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A module for energy and pulse shape data acquisition
Nuclear Instruments and Methods in Physics Research A 422 (1999) 411—416
A module for energy and pulse shape data acquisition Bradley Hubbard-Nelson, Michael Momayezi*, William K. Warburton X-Ray Instrumentation Associates (XIA), 2513 Charleston Road, Suite 207, Mountain View, CA 94043, USA
Abstract This paper describes a 4-channel data acquisition module designed for use with multi-electrode semiconductor detectors of X-ray and c-rays. It combines high-speed waveform sampling with digital filtering to acquire accurate energy spectra at high rates and, at the same time, capture and store precisely measured waveforms. 1999 Elsevier Science B.V. All rights reserved. Keywords: Pulse shape; Data acquistion; Ge-detectors; GRETA
1. Introduction Researchers in the field of X-ray and c-ray detection and measurement have traditionally been interested mainly in obtaining the best possible energy resolution with a given detector. The advent of digital data acquisition and filtering has greatly benefited the field. Programmable digital filters are a very flexible and compact alternative to bulky analog electronics, and can achieve feats which are impossible to mimic in analog filtering. Finite impulse response filters, for example, allow much higher data rates, and trapezoidal filters yield optimal energy resolution in systems with widely varying signal rise times. Further, todays fast digital signal processors have also opened wide the doors to pulse shape analysis.
* Corresponding author. Tel.: #1 650 903 9980; e-mail: [email protected].
It has been observed that precise knowledge of the signal forms can be used advantageously in a number of applications. In multi-electrode detectors this information can be combined with the event energy to obtain position information about the photon conversion point. With sufficient precision in the measured pulse shape, position resolutions can be achieved that are at least a factor of 10 finer than the electrode size. With adequate computing power, one can thereby obtain good position and energy resolution using bigger electrodes and fewer discrete electronics channels. Typical applications are to X-ray imaging and the large-volume Ge-detectors used in nuclear physics. We describe below a data acquisition module that acquires energy and pulse shape information at the same time. It uses finite impulse response, trapezoidal, digital filters to produce very good energy resolution at high throughput rates and also captures individual signal traces which can be used to implement position reconstruction algorithms.
0168-9002/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 1 0 4 5 - 6
IV. INFORMATION PROCESSING
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B. Hubbard-Nelson et al. /Nucl. Instr. and Meth. in Phys. Res. A 422 (1999) 411—416
2. Module description The 4-channel Digital Gamma Finder (DGF-4C) was designed specifically for nuclear physics applications involving large-volume Ge-detectors using resistive feedback preamplifiers. It is, however, flexible enough to be useful in many other applications as well. Fig. 1 shows a block diagram of the DGF-4C. It consists of four independent acquisition channels. Each features finely programmable gain and offset in its amplifier chain, which is followed by
a 40MSPS, 12-bit ADC. For each channel there is programmable digital filter and trigger logic as well as a FIFO that stores 1024 consecutive ADC values for a single event. 2.1. Digital filtering The digital filter and trigger logic for each channel is combined into one Field ProGrammable Array (FPGA). Data are continuously processed at the system clock rate of 40 MHz. The energy filter is a trapezoidal filter whose length (peaking time)
Fig. 1. Simplified block diagram of the DGF-4C; entire module and amplifier chain (bottom).
B. Hubbard-Nelson et al. /Nucl. Instr. and Meth. in Phys. Res. A 422 (1999) 411—416
and gap (flat top) are user programmable. The sum of length and gap can vary between 1 and 31 data input values, i.e 25 to 775 ns. As the capacity of the FPGA is limited, longer filter times are created by preceding the energy filter with a times 4,16, or 64 decimator, implemented in the same FPGA. The decimator computes sums of 4,16, or 64 ADC values and the data input rate to the filter is reduced by the decimator setting. There are thus four coarse ranges of filter times available, and peaking times from 25 ns to 49.6 ls can be achieved. The energy filter length will in general be either chosen to optimize the energy resolution or will be a compromise between the latter and the attainable data acquisition rate. The energy filter gap will be chosen to be wider than the anticipated spread in signal rise times, i.e. charge collection times. Beyond that is has no influence on the obtainable energy resolution. 2.2. Triggering The trigger logic uses a trapezoidal filter, identical to the energy filter, but it is not preceded by a decimator. When a fast trigger is generated, pileup inspection logic in the FPGA ensures that a final trigger will be issued only if no new fast trigger is generated within a time equal to the sum of the energy filter length plus its gap. This avoids erroneous energy measurements due to pile up in the slower energy filter. The module’s triggering scheme is very flexible. First, any of the four channels can act as a trigger source. Second, a sum energy trigger is available with a 16-bit programmable threshold. Third, the trigger may be derived from an external trigger
413
(NIM-level input) or, fourth, be communicated from another module in the system. The latter is necessary when a number of modules are to be combined in instrumenting a detector with more than four electrodes. In such a system the modules can be connected via an auxilliary bus to synchronize the sampling clock and distribute triggers. 2.3. Technical summary The following data, organized in Tables 1 and 2 characterize more fully the capabilities of the DGG-4C
3. Performance While a complete performance evaluation has not yet been made, the DGF-4C has been tested in the home laboratory and with a multi-electrode Ge-detector at Lawrence Berkeley National Laboratory.
Table 2 Filters, FIFO and trigger Energy filter Peaking time Gap time Trigger filter Peaking time Gap time Trigger threshold FIFO write speed
trapezoidal 25 ns,2,49.6 ls 0, ns,2,48.0 ls trapezoidal 25 ns,2,775 ns 0 ns,2,750 ns 02255 ADC counts 40 MSPS
In 4 ranges In 4 ranges
Stops on trigger
Table 1 Analog conditioning Input impedance Input attenuation Voltage gain Offset adjustment ADC full scale input Nyquist filter fourth order Noise at ADC input
50 ) 1 : 1, 3 : 1, 10 : 1 !14 dB,2,#27 dB $3 V 1V !12 dB at 20 MHz 41,5 LSB
Jumper selectable In steps of 40 dB/2 In setps of 6 V/2 Gaussian filter, no overshoot
IV. INFORMATION PROCESSING
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B. Hubbard-Nelson et al. /Nucl. Instr. and Meth. in Phys. Res. A 422 (1999) 411—416
3.1. Laboratory test Fig. 2 demonstrates noise, crosstalk and pulse shaping. Four traces acquired simultaneously are displayed. The bottom channel received a full scale input pulse with 10 ns rise time. The fourth order Gaussian filter responds nicely without overshoot and a 10 to 90% rise time of about 50 ns. At the time when the signal in channel 3 rises there is no detectable crosstalk signal in the neighbouring channels, to a precision of better than 4 LSB. It follows that, over the system bandwidth, crosstalk rejection exceeds 60 dB. In the three channels without input signal, the traces show the noise present at the ADC input,
which amounts to less than 1.5 LSB (rms) (370 lV). Due to the specifics of the design, the noise at the ADC input which is generated by the on-board electronics itself is largely independent of the programmable voltage gain. 3.2. Tests with a GRETA-prototype detector Among other applications, the DGF-4C was particularly designed to meet the needs of the GRETA-project. GRETA is a proposed c-ray detector array to succeed GammaSphere. As presently conceived, it will comprise of about 100 Gedetectors, each with one anode and 36 cathode segments. The detectors are shaped as truncated,
Fig. 2. Four FIFO traces acquired simultaneously. The noise is 1.5 LSB (rms) or 370 lV (rms). The bottom trace shows the response to a full-range input signal with 10 ns rise time. Observe that there is neither an overshot in the signal nor any detectable crosstalk into the other channels.
B. Hubbard-Nelson et al. /Nucl. Instr. and Meth. in Phys. Res. A 422 (1999) 411—416
hexagonal pyramids with a minimum and maximum diameter of approximately 4.0 and 7.0 cm. The detector length is 9 cm. For testing the DGF-4C performance, a GRETA-prototype detector of the same size, but with only 12 cathode segments, was made available to us by the GRETA collaboration. The central bore, ending some 15 mm behind the entrance surface, carries the anode. On the outside, the cathode is segmented six-fold around the crystal and twofold in length. Fig. 3 shows signal traces acquired during experimentation at Lawrence Berkeley Laboratory, using this detector. The source was Cs and the traces are for a fully absorbed 662 keV c-ray. The anode, which always collects the full charge is shown in trace (a). The particular cathode segment that collected this event’s full charge is shown in trace (b), and the induced signal on one of its neighbours in trace (c). Note that the amplifiers show considerable overshoots in their response to the current pulse. An inspection of Fig. 2 shows that this overshoot is not an artefact introduced by the DGF-4C electronics. It is nice to see, though, how the signals in traces (a) and (b) rise monotonically during charge collection, while the signal in (c) reaches a maximum and returns to zero when all charge has finally been collected onto the respective electrodes. AT LBNL and at XIA there is work in progress on algorithms, which use pulse shape data such as shown in Fig. 3, to reconstruct the positions within the detector of major energy depositions from a c-ray. Results from this work will be presented elsewhere. In order to study the effect of the energy filter parameters on the obtainable energy resolution we acquired a set of traces from over 3000 c-interactions. In an off-line analysis we varied the filter length and gap over a wide range of values and determined the width of the full energy peak from the 662 keV c’s. Starting with very small gap values, we found that the energy resolution improved with increasing gaps until the gap was about 500 ns. Gap values beyond that yielded no further improvement. From the recorded traces it could be seen that the signal rise times from the Ge-detector varied in between 100 and 250 ns. After the initial signal rise, the preamplifier ringing subsided within 200 to 400 ns.
415
Fig. 3. FIFO traces from a fully absorbed Cs, 662 keV c-ray in a Ge-detector. (a) Anode, (b) cathode with full-charge collection, (c) neighbouring cathode. Note that the gains are different on the three channels.
The observed ringing was less severe for slower signals, so that as a result, the overall settling time was about 500 ns. A gap region of 500 ns or more excludes the variant part of the signal pulse forms, and thereby leads to a much improved energy resolution. In the second part of the analysis, we studied the influence of the filter length on the energy resolution, using fixed gap values of 1 ls. As the Table 3 below shows, the FWHM of the full-energy peak initially falls as the filter length is increased, but levels off at peaking times beyond 2 ls. The smallest observed peak width is much bigger than the Fano contribution or the contribution from the electronics noise. We tentatively assume that the observed limit in energy resolution is due to ballistic deficit. While the use of trapezoidal filters helps to avoid the region in which the signal shapes vary a lot, it does not correct for ballistic deficit. The GRETA-prototype detector used preamplifiers with a reset time constant of q"50 ls. At a given time after the c-interaction, the preamplifier output voltage is a function of the collected charge and the time since the charge was collected. For an event in which the charge was collected in a short time, the preamplifier voltage had more time to decay, than for an event with slow charge collection. As a result, the energy peak broadens in proportion to the ratio of charge collection time
IV. INFORMATION PROCESSING
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B. Hubbard-Nelson et al. /Nucl. Instr. and Meth. in Phys. Res. A 422 (1999) 411—416
Table 3 T
0.5 ls
1.0 ls
2.0 ls
4.0 ls
8.0 ls
FWHM
3.5 keV
2.6 keV
2.1 keV
2.2 keV
2.1 keV
variation over preamplifier decay constant. If the filter peaking time is short compared to q, the ballistic deficit-induced peak broadening is 150 ns/ 50 ls"0.003 or 2 keV/662 keV. Of course, one expects that knowledge of the pulse shapes could be used to correct for the ballistic deficit. However, the long rise times and the ringing of the preamplifiers conspired to make this correction impossible.
4. Conclusion We have described the design and performance of a data acquisition module that provides simultaneous energy and pulse shape measurements. Tests with GRETA-prototype Ge-detector have shown, that good energy resolutions for c-rays can be
achieved at fairly short peaking times, which is accomplished through the use of digital, trapezoidal filtering. Though being designed primarily with nuclear physics applications in mind, the module is flexible enough to be of use in other fields as well.
Acknowledgements We wish to express our gratitude to Ms. I.Yang Lee, Kai Vetter, Michael Maier and other members of the GRETA-collaboration whose continued support and hospitality contributed no little to the success of this effort. This work was funded by the US Department of Energy with SBIR-grant no. DE-FG0397ER82510, which is greatly appreciated.
A module for energy and pulse shape data acquisition Bradley Hubbard-Nelson, Michael Momayezi*, William K. Warburton X-Ray Instrumentation Associates (XIA), 2513 Charleston Road, Suite 207, Mountain View, CA 94043, USA
Abstract This paper describes a 4-channel data acquisition module designed for use with multi-electrode semiconductor detectors of X-ray and c-rays. It combines high-speed waveform sampling with digital filtering to acquire accurate energy spectra at high rates and, at the same time, capture and store precisely measured waveforms. 1999 Elsevier Science B.V. All rights reserved. Keywords: Pulse shape; Data acquistion; Ge-detectors; GRETA
1. Introduction Researchers in the field of X-ray and c-ray detection and measurement have traditionally been interested mainly in obtaining the best possible energy resolution with a given detector. The advent of digital data acquisition and filtering has greatly benefited the field. Programmable digital filters are a very flexible and compact alternative to bulky analog electronics, and can achieve feats which are impossible to mimic in analog filtering. Finite impulse response filters, for example, allow much higher data rates, and trapezoidal filters yield optimal energy resolution in systems with widely varying signal rise times. Further, todays fast digital signal processors have also opened wide the doors to pulse shape analysis.
* Corresponding author. Tel.: #1 650 903 9980; e-mail: [email protected].
It has been observed that precise knowledge of the signal forms can be used advantageously in a number of applications. In multi-electrode detectors this information can be combined with the event energy to obtain position information about the photon conversion point. With sufficient precision in the measured pulse shape, position resolutions can be achieved that are at least a factor of 10 finer than the electrode size. With adequate computing power, one can thereby obtain good position and energy resolution using bigger electrodes and fewer discrete electronics channels. Typical applications are to X-ray imaging and the large-volume Ge-detectors used in nuclear physics. We describe below a data acquisition module that acquires energy and pulse shape information at the same time. It uses finite impulse response, trapezoidal, digital filters to produce very good energy resolution at high throughput rates and also captures individual signal traces which can be used to implement position reconstruction algorithms.
0168-9002/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 1 0 4 5 - 6
IV. INFORMATION PROCESSING
412
B. Hubbard-Nelson et al. /Nucl. Instr. and Meth. in Phys. Res. A 422 (1999) 411—416
2. Module description The 4-channel Digital Gamma Finder (DGF-4C) was designed specifically for nuclear physics applications involving large-volume Ge-detectors using resistive feedback preamplifiers. It is, however, flexible enough to be useful in many other applications as well. Fig. 1 shows a block diagram of the DGF-4C. It consists of four independent acquisition channels. Each features finely programmable gain and offset in its amplifier chain, which is followed by
a 40MSPS, 12-bit ADC. For each channel there is programmable digital filter and trigger logic as well as a FIFO that stores 1024 consecutive ADC values for a single event. 2.1. Digital filtering The digital filter and trigger logic for each channel is combined into one Field ProGrammable Array (FPGA). Data are continuously processed at the system clock rate of 40 MHz. The energy filter is a trapezoidal filter whose length (peaking time)
Fig. 1. Simplified block diagram of the DGF-4C; entire module and amplifier chain (bottom).
B. Hubbard-Nelson et al. /Nucl. Instr. and Meth. in Phys. Res. A 422 (1999) 411—416
and gap (flat top) are user programmable. The sum of length and gap can vary between 1 and 31 data input values, i.e 25 to 775 ns. As the capacity of the FPGA is limited, longer filter times are created by preceding the energy filter with a times 4,16, or 64 decimator, implemented in the same FPGA. The decimator computes sums of 4,16, or 64 ADC values and the data input rate to the filter is reduced by the decimator setting. There are thus four coarse ranges of filter times available, and peaking times from 25 ns to 49.6 ls can be achieved. The energy filter length will in general be either chosen to optimize the energy resolution or will be a compromise between the latter and the attainable data acquisition rate. The energy filter gap will be chosen to be wider than the anticipated spread in signal rise times, i.e. charge collection times. Beyond that is has no influence on the obtainable energy resolution. 2.2. Triggering The trigger logic uses a trapezoidal filter, identical to the energy filter, but it is not preceded by a decimator. When a fast trigger is generated, pileup inspection logic in the FPGA ensures that a final trigger will be issued only if no new fast trigger is generated within a time equal to the sum of the energy filter length plus its gap. This avoids erroneous energy measurements due to pile up in the slower energy filter. The module’s triggering scheme is very flexible. First, any of the four channels can act as a trigger source. Second, a sum energy trigger is available with a 16-bit programmable threshold. Third, the trigger may be derived from an external trigger
413
(NIM-level input) or, fourth, be communicated from another module in the system. The latter is necessary when a number of modules are to be combined in instrumenting a detector with more than four electrodes. In such a system the modules can be connected via an auxilliary bus to synchronize the sampling clock and distribute triggers. 2.3. Technical summary The following data, organized in Tables 1 and 2 characterize more fully the capabilities of the DGG-4C
3. Performance While a complete performance evaluation has not yet been made, the DGF-4C has been tested in the home laboratory and with a multi-electrode Ge-detector at Lawrence Berkeley National Laboratory.
Table 2 Filters, FIFO and trigger Energy filter Peaking time Gap time Trigger filter Peaking time Gap time Trigger threshold FIFO write speed
trapezoidal 25 ns,2,49.6 ls 0, ns,2,48.0 ls trapezoidal 25 ns,2,775 ns 0 ns,2,750 ns 02255 ADC counts 40 MSPS
In 4 ranges In 4 ranges
Stops on trigger
Table 1 Analog conditioning Input impedance Input attenuation Voltage gain Offset adjustment ADC full scale input Nyquist filter fourth order Noise at ADC input
50 ) 1 : 1, 3 : 1, 10 : 1 !14 dB,2,#27 dB $3 V 1V !12 dB at 20 MHz 41,5 LSB
Jumper selectable In steps of 40 dB/2 In setps of 6 V/2 Gaussian filter, no overshoot
IV. INFORMATION PROCESSING
414
B. Hubbard-Nelson et al. /Nucl. Instr. and Meth. in Phys. Res. A 422 (1999) 411—416
3.1. Laboratory test Fig. 2 demonstrates noise, crosstalk and pulse shaping. Four traces acquired simultaneously are displayed. The bottom channel received a full scale input pulse with 10 ns rise time. The fourth order Gaussian filter responds nicely without overshoot and a 10 to 90% rise time of about 50 ns. At the time when the signal in channel 3 rises there is no detectable crosstalk signal in the neighbouring channels, to a precision of better than 4 LSB. It follows that, over the system bandwidth, crosstalk rejection exceeds 60 dB. In the three channels without input signal, the traces show the noise present at the ADC input,
which amounts to less than 1.5 LSB (rms) (370 lV). Due to the specifics of the design, the noise at the ADC input which is generated by the on-board electronics itself is largely independent of the programmable voltage gain. 3.2. Tests with a GRETA-prototype detector Among other applications, the DGF-4C was particularly designed to meet the needs of the GRETA-project. GRETA is a proposed c-ray detector array to succeed GammaSphere. As presently conceived, it will comprise of about 100 Gedetectors, each with one anode and 36 cathode segments. The detectors are shaped as truncated,
Fig. 2. Four FIFO traces acquired simultaneously. The noise is 1.5 LSB (rms) or 370 lV (rms). The bottom trace shows the response to a full-range input signal with 10 ns rise time. Observe that there is neither an overshot in the signal nor any detectable crosstalk into the other channels.
B. Hubbard-Nelson et al. /Nucl. Instr. and Meth. in Phys. Res. A 422 (1999) 411—416
hexagonal pyramids with a minimum and maximum diameter of approximately 4.0 and 7.0 cm. The detector length is 9 cm. For testing the DGF-4C performance, a GRETA-prototype detector of the same size, but with only 12 cathode segments, was made available to us by the GRETA collaboration. The central bore, ending some 15 mm behind the entrance surface, carries the anode. On the outside, the cathode is segmented six-fold around the crystal and twofold in length. Fig. 3 shows signal traces acquired during experimentation at Lawrence Berkeley Laboratory, using this detector. The source was Cs and the traces are for a fully absorbed 662 keV c-ray. The anode, which always collects the full charge is shown in trace (a). The particular cathode segment that collected this event’s full charge is shown in trace (b), and the induced signal on one of its neighbours in trace (c). Note that the amplifiers show considerable overshoots in their response to the current pulse. An inspection of Fig. 2 shows that this overshoot is not an artefact introduced by the DGF-4C electronics. It is nice to see, though, how the signals in traces (a) and (b) rise monotonically during charge collection, while the signal in (c) reaches a maximum and returns to zero when all charge has finally been collected onto the respective electrodes. AT LBNL and at XIA there is work in progress on algorithms, which use pulse shape data such as shown in Fig. 3, to reconstruct the positions within the detector of major energy depositions from a c-ray. Results from this work will be presented elsewhere. In order to study the effect of the energy filter parameters on the obtainable energy resolution we acquired a set of traces from over 3000 c-interactions. In an off-line analysis we varied the filter length and gap over a wide range of values and determined the width of the full energy peak from the 662 keV c’s. Starting with very small gap values, we found that the energy resolution improved with increasing gaps until the gap was about 500 ns. Gap values beyond that yielded no further improvement. From the recorded traces it could be seen that the signal rise times from the Ge-detector varied in between 100 and 250 ns. After the initial signal rise, the preamplifier ringing subsided within 200 to 400 ns.
415
Fig. 3. FIFO traces from a fully absorbed Cs, 662 keV c-ray in a Ge-detector. (a) Anode, (b) cathode with full-charge collection, (c) neighbouring cathode. Note that the gains are different on the three channels.
The observed ringing was less severe for slower signals, so that as a result, the overall settling time was about 500 ns. A gap region of 500 ns or more excludes the variant part of the signal pulse forms, and thereby leads to a much improved energy resolution. In the second part of the analysis, we studied the influence of the filter length on the energy resolution, using fixed gap values of 1 ls. As the Table 3 below shows, the FWHM of the full-energy peak initially falls as the filter length is increased, but levels off at peaking times beyond 2 ls. The smallest observed peak width is much bigger than the Fano contribution or the contribution from the electronics noise. We tentatively assume that the observed limit in energy resolution is due to ballistic deficit. While the use of trapezoidal filters helps to avoid the region in which the signal shapes vary a lot, it does not correct for ballistic deficit. The GRETA-prototype detector used preamplifiers with a reset time constant of q"50 ls. At a given time after the c-interaction, the preamplifier output voltage is a function of the collected charge and the time since the charge was collected. For an event in which the charge was collected in a short time, the preamplifier voltage had more time to decay, than for an event with slow charge collection. As a result, the energy peak broadens in proportion to the ratio of charge collection time
IV. INFORMATION PROCESSING
416
B. Hubbard-Nelson et al. /Nucl. Instr. and Meth. in Phys. Res. A 422 (1999) 411—416
Table 3 T
0.5 ls
1.0 ls
2.0 ls
4.0 ls
8.0 ls
FWHM
3.5 keV
2.6 keV
2.1 keV
2.2 keV
2.1 keV
variation over preamplifier decay constant. If the filter peaking time is short compared to q, the ballistic deficit-induced peak broadening is 150 ns/ 50 ls"0.003 or 2 keV/662 keV. Of course, one expects that knowledge of the pulse shapes could be used to correct for the ballistic deficit. However, the long rise times and the ringing of the preamplifiers conspired to make this correction impossible.
4. Conclusion We have described the design and performance of a data acquisition module that provides simultaneous energy and pulse shape measurements. Tests with GRETA-prototype Ge-detector have shown, that good energy resolutions for c-rays can be
achieved at fairly short peaking times, which is accomplished through the use of digital, trapezoidal filtering. Though being designed primarily with nuclear physics applications in mind, the module is flexible enough to be of use in other fields as well.
Acknowledgements We wish to express our gratitude to Ms. I.Yang Lee, Kai Vetter, Michael Maier and other members of the GRETA-collaboration whose continued support and hospitality contributed no little to the success of this effort. This work was funded by the US Department of Energy with SBIR-grant no. DE-FG0397ER82510, which is greatly appreciated.