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Array of Virtual Frisch-Grid CZT Detectors With Common Cathode Readout for Correcting Charge Signals and Rejection of Incomplete Charge-Collection Events A. E. Bolotnikov, Member, IEEE, J. Butcher, G. S. Camarda, Y. Cui, Member, IEEE, G. De Geronimo, J. Fried, R. Gul, P. M. Fochuk, M. Hamade, A. Hossain, Member, IEEE, K. H. Kim, Member, IEEE, O. V. Kopach, M. Petryk, E. Vernon, G. Yang, and R. B. James, Fellow, IEEE
Abstract—New results from testing an array of 6 6 15 mm virtual Frisch-grid CdZnTe (CZT) detectors with common-cathode readout for charge signals correction and rejection of incomplete charge collection events (ICC) are presented. The array employs parallelepiped-shaped crystals of a large geometrical aspect ratio with two planar contacts on the top and bottom surfaces (anode and cathode) and an additional shielding electrode placed on the sides to create the virtual Frisch-grid effect. The detectors are arranged in 2 2 or 3 3 detector modules with the common cathode readout by a single electronic channel. Because of the common cathode, the length of the shielding electrode can be further reduced with no adverse effects on the device performance. By implementing a novel technique for rejecting ICC events caused by the extended defects, we can achieve good spectral responses from ordinary CZT crystals, which can be produced with higher yield and at lower cost. For such crystals, the resolution of individual detectors is expected to be in the range of 0.8–1.5% FWHM at 662 keV with an average value of 1.3%. Arrays of virtual Frisch-grid detectors offer a robust and low-cost approach for making large-area detection modules that can potentially substitute for more advanced, but also more expensive and less available, pixel detectors in applications with slightly relaxed requirements on position- and energy-resolution (e.g., for coded aperture telescopes). In addition, such virtual Frisch-grid arrays will require a comparably smaller number of readout channels, which allows for lower power consumption. Index Terms—CdZnTe, crystal defects, virtual Frisch-grid detectors.
I. INTRODUCTION
T
HE high cost and low availability of CdZnTe (CZT) crystals suitable for large-volume detectors having an energy resolution of 1% FWHM at 662 keV motivated the develop-
Manuscript received December 02, 2011; revised January 12, 2012 and February 06, 2012; accepted February 07, 2012. Date of publication March 15, 2012; date of current version August 14, 2012. This work was supported in part by the U.S. Department of Energy, Office of Nonproliferation Research and Development, NA-22, 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, R. Gul, A. Hossain, 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 from Chernivtsi National University, Chernivtsi, Ukraine. M. Hamade is with Stony Brook University, Stony Brook, NY 11794 USA. Digital Object Identifier 10.1109/TNS.2012.2187932
ment of CZT arrays consisting of small but higher yield acceptable crystals that would provide comparable functionality and performance. Recently, we proposed to develop a position-sensitive array consisting of 6 6 15 mm virtual-Frisch-grid detectors for spectroscopy and gamma-rays imaging [1]. This compact, robust, and low-cost system will provide a good energy resolution, % (typical 1.3% FWHM at 662 keV) and a large effective area. When coupled to the coded aperture mask, it can be used as a compact gamma-ray imaging camera in many areas of applications. Today, there is a very limited supply of commercial CZT crystals suitable for making large detectors, e.g., 20 20 10 and 20 20 15-mm-thick 3-D detectors [2], [3]. However, small cross section area, 6 6 mm , but long detectors, over 15 mm in length can be easily fabricated from relatively thin, 7-mm long, CZT wafers that also allow for better screening and much more flexibility in cutting. Another advantage of the proposed array is its capability to correct continuous charge-losses due to trapping and to reject incomplete collection-charge (ICC) events caused by crystal defects. These features allow for use of off-the-shelf crystals with moderate requirements to their quality. The yield of acceptable crystals determines the cost of a CZT device. The crystals with little or no major defects come at the greatest cost. The major advantage of the array is that it can be built of average quality crystals with a higher content of extended defects. Moreover, if we raise the acceptable level of the energy resolution up to 1.5%, which is adequate for most isotope identification applications, this drastically increases the yield and reduces the cost of the crystals. By implementing a novel technique [4] for rejecting the ICC events, we will still be able to achieve good spectral responses from the typical commercial crystals. For such crystals, the resolution of individual detectors is expected to be in the range of 0.8–1.5% FWHM at 662 keV with an average value of 1.3%. In the past, a number of techniques has been proposed for rejecting the events affected by crystal defects to improve the spectral responses of CZT detectors. These techniques (see e.g., [5] and references therein) rely on the digitization of the output signals and digital pulse-shape analysis. In contrast, our technique [4] uses a robust and simple merit function to reject the
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ICC events based on the amplitudes of the signals and the drift times only. Such information is provided by the 3-D ASIC. It should be noted that the use of these low-cost CZT crystals with many defects will inevitably result in some loss of their active volumes. Based on testing over 50 detectors, we estimate that the average effective volume of the 6 6 15 mm devices made of the typical quality material would be in the range of 50–90% of their total geometrical volumes. However, efficiency losses can be offset by making larger area arrays of such low-cost and readily available crystals. Furthermore, rejection of the ICC events is equivalent to vetoing “bad” regions of the crystals, meaning that the background noise caused by the Compton scattering inside the “bad” regions could be significantly suppressed. The array consisted of easily replaceable virtual Frisch-grid detectors (bars) encased in a honeycomb-shaped plastic case with a 0.5-mm-thick wall. The case was mounted onto the motherboard with spring load connectors for each detector. We grouped the detectors in a 2 2 (or 3 3 depending on the total leakage current) sub-arrays having a common cathode. To read the anode and cathode signals, we used a modified ASIC and data acquisition system originally developed by BNL’s Instrumentation Division for 3-D pixel detectors [6], [7]. The 64-detector array utilized 64 anode channels and 16 cathode channels for a total of 80 channels. This number is considerably smaller than the total number of channels that would have been required for the 3-D detectors when total volume is equal to the array (4 428 versus 80). Without cooling, the 3-D ASICs generate enough heat to raise the temperature of the CZT detectors up to 40 C. For example, 18 3D-ASICs are required for Polaris detectors with a total volume of 108 cm , while a single ASIC will be sufficient for a 64-detector array with a total volume of 207 cm . Taking 2 mW per channel [6], [7], the dissipated power would be 4.4 and 0.16 W, respectively, with considerable savings in cooling requirements and more importantly, in power dissipation. The virtual Frisch-grid arrays require a relatively smaller number of readout channels, which allows for lower power consumption and less heat dissipation per cm of the array. In this paper, we present the results from testing the first prototype of the 2 2 sub-array of 6 6 15 mm virtual Frisch-grid detectors with common-cathode readout for correcting charge signals and rejection of the ICC events. II. EXPERIMENTAL A 2 2 sub-array of 6 6 15 mm virtual Frisch-grid detectors was assembled and tested using an existing data acquisition system developed originally for 3-D devices. A common feature of virtual Frisch-grid devices is a shielding electrode (or several electrodes) that simulates the same effect of a real shielding grid placed inside a detector. There are several types of CZT devices which rely on this effect: CAPture™, hemispherical, Frisch-ring, capacitive Frisch-grid and pixel detectors in which pixel contacts act like shielding electrodes [8]–[13]. The particular features of the virtual Frisch-grid design and the advantages of our approach are summarized in [1]. The parallelepiped-shaped CZT crystals were first encapsulated in the
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Fig. 1. A 6 6 15 mm virtual Frisch-grid detector (a) and a 3D-ASIC compatible interposing substrate (b) supporting four detectors with interconnected cathodes.
ultrathin polyester tubes. Then a 5-mm wide shielding electrode (aluminum tape with a thin adhesive layer) was placed around the sides of the detector. The electrode provides high shielding efficiency of the anode and, at the same time, ensures that the cathode senses events interacting over the entire thickness of the crystal, even those close to the anode. For the 2 2 sub-array we selected four detectors, representing the performance range of 30 individually characterized and tested detectors [1], and mounted them on the interposing substrate compatible to the 3-D readout motherboard. The detectors were fabricated using the high-resistivity, Ohm-cm, high- -product, cm /V, crystals acquired from Endicott Interconnect. Fig. 1 shows one of the virtual Frisch-grid detectors (a) and a 3D-ASIC compatible interposing substrate with four mounted detectors (b). For these tests, the detectors were glued to the substrate’s pads by using the conductive epoxy. The same epoxy was also used to connect the detectors’ cathodes. By using long CZT crystals (up to 20 mm), we minimized a fraction of the events generated near the anode, below the location of the virtual-grid, in which the shielding effect is significantly reduced. Such events contribute to a flat continuum in the pulse-height spectrum extending up to the photopeak and Compton scattering events. In fact, 15 mm is probably the smallest acceptable size for the virtual Frisch-grid detectors to achieve the reasonable peak-to-Compton ratio and small Compton continuum. In addition, long detectors provide a higher stopping power for gamma rays, which will also improve the device spectral responses. We connected together the cathodes of several detectors, and employed the common cathode readout to correct charge losses and reject the ICC events which otherwise increase continua in the pulse-height spectra. The implementation of a correction scheme for electron trapping using the cathode readout is necessary for long-drift detectors. Also, rejecting the events interacting near the anode, which contribute to the continuum in the pulse-height spectra, is important for achieving the high peak-to-Compton ratio. The array was plugged into the detector board located inside the test box originally built for testing and evaluation of the readout ASIC for 3-D position-sensitive detectors [6], [7]. The test box contained readout electronics, FPGA, and USB communications port. The identical 1- s peaking times were used to shape the cathode and anodes output signals [6], [7]. During the measurement, we cooled the entire box with electronics and
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Fig. 2. Dependencies measured for each of the detectors (columns) and used to characterize the performance of the array: the anode versus drift times (first row), the anode versus cathode (second row), the cathode versus drift times (third row) and the ratio of cathode to anode signals versus drift times (fourth row). The versus and versus were measured before applying the charge-loss amplitudes and drift times are given in channels. The dependencies of versus correction. For the versus we used the corrected charge signal normalized to the photopeak position at a channel of 1300. To keep the same scale in the plots, were multiplied by 1300. the ratios of
detectors inside the environmental chamber and took the measurements in a temperature range 10–20 C. Without cooling, a single ASIC generated enough heat to raise the temperature of CZT detectors up to 40 C, which increased the cathode leakage current above the acceptable level. The array was illuminated with the uncollimated Cs source placed 3 cm above the cathode. The cathode bias was 2000 V. We used the full readout mode: any of the four detectors could initiate the readout sequence of the measured signals from all detectors. For each event, the system sends out the amplitudes and drift times associated with the electron clouds generated by an incident photon in all four detectors. It should be mentioned that the ASIC used in these measurements was designed for 3-D position-sensitive detectors. It is a complex and multifunctional device that can support different
operation modes for pixel detectors. To ensure its high performance it requires thoughtful calibrations and additional corrections for measured drift times. Due to the high complexity, in these tests we did not apply such corrections which would have a second-order effect on the drift time measurements. The only correction we implemented here was the correction of charge losses due to continuous trapping. III. RESULTS A. Correlation Curves Fig. 2 shows the dependencies measured for each of the detectors (columns) that characterize the performance of the array: the anode, , versus drift times, , (first row), the anode versus cathode, , signals (second row), the cathode versus drift times
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Fig. 3. Distributions measured for the individual detectors by employing two hybrid preamplifiers and digitizing the output signals with a LeCroy WaveRunner.
(third row) and the ratio, , of cathode to anode signals versus drift times (fourth row). We measured the amplitudes and drift times in channels. The dependencies of vs. vs. and vs. were measured before applying the charge-loss correction. For the vs. , we used the corrected charge signals normalized to the photopeak position at a channel of 1300. The ratio was multiplied by 1300. The points in the above plots represent individual interaction events. The points’ distributions follow the patterns corresponding to the features normally seen in the pulse-height spectra. The photoabsorption and Compton edge events are concentrated along the lines clearly seen in the plots presented in the vs. vs. and vs. plots (first three rows). We note that the dependencies of vs. (third row) exhibit very broad point distributions corresponding to the photoabsorption events in comparison to the analogous distributions (Fig. 3) measured for the individual detectors [1] when employing two hybrid preamplifiers (eV-5093) and digitizing the output signals with a LeCroy WaveRunner. The broadening is caused by some systematic error in the determinations of drift times when using the ASIC readout. The origin of these errors is not fully understood at the moment; it is most probably related to a weak dependence of the measured drift time versus the signal amplitudes which, in general, could be corrected after ASIC calibration. The same effect caused the broadening of vs. (fourth row). The dependencies of vs. and vs. are used to correct the continuous charge loss in CZT detectors. However, one should bear in mind that the continuous charge loss due to point defects (impurities) is proportional to the drift time, while the continuous charge loss due to extended defects (e.g., Te inclusions) is proportional to the distance, and that the measured drift times may fluctuate significantly for the same drift distances. In this analysis we used the dependencies vs. for charge loss corrections. The dependencies vs. and vs. indicate the positions of the virtual Frisch-grids from the anodes; in the cathode-to-grid region (also called drift region) the cathode signal changes linearly with the distance measured from the grid position, while the anode-to-grid region (also called collection region) is shielded from the cathode. Correspondingly, the anode signal remains practically unchanged in the drift region and rises linearly in the collection region. This is a full analog of the classic ionization chamber with a Frisch-grid.
Fig. 4. Raw spectra plotted for all events from each of the detectors in the array.
The dependence vs. relates three correlated variables measured for each interaction event in each detector: the anode and cathode signals and the drift times. In the 2-coordinate space, the points corresponding to the normal events, for which the collected charge signals are affected by the continuous charge lose due to the trapping by impurities, concentrate along the curve (we call it the correlation function, ) within a narrow corridor determined by the electronic noise only, while the ICC events fall outside the corridor. By choosing a certain acceptance band around the correlation curve, it is possible to identify and veto the ICC events, including the edge-affected events and the events interacting in the collection region near the anode. The correlation curve, , can be evaluated by fitting the center of gravity of the distribution vs. for the events under the photopeaks. The results of the fitting procedure applied to each of the detectors are shown with solid lines: two lines defined the 200-channel acceptance band around the middle line representing the correlation curve, . B. Pulse-Height Spectra Fig. 4 shows the raw (without corrections) spectra evaluated for all events measured by each of the detectors. As seen, Detector 1 shows a poor spectral response which can be attributed to the high dislocation density sub-grain boundary (revealed with white beam X-ray diffraction topography) located in the middle of the crystal parallel to its side surfaces. The wide photopeak correlates with the broad dependence of vs. shown previously in Fig. 2; in fact the points representing the photopeak events are concentrated along the two close-located lines seemingly as if two domains exist within a single crystal. Fig. 5 shows the pulse-height spectra after charge loss correction. We employed the dependencies vs. to correct the output signal and normalized the spectra to the same peak position at channel 1300. As expected, Detector 1 shows poor performance, which was not possible to correct: such detectors should not be accepted for making arrays. Detectors 2 and 4 look more typical, while Detector 3 demonstrates an exceptional level of performance with the energy resolution of 0.65% FWHM at 662 keV. We note that the dependencies vs.
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Fig. 5. Pulse-height distributions shown in Fig. 4 after correcting for charge losses. The spectra were normalized to the same peak position at channel 1300.
Fig. 7. Combined spectra after adding together the events from all detectors: (a) all events from spectra in Fig. 5, (b) single-detector events from spectra in Fig. 6, and (c) two-detector events from spectra in Fig. 6.
Fig. 6. Pulse-height distributions shown in Fig. 5 after rejecting the events interacting inside the collection regions near the anodes.
plotted for Detectors 2–4 in Fig. 2 have narrowband distribution of the photoabsorption events, which indicate the low defect concentrations in these crystals. Further improvement of the pulse-height spectra can be achieved by rejecting the events interacting inside the collection region near the anode. Such events primarily contribute to continua and can be rejected by setting a threshold on the drift times. Fig. 6 shows the pulse-height distributions of the events for which the normalized drift times were above 400 channels (as seen from Fig. 2, this gives an approximate location of the virtual Frisch-grid). Spectra in Fig. 6 show notable reductions of continua especially in the low-channel region. The ICC events contributed in the peak-to-Compton valley (the region between the Compton edge and the photopeak) were also reduced. Fig. 7 shows the combined spectra after adding together the interaction events from all four detectors: (a) all events from spectra in Fig. 5, (b) single-detector events from pulse-height distributions in Fig. 6, and (c) two-detector events from Fig. 6. We defined a single-detector event to be when the incident photon interacts in a single detector only. Accordingly, a
Fig. 8. Pulse-height distributions of the two-detector events.
two-detector event means that the incident photon interacts in any two detectors out of four. For the two-detector events the output signals were added together before plotting the combined spectrum. For the single- and two-detector events the overall energy resolutions were found to be 1.1% and 1.3% at 662 keV, correspondingly. To complete the description of the array performance, Fig. 8 depicts the pulse-height distributions of the two-detector events evaluated for each detector. We note that the total number of the events in the spectra of Fig. 8 is about three times greater
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Fig. 9. Pulse-height distributions of the events found within the acceptance bands (accepted events).
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Fig. 10. Pulse-height distributions of the events located outside the acceptance bands (rejected events).
than that in the corresponding combined spectrum in Fig. 7(c), because each two-detector event was accounted for three times. C. Rejecting ICC Events In the case of the single-detector events, rejecting the ICC events is straightforward. We used the normalized distributions vs. , shown in Fig. 2 (last row), to reject all the events outside the acceptance bands of 200 channels around the curve evaluated by fitting the center of gravity of the vs. distribution for the events under the photopeaks. The resulting pulseheight spectra of the accepted events are shown in Fig. 9. keV (channel 200), peaks were As seen, the low energy, completely removed from the spectrum due to noise-related limitations of the ASIC as mentioned in the introduction, incomplete calibration, and not optimal settings of the ASIC. When we tested the detectors with hybrid preamplifiers, no low-energy events were lost [4], [14], at least down to 50 keV. Theoretically, the technique should work for all energies. Using a wider acceptance corridor for the long-drift-time events (low energy events interacting near the cathode) one can offset the uncertainties in the amplitudes and drift times caused by the electronic noise. The main problem related to the noisy signals generated by low-energy photons is that ASIC. This problem needs to be better understood and corrected in the future. To illustrate reduction of the continua in the pulse-height spectra, Fig. 10 shows the distributions of the events located outside the acceptance bands. A notable reduction of the continuum can be seen in the case of Detector 1; for Detectors 2–4 the decrease is less pronounced because of the smaller content of defects in these detectors. In the case of the two-detector events, a direct application of the above algorithm obviously will not work. To illustrate this, we selected the two-detector events used previously to evaluate the pulse-height spectra in Fig. 8 and plotted the ratio of the cathode to the sum of the two anodes signals versus the drift time vs. corresponding to the maximum deposited charge, . As seen in Fig. 11, the plots represent the featurelessdots continua that are useless in rejecting the ICC events. For
Fig. 11. Ratio of the cathode signal, , and the sum of the two anode sigversus the drift time corresponding to the maximum deposited charge, nals, . The solid lines represent the correlation functions evaluated for these detectors.
comparison, the solid lines represent the correlation functions evaluated for these detectors. For the multiple interaction points (detectors) events, we introduced a modified algorithm that takes into account the fractions of the charge signals induced by each electron clouds on the common cathode. It is clear that the fraction of the cathode induced by the charge generated in the ith detector signal is given by (1) where are the correlation functions evaluated for each of the detector and are the measured anode signals. From here, one can find the estimates for the ratios : (2)
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Fig. 12. Distributions vs. evaluated for two-detector events using (2). In contrast to the distributions in Fig. 11, the correlations are clearly seen in these plots.
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Fig. 14. vs. distribution plotted for the events selected from peak areas % range from peak positions). All the selected in the spectra of Fig. 7 (within events fall within the 200-channel bands around the correlation curves.
IV. CONCLUSION
Fig. 13. Pulse-height distributions of the two-detector events after rejecting the ICC events.
We integrated and evaluated a 2 2 prototype of arrays of 6 6 15 mm virtual Frisch-grid detectors with common-cathode readout for correcting charge signals and rejecting incomplete charge collection events. Such arrays can potentially substitute for more advanced, but also more expensive, 3-D pixel detectors in applications with relaxed requirements for position- and energy-resolution. In addition, they can provide good energy resolution 1–1.5% FWHM at 662 keV resolution, high stopping power, and position resolution suitable for coded aperture telescopes. We demonstrated that by using the cathode signals, it is possible to reject the incomplete charge collection events caused by crystal defects. This means that less expensive crystals with some content of defects can be used for making such arrays. REFERENCES
We used this equation to reevaluate the data collected for the two-detector shown events in Fig. 11. Fig. 12 presents the re-plotted distributions vs. , which now look similar to those plotted in Fig. 2 (fourth row) for the single-detector events (see Fig. 12). Again selecting the 200-channel acceptance bands around the correlation curves, we discriminated the ICC events from the pulse-height distributions of the two-detector events plotted in Fig. 13. The spectra in Fig. 13 demonstrate a reduction of the continua in comparison to the original spectra in Fig. 8; the remaining continua are associated with the Compton-escape events. To verify that no photopeak events had been lost when the vs. distrimodified algorithm was used, we plotted the bution for the events selected from peak areas in the spectra of Fig. 8 (within % range from the peak positions). As shown in Fig. 14, all the selected events fall within the 200-channel bands around the correlation curves.
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