold, , and low electronic noise, FWHM. However, we noticed some limitations of the ASIC (when used with CZT detectors) related to its design’s architecture implementing the analog processing of the input signals. One of them was the excessive spread of the drift times measured for some of the detectors. We traced the origin of this time jitter to uncertainties in the measurements of the starting times of the cathode signals. By correlating the drift times evaluated by the analog and digital processing of the cathode signals, we found that an approach based on using fast shaping and threshold crossing to determine the drift times is not accurate for CZT detectors because of local nonuniformities of the electric field; digitizing of the cathode signals would afford better accuracy in the drift-time measurements. This approach should be considered for the next generation of readout ASICs. These measurements also allowed us to identify the requirements for the new ASIC, which we will develop specifically for virtual Frisch-grid arrays. III. RESULTS AND DISCUSSION Three array configurations were tested: — 2 2 and 3 3 arrays with single common cathodes, and — 2 4 array with two common cathodes. The results from testing the 2 2 array were similar to those previously obtained from our first array prototype [1] and will not be discussed further here except for noting the fact that we did not observe any interference between the cathode wire and the detectors’ anodes. The results from testing the 2 4 and 3 3 arrays are presented next. A. Results From Testing a 2 Cathodes

4 Array With two Common

Fig. 6 schematically illustrates the 2 4 array with two common cathodes. The cross-talk between the common cathodes potentially can cause incorrect determination of the cathode’s amplitudes that are used in evaluating the interaction depth and drift time. Figs. 7(a), 7(b) shows the pulse-height spectra measured from a source for all the detectors before and after applying the charge-loss corrections and rejecting ICC events. As shown after these corrections, the energy resolution for all the detectors, except one, lays in a range of 1.2–1.7% FWHM at 662 keV; this is an indication that the cross-talk between the cathode wires and the cathode has no impact on the detector’s spectral


Fig. 6. Schematics of 2

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4 array with two common cathodes. Fig. 8. Correlations between cathode signals: (a) Interactions take place in any of four detectors located apart from each other (3, 4, 7, and 8); (b) interactions take place in adjacent detectors (1, 2, 5, and 6).

Fig. 9. Application of charge-loss corrections for a 3 3 array with the common cathode (only 3 detectors are shown): (a) Raw spectra, (b) corrected spectra based on the drift times, (c) corrected spectra based on the cathode signals (interaction depths), and, (d), and (e) dependencies of the anode’s amplitudes versus the drift times and the cathode amplitudes, respectively. Fig. 7. Pulse-height spectra measured from a 137Cs source for all the detectors from the 2 4 array before and after applying charge-loss corrections and rejecting ICC events.

responses. We discuss these findings in detail in the next sections where we describe the results from testing the 3 3 array. Figs. 8(a), 8(b) illustrates the correlations between the cathode signals when interactions take place in: (a) any of the four detectors separated from each other 3, 4, 7, and 8 and (b) any of the adjacent detectors 1, 2, 5, and 6 (see Fig. 6 for reference). No cross-talk is seen in the first case (a). In the second case (b), an interaction in the detector of one group causes a small induced signal on the cathode of the second group. The impact of this cross-talk seems to be insignificant but more careful measurements are required to confirm this conclusion. B. Results From Testing a 3 Cathode

3 Array With a Single Common

Our first task was to demonstrate the application of chargeloss correction techniques for a 3 3 array with a common cathode. As illustrated in Fig. 9, corrections to both the drift time and interaction depth equally can be applied in the cases where the electric field inside the detector is uniform and, hence, the drift time is a linear function of the interaction depth. The first three columns in Fig. 9 represent the raw and corrected data based on the drift time and interaction depth pulse-height

spectra, correspondingly, plotted for three representative detectors. The fourth and fifth column show the dependency of the amplitude of the anode signal versus drift time and the amplitude of the cathode signal. The solid lines represent the fitted curves used to correct the anodes’ amplitudes. Fig. 10 shows the correlations between the anode signals readout from any two detectors whose amplitudes exceed noise thresholds. In these plots, the photopeak events follow straight lines as would be expected for multiple interaction events with no charge sharing. We note that for pixel detectors, such lines would have curvature indicating the signal losses. The Compton edges clearly are evident in these plots. The possibility of rejecting ICC events including events interacting near the anode (inside the induction region) is another important feature of the proposed array employing unselected CZT crystals containing some defects. Fig. 11 demonstrates the improvements of the raw pulse-height spectra (a) measured from nine detectors of the 3 3 array after applying the charge-loss correction and rejecting ICC events from single interaction point events (b). All the spectra are plotted using the same scale to emphasize the detectors’ different photopeak efficiencies. The corrected spectra show good or acceptable responses (except for one of them) with energy resolution in the range 1.0-1.5% FWHM at 662 keV. The numbers in the top left-hand corners indicate the detectors’ ratings that we assigned

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Fig. 10. Correlations between the anode signals readout from a particular detectors (vertical axis) and other detectors (horizontal axis) from the 3 3 array in which the amplitudes exceed the noise thresholds.

based on their spectral resolution and relative photo-peak efficiency. In fact, all spectra look similarly when normalized to the same height of the photopeaks as illustrated in Fig. 12, but the detectors have different photo-peak efficiencies; the numbers in the top left corners indicate the relative number of events under the photopeaks in percentages of the number found for the best detector. The reduction in photopeak events is caused by defects in the crystal that “move” ICC events from a photopeak area into the Compton continuum. Such events can be identified and rejected from the pulse-height spectra, so peak-to-Compton ratios stay the same for all detectors. In other words, we effectively reduce the detectors’ sensitive volumes by vetoing the defective regions inside which otherwise contribute to the continuous background. On the other hand, as seen in Fig. 12, the pulse-height spectra plotted before (top lines) and after rejection (bottom lines) have the same heights meaning the rejection affects ICC events only. The rejection of ICC events allows us to employ practically every detector (without compromising an overall array’s performance) as long as it provides an acceptable energy resolution even though only a fraction of the crystal is actually used. The array’s effective area can be maintained by selecting detectors with smaller fractions of defective regions or using a larger number of less expensive and easily available detectors; it is a tradeoff between the cost and effective area. The feasibility of employing the ICC events algorithm was demonstrated previously for single interaction point-events [1]. Here, we validated the algorithm for single- and multiple-interaction point events taking place in two or more detectors. For completeness, first we illustrate the rejection algorithm in the case of single-interaction point events. The first column in Fig. 13 shows the pulse-height spectra from the source after the charge-loss correction for three representative detectors. The second and third columns,

Fig. 11. Improvements of the raw pulse-height spectra (a) measured from nine detectors of the 3 3 array after applying the charge-loss correction and rejecting ICC events from single-interaction point events (b). All the spectra are plotted using the same scale to emphasize the detectors’ different photopeak efficiencies. The numbers in the top left corners indicate the detectors’ ratings that we assigned based on their relative photoefficiency.

respectively, show spectra containing only accepted and rejected events. The last column contains scattered plots of all events in coordinates: Ratio of cathode to anode, , versus drift times, . As is evident, the majority of the events (dots) concentrate along the curves whose shapes slightly vary from one detector to another. These curves, which we call the correlation functions, are evaluated by selecting the photoabsorption events that are not affected by the crystal’s defects, and fitting their distributions with spline functions (the solid lines going in the middle of the dots distributions in Fig. 13). Two parallel lines represent the wide acceptance intervals; all events outside these intervals were rejected. As


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Fig. 14. Same distributions as in Fig. 13 but plotted for the low-energy gamma source. rays from the

Fig. 12. Pulse-height spectra from nine detectors of the 3 3 array plotted before and after rejecting ICC events. The top lines represent spectra before rejecting.

Fig. 13. Illustration of the ICC events-rejection algorithm. The first column source after the charge-loss shows the pulse-height spectra from the correction for three representative detectors. The second and third columns, respectively, show the spectra containing only accepted and rejected events. The last column contains scatter plots of all events in coordinates: Ratio of cathode , versus drift times, . to anode,

seen, the majority of the rejected events are from the Compton continuum region while the photopeaks remain uncut. The same correlation functions, , also can be used to reject the ICC events over the entire energy range starting from very low energies. Fig. 14 shows similar plots as in Fig. 13 but for the events generated by the low-energy photons from the source. Again, as previously, we illustrate the results only from three detectors. As is evident, events unaffected by the crystal defects follow the same correlation curves used in the previous case (Fig. 13). Also apparent are the small event losses from the low-energy photopeaks, . Thus, in the case of the low-energy gamma-lines, we need to modify the algorithm.

Fig. 15. Same distributions as in Figs. 13 but plotted for multiple interactionpoint events: (a) two points of interactions; (b) three points of interactions.

For events with multiple interaction points, the adjusted ratio, must be used instead of [12]:

(2)

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where is the correlation function of the single-interaction point event evaluated for each of the detectors, while and stand for the amplitudes measured from the detector and the cathode. Figs. 15(a) and 15(b), respectively, show pulse-height spectra similar to those in Figs. 13 and 14 but plotted for two (a) and three (b) interaction-point events. As follows from these plots, the algorithm works well for multiple interaction point events. We note that the rejection algorithm also should be applicable for pixelated detectors. IV. CONCLUSIONS We validated our design for arrays of virtual Frisch-grid detectors and tested the performance for 2 2, 2 4 and 3 3 arrays with common cathodes. The arrays were tested with the currently existing readout system based on the H3D ASIC, which provides information on the amplitude and timing of interaction events. A new ASIC optimized for the array is currently under development. It will retain the same functionality to provide both the amplitude and timing information as the original H3D ASIC. Amplitude and timing information are critical for the detector’s response corrections and achieving the best performance from currently available CZT crystals. The key elements of the array design are simplicity and the ability to easily replace individual detectors in the array. For this reason we used Cu-Be spring-loaded contacts (instead of using conductive epoxy) to provide electrical connections for the cathodes, anodes and shielding electrodes. We also validated the algorithm for rejecting incomplete charge-collection events for single- and multiple interactionpoint events, which will allow for additional improvements of the array’s spectral response. REFERENCES [1] A. E. Bolotnikov, G. S. Camarda, Y. Cui, G. D. Geronimo, J. Fried, R. Gul, A. Hossain, K. H. Kim, E. Vernon, G. Yang, and R. B. James, “Array of virtual Frisch-grid CZT detectors with common cathode readout for correcting charge signals and rejecting incomplete charge-collection events,” IEEE Trans. Nucl. Sci., vol. 59, no. 4, pp. 1544–1551, Aug. 2012. [2] J. K. Polack, M. Hirt, J. Sturgess, N. D. Sferrazza, A. E. Bolotnikov, S. Babalola, G. S. Camarda, Y. Cui, S. U. Egarievwe, P. M. Fochuk, R. Gul, A. Hossain, K. Kim, O. V. Kopach, L. Marchini, G. Yang, L. Xu, and R. B. James, “Variation of electric shielding on virtual Frisch-grid detectors,” Nucl. Instrum. Methods Phys. Res. A, vol. 621, pp. 424–430, 2010. [3] A. E. Bolotnikov, N. M. Abdul-Jabbar, S. Babalola, G. S. Camarda, Y. Cui, A. Hossain, E. Jackson, H. Jackson, J. R. James, A. L. Luryi, and R. B. James, “Optimization of virtual Frisch-grid CdZnTe detector designs for imaging and spectroscopy of gamma rays,” in Proc. SPIE Hard X-Ray and Gamma-Ray Detector Physics VIII, Invited Paper, L. A. Franks, A. Burger, R. B. James, H. B. Barber, F. P. Doty, and H. Roehrig, Eds. Bellingham, WA, USA: SPIE, 2007, vol. 6702.

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