- Email: [email protected]
CZT performance for different anode pixel geometries and data corrections
Nuclear Instruments and Methods in Physics Research A 648 (2011) S37–S41
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
CZT performance for different anode pixel geometries and data corrections Kristen Wangerin n, Yanfeng Du, Floris Jansen GE Global Research, 1 Research Circle, Niskayuna, NY 12309, USA
a r t i c l e i n f o
abstract
Available online 7 January 2011
We have investigated the energy resolution and absolute photopeak efficiency of CZT detectors with three anode pixel geometries after three data correction steps. Before data corrections, the energy resolution is 4–5% at 122 keV, and the efficiency is 46–70% for a 21 keV photopeak window. Charge sharing events, which make up 10–25% of all events, are a significant cause of the poor efficiency. We discovered that large gaps between anode pixels have a strong negative impact on both charge sharing and absolute photopeak efficiency. Using three-dimensional position-dependent energy corrections, we were able to compensate for some of this reduced sensitivity, increasing the efficiency by up to 35% to a final efficiency of 62–80% of incoming gamma events. & 2011 Elsevier B.V. All rights reserved.
Keywords: CZT Corrections Charge sharing Charge loss Depth of interaction Photopeak efficiency
1. Introduction CZT detectors have the potential to replace other radiation detector technologies, such as NaI, in areas ranging from medical (nuclear medicine) to security and space applications [1,2]. Our focus is nuclear medicine and the development of CZT-based SPECT systems. In SPECT imaging, CZT’s excellent energy resolution enables dual isotope capabilities [3], and its spatial resolution and compact form factor enable flexible and adaptable system geometries. Our goal is to optimize the detector geometry by using software corrections to achieve better detector performance in terms of energy resolution and absolute photopeak efficiency with today’s commercially available CZT crystal quality. Due to the poor hole mobility-lifetime with today’s CZT crystal quality, the hole mean travel distance in CZT is only on the order of 1 mm under the typical bias electric field of 1000 V/cm. If the CZT detector thickness is larger than this short hole travel distance, the induced charge on the anode electrode is mainly due to the electron contribution. For a planar detector electrode, the induced signal on the anode electrode strongly depends on the gamma ray interaction depth in the detector. For pixilated detectors, due to the small pixel effect, most of the anode pixel signal is induced when the electrons are traveling very close to the anode surface; thus the induced charge is much more uniform than a planar detector. The residual weak dependence of the induced charge on the interaction depth, due to the non-zero weighting potential distribution in the bulk with finite pixel to detector thickness ratio and small electron trapping, can be corrected by applying a depth of interaction (DOI) correction [4]. For pixilated detectors, due to the Compton scatter, photoelectron travel, fluorescence X-ray travel, and electron diffusion, more than one pixel could share the charge for a single gamma ray [5]. For a
n
Corresponding author. Tel.: +1 518 387 5414; fax: + 1 518 387 5975. E-mail address: [email protected] (K. Wangerin).
0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.12.240
detector with an ideal dielectric surface, all the bias electric field lines should end-up on the anode electrodes [6]. In this case, if charge is summed from all charge sharing pixels, the total signal should be equal to a single pixel event. The only drawback is the electronic noise also is summed together quadratically. However, if the readout electronic noise is small from each channel, the total electronic noise for two-pixel events should not dominate the final detector energy resolution. In real detectors with a large gap between anode pixels, if the surface conductivity is lower than the bulk CZT, some electric field lines may end-up on the gap [7]. In this case, some of the electrons may not get collected by anode electrodes [7,8]. The summed signal will have smaller amplitude than it should have, and these events will appear as low energy tail in the energy spectrum, similar to the effects caused by the non-zero weighting potential distribution and electron trapping in bulk for single pixel events. In order to distinguish these three effects that can lead to the charge loss, we refer the charge loss for charge sharing events due to low or distorted electric field between anode electrodes as ‘‘electron loss’’. In this paper, we first study the electron loss of a baseline detector design that has already been used in the medical imaging scanner today. We then develop a correction algorithm to overcome the electron loss. Finally, we evaluate the performance of two new anode designs with smaller pixel pitch and narrower gap.
2. Methods We acquired energy spectra using a RENA3 system (NOVA R&D, Riverside California, USA) and a collimated 57Co point source incident on the cathode surface. The collimated beam includes approximately 16 central anode pixels. We used 5 mm thick CZT detectors (Redlen, Vancouver BC, Canada) with three anode patterns shown in Table 1. We measured the energy resolution of the 122 keV peak, charge sharing event fraction, and absolute photopeak efficiency for a 21 keV
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K. Wangerin et al. / Nuclear Instruments and Methods in Physics Research A 648 (2011) S37–S41
energy window. The energy resolution is calculated by fitting a Gaussian to the photopeak over an asymmetric interval from 70% (lower level) to 50% (upper level) of the peak maximum. The electronic noise and thresholds are channel-dependent and range from 3 to 5 keV and 20 to 40 keV, respectively. The charge sharing event fraction is defined as the ratio of detected two-pixel events to the sum of detected one- and two-pixel events. The absolute photopeak efficiency is calculated from 14 to +7 keV of the 122 keV photopeak divided by incident counts and is verified using a large NaI(Tl) crystal+PM tube that detects all of the incident collimated photons. The incident counts are determined based on a source-detector distance of 17 mm and solid angle fraction calculated to be 1.6 10 5. Detector performance is compared for different pixel configurations without and with each data correction step in order to distinguish the impact of each correction and the dominant effect for each form factor. Design C is the standard CZT configuration used in medical imaging scanners today. Designs A and B seek to improve upon this design with smaller pixel pitch and a narrower gap between anode pads (road). First, single pixel events are corrected for DOI. Then, anode signals from the same charge sharing event are added together. Finally, the electron loss is determined and corrected based on the 3D interaction location of the event (DOI and lateral position relative to two neighboring electrodes) and combined with the single pixel DOI correction. 3. Results Events are separated according to the number of anode pixels per event. Table 2 shows the distribution of detected events. In Table 1 Anode geometries for 5 mm thick CZT crystals. Design
A
B
C
Size (mm2) Pixel pitch (mm) Road (mm)
15 15 15 15 20 20
1.3 1.8 2.46
0.1 0.1 0.6
Table 2 Distribution of events.
1 1 2 3
Pixel, no cathode (%) Pixel+ cathode (%) Pixel+ cathode (%) Pixel+ cathode (%)
some cases, the cathode signal is not detected (1 pixel, no cathode), which is the result of a small cathode signal falling below the lower energy threshold of the channel. These events are added to the spectrum without any corrections. Single pixel events with cathode signal are corrected for DOI based on the cathode/anode signal ratio [4]. Fig. 1 shows the cathode versus anode plots and spectra for Design C. The improvement of the energy resolution and photopeak efficiency in all three designs are summarized in Table 3. DOI correction gives biggest benefits for Design C with the biggest pixel and weakest small pixel effect. Even with this biggest pixel size, the efficiency improvement is only from 46% to 49%, indicating the charge loss due to non-zero weighting potential and electron trapping in the bulk marginally influence detector efficiency. For two-pixel charge sharing events, the correlation between two pixels is shown in Fig. 2 for Designs A and C. This graph tells both the depth of interaction and lateral location with respect to the two pixels and the road. If there is no electron loss, the cluster of the distribution would follow the diagonal line for 122 keV events, which is the case for Design A with only 0.1 mm gap. The events above the diagonal white line near the electronic threshold are due to weighting potential crosstalk [9]. For Design C, there is a large discrepancy between the measured (curved distribution) and expected signals (diagonal white line), which shows positiondependent electron loss; the maximum electron loss occurs for events interacting near the anode surface and above the road, and the detected energy loss is as high as 55%. Events occurring closer to pixel 1 or pixel 2 have less electron loss. We correct two-pixel events for charge sharing and electron loss as a function of depth and lateral interaction position. Fig. 3 shows the cathode versus anode plot (a) and highlights the electron loss correction steps. We first separate events based on the depth of interaction (cathode/(anode 1+ anode 2)) (b), and for each subset of events, we plot pixel 1 versus pixel 2 (c). Fitting a correlation to the measured data in each subset generates a family of curves (d). The ratio between the expected energy (122 keV) and the correlation generates an energy correction factor as a function of depth and lateral interaction location.
Table 3 Performance of single pixel events. Design A
Design B
Design C
7 68 22 3
5 85 10 0
5 77 17 1
FWHM (keV) Efficiency (%)
DOI correction
Design A
Design B
Design C
Before After Before After
5.2 4.8 61 62
5.9 5.1 70 73
6.5 5.2 46 49
Fig. 1. For Design C, the depth of interaction index versus anode signal (left), the corrected depth of interaction index versus anode signal (center), and uncorrected and corrected anode spectra (right).
K. Wangerin et al. / Nuclear Instruments and Methods in Physics Research A 648 (2011) S37–S41
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Fig. 2. For two-pixel events, anode signals for pixel 1 versus pixel 2. Curvature in the distribution for Design C indicates electron loss.
Fig. 3. Cathode versus anode plot (a) and charge loss correction steps: depth of interaction plot (b), subdivision of events from one depth bin to perform each depth bin correction (c), correlations for different depth bins (d), shifting of anode spectrum (e), and two-pixel event spectrum before and after correction (f).
The correction factors are multiplied to the subsets to correct the difference between the expected and measured values by shifting each sub-spectrum (e). Fig. 3(f) shows the dramatic improvement in the charge sharing spectrum after this correction. We compare the performance of the detector designs at three steps. The first is raw data without corrections, the second is with the charge sharing event sum, and the third is with electron loss and DOI corrections. Fig. 4 shows the histograms of the one- and two-pixel events and the combined spectrum along with the FWHM and absolute efficiency.
4. Discussion We expect detectors with smaller pixel pitch to have better charge collection properties as a result of the small pixel effect at the expense of increased charge sharing [5], and our measurements confirm this. Before corrections, Design A has the best energy resolution (5.2 keV) and the most charge sharing events (22%). Design B, which has bigger pixels, has 6.3 keV energy resolution and only 10% charge sharing events. Design C, with the biggest pixels, has the worst energy resolution (6.5 keV) as
expected, but the high charge sharing fraction of 17% is unexpected; the charge sharing event fraction should decrease with increasing pixel pitch. Additionally, we calculate as much as 55% signal loss for these charge sharing events, particularly for events occurring above the gap between anode pixels. The efficiency for Design C is only 46%. The photopeak efficiency is a function of the overall detector response, and we distinguish the effects of depth of interaction and charge sharing by separating one- and two-pixel events. The charge collection efficiency of single pixel events is mostly a function of the depth of interaction. The road and electron loss effects are not dominate factors because these events are not typically occurring over a pixel boundary. For all three anode designs, the photopeak location for single pixel events indicates complete charge collection for a majority of detected single pixel events; only depth of interaction charge loss effects can be seen in the low energy tail. The charge collection efficiency of two-pixel events is both a function of the depth of interaction and lateral interaction location above the anode pads. For Designs A and B, plotting pixel 1 versus pixel 2 shows no electron loss as a function of lateral interaction location, only a depth of interaction dependence. For Design C,
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Fig. 4. The spectra and performance of each form factor: without charge sharing or electron loss corrections (first row), with the sum of two-pixel events (middle row), and events after electron loss and DOI corrections (bottom row). The y-axis is scaled accordingly by the incident counts on each detector to better illustrate improvements in the photopeak efficiency.
however, there are both charge loss and electron loss effects; both the spectrum shape and photopeak location are affected, particularly for events occurring closer to the road. There is in fact a strong correlation of electron loss as a function of lateral interaction position, indicating the influence of the road on charge collection efficiency in addition to the depth of interaction effects. We conclude that a wide gap between anode pixels dominates the detector response and negatively influences charge collection, increasing both charge sharing event fraction and electron loss. The primary explanation for these negative effects is the surface condition of the road [10]. Surface states alter the bulk electric field, thus affecting the charge collection, particularly for events interacting near the anode surface. Due to the poor charge collection efficiency, some of the charge is never collected by the anode pixels, explaining the observed signal loss [6,8]. This charge loss is particularly problematic for charge sharing events, because there could be so much charge loss that the induced anode signal falls below the low energy threshold of the ASIC. Such events would not be detected at all, further lowering the detection and absolute photopeak efficiency. It is, therefore, preferable to have narrow roads to minimize the electron loss. Designs A and B have narrower gaps of only 0.1 mm, and these designs show very little electron loss for charge sharing events. With narrow roads, the signal loss is mostly due to depth of interaction effects, which can be corrected using the cathode/anode ratio. With minimal electron loss, the charge sharing events can simply be added back together, and a significant fraction of events will be shifted back into the
photopeak window. In addition to the narrower roads, Designs A and B also have a smaller pixel pitch, which improves the energy resolution and photopeak efficiency separately from the benefits of reducing the width of the road.
5. Conclusion We have improved the performance of CZT detectors through data corrections by utilizing event information and triangulating the interaction location. The depth of interaction was determined using the cathode/anode ratio, and the lateral position was determined through the relationship between the two-pixel event anode signals. We studied three anode geometries and compared the energy resolution and absolute photopeak efficiency before and after data corrections. After corrections, both the energy resolution and the efficiency improved. The detector with 1.3 mm pixel pitch and therefore greatest small pixel effect had the best resolution of 5.1 keV. The 1.8/0.1 form factor had the highest efficiency of 80%. There was higher than expected charge sharing event fraction as well as signal loss for the 2.46/0.6 form factor. Although some signal loss was due to reduced small pixel effect and depth of interaction effects, the dominate effect was electron loss. The electron loss was a strong function of the lateral and depth of interaction location and was attributed to the large road size and corresponding surface effects that could alter the electric field distribution and negatively influence charge collection.
K. Wangerin et al. / Nuclear Instruments and Methods in Physics Research A 648 (2011) S37–S41
References [1] H. Chen et al., High-performance, large-volume THM CdZnTe detectors for medical imaging and homeland security applications, in: Proceedings of the IEEE NSS, 2006, pp. 3629-3637. [2] B.D. Ramsey, (Fine-pixel) Imaging CdZnTe arrays for space applications, in: Proceedings of the IEEE NSS, 2001, pp. 2377–2381. [3] I. Blevis et al., CZT gamma camera with pinhole collimator: spectral measurements, in: Proceedings of the IEEE NSS, 2008, pp. 4931–4932. [4] Z. He, et al., Nucl. Instr. and Meth. A 422 (1999) 173.
[5] [6] [7] [8]
S41
E. Gros D’Aillon, et al., Nucl. Instr. and Meth. A 563 (2006) 124. E. Kalemci, J.L. Matteson., Nucl. Instr. And Meth. A 478 (2008) 527. A.E. Bolotnikov, et al., Nucl. Instr. and Meth. A 432 (1999) 326. I. Kuvvetli, C. Budtz-Jorgensen, Measurements of charge sharing effects in pixilated CZT/CdTe detectors, in: Proceedings of the IEEE NSS, 2007, pp. 2252–2257. [9] P.N. Luke., Appl. Phys. Lett. 65 (22) (1994) 2884. [10] K. Iniewski et al., Modeling charge-sharing effects in pixellated CZT detectors, in: Proceedings of the IEEE NSS, 2007, pp. 4608–4611.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
CZT performance for different anode pixel geometries and data corrections Kristen Wangerin n, Yanfeng Du, Floris Jansen GE Global Research, 1 Research Circle, Niskayuna, NY 12309, USA
a r t i c l e i n f o
abstract
Available online 7 January 2011
We have investigated the energy resolution and absolute photopeak efficiency of CZT detectors with three anode pixel geometries after three data correction steps. Before data corrections, the energy resolution is 4–5% at 122 keV, and the efficiency is 46–70% for a 21 keV photopeak window. Charge sharing events, which make up 10–25% of all events, are a significant cause of the poor efficiency. We discovered that large gaps between anode pixels have a strong negative impact on both charge sharing and absolute photopeak efficiency. Using three-dimensional position-dependent energy corrections, we were able to compensate for some of this reduced sensitivity, increasing the efficiency by up to 35% to a final efficiency of 62–80% of incoming gamma events. & 2011 Elsevier B.V. All rights reserved.
Keywords: CZT Corrections Charge sharing Charge loss Depth of interaction Photopeak efficiency
1. Introduction CZT detectors have the potential to replace other radiation detector technologies, such as NaI, in areas ranging from medical (nuclear medicine) to security and space applications [1,2]. Our focus is nuclear medicine and the development of CZT-based SPECT systems. In SPECT imaging, CZT’s excellent energy resolution enables dual isotope capabilities [3], and its spatial resolution and compact form factor enable flexible and adaptable system geometries. Our goal is to optimize the detector geometry by using software corrections to achieve better detector performance in terms of energy resolution and absolute photopeak efficiency with today’s commercially available CZT crystal quality. Due to the poor hole mobility-lifetime with today’s CZT crystal quality, the hole mean travel distance in CZT is only on the order of 1 mm under the typical bias electric field of 1000 V/cm. If the CZT detector thickness is larger than this short hole travel distance, the induced charge on the anode electrode is mainly due to the electron contribution. For a planar detector electrode, the induced signal on the anode electrode strongly depends on the gamma ray interaction depth in the detector. For pixilated detectors, due to the small pixel effect, most of the anode pixel signal is induced when the electrons are traveling very close to the anode surface; thus the induced charge is much more uniform than a planar detector. The residual weak dependence of the induced charge on the interaction depth, due to the non-zero weighting potential distribution in the bulk with finite pixel to detector thickness ratio and small electron trapping, can be corrected by applying a depth of interaction (DOI) correction [4]. For pixilated detectors, due to the Compton scatter, photoelectron travel, fluorescence X-ray travel, and electron diffusion, more than one pixel could share the charge for a single gamma ray [5]. For a
n
Corresponding author. Tel.: +1 518 387 5414; fax: + 1 518 387 5975. E-mail address: [email protected] (K. Wangerin).
0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.12.240
detector with an ideal dielectric surface, all the bias electric field lines should end-up on the anode electrodes [6]. In this case, if charge is summed from all charge sharing pixels, the total signal should be equal to a single pixel event. The only drawback is the electronic noise also is summed together quadratically. However, if the readout electronic noise is small from each channel, the total electronic noise for two-pixel events should not dominate the final detector energy resolution. In real detectors with a large gap between anode pixels, if the surface conductivity is lower than the bulk CZT, some electric field lines may end-up on the gap [7]. In this case, some of the electrons may not get collected by anode electrodes [7,8]. The summed signal will have smaller amplitude than it should have, and these events will appear as low energy tail in the energy spectrum, similar to the effects caused by the non-zero weighting potential distribution and electron trapping in bulk for single pixel events. In order to distinguish these three effects that can lead to the charge loss, we refer the charge loss for charge sharing events due to low or distorted electric field between anode electrodes as ‘‘electron loss’’. In this paper, we first study the electron loss of a baseline detector design that has already been used in the medical imaging scanner today. We then develop a correction algorithm to overcome the electron loss. Finally, we evaluate the performance of two new anode designs with smaller pixel pitch and narrower gap.
2. Methods We acquired energy spectra using a RENA3 system (NOVA R&D, Riverside California, USA) and a collimated 57Co point source incident on the cathode surface. The collimated beam includes approximately 16 central anode pixels. We used 5 mm thick CZT detectors (Redlen, Vancouver BC, Canada) with three anode patterns shown in Table 1. We measured the energy resolution of the 122 keV peak, charge sharing event fraction, and absolute photopeak efficiency for a 21 keV
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K. Wangerin et al. / Nuclear Instruments and Methods in Physics Research A 648 (2011) S37–S41
energy window. The energy resolution is calculated by fitting a Gaussian to the photopeak over an asymmetric interval from 70% (lower level) to 50% (upper level) of the peak maximum. The electronic noise and thresholds are channel-dependent and range from 3 to 5 keV and 20 to 40 keV, respectively. The charge sharing event fraction is defined as the ratio of detected two-pixel events to the sum of detected one- and two-pixel events. The absolute photopeak efficiency is calculated from 14 to +7 keV of the 122 keV photopeak divided by incident counts and is verified using a large NaI(Tl) crystal+PM tube that detects all of the incident collimated photons. The incident counts are determined based on a source-detector distance of 17 mm and solid angle fraction calculated to be 1.6 10 5. Detector performance is compared for different pixel configurations without and with each data correction step in order to distinguish the impact of each correction and the dominant effect for each form factor. Design C is the standard CZT configuration used in medical imaging scanners today. Designs A and B seek to improve upon this design with smaller pixel pitch and a narrower gap between anode pads (road). First, single pixel events are corrected for DOI. Then, anode signals from the same charge sharing event are added together. Finally, the electron loss is determined and corrected based on the 3D interaction location of the event (DOI and lateral position relative to two neighboring electrodes) and combined with the single pixel DOI correction. 3. Results Events are separated according to the number of anode pixels per event. Table 2 shows the distribution of detected events. In Table 1 Anode geometries for 5 mm thick CZT crystals. Design
A
B
C
Size (mm2) Pixel pitch (mm) Road (mm)
15 15 15 15 20 20
1.3 1.8 2.46
0.1 0.1 0.6
Table 2 Distribution of events.
1 1 2 3
Pixel, no cathode (%) Pixel+ cathode (%) Pixel+ cathode (%) Pixel+ cathode (%)
some cases, the cathode signal is not detected (1 pixel, no cathode), which is the result of a small cathode signal falling below the lower energy threshold of the channel. These events are added to the spectrum without any corrections. Single pixel events with cathode signal are corrected for DOI based on the cathode/anode signal ratio [4]. Fig. 1 shows the cathode versus anode plots and spectra for Design C. The improvement of the energy resolution and photopeak efficiency in all three designs are summarized in Table 3. DOI correction gives biggest benefits for Design C with the biggest pixel and weakest small pixel effect. Even with this biggest pixel size, the efficiency improvement is only from 46% to 49%, indicating the charge loss due to non-zero weighting potential and electron trapping in the bulk marginally influence detector efficiency. For two-pixel charge sharing events, the correlation between two pixels is shown in Fig. 2 for Designs A and C. This graph tells both the depth of interaction and lateral location with respect to the two pixels and the road. If there is no electron loss, the cluster of the distribution would follow the diagonal line for 122 keV events, which is the case for Design A with only 0.1 mm gap. The events above the diagonal white line near the electronic threshold are due to weighting potential crosstalk [9]. For Design C, there is a large discrepancy between the measured (curved distribution) and expected signals (diagonal white line), which shows positiondependent electron loss; the maximum electron loss occurs for events interacting near the anode surface and above the road, and the detected energy loss is as high as 55%. Events occurring closer to pixel 1 or pixel 2 have less electron loss. We correct two-pixel events for charge sharing and electron loss as a function of depth and lateral interaction position. Fig. 3 shows the cathode versus anode plot (a) and highlights the electron loss correction steps. We first separate events based on the depth of interaction (cathode/(anode 1+ anode 2)) (b), and for each subset of events, we plot pixel 1 versus pixel 2 (c). Fitting a correlation to the measured data in each subset generates a family of curves (d). The ratio between the expected energy (122 keV) and the correlation generates an energy correction factor as a function of depth and lateral interaction location.
Table 3 Performance of single pixel events. Design A
Design B
Design C
7 68 22 3
5 85 10 0
5 77 17 1
FWHM (keV) Efficiency (%)
DOI correction
Design A
Design B
Design C
Before After Before After
5.2 4.8 61 62
5.9 5.1 70 73
6.5 5.2 46 49
Fig. 1. For Design C, the depth of interaction index versus anode signal (left), the corrected depth of interaction index versus anode signal (center), and uncorrected and corrected anode spectra (right).
K. Wangerin et al. / Nuclear Instruments and Methods in Physics Research A 648 (2011) S37–S41
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Fig. 2. For two-pixel events, anode signals for pixel 1 versus pixel 2. Curvature in the distribution for Design C indicates electron loss.
Fig. 3. Cathode versus anode plot (a) and charge loss correction steps: depth of interaction plot (b), subdivision of events from one depth bin to perform each depth bin correction (c), correlations for different depth bins (d), shifting of anode spectrum (e), and two-pixel event spectrum before and after correction (f).
The correction factors are multiplied to the subsets to correct the difference between the expected and measured values by shifting each sub-spectrum (e). Fig. 3(f) shows the dramatic improvement in the charge sharing spectrum after this correction. We compare the performance of the detector designs at three steps. The first is raw data without corrections, the second is with the charge sharing event sum, and the third is with electron loss and DOI corrections. Fig. 4 shows the histograms of the one- and two-pixel events and the combined spectrum along with the FWHM and absolute efficiency.
4. Discussion We expect detectors with smaller pixel pitch to have better charge collection properties as a result of the small pixel effect at the expense of increased charge sharing [5], and our measurements confirm this. Before corrections, Design A has the best energy resolution (5.2 keV) and the most charge sharing events (22%). Design B, which has bigger pixels, has 6.3 keV energy resolution and only 10% charge sharing events. Design C, with the biggest pixels, has the worst energy resolution (6.5 keV) as
expected, but the high charge sharing fraction of 17% is unexpected; the charge sharing event fraction should decrease with increasing pixel pitch. Additionally, we calculate as much as 55% signal loss for these charge sharing events, particularly for events occurring above the gap between anode pixels. The efficiency for Design C is only 46%. The photopeak efficiency is a function of the overall detector response, and we distinguish the effects of depth of interaction and charge sharing by separating one- and two-pixel events. The charge collection efficiency of single pixel events is mostly a function of the depth of interaction. The road and electron loss effects are not dominate factors because these events are not typically occurring over a pixel boundary. For all three anode designs, the photopeak location for single pixel events indicates complete charge collection for a majority of detected single pixel events; only depth of interaction charge loss effects can be seen in the low energy tail. The charge collection efficiency of two-pixel events is both a function of the depth of interaction and lateral interaction location above the anode pads. For Designs A and B, plotting pixel 1 versus pixel 2 shows no electron loss as a function of lateral interaction location, only a depth of interaction dependence. For Design C,
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K. Wangerin et al. / Nuclear Instruments and Methods in Physics Research A 648 (2011) S37–S41
Fig. 4. The spectra and performance of each form factor: without charge sharing or electron loss corrections (first row), with the sum of two-pixel events (middle row), and events after electron loss and DOI corrections (bottom row). The y-axis is scaled accordingly by the incident counts on each detector to better illustrate improvements in the photopeak efficiency.
however, there are both charge loss and electron loss effects; both the spectrum shape and photopeak location are affected, particularly for events occurring closer to the road. There is in fact a strong correlation of electron loss as a function of lateral interaction position, indicating the influence of the road on charge collection efficiency in addition to the depth of interaction effects. We conclude that a wide gap between anode pixels dominates the detector response and negatively influences charge collection, increasing both charge sharing event fraction and electron loss. The primary explanation for these negative effects is the surface condition of the road [10]. Surface states alter the bulk electric field, thus affecting the charge collection, particularly for events interacting near the anode surface. Due to the poor charge collection efficiency, some of the charge is never collected by the anode pixels, explaining the observed signal loss [6,8]. This charge loss is particularly problematic for charge sharing events, because there could be so much charge loss that the induced anode signal falls below the low energy threshold of the ASIC. Such events would not be detected at all, further lowering the detection and absolute photopeak efficiency. It is, therefore, preferable to have narrow roads to minimize the electron loss. Designs A and B have narrower gaps of only 0.1 mm, and these designs show very little electron loss for charge sharing events. With narrow roads, the signal loss is mostly due to depth of interaction effects, which can be corrected using the cathode/anode ratio. With minimal electron loss, the charge sharing events can simply be added back together, and a significant fraction of events will be shifted back into the
photopeak window. In addition to the narrower roads, Designs A and B also have a smaller pixel pitch, which improves the energy resolution and photopeak efficiency separately from the benefits of reducing the width of the road.
5. Conclusion We have improved the performance of CZT detectors through data corrections by utilizing event information and triangulating the interaction location. The depth of interaction was determined using the cathode/anode ratio, and the lateral position was determined through the relationship between the two-pixel event anode signals. We studied three anode geometries and compared the energy resolution and absolute photopeak efficiency before and after data corrections. After corrections, both the energy resolution and the efficiency improved. The detector with 1.3 mm pixel pitch and therefore greatest small pixel effect had the best resolution of 5.1 keV. The 1.8/0.1 form factor had the highest efficiency of 80%. There was higher than expected charge sharing event fraction as well as signal loss for the 2.46/0.6 form factor. Although some signal loss was due to reduced small pixel effect and depth of interaction effects, the dominate effect was electron loss. The electron loss was a strong function of the lateral and depth of interaction location and was attributed to the large road size and corresponding surface effects that could alter the electric field distribution and negatively influence charge collection.
K. Wangerin et al. / Nuclear Instruments and Methods in Physics Research A 648 (2011) S37–S41
References [1] H. Chen et al., High-performance, large-volume THM CdZnTe detectors for medical imaging and homeland security applications, in: Proceedings of the IEEE NSS, 2006, pp. 3629-3637. [2] B.D. Ramsey, (Fine-pixel) Imaging CdZnTe arrays for space applications, in: Proceedings of the IEEE NSS, 2001, pp. 2377–2381. [3] I. Blevis et al., CZT gamma camera with pinhole collimator: spectral measurements, in: Proceedings of the IEEE NSS, 2008, pp. 4931–4932. [4] Z. He, et al., Nucl. Instr. and Meth. A 422 (1999) 173.
[5] [6] [7] [8]
S41
E. Gros D’Aillon, et al., Nucl. Instr. and Meth. A 563 (2006) 124. E. Kalemci, J.L. Matteson., Nucl. Instr. And Meth. A 478 (2008) 527. A.E. Bolotnikov, et al., Nucl. Instr. and Meth. A 432 (1999) 326. I. Kuvvetli, C. Budtz-Jorgensen, Measurements of charge sharing effects in pixilated CZT/CdTe detectors, in: Proceedings of the IEEE NSS, 2007, pp. 2252–2257. [9] P.N. Luke., Appl. Phys. Lett. 65 (22) (1994) 2884. [10] K. Iniewski et al., Modeling charge-sharing effects in pixellated CZT detectors, in: Proceedings of the IEEE NSS, 2007, pp. 4608–4611.