- Email: [email protected]
A full digital approach to the TDCR method
Applied Radiation and Isotopes 87 (2014) 166–170
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
A full digital approach to the TDCR method Giuliano Mini a,n, Francesco Pepe a, Carlo Tintori a, Marco Capogni b a b
CAEN S.p.A., Via Vetraia 11, I-55049 Viareggio (LU), Italy ENEA – Istituto Nazionale di Metrologia delle Radiazioni Ionizzanti (INMRI), C.R. Casaccia – Via Anguillarese 301, I-00123 Roma, Italy
H I G H L I G H T S
CAEN Desktop Digitizers used to emulate the MAC3 analog board in TDCR acquisition. Spectroscopic application of the CAEN digitizers to the TDCR for charge spectra. Development of two different softwares by CAEN and ENEA-INMRI for TDCR analysis. Single electron peak obtained by CAEN digitizer and ENEA-INMRI portable TDCR. Measurements of 90Sr/90Y by the new TDCR device equipped with CAEN digitizers.
art ic l e i nf o
a b s t r a c t
Available online 2 December 2013
Current state of the art solutions based on the Triple to Double Coincidence Ratio method are generally large size, heavy-weight and not transportable systems. This is due, on one side, to large detectors and scintillation chambers and, on the other, to bulky analog electronics for data acquisition. CAEN developed a new, full digital approach to TDCR technique based on a portable, stand-alone, high-speed multichannel digitizer, on-board Digital Pulse Processing and dedicated DAQ software that emulates the wellknown MAC3 analog board. & 2013 Elsevier Ltd. All rights reserved.
Keywords: TDCR method Radionuclide metrology Liquid scintillation Digital pulse processing
2. The analog chain for TDCR counting
are amplified by a linear amplifier whose outputs feed a discriminator. The thresholds of the discriminator are usually set between the noise peak and the single photoelectron peak in order to guarantee the single photon counting capabilities of the system; this setting is required to obtain an optimal match between the statistical model and the detection setup. Finally, by means of several logic modules, the single, double and triple coincidences are obtained and eventually the numerical TDCR analysis can be applied. Furthermore, a dedicated management of the dead time has to be implemented (Bouchard, 2000) to take into account several effects including scintillator and PMT afterpulses. The MAC3 coincidence and dead-time unit (Bouchard and Cassette, 2000) represents the current state of the art for TDCR systems and is widely used to process the signals coming from the three PMTs. Even if this approach based on analog modules is well established and effective, it foresees the use of bulky electronics, excluding de facto the possibility to realize a compact, lightweight TDCR system; moreover, the use of several different modules makes the setup quite complex.
The classical acquisition chain for the TDCR setup is made of several analog modules; the pulses coming from the three PMTs
3. A digital approach to the TDCR-counting instrumentation
1. Introduction The Triple to Double Coincidence Ratio (TDCR) method has been established as a primary technique of standardization of beta emitting radioactive samples (Pochwalski et al., 1988; Broda et al., 2007). This method is based on liquid scintillation counting: the radioactive sample is mixed with a liquid scintillator that is excited by the beta emission. The resulting light, that can be as low as the single photon level if low-energy beta emitters are measured, is then observed by three photo detectors, usually photomultiplier tubes (PMTs). Relying on three fundamental assumptions concerning the statistics of the emitted light, the detection threshold of the counter and the description of the scintillator non-linearity (Cassette and Vatin, 1992; Broda et al., 2007), the counter detection efficiency is deduced from the triple to double coincidence ratio using a statistical model depending on the energy spectrum of the radionuclide to standardized.
n
Corresponding author. Tel.: þ 39 0584 388 398; fax: þ39 0584 388 959. E-mail address: [email protected] (G. Mini).
0969-8043/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apradiso.2013.11.103
In recent years, the developments in digital electronics have made available both fast and precise analog to digital converters
G. Mini et al. / Applied Radiation and Isotopes 87 (2014) 166–170
(ADCs) from one hand and flexible and powerful field programmable gate arrays (FPGAs) on the other, making possible to effectively reduce the amount of analog modules needed by an electronic acquisition chain and to replace the traditional boards implementing their functionalities in dedicated firmware. Taking advantage of these devices, it is then possible to realize portable TDCR instruments adapted to in-situ activity measurements, as the ones requested by the EMRP JRP1 ENG08 MetroFission that started the CAEN involvements in TDCR method by the ENEA partnership for which the affiliated author (Marco Capogni) is the Scientific Responsible of the project.
3.1. Hardware description CAEN decided to base its TDCR digital solution on a stand-alone desktop digitizer. The choice of this board form factor relies on the small size (154 50 164 mm3), light weight (700 g), multichannel capabilities and operation without any crate, making the resulting solution really portable. For this application, two possible board models, namely DT5720 and DT5751, have been selected. Both of them house a 4-channel fast digitizer with different characteristics: the two boards are provided respectively with 250 MS/s, 12 bit and 1 GS/s, 10 bit Flash ADCs. The analog bandwidth is 125 MHz for DT5720 and 500 MHz for DT5751 while the standard input dynamic range is 2 V peak-to-peak for the former and 1 V peak-to-peak for the latter although it can be customized by changing the front-end circuitry of the digitizers. The features of the selected digitizers cope well with the typical signals coming from photomultipliers in terms of dynamic range and time behavior even with fast detectors making the two boards suitable for the data acquisition of a TDCR system. The digitizers follow the architecture shown in Fig. 1. Both the mentioned modules house a mother board and mezzanine cards plugged on it. The mother board is equipped with a readout and control (ROC) FPGA that manages the digital input and output signals, the communication links (USB or optical link) and the local bus to control the daughter boards. The mezzanines are provided with the input connectors and an analog front-end stage followed by the ADCs and the ADC & memory controller (AMC) FPGAs. These FPGAs continuously catch the samples from the ADCs and store the data in the digitizer memories. A digital to analog converter (DAC) is also available to change the DC level of the input signals, fitting the dynamic range of the ADCs.
3.2. Firmware The aim of a traditional waveform digitizer is to record chunks of digitized input waveforms, to store them in its local memories and eventually to transfer all the samples via a readout link. In this way, all the information of the input pulses is preserved and can be analyzed applying off-line algorithms. This mode of operation is called waveform mode. The drawbacks of this approach are the large amount of data that has to be transferred and the resulting risk to saturate the readout link and lose events as soon as the input rate increases. To avoid this issue, an on-board data reduction has to be provided. Since the TDCR systems are based on PMTs, an on-board charge integration of the input pulses coming from the detector is convenient. The selected modules can run firmware to perform an on-board digital pulse processing (DPP) to evaluate the area of the input 1 European Metrology Research Programme Joint Research Project 〈http:// projects.npl.co.uk/metrofission/〉.
167
pulses. The principles of operation of the DPP are the following, as shown in Fig. 2. Each digitizer channel behaves independently to the others; the ADCs continuously sample the analog input signal and the resulting stream of samples is sent to the FPGAs. The firmware evaluates the input signal baseline, refers a trigger threshold to it and as soon as a pulse crosses the threshold, a trigger is issued. At this point, an integration gate of adjustable width is opened and, thanks to the digital processing, it can be moved back in time in order to fit the input pulse within it, setting a gate offset accordingly. Once the gate is over, the digitizer is able to associate to each triggered pulse a trigger time tag (TTT) with the resolution of the sampling period (i.e. 4 ns in case of DT5720 or 1 ns in case of DT5751) and a charge value. Considering the sampling period and the least significant bit (LSB) of the two boards, a charge sensitivity of 40 fC for DT5720 and 20 fC for DT5751 is provided. This firmware digitally implements the functions of an analog chain based on discriminator, gate and delay generator and charge to digital converter (QDC) without the need of any analog delay, furthermore adding precise time information useful for event correlations. As a result of a data acquisition with DPP it is possible to obtain a stream of TTTs and charge values from each enabled acquisition channel, working, therefore, in List Mode. In this way, the raw samples are no more needed having dramatically reduced the amount of data to be transferred from the digitizer, as shown in Fig. 3. Since the charge values are expressed as 2 byte numbers and the TTTs are 4 byte ones, each event is associated to a few byte of information. 3.3. Software In order to perform a TDCR analysis on the collected event lists, a customized DAQ software has been implemented. Since all the acquisition channels of a digitizer are synchronized, it is possible to correlate events from different input channels comparing their TTTs. The DAQ software is then able to configure the digitizers, set all the DPP parameters (i.e. trigger threshold, gate and pre-gate width of each acquisition channel) and handle the data acquisition. Once the run is over, the DAQ software scans the resulting event lists looking for coincidences. Moreover, the DAQ software implements the same algorithms of the well-known MAC3 analog module emulating its operations in terms of coincidence definition and extendable-type dead time management. In particular, the software ignores all those events found in the lists whose TTTs lie in a dead time window, common for the three PMTs, that was opened by a previous pulse independently triggered by one of the channels; the dead time is then extended according to the MAC3 logic. As a result of this analysis, the DAQ software can provide the counting from the three photomultipliers and the related coincidences (A, B, C, AB, BC, AC, S, D and T) as well as real, dead and live time, balance equations, useful to check the different coincidence and counting values, and TDCR parameter. Another important feature of this DAQ software is that it is able to run the TDCR analysis both on real-time measurements and previously acquired data. This makes possible to perform a parameter sweep analysis repeating the TDCR technique on the same data set changing automatically for example the coincidence resolving time and/or dead time. It is possible, therefore, to study the behavior of the results obtained with different parameter values as shown in (Steele et al., 2009; Bobin et al., 2010) applying them on the same data, without repeating the measurements several times and just by changing the parameter values in the software configuration, with no hardware operations.
168
G. Mini et al. / Applied Radiation and Isotopes 87 (2014) 166–170
Fig. 1. Caen desktop digitizer architecture.
Fig. 2. Principles of operation of DPP.
perform a complete data acquisition of the signals coming from a TDCR detector, providing a charge value and a trigger time tag of each pulse; the TDCR analysis can be run on the resulting data set providing also spectroscopic information of the measured sample.
4. Tests 4.1. Single photoelectron
Fig. 3. Typical signal processed and recorded by the DPP. The main parameters of DPP are shown.
Finally, taking advantage of the charge values provided by the two DPPs, it is possible to obtain spectroscopic information about the measured beta emitter analyzing for instance the charge spectra of the triple coincident events. Independently from CAEN, another TDCR analysis software was developed at ENEA-INMRI by implementing the MAC3 philosophy described above in the CERN ROOT framework, an object oriented package for physics analysis. The algorithm for the TDCR coincidence analysis was then developed by following the main idea described in Bouchard and Cassette (2000) corresponding to the MAC3 “philosophy” and by taking into account the powerful CERN ROOT resources for analyzing data coming from complex detectors. Both codes can run either on Windows or Linux machine. In summary, it can be said therefore that the described portable system made of digitizer, DPP and dedicated software is able to
The first test of this digital approach for TDCR has been carried out at ENEA-INMRI laboratories to verify the capability of the system to perform measurements at the single photoelectron level. The ENEA-INMRI TDCR portable detector built in the framework of the MetroFission project was used for these tests. The PMTs housed in this detector are three Hamamatsu R7600-U200 series. In order to work in the single photoelectron (SPE) regime, the PMTs are biased around 900 V, the HV value supplied to each PMT being fine-tuned to equalize the gain of the three devices in order to match the PMTs. The first acquisition has been made using the dark current of the PMTs (i.e. no vial in the optical chamber of the TDCR) in order to check the capability to distinguish the single photoelectron peaks. The PMTs outputs were connected directly to the input channels of the digitizer with no amplifier in between. In Fig. 4 two single photoelectron pulses coming from one of the PMTs of the ENEA-INMRI portable TDCR and processed by the CAEN Desktop Digitizer DT5720 and DT5751 are shown. It is possible to notice how the reduced analog bandwidth of DT5720 slightly shapes the pulse behavior preserving its area.
G. Mini et al. / Applied Radiation and Isotopes 87 (2014) 166–170
169
Fig. 4. Waveforms of SPE signals generated by one PMT of the ENEA portable TDCR and processed by the two CAEN Desktop Digitizers DT5720 and DT5751.
Fig. 5. SPE recorded by the CAEN Desktop Digitizers DT5720 (top) and DT5751 (bottom).
In Fig. 5 well separated SPE peaks recorded by the both CAEN Desktop Digitizer DT5720 and DT5751 are shown; this guarantees the capability to set a suitable threshold removing the noise and triggering the acquisition of the single photoelectron pulses. 4.2. Preliminary comparison between the two TDCR analysis software As before mentioned, two TDCR analysis softwares were developed independently by CAEN and ENEA taking into account the MAC3 philosophy. The results obtained by the two software
have been compared by measuring a standard solution of 90Sr–90Y with an activity concentration of A0 ¼ (8.023 70.041) kBq g 1at the reference date of 03 April 2013 07.00 UT. The solution was standardized by using two TDCR counters available at ENEAINMRI: the Hidex 300SL “Metro” version (Capogni et al., 2013) and the new fixed home-made TDCR detector built in the framework of the EMRP ENG08 MetroFission project (Capogni and Antohe, this issue) equipped with the same digitizers used in these tests. Typical TDCR value obtained in these measurements is 0.99 with a corresponding efficiency for logical sum of double coincidences equal to 0.9924 and for triple coincidences of 0.984.
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G. Mini et al. / Applied Radiation and Isotopes 87 (2014) 166–170
Fig. 6. Spectrum of the triple coincidences recorded by the ENEA portable TDCR equipped with the CAEN Desktop Digitizer DT5720 measuring a
The reference activity was computed as mean value of the final results obtained by the previous TDCR measurements carried out with the two counter systems. A coincidence window of 40 ns and a dead time of 50 μs were applied on the set of recorded data to perform the TDCR analysis. A deviation in percent, Δ(%)¼ 100 (1 AENEA/A0), between the activity concentration, AENEA, computed with the ENEA code and the reference value of 0.01% with the DT5720 and þ0.41% with the DT5751 Digitizer was found. The deviation in percent, Δ(%) ¼100(1 ACAEN/A0), between the activity concentration value, ACAEN, computed by the CAEN code and the reference value for the same standard 90Sr–90Y source is þ0.22% with the DT5720 and þ0.51% with the DT5751 Digitizer. The results obtained in this preliminary comparison between the two software codes developed independently by CAEN and ENEAINMRI and based on the same MAC3 philosophy require further investigations to understand the dependence of the results from the new front-end electronics (sampling period, digitizer parameters, coincidence windows, threshold, etc.) and from the physical parameters (radionuclide, energy spectrum, etc.). To analyze the TDCR data for the activity computation the code TDCR07c.for written in Fortan77 was used and provided by the Laboratoire National Henry Becquerel (LNHB). All the details of this analysis will be given in a further paper. Finally, in Fig. 6 the spectra for triple coincident events of the same 90Sr–90Y solution obtained by the ENEA portable TDCR system equipped with the CAEN Desktop DT5720 Digitizer is shown.
5. Conclusions A new total digital approach was applied for processing pulses delivered by three photomultipliers in a TDCR system in which for the first time at ENEA-INMRI the CAEN digitizers belonging to the family of Desktop digitizers DT57XX were used to link directly the PMTs of home-made TDCR counters to these new front-end electronics devices. New very interesting perspectives were then opened in the TCDR acquisition and analysis by using new software codes that take into account the digitized information recorded by compact digitizer modules directly linked to the detector. Deeper investigations are still in progress to meet the required metrological accuracy in the results obtained also taking
90
Sr–90Y solution.
into account different parameters involved in these complex measurements. Acknowledgment The authors are deeply grateful to Mr. Massimo Pagliari who provided the necessary assistance in the realization of the portable TDCR system thanks to his technical skills in the field of the highprecision mechanics. A special thanks also goes to Dr. Maria Letizia Cozzella, who prepared the 90Sr–90Y solution for liquid scintillation measurements, and to Mr. Aldo Fazio, for gamma impurity check performed on the same solution by high-energy resolution HPGe spectrometry. The authors would also like to thank Dr. Philippe Cassette from LNHB-CEA, who coordinates the part of the MetroFission project devoted to the development of a prototype for a portable TDCR counter, for the useful discussions about the TDCR method, the MAC3 board and for having provided the TDCR07c.for code. This work has been supported by the European Metrology Research Programme (EMRP) within the framework of the EMRP ENG08 MetroFission project. References Bobin, C., Bouchard, J., Censier, B., 2010. First results in the development of an online digital counting platform dedicated to primary measurements. Appl. Radiat. Isot. 68, 1519–1522. Bouchard, J., 2000. MTR2: a discriminator and dead-time module used in counting systems. Appl. Radiat. Isot. 52 (3), 441–446. Bouchard, J., Cassette, P., 2000. MAC3: an electronic module for the processing of pulses delivered by a three photomultiplier liquid scintillation counting system. Appl. Radiat. Isot. 52 (3), 669–672. Broda, R., Cassette, P., Kossert, K., 2007. Radionuclide metrology using liquid scintillation counting. Metrologia 44, S36–S52. Capogni M., Cozzella M.L., Rusu O.A., 2013. Comparison between two liquid scintillation counting primary techniques for activity measurements of pure beta radionuclides at ENEA-INMRI. Poster presentation at the ICRM 2013 Conference. Antwerp, Belgium, 17–21 June 2013. Capogni and Antohe, Construction and implementation of a TDCR system at ENEA, Appl. Radiat. Isot., http://dx.doi.org/10.1016/j.apradiso.2013.11.014, this issue. Cassette, P., Vatin, R., 1992. Experimental evaluation of TDCR models for the 3 PM liquid scintillation counter. Nucl. Instrum. Methods Phys. Res. A 312 (1–2), 95–99. Pochwalski, K., Broda, R., Radoszewski, T., 1988. Standardization of pure beta emitters by liquid-scintillation counting. Appl. Radiat. Isot. 39 (2), 165–172. Steele, T., Mo, L., Bignell, L., Smith, M., Alexiev, D., 2009. FASEA: a FPGA acquisition system and software event analysis for liquid scintillation counting. Nucl. Instrum. Methods Phys. Res. A 609, 217–220.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
A full digital approach to the TDCR method Giuliano Mini a,n, Francesco Pepe a, Carlo Tintori a, Marco Capogni b a b
CAEN S.p.A., Via Vetraia 11, I-55049 Viareggio (LU), Italy ENEA – Istituto Nazionale di Metrologia delle Radiazioni Ionizzanti (INMRI), C.R. Casaccia – Via Anguillarese 301, I-00123 Roma, Italy
H I G H L I G H T S
CAEN Desktop Digitizers used to emulate the MAC3 analog board in TDCR acquisition. Spectroscopic application of the CAEN digitizers to the TDCR for charge spectra. Development of two different softwares by CAEN and ENEA-INMRI for TDCR analysis. Single electron peak obtained by CAEN digitizer and ENEA-INMRI portable TDCR. Measurements of 90Sr/90Y by the new TDCR device equipped with CAEN digitizers.
art ic l e i nf o
a b s t r a c t
Available online 2 December 2013
Current state of the art solutions based on the Triple to Double Coincidence Ratio method are generally large size, heavy-weight and not transportable systems. This is due, on one side, to large detectors and scintillation chambers and, on the other, to bulky analog electronics for data acquisition. CAEN developed a new, full digital approach to TDCR technique based on a portable, stand-alone, high-speed multichannel digitizer, on-board Digital Pulse Processing and dedicated DAQ software that emulates the wellknown MAC3 analog board. & 2013 Elsevier Ltd. All rights reserved.
Keywords: TDCR method Radionuclide metrology Liquid scintillation Digital pulse processing
2. The analog chain for TDCR counting
are amplified by a linear amplifier whose outputs feed a discriminator. The thresholds of the discriminator are usually set between the noise peak and the single photoelectron peak in order to guarantee the single photon counting capabilities of the system; this setting is required to obtain an optimal match between the statistical model and the detection setup. Finally, by means of several logic modules, the single, double and triple coincidences are obtained and eventually the numerical TDCR analysis can be applied. Furthermore, a dedicated management of the dead time has to be implemented (Bouchard, 2000) to take into account several effects including scintillator and PMT afterpulses. The MAC3 coincidence and dead-time unit (Bouchard and Cassette, 2000) represents the current state of the art for TDCR systems and is widely used to process the signals coming from the three PMTs. Even if this approach based on analog modules is well established and effective, it foresees the use of bulky electronics, excluding de facto the possibility to realize a compact, lightweight TDCR system; moreover, the use of several different modules makes the setup quite complex.
The classical acquisition chain for the TDCR setup is made of several analog modules; the pulses coming from the three PMTs
3. A digital approach to the TDCR-counting instrumentation
1. Introduction The Triple to Double Coincidence Ratio (TDCR) method has been established as a primary technique of standardization of beta emitting radioactive samples (Pochwalski et al., 1988; Broda et al., 2007). This method is based on liquid scintillation counting: the radioactive sample is mixed with a liquid scintillator that is excited by the beta emission. The resulting light, that can be as low as the single photon level if low-energy beta emitters are measured, is then observed by three photo detectors, usually photomultiplier tubes (PMTs). Relying on three fundamental assumptions concerning the statistics of the emitted light, the detection threshold of the counter and the description of the scintillator non-linearity (Cassette and Vatin, 1992; Broda et al., 2007), the counter detection efficiency is deduced from the triple to double coincidence ratio using a statistical model depending on the energy spectrum of the radionuclide to standardized.
n
Corresponding author. Tel.: þ 39 0584 388 398; fax: þ39 0584 388 959. E-mail address: [email protected] (G. Mini).
0969-8043/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apradiso.2013.11.103
In recent years, the developments in digital electronics have made available both fast and precise analog to digital converters
G. Mini et al. / Applied Radiation and Isotopes 87 (2014) 166–170
(ADCs) from one hand and flexible and powerful field programmable gate arrays (FPGAs) on the other, making possible to effectively reduce the amount of analog modules needed by an electronic acquisition chain and to replace the traditional boards implementing their functionalities in dedicated firmware. Taking advantage of these devices, it is then possible to realize portable TDCR instruments adapted to in-situ activity measurements, as the ones requested by the EMRP JRP1 ENG08 MetroFission that started the CAEN involvements in TDCR method by the ENEA partnership for which the affiliated author (Marco Capogni) is the Scientific Responsible of the project.
3.1. Hardware description CAEN decided to base its TDCR digital solution on a stand-alone desktop digitizer. The choice of this board form factor relies on the small size (154 50 164 mm3), light weight (700 g), multichannel capabilities and operation without any crate, making the resulting solution really portable. For this application, two possible board models, namely DT5720 and DT5751, have been selected. Both of them house a 4-channel fast digitizer with different characteristics: the two boards are provided respectively with 250 MS/s, 12 bit and 1 GS/s, 10 bit Flash ADCs. The analog bandwidth is 125 MHz for DT5720 and 500 MHz for DT5751 while the standard input dynamic range is 2 V peak-to-peak for the former and 1 V peak-to-peak for the latter although it can be customized by changing the front-end circuitry of the digitizers. The features of the selected digitizers cope well with the typical signals coming from photomultipliers in terms of dynamic range and time behavior even with fast detectors making the two boards suitable for the data acquisition of a TDCR system. The digitizers follow the architecture shown in Fig. 1. Both the mentioned modules house a mother board and mezzanine cards plugged on it. The mother board is equipped with a readout and control (ROC) FPGA that manages the digital input and output signals, the communication links (USB or optical link) and the local bus to control the daughter boards. The mezzanines are provided with the input connectors and an analog front-end stage followed by the ADCs and the ADC & memory controller (AMC) FPGAs. These FPGAs continuously catch the samples from the ADCs and store the data in the digitizer memories. A digital to analog converter (DAC) is also available to change the DC level of the input signals, fitting the dynamic range of the ADCs.
3.2. Firmware The aim of a traditional waveform digitizer is to record chunks of digitized input waveforms, to store them in its local memories and eventually to transfer all the samples via a readout link. In this way, all the information of the input pulses is preserved and can be analyzed applying off-line algorithms. This mode of operation is called waveform mode. The drawbacks of this approach are the large amount of data that has to be transferred and the resulting risk to saturate the readout link and lose events as soon as the input rate increases. To avoid this issue, an on-board data reduction has to be provided. Since the TDCR systems are based on PMTs, an on-board charge integration of the input pulses coming from the detector is convenient. The selected modules can run firmware to perform an on-board digital pulse processing (DPP) to evaluate the area of the input 1 European Metrology Research Programme Joint Research Project 〈http:// projects.npl.co.uk/metrofission/〉.
167
pulses. The principles of operation of the DPP are the following, as shown in Fig. 2. Each digitizer channel behaves independently to the others; the ADCs continuously sample the analog input signal and the resulting stream of samples is sent to the FPGAs. The firmware evaluates the input signal baseline, refers a trigger threshold to it and as soon as a pulse crosses the threshold, a trigger is issued. At this point, an integration gate of adjustable width is opened and, thanks to the digital processing, it can be moved back in time in order to fit the input pulse within it, setting a gate offset accordingly. Once the gate is over, the digitizer is able to associate to each triggered pulse a trigger time tag (TTT) with the resolution of the sampling period (i.e. 4 ns in case of DT5720 or 1 ns in case of DT5751) and a charge value. Considering the sampling period and the least significant bit (LSB) of the two boards, a charge sensitivity of 40 fC for DT5720 and 20 fC for DT5751 is provided. This firmware digitally implements the functions of an analog chain based on discriminator, gate and delay generator and charge to digital converter (QDC) without the need of any analog delay, furthermore adding precise time information useful for event correlations. As a result of a data acquisition with DPP it is possible to obtain a stream of TTTs and charge values from each enabled acquisition channel, working, therefore, in List Mode. In this way, the raw samples are no more needed having dramatically reduced the amount of data to be transferred from the digitizer, as shown in Fig. 3. Since the charge values are expressed as 2 byte numbers and the TTTs are 4 byte ones, each event is associated to a few byte of information. 3.3. Software In order to perform a TDCR analysis on the collected event lists, a customized DAQ software has been implemented. Since all the acquisition channels of a digitizer are synchronized, it is possible to correlate events from different input channels comparing their TTTs. The DAQ software is then able to configure the digitizers, set all the DPP parameters (i.e. trigger threshold, gate and pre-gate width of each acquisition channel) and handle the data acquisition. Once the run is over, the DAQ software scans the resulting event lists looking for coincidences. Moreover, the DAQ software implements the same algorithms of the well-known MAC3 analog module emulating its operations in terms of coincidence definition and extendable-type dead time management. In particular, the software ignores all those events found in the lists whose TTTs lie in a dead time window, common for the three PMTs, that was opened by a previous pulse independently triggered by one of the channels; the dead time is then extended according to the MAC3 logic. As a result of this analysis, the DAQ software can provide the counting from the three photomultipliers and the related coincidences (A, B, C, AB, BC, AC, S, D and T) as well as real, dead and live time, balance equations, useful to check the different coincidence and counting values, and TDCR parameter. Another important feature of this DAQ software is that it is able to run the TDCR analysis both on real-time measurements and previously acquired data. This makes possible to perform a parameter sweep analysis repeating the TDCR technique on the same data set changing automatically for example the coincidence resolving time and/or dead time. It is possible, therefore, to study the behavior of the results obtained with different parameter values as shown in (Steele et al., 2009; Bobin et al., 2010) applying them on the same data, without repeating the measurements several times and just by changing the parameter values in the software configuration, with no hardware operations.
168
G. Mini et al. / Applied Radiation and Isotopes 87 (2014) 166–170
Fig. 1. Caen desktop digitizer architecture.
Fig. 2. Principles of operation of DPP.
perform a complete data acquisition of the signals coming from a TDCR detector, providing a charge value and a trigger time tag of each pulse; the TDCR analysis can be run on the resulting data set providing also spectroscopic information of the measured sample.
4. Tests 4.1. Single photoelectron
Fig. 3. Typical signal processed and recorded by the DPP. The main parameters of DPP are shown.
Finally, taking advantage of the charge values provided by the two DPPs, it is possible to obtain spectroscopic information about the measured beta emitter analyzing for instance the charge spectra of the triple coincident events. Independently from CAEN, another TDCR analysis software was developed at ENEA-INMRI by implementing the MAC3 philosophy described above in the CERN ROOT framework, an object oriented package for physics analysis. The algorithm for the TDCR coincidence analysis was then developed by following the main idea described in Bouchard and Cassette (2000) corresponding to the MAC3 “philosophy” and by taking into account the powerful CERN ROOT resources for analyzing data coming from complex detectors. Both codes can run either on Windows or Linux machine. In summary, it can be said therefore that the described portable system made of digitizer, DPP and dedicated software is able to
The first test of this digital approach for TDCR has been carried out at ENEA-INMRI laboratories to verify the capability of the system to perform measurements at the single photoelectron level. The ENEA-INMRI TDCR portable detector built in the framework of the MetroFission project was used for these tests. The PMTs housed in this detector are three Hamamatsu R7600-U200 series. In order to work in the single photoelectron (SPE) regime, the PMTs are biased around 900 V, the HV value supplied to each PMT being fine-tuned to equalize the gain of the three devices in order to match the PMTs. The first acquisition has been made using the dark current of the PMTs (i.e. no vial in the optical chamber of the TDCR) in order to check the capability to distinguish the single photoelectron peaks. The PMTs outputs were connected directly to the input channels of the digitizer with no amplifier in between. In Fig. 4 two single photoelectron pulses coming from one of the PMTs of the ENEA-INMRI portable TDCR and processed by the CAEN Desktop Digitizer DT5720 and DT5751 are shown. It is possible to notice how the reduced analog bandwidth of DT5720 slightly shapes the pulse behavior preserving its area.
G. Mini et al. / Applied Radiation and Isotopes 87 (2014) 166–170
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Fig. 4. Waveforms of SPE signals generated by one PMT of the ENEA portable TDCR and processed by the two CAEN Desktop Digitizers DT5720 and DT5751.
Fig. 5. SPE recorded by the CAEN Desktop Digitizers DT5720 (top) and DT5751 (bottom).
In Fig. 5 well separated SPE peaks recorded by the both CAEN Desktop Digitizer DT5720 and DT5751 are shown; this guarantees the capability to set a suitable threshold removing the noise and triggering the acquisition of the single photoelectron pulses. 4.2. Preliminary comparison between the two TDCR analysis software As before mentioned, two TDCR analysis softwares were developed independently by CAEN and ENEA taking into account the MAC3 philosophy. The results obtained by the two software
have been compared by measuring a standard solution of 90Sr–90Y with an activity concentration of A0 ¼ (8.023 70.041) kBq g 1at the reference date of 03 April 2013 07.00 UT. The solution was standardized by using two TDCR counters available at ENEAINMRI: the Hidex 300SL “Metro” version (Capogni et al., 2013) and the new fixed home-made TDCR detector built in the framework of the EMRP ENG08 MetroFission project (Capogni and Antohe, this issue) equipped with the same digitizers used in these tests. Typical TDCR value obtained in these measurements is 0.99 with a corresponding efficiency for logical sum of double coincidences equal to 0.9924 and for triple coincidences of 0.984.
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Fig. 6. Spectrum of the triple coincidences recorded by the ENEA portable TDCR equipped with the CAEN Desktop Digitizer DT5720 measuring a
The reference activity was computed as mean value of the final results obtained by the previous TDCR measurements carried out with the two counter systems. A coincidence window of 40 ns and a dead time of 50 μs were applied on the set of recorded data to perform the TDCR analysis. A deviation in percent, Δ(%)¼ 100 (1 AENEA/A0), between the activity concentration, AENEA, computed with the ENEA code and the reference value of 0.01% with the DT5720 and þ0.41% with the DT5751 Digitizer was found. The deviation in percent, Δ(%) ¼100(1 ACAEN/A0), between the activity concentration value, ACAEN, computed by the CAEN code and the reference value for the same standard 90Sr–90Y source is þ0.22% with the DT5720 and þ0.51% with the DT5751 Digitizer. The results obtained in this preliminary comparison between the two software codes developed independently by CAEN and ENEAINMRI and based on the same MAC3 philosophy require further investigations to understand the dependence of the results from the new front-end electronics (sampling period, digitizer parameters, coincidence windows, threshold, etc.) and from the physical parameters (radionuclide, energy spectrum, etc.). To analyze the TDCR data for the activity computation the code TDCR07c.for written in Fortan77 was used and provided by the Laboratoire National Henry Becquerel (LNHB). All the details of this analysis will be given in a further paper. Finally, in Fig. 6 the spectra for triple coincident events of the same 90Sr–90Y solution obtained by the ENEA portable TDCR system equipped with the CAEN Desktop DT5720 Digitizer is shown.
5. Conclusions A new total digital approach was applied for processing pulses delivered by three photomultipliers in a TDCR system in which for the first time at ENEA-INMRI the CAEN digitizers belonging to the family of Desktop digitizers DT57XX were used to link directly the PMTs of home-made TDCR counters to these new front-end electronics devices. New very interesting perspectives were then opened in the TCDR acquisition and analysis by using new software codes that take into account the digitized information recorded by compact digitizer modules directly linked to the detector. Deeper investigations are still in progress to meet the required metrological accuracy in the results obtained also taking
90
Sr–90Y solution.
into account different parameters involved in these complex measurements. Acknowledgment The authors are deeply grateful to Mr. Massimo Pagliari who provided the necessary assistance in the realization of the portable TDCR system thanks to his technical skills in the field of the highprecision mechanics. A special thanks also goes to Dr. Maria Letizia Cozzella, who prepared the 90Sr–90Y solution for liquid scintillation measurements, and to Mr. Aldo Fazio, for gamma impurity check performed on the same solution by high-energy resolution HPGe spectrometry. The authors would also like to thank Dr. Philippe Cassette from LNHB-CEA, who coordinates the part of the MetroFission project devoted to the development of a prototype for a portable TDCR counter, for the useful discussions about the TDCR method, the MAC3 board and for having provided the TDCR07c.for code. This work has been supported by the European Metrology Research Programme (EMRP) within the framework of the EMRP ENG08 MetroFission project. References Bobin, C., Bouchard, J., Censier, B., 2010. First results in the development of an online digital counting platform dedicated to primary measurements. Appl. Radiat. Isot. 68, 1519–1522. Bouchard, J., 2000. MTR2: a discriminator and dead-time module used in counting systems. Appl. Radiat. Isot. 52 (3), 441–446. Bouchard, J., Cassette, P., 2000. MAC3: an electronic module for the processing of pulses delivered by a three photomultiplier liquid scintillation counting system. Appl. Radiat. Isot. 52 (3), 669–672. Broda, R., Cassette, P., Kossert, K., 2007. Radionuclide metrology using liquid scintillation counting. Metrologia 44, S36–S52. Capogni M., Cozzella M.L., Rusu O.A., 2013. Comparison between two liquid scintillation counting primary techniques for activity measurements of pure beta radionuclides at ENEA-INMRI. Poster presentation at the ICRM 2013 Conference. Antwerp, Belgium, 17–21 June 2013. Capogni and Antohe, Construction and implementation of a TDCR system at ENEA, Appl. Radiat. Isot., http://dx.doi.org/10.1016/j.apradiso.2013.11.014, this issue. Cassette, P., Vatin, R., 1992. Experimental evaluation of TDCR models for the 3 PM liquid scintillation counter. Nucl. Instrum. Methods Phys. Res. A 312 (1–2), 95–99. Pochwalski, K., Broda, R., Radoszewski, T., 1988. Standardization of pure beta emitters by liquid-scintillation counting. Appl. Radiat. Isot. 39 (2), 165–172. Steele, T., Mo, L., Bignell, L., Smith, M., Alexiev, D., 2009. FASEA: a FPGA acquisition system and software event analysis for liquid scintillation counting. Nucl. Instrum. Methods Phys. Res. A 609, 217–220.