Applied Radiation and Isotopes 87 (2014) 249–253
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
A portable TDCR system Ole Nähle a,n, Qi Zhao a, Carsten Wanke a,b, Mathias Weierganz a, Karsten Kossert a a
Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Braunschweig, Germany Medizinische Hochschule Hannover (MHH), Stabsstelle Strahlenschutz und Abteilung Medizinische Physik—OE 0020, Carl-Neuberg-Str. 1, 30625 Hannover, Germany b
H I G H L I G H T S
A portable TDCR system for activity measurements was developed at PTB. A FPGA based module to realize the coincidence and dead-time logic was developed. Activity determination with uncertainties of less than 1% is possible. Uncertainties increase for nuclides with lower detection efficiency.
art ic l e i nf o
a b s t r a c t
Available online 1 December 2013
The triple-to-double-coincidence ratio method (TDCR) is an important method for activity standardization in metrology institutes worldwide. There is an increasing interest in portable systems that allow activity determination outside of specialized laboratories with high accuracy. Within the framework of the EMRP “MetroFission” project, several portable systems using different designs were developed. The PTB system described here is based on channel photomultipliers incorporated in a portable detection module, a separate electronics bin and a computer for data acquisition and storage. This miniature TDCR system was extensively tested and compared to the PTB reference TDCR system that is very well characterized and has been used in several intercomparisons. & 2013 Elsevier Ltd. All rights reserved.
Keywords: TDCR Portable TDCR Activity standardization Free parameter model Coincidence and dead-time logic EMRP
1. Introduction TDCR as a liquid scintillation counting method for primary activity standardization is well established now and used in many national metrology institutes worldwide (Broda et al., 2007). Usually the detector systems are optimized for maximum counting efficiency to achieve uncertainties as low as possible. However, for some applications inexpensive portable TDCR systems are desirable, which are not yet commercially available. Such systems could be used for reliable on-site activity measurements in the nuclear power industry and also for measurements at production sites of short-lived isotopes used in nuclear medicine. There are some prototypes of portable systems (cf. Ivan et al., 2010) but the inherent lower efficiency of these systems also requires at the very least a verification of the analysis procedures. The free parameter range is different from standard systems with higher efficiencies and, thus, the measurements are a good test for the assumptions and parameterizations used in the analysis software for the efficiency calculation. At PTB, a portable system was developed that consists of a carrying case containing an optical chamber to position the source and three channel photomultipliers (CPMT) for photon detection. The front-end
electronics can be placed in a portable mini NIM bin and a laptop is sufficient for data taking. It was possible to achieve this by developing an FPGA-based NIM module that comprises all coincidence and deadtime logic as well as counters for all relevant channels. The sources which were measured in the new mini TDCR system comprise alpha- and beta emitting radionuclides as well as isotopes decaying by electron capture. In addition, some radionuclides with complex decay schemes were measured. The counting efficiency in the portable TDCR can, in extreme cases like the standardization of 241 Pu, be by about one order of magnitude lower than in a standard TDCR system. Despite this considerable difference, the agreement between the activities determined was reasonable. Since this approach covers a wide range of detection efficiencies, consistent results indicate that the theoretical calculations have a valid basis. For low-energy beta emitters like 3H, however, some deviations have been observed suggesting that the theoretical model still needs improvement and the low efficiency will also introduce a strong background influence.
2. Set-up 2.1. Design considerations
n
Corresponding author. Tel.: þ 49 531 592 6322; fax: þ 49 531 592 6305. E-mail address: [email protected] (O. Nähle).
0969-8043/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apradiso.2013.11.084
In the first phase of the EMRP “MetroFission” project WP 6 (EMRP, 2009) the collaborating institutes (ENEA, PTB, NPL and CEA)
250
O. Nähle et al. / Applied Radiation and Isotopes 87 (2014) 249–253
Fig. 1. Design of the optical chamber based on a cylinder with a height of 85 mm and a diameter of 100 mm. Fig. 2. Complete set-up of the system including the detector, readout electronics and the PC for data acquisition including the GPS antenna.
agreed on a protocol defining the requirements for a portable TDCR system (Capogni et al., 2011) and it was decided to build four systems employing different designs and characteristics. The design of each system is based on the requirement that it has to be portable, at least in the sense that it can be transported in a standard car and set up by one person, thus limiting the dimensions to about 50 cm 50 cm 50 cm and the weight to about 30 kg for each single component. It is also necessary to select rather robust components, especially for the optical detection system. We decided to use a carrying case with a foam inset to house the detection system including the optical chamber and the CPMTs, and to have two separate modules namely an electronic bin and the PC for data taking. 2.2. Optical system The optical chamber was designed to optimize the light transport from the liquid scintillation sample to the photocathodes of the CPMTs to improve the efficiency of the detection system. This is especially important for a portable system because the smaller size of the system implies the use of miniaturized PMTs with inherent lower overall efficiency and a smaller total cathode surface area. Special care must be taken in the design of the optical chamber to keep reasonable detection efficiency in order to optimize counting statistics. It was made using the diffuse reflecting material OP.DI.MA (ODM98) produced by Gigahertz Optik GmbH with a reflectivity of more than 98% over a wide wavelength range (typically 250 nm to 2500 nm). It is a volume reflector requiring a minimum thickness of 10 mm of the reflecting surface, thus increasing the isotropy of the collected light which is considered to be important for the theoretical model involved in free parameter methods. Specular reflections might also cause light trapping in the chamber by total reflection. It was decided to design the system at PTB in such a way, that sources in standard geometry with a total of 16 mL of scintillator and active solution in 20 mL glass vials can be measured. This implies a larger optical chamber and therefore the chamber is designed as a cylinder (height 85 mm, diameter 100 mm) with three bores for the CPMTs with an angle of 1201 between any two of them and a bore from the top to place the liquid scintillation vials. The distance of the photocathodes to the vial is minimized and they are centered towards the level of the scintillator in the vial to make use of the meniscus effect for glass vials yielding a higher light output in this area (Nähle et al., 2009). The configuration of the optical system is illustrated in Fig. 1.
3. Electronics The readout chain for the working system is reduced to a minimum to have all the necessary electronics included in a portable NIM bin with 5 available slots. The high voltage is supplied by
3 PerkinElmer CHV 30N modules placed close to the CPMTs within the carrying case. To set the high voltage, the modules include an additional input that translates a reference voltage of 0 V to 2.9 V into an output voltage of up to 3 kV. The reference voltage is set by a National Instruments PCI-6703 Digital-to-Analogue-Converter (DAC) in the PC. The necessary operating voltage of 5 V for the HV modules is derived from the backplane of the NIM bin using a voltage regulator in a dedicated NIM module that also adapts the SCSI connector from the DAC to a LEMO 19-pin connector and features contact points to measure the monitoring voltage from the HV modules. The analogue output of the CPMT is fed into a CAEN N978 fast amplifier and digitized by an Ortec 935 4-fold constant fraction discriminator (CFD). The time basis for all measurements is realized by a GPS receiver board in the PC. The board generates a reference frequency of 10 MHz while date and time of the start of the measurement is taken from the PC clock. All coincidence, dead-time and counting operations are made by a special module developed at PTB called 4KAM, which is based on FPGA chip. The whole system including all the components is shown in Fig. 2.
4. 4KAM module The 4KAM module is designed to realize all processing of the digitized signals from the SCA and is realized as a double width NIM module. It is based on the Spartan-3 FPGA chip (Xilinx, 2012) incorporated into the commercial board TE0140 from trenz elektronic including the FPGA and interfaces for programming and input/output operations (trenz, 2011). The programming language is VHDL and the program code is uploaded to the FPGA using a dedicated Xilinx interface. The software code and the electronics to include the FPGA module was developed at PTB. The 4KAM module has 5 inputs with upstream comparators with adjustable thresholds between 3 V and þ 3 V. In the present configuration, 4 input channels are used for the detector, and one input is used to feed in an external reference frequency of 10 MHz generated by a GPS or radio controlled receiver. If there is no reference frequency available, an internal quartz oscillator will be used as a fallback solution. There are also numerous digital lines which can be configured as input or as output. A microcontroller of the type ATmega8515 (Atmel, 2006) ensures communication between the FPGA and the computer via a RS-232 serial bus. This interface is used to set the time windows, to read out the internal counters and to start a measurement. Nine programmable delay lines serve to generate variable delay times. A block diagram is shown in Fig. 3. The dead-time and coincidence logic follows the principle as described by Bouchard and Cassette (2000) with the option to add
O. Nähle et al. / Applied Radiation and Isotopes 87 (2014) 249–253
251
Fig. 3. Block diagram of the 4KAM module. The scheme was simplified to show only those parts relevant for TDCR measurements without additional channels.
a fourth channel to read out a γ-detector with adjustable delay and coincidence time. The first incident pulse of a photomultiplier starts a dead-time window which is realized by a triggering of the dead time which can be chosen to be between 10 ms and 100 ms as well as starting a coincidence window with a duration of typically 10 ns to 100 ns. Each subsequent input signal of the other two photomultipliers incident in this coincidence window is registered. Pulses occurring outside this window are ignored and simply lead to a re-triggering of the dead time. About 2.5 ms after the first coincidence window, the time window for the gamma detector channel is open for 100 ns. After this time has elapsed, the multiplicity of the coincidences is determined and the corresponding counters are incremented. All in all, there are 14 counters implemented to record single events, double and triple coincidences as well as live time and real time. The whole cycle of this measurement principle is controlled by the FPGA. The first coincidence window is generated by means of a programmable delay line and can be adjusted with a resolution of 250 ps. The dead time as well as the delay time and the width of the window for the gamma channel can be adjusted with a resolution of 5 ns. The time base can be generated either internally by means of a quartz oscillator or by a standard frequency of 10 MHz fed in externally. Further applications can be realized in the short term by reprogramming the FPGA.
5. Measurement procedures and system checks 5.1. Threshold adjustments Although CPMTs do not provide spectral information and, consequently, do not resolve the single electron peak like conventional PMTs, correct HV settings and proper threshold adjustments are crucial to obtain consistent and reliable measurement data. The HV values were adjusted to reach comparable amplification for all three CPMTs and reasonable threshold values within the limits of the discriminator but without overloading the amplifier. There is no additional signal output to obtain spectral information recordable with an ADC and, therefore, the correct threshold settings were determined by varying the threshold of all channels simultaneously on the discriminator hardware. Observing the double count rates for each pair of CPMTs, a common threshold was selected to reach maximum efficiency and to be well above the noise (Fig. 4). 5.2. Linearity checks A check of the system linearity, i.e. plotting the measured specific activity as a function of the known source activity, is a
Fig. 4. Count rates of any pair of double coincidences as a function of the common threshold, which was finally set at 60 mV to ensure a discrimination of noise.
good test for a working dead-time measurement and an appropriate correction. Since the detection efficiency of alpha emitters is essentially 1 (Kossert et al., 2009), there is no model uncertainty for the efficiency calculation involved in this test. From well standardized PTB solutions of 241Am, a series of sources was prepared to check the linearity of LSC counters at PTB (see e.g. Wanke et al., 2012). The samples measured had expected count rates of approx. 106, 5 105, 2 105, 1.4 105, 5 104, 2 104 counts per minute and were prepared in PTB's standard geometry in 20 mL borosilicate glass vials with 15 mL of Ultima Gold™ AB as a cocktail, the radioactive solution and an appropriate amount of distilled water to reach a total volume of 16 mL. The result of the linearity check is shown in Fig. 5 and confirms a very good linearity of the system of better than 0.1%. This includes the whole detection chain from CPMT to amplifier, discriminator and the final digital signal processing in the 4KAM module.
5.3. Dead time and coincidence resolving time Recently there have been some discussions about the appropriate setting of the coincidence resolving time and there are hints that a coincidence time of 40 ns as used by most laboratories might be too short (Bobin et al., 2012). The 4KAM allows a variation of the coincidence, time but since the reference system is a TDCR counter connected to a MAC3 module with 40 ns implemented, all the measurements presented in this work were carried out using the same dead time of 30 ms and a coincidence time of 40 ns.
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6. Results After the adjustment of HV and thresholds, the mini TDCR system was used to measure samples that were available at PTB and that were standardized in the reference system at the same time. All in all, 13 nuclides in glass and PE vials were standardized and compared to the PTB reference (Nähle et al., 2010). The results
0.15
Residuals in %
0.10 0.05 0.00 -0.05 -0.10 -0.15
1000
10000
Activity in Bq
Fig. 5. Test of the system linearity with a set of well calibrated 241Am sources. The deviation of the measured from the expected count rate is shown.
Table 1 Summary of the measurements made with the mini TDCR system and the reference TDCR system of PTB. For each nuclide the vial type and the deviation Δ of the mini TDCR from the reference system is given. The uncertainty stated does not take correlation, for example due to weighing, of the two results into account.
are summarized in Table 1 showing the relative deviations for each radionuclide and the combined uncertainty without taking into account correlations due to weighing and without uncertainty components due to the efficiency calculations. This is expected to be low for radionuclides with high counting efficiency but is important for low counting efficiencies (Table 2). For efficiencies of more than 90%, an uncertainty of less than 0.5% can be expected and alpha emitters can be standardized with even higher precision proving a working signal processing from the PMT output to the digital counters. It is interesting to note that all mini TDCR results are lower than those of the reference system and the relative difference increases with decreasing efficiency. So far, there is no explanation for this observation but since count rates are usually low for these measurements, it cannot be attributed to problems with the electronics, such as for example pile-up effects or deadtime circuitry. But there are indications that the model used in the TDCR calculations is not valid in case of low photon statistics (Bergeron and Zimmerman, 2011; Bobin et al., 2012). The detection efficiency for the logical sum of double coincidences can be as low as 5% for the measurement of 241Pu. With a TDCR value of about 0.1 this gives a detection efficiency for triple coincidences of less than 1%, which results, for a source in the kBq range, in count rates that are comparable to the background count rate. Therefore, background gives a considerable contribution to the uncertainty of the experimental TDCR value and, thus, also to the final result (Table 2, 3H uncertainty budget). With such low efficiencies there is also a large model influence on the determined activity. The influence of the kB value and the ionization quenching function, the calculated spectra and other decay data can add up to several percent.
Nuclide
Source type
Δ in %
u(Δ) in %
7. Summary and outlook
H-3 Cl-36 Ca-45 Fe-55 Fe-59 Fe-59 Ni-63 Sr-89 Sr-90/Y-90 Y-90 Tc-99 Pm-147 Th-229 Pu-241
PE Glass Glass PE PE Glass PE Glass Glass Glass PE Glass Glass PE
1.74 0.79 0.55 1.98 0.047 0.271 1.97 0.068 0.163 0.089 0.163 0.85 0.0125 1.42
5.4 0.153 0.31 0.56 0.128 0.086 0.257 0.095 0.032 0.128 0.090 0.039 0.091 7.2
The work presented in this paper proves that a miniaturized TDCR system can be used to standardize nuclides with rather high beta energies with an uncertainty of less than 1%. This is still unsatisfactory for metrological applications but would be sufficient for measurements outside of a national metrology institute where large uncertainties are less critical. For nuclides with lower energies such as 3H, 55Fe and 241Pu where uncertainties due to the theoretical model, the nuclide data and due to low statistics become more important, the uncertainty could be as high as 10% limiting the use of a mini TDCR system for these applications. From the experimental side it seems desirable to test a different type of PMT with higher intrinsic efficiency and a larger cathode
Table 2 Uncertainty budgets for the activity concentration as a measure for the system performance The uncertainties are given as standard uncertainties (k ¼ 1). Component
u(a)/a in % 3
89
241
3.0 0.02 0.03 4.0 0.01 0.05 0.01 o 0.25 0.2 0.10 2.0 0.25 5.4
0.03 0.01 0.01 0.05 0.01 0.05 0.01 0.15 0.1 0.05 0.1 0.1 0.25
0.03 0.02 0.01 0.05 0.01 0.05 0.01 o0.10 0.1 –/– –/– –/– 0.13
H
Counting statistics Weighing Dead time Background Counting time Adsorption Decay correction (half-life) Impurities TDCR value and interpolation of efficiency curve Nuclear and atomic data, model excluding ionization quenching Ionization quenching and kB value PMT asymmetry Square root of the sum of quadratic components (correlation coefficients are taken into account in the summation)
Sr
Am
O. Nähle et al. / Applied Radiation and Isotopes 87 (2014) 249–253
surface. An option to gain some spectral information would also improve the understanding of the results. The whole system could be made smaller by replacing the portable PC with a laptop. This would require a different approach for the setting of the HV along with the use of a calibrated oscillator in the coincidence module instead of a GPS board with an antenna. Acknowledgements This work has been carried out within the scope of the European Metrology Research Programme (EMRP) Joint Research Project ENG08, entitled “MetroFission”. References Atmel, 2006. ATmega8515(L) Summary datasheet. 〈http://www.atmel.com/Images/ 2512S.pdf〉. Bergeron, D.E., Zimmerman, B.E., 2011. TDCR measurements on 241Pu at NIST. In: Cassette, P. (Ed.), LSC2010, Advances in Liquid Scintillation Spectrometry: Proceedings of the International Conference on Liquid Scintillation Spectrometry, 6–10 September 2010, Paris, France. Radiocarbon, The University of Arizona, Tucson, Arizona, USA, ISBN 978-0-9638314-7-7, pp. 171–179. Bobin, C., Thiam, C., Chauvenet, B., Bouchard, J., 2012. On the stochastic dependence between photomultipliers in the TDCR method. Appl. Radiat. Isot 70, 770–780. 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, 669–672.
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Broda, R., Cassette, P., Kossert, K., 2007. Radionuclide metrology using liquid scintillation counting. Metrologia 44, S36–S52. Capogni, M., Cassette, P., Johansson, L., Kossert, K., Nähle, O., Sephton, J., Wanke, C.., 2011. Protocol for the Design of a Portable Liquid Scintillation Counter Based on the Triple to Double Coincidence Ratio Method. EMRP WP6, Deliverable 6.1.1. P. Cassette, Private Communication. EMRP, 2009. Energy – Joint Research Project Summary – Metrology for New Generation Nuclear Power Plants MetroFission, 2009. See 〈http://www.eura met.org/index.php?id=a169jrps〉. Ivan, C., Wätjen, A.C., Cassette, P., Sahagia, M., Antohe, A., Grigorescu, E.L., 2010. Participation in the CCRI(II)-K2.H-3 comparison and study of the new TDCR-LS counter with 6 CPMs. Appl. Radiat. Isot 68, 1543–1545. Kossert, K., Jörg, G., Nähle, O., Lierse, v., Gostomski, C., 2009. High-precision measurement of the half-life of 147Sm. Appl. Radiat. Isot 67, 1702–1706. Nähle, O., Kossert, K., Brunzendorf, J., 2009. Study of light emission processes for the design of liquid scintillation counters. In: Eikenberg, J., Jäggi, M., Beer, H., Baehrle, H. (Eds.), LSC2008, Advances in Liquid Scintillation Spectrometry: Proceedings of the Liquid Scintillation Spectrometry Conference, 25–30 May 2008, Davos, Switzerland. Radiocarbon, The University of Arizona, Tucson, Arizona, USA, pp. 87–95. Nähle, O., Kossert, K., Cassette, P., 2010. Activity standardization of 3H with the new TDCR system at PTB. Appl. Radiat. Isot 68, 1534–1536. trenz, 2011. trenz electronic, Spartan-3 Micromodule. 〈http://www.trenz-electronic. de/de/produkte/fpga-boards/trenz-electronic/te0140-spartan-3-series.html〉. Wanke, C., Kossert, K., Nähle, O.J., 2012. Investigations on TDCR measurements with the HIDEX 300 SL using a free parameter model. Appl. Radiat. Isot 70, 2176–2183. Xilinx, 2012. Xilinx Inc., Spartan-3 FPGA Family Data Sheet. 〈http://www.xilinx. com/support/documentation/data_sheets/ds099.pdf〉.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
A portable TDCR system Ole Nähle a,n, Qi Zhao a, Carsten Wanke a,b, Mathias Weierganz a, Karsten Kossert a a
Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Braunschweig, Germany Medizinische Hochschule Hannover (MHH), Stabsstelle Strahlenschutz und Abteilung Medizinische Physik—OE 0020, Carl-Neuberg-Str. 1, 30625 Hannover, Germany b
H I G H L I G H T S
A portable TDCR system for activity measurements was developed at PTB. A FPGA based module to realize the coincidence and dead-time logic was developed. Activity determination with uncertainties of less than 1% is possible. Uncertainties increase for nuclides with lower detection efficiency.
art ic l e i nf o
a b s t r a c t
Available online 1 December 2013
The triple-to-double-coincidence ratio method (TDCR) is an important method for activity standardization in metrology institutes worldwide. There is an increasing interest in portable systems that allow activity determination outside of specialized laboratories with high accuracy. Within the framework of the EMRP “MetroFission” project, several portable systems using different designs were developed. The PTB system described here is based on channel photomultipliers incorporated in a portable detection module, a separate electronics bin and a computer for data acquisition and storage. This miniature TDCR system was extensively tested and compared to the PTB reference TDCR system that is very well characterized and has been used in several intercomparisons. & 2013 Elsevier Ltd. All rights reserved.
Keywords: TDCR Portable TDCR Activity standardization Free parameter model Coincidence and dead-time logic EMRP
1. Introduction TDCR as a liquid scintillation counting method for primary activity standardization is well established now and used in many national metrology institutes worldwide (Broda et al., 2007). Usually the detector systems are optimized for maximum counting efficiency to achieve uncertainties as low as possible. However, for some applications inexpensive portable TDCR systems are desirable, which are not yet commercially available. Such systems could be used for reliable on-site activity measurements in the nuclear power industry and also for measurements at production sites of short-lived isotopes used in nuclear medicine. There are some prototypes of portable systems (cf. Ivan et al., 2010) but the inherent lower efficiency of these systems also requires at the very least a verification of the analysis procedures. The free parameter range is different from standard systems with higher efficiencies and, thus, the measurements are a good test for the assumptions and parameterizations used in the analysis software for the efficiency calculation. At PTB, a portable system was developed that consists of a carrying case containing an optical chamber to position the source and three channel photomultipliers (CPMT) for photon detection. The front-end
electronics can be placed in a portable mini NIM bin and a laptop is sufficient for data taking. It was possible to achieve this by developing an FPGA-based NIM module that comprises all coincidence and deadtime logic as well as counters for all relevant channels. The sources which were measured in the new mini TDCR system comprise alpha- and beta emitting radionuclides as well as isotopes decaying by electron capture. In addition, some radionuclides with complex decay schemes were measured. The counting efficiency in the portable TDCR can, in extreme cases like the standardization of 241 Pu, be by about one order of magnitude lower than in a standard TDCR system. Despite this considerable difference, the agreement between the activities determined was reasonable. Since this approach covers a wide range of detection efficiencies, consistent results indicate that the theoretical calculations have a valid basis. For low-energy beta emitters like 3H, however, some deviations have been observed suggesting that the theoretical model still needs improvement and the low efficiency will also introduce a strong background influence.
2. Set-up 2.1. Design considerations
n
Corresponding author. Tel.: þ 49 531 592 6322; fax: þ 49 531 592 6305. E-mail address: [email protected] (O. Nähle).
0969-8043/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apradiso.2013.11.084
In the first phase of the EMRP “MetroFission” project WP 6 (EMRP, 2009) the collaborating institutes (ENEA, PTB, NPL and CEA)
250
O. Nähle et al. / Applied Radiation and Isotopes 87 (2014) 249–253
Fig. 1. Design of the optical chamber based on a cylinder with a height of 85 mm and a diameter of 100 mm. Fig. 2. Complete set-up of the system including the detector, readout electronics and the PC for data acquisition including the GPS antenna.
agreed on a protocol defining the requirements for a portable TDCR system (Capogni et al., 2011) and it was decided to build four systems employing different designs and characteristics. The design of each system is based on the requirement that it has to be portable, at least in the sense that it can be transported in a standard car and set up by one person, thus limiting the dimensions to about 50 cm 50 cm 50 cm and the weight to about 30 kg for each single component. It is also necessary to select rather robust components, especially for the optical detection system. We decided to use a carrying case with a foam inset to house the detection system including the optical chamber and the CPMTs, and to have two separate modules namely an electronic bin and the PC for data taking. 2.2. Optical system The optical chamber was designed to optimize the light transport from the liquid scintillation sample to the photocathodes of the CPMTs to improve the efficiency of the detection system. This is especially important for a portable system because the smaller size of the system implies the use of miniaturized PMTs with inherent lower overall efficiency and a smaller total cathode surface area. Special care must be taken in the design of the optical chamber to keep reasonable detection efficiency in order to optimize counting statistics. It was made using the diffuse reflecting material OP.DI.MA (ODM98) produced by Gigahertz Optik GmbH with a reflectivity of more than 98% over a wide wavelength range (typically 250 nm to 2500 nm). It is a volume reflector requiring a minimum thickness of 10 mm of the reflecting surface, thus increasing the isotropy of the collected light which is considered to be important for the theoretical model involved in free parameter methods. Specular reflections might also cause light trapping in the chamber by total reflection. It was decided to design the system at PTB in such a way, that sources in standard geometry with a total of 16 mL of scintillator and active solution in 20 mL glass vials can be measured. This implies a larger optical chamber and therefore the chamber is designed as a cylinder (height 85 mm, diameter 100 mm) with three bores for the CPMTs with an angle of 1201 between any two of them and a bore from the top to place the liquid scintillation vials. The distance of the photocathodes to the vial is minimized and they are centered towards the level of the scintillator in the vial to make use of the meniscus effect for glass vials yielding a higher light output in this area (Nähle et al., 2009). The configuration of the optical system is illustrated in Fig. 1.
3. Electronics The readout chain for the working system is reduced to a minimum to have all the necessary electronics included in a portable NIM bin with 5 available slots. The high voltage is supplied by
3 PerkinElmer CHV 30N modules placed close to the CPMTs within the carrying case. To set the high voltage, the modules include an additional input that translates a reference voltage of 0 V to 2.9 V into an output voltage of up to 3 kV. The reference voltage is set by a National Instruments PCI-6703 Digital-to-Analogue-Converter (DAC) in the PC. The necessary operating voltage of 5 V for the HV modules is derived from the backplane of the NIM bin using a voltage regulator in a dedicated NIM module that also adapts the SCSI connector from the DAC to a LEMO 19-pin connector and features contact points to measure the monitoring voltage from the HV modules. The analogue output of the CPMT is fed into a CAEN N978 fast amplifier and digitized by an Ortec 935 4-fold constant fraction discriminator (CFD). The time basis for all measurements is realized by a GPS receiver board in the PC. The board generates a reference frequency of 10 MHz while date and time of the start of the measurement is taken from the PC clock. All coincidence, dead-time and counting operations are made by a special module developed at PTB called 4KAM, which is based on FPGA chip. The whole system including all the components is shown in Fig. 2.
4. 4KAM module The 4KAM module is designed to realize all processing of the digitized signals from the SCA and is realized as a double width NIM module. It is based on the Spartan-3 FPGA chip (Xilinx, 2012) incorporated into the commercial board TE0140 from trenz elektronic including the FPGA and interfaces for programming and input/output operations (trenz, 2011). The programming language is VHDL and the program code is uploaded to the FPGA using a dedicated Xilinx interface. The software code and the electronics to include the FPGA module was developed at PTB. The 4KAM module has 5 inputs with upstream comparators with adjustable thresholds between 3 V and þ 3 V. In the present configuration, 4 input channels are used for the detector, and one input is used to feed in an external reference frequency of 10 MHz generated by a GPS or radio controlled receiver. If there is no reference frequency available, an internal quartz oscillator will be used as a fallback solution. There are also numerous digital lines which can be configured as input or as output. A microcontroller of the type ATmega8515 (Atmel, 2006) ensures communication between the FPGA and the computer via a RS-232 serial bus. This interface is used to set the time windows, to read out the internal counters and to start a measurement. Nine programmable delay lines serve to generate variable delay times. A block diagram is shown in Fig. 3. The dead-time and coincidence logic follows the principle as described by Bouchard and Cassette (2000) with the option to add
O. Nähle et al. / Applied Radiation and Isotopes 87 (2014) 249–253
251
Fig. 3. Block diagram of the 4KAM module. The scheme was simplified to show only those parts relevant for TDCR measurements without additional channels.
a fourth channel to read out a γ-detector with adjustable delay and coincidence time. The first incident pulse of a photomultiplier starts a dead-time window which is realized by a triggering of the dead time which can be chosen to be between 10 ms and 100 ms as well as starting a coincidence window with a duration of typically 10 ns to 100 ns. Each subsequent input signal of the other two photomultipliers incident in this coincidence window is registered. Pulses occurring outside this window are ignored and simply lead to a re-triggering of the dead time. About 2.5 ms after the first coincidence window, the time window for the gamma detector channel is open for 100 ns. After this time has elapsed, the multiplicity of the coincidences is determined and the corresponding counters are incremented. All in all, there are 14 counters implemented to record single events, double and triple coincidences as well as live time and real time. The whole cycle of this measurement principle is controlled by the FPGA. The first coincidence window is generated by means of a programmable delay line and can be adjusted with a resolution of 250 ps. The dead time as well as the delay time and the width of the window for the gamma channel can be adjusted with a resolution of 5 ns. The time base can be generated either internally by means of a quartz oscillator or by a standard frequency of 10 MHz fed in externally. Further applications can be realized in the short term by reprogramming the FPGA.
5. Measurement procedures and system checks 5.1. Threshold adjustments Although CPMTs do not provide spectral information and, consequently, do not resolve the single electron peak like conventional PMTs, correct HV settings and proper threshold adjustments are crucial to obtain consistent and reliable measurement data. The HV values were adjusted to reach comparable amplification for all three CPMTs and reasonable threshold values within the limits of the discriminator but without overloading the amplifier. There is no additional signal output to obtain spectral information recordable with an ADC and, therefore, the correct threshold settings were determined by varying the threshold of all channels simultaneously on the discriminator hardware. Observing the double count rates for each pair of CPMTs, a common threshold was selected to reach maximum efficiency and to be well above the noise (Fig. 4). 5.2. Linearity checks A check of the system linearity, i.e. plotting the measured specific activity as a function of the known source activity, is a
Fig. 4. Count rates of any pair of double coincidences as a function of the common threshold, which was finally set at 60 mV to ensure a discrimination of noise.
good test for a working dead-time measurement and an appropriate correction. Since the detection efficiency of alpha emitters is essentially 1 (Kossert et al., 2009), there is no model uncertainty for the efficiency calculation involved in this test. From well standardized PTB solutions of 241Am, a series of sources was prepared to check the linearity of LSC counters at PTB (see e.g. Wanke et al., 2012). The samples measured had expected count rates of approx. 106, 5 105, 2 105, 1.4 105, 5 104, 2 104 counts per minute and were prepared in PTB's standard geometry in 20 mL borosilicate glass vials with 15 mL of Ultima Gold™ AB as a cocktail, the radioactive solution and an appropriate amount of distilled water to reach a total volume of 16 mL. The result of the linearity check is shown in Fig. 5 and confirms a very good linearity of the system of better than 0.1%. This includes the whole detection chain from CPMT to amplifier, discriminator and the final digital signal processing in the 4KAM module.
5.3. Dead time and coincidence resolving time Recently there have been some discussions about the appropriate setting of the coincidence resolving time and there are hints that a coincidence time of 40 ns as used by most laboratories might be too short (Bobin et al., 2012). The 4KAM allows a variation of the coincidence, time but since the reference system is a TDCR counter connected to a MAC3 module with 40 ns implemented, all the measurements presented in this work were carried out using the same dead time of 30 ms and a coincidence time of 40 ns.
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6. Results After the adjustment of HV and thresholds, the mini TDCR system was used to measure samples that were available at PTB and that were standardized in the reference system at the same time. All in all, 13 nuclides in glass and PE vials were standardized and compared to the PTB reference (Nähle et al., 2010). The results
0.15
Residuals in %
0.10 0.05 0.00 -0.05 -0.10 -0.15
1000
10000
Activity in Bq
Fig. 5. Test of the system linearity with a set of well calibrated 241Am sources. The deviation of the measured from the expected count rate is shown.
Table 1 Summary of the measurements made with the mini TDCR system and the reference TDCR system of PTB. For each nuclide the vial type and the deviation Δ of the mini TDCR from the reference system is given. The uncertainty stated does not take correlation, for example due to weighing, of the two results into account.
are summarized in Table 1 showing the relative deviations for each radionuclide and the combined uncertainty without taking into account correlations due to weighing and without uncertainty components due to the efficiency calculations. This is expected to be low for radionuclides with high counting efficiency but is important for low counting efficiencies (Table 2). For efficiencies of more than 90%, an uncertainty of less than 0.5% can be expected and alpha emitters can be standardized with even higher precision proving a working signal processing from the PMT output to the digital counters. It is interesting to note that all mini TDCR results are lower than those of the reference system and the relative difference increases with decreasing efficiency. So far, there is no explanation for this observation but since count rates are usually low for these measurements, it cannot be attributed to problems with the electronics, such as for example pile-up effects or deadtime circuitry. But there are indications that the model used in the TDCR calculations is not valid in case of low photon statistics (Bergeron and Zimmerman, 2011; Bobin et al., 2012). The detection efficiency for the logical sum of double coincidences can be as low as 5% for the measurement of 241Pu. With a TDCR value of about 0.1 this gives a detection efficiency for triple coincidences of less than 1%, which results, for a source in the kBq range, in count rates that are comparable to the background count rate. Therefore, background gives a considerable contribution to the uncertainty of the experimental TDCR value and, thus, also to the final result (Table 2, 3H uncertainty budget). With such low efficiencies there is also a large model influence on the determined activity. The influence of the kB value and the ionization quenching function, the calculated spectra and other decay data can add up to several percent.
Nuclide
Source type
Δ in %
u(Δ) in %
7. Summary and outlook
H-3 Cl-36 Ca-45 Fe-55 Fe-59 Fe-59 Ni-63 Sr-89 Sr-90/Y-90 Y-90 Tc-99 Pm-147 Th-229 Pu-241
PE Glass Glass PE PE Glass PE Glass Glass Glass PE Glass Glass PE
1.74 0.79 0.55 1.98 0.047 0.271 1.97 0.068 0.163 0.089 0.163 0.85 0.0125 1.42
5.4 0.153 0.31 0.56 0.128 0.086 0.257 0.095 0.032 0.128 0.090 0.039 0.091 7.2
The work presented in this paper proves that a miniaturized TDCR system can be used to standardize nuclides with rather high beta energies with an uncertainty of less than 1%. This is still unsatisfactory for metrological applications but would be sufficient for measurements outside of a national metrology institute where large uncertainties are less critical. For nuclides with lower energies such as 3H, 55Fe and 241Pu where uncertainties due to the theoretical model, the nuclide data and due to low statistics become more important, the uncertainty could be as high as 10% limiting the use of a mini TDCR system for these applications. From the experimental side it seems desirable to test a different type of PMT with higher intrinsic efficiency and a larger cathode
Table 2 Uncertainty budgets for the activity concentration as a measure for the system performance The uncertainties are given as standard uncertainties (k ¼ 1). Component
u(a)/a in % 3
89
241
3.0 0.02 0.03 4.0 0.01 0.05 0.01 o 0.25 0.2 0.10 2.0 0.25 5.4
0.03 0.01 0.01 0.05 0.01 0.05 0.01 0.15 0.1 0.05 0.1 0.1 0.25
0.03 0.02 0.01 0.05 0.01 0.05 0.01 o0.10 0.1 –/– –/– –/– 0.13
H
Counting statistics Weighing Dead time Background Counting time Adsorption Decay correction (half-life) Impurities TDCR value and interpolation of efficiency curve Nuclear and atomic data, model excluding ionization quenching Ionization quenching and kB value PMT asymmetry Square root of the sum of quadratic components (correlation coefficients are taken into account in the summation)
Sr
Am
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surface. An option to gain some spectral information would also improve the understanding of the results. The whole system could be made smaller by replacing the portable PC with a laptop. This would require a different approach for the setting of the HV along with the use of a calibrated oscillator in the coincidence module instead of a GPS board with an antenna. Acknowledgements This work has been carried out within the scope of the European Metrology Research Programme (EMRP) Joint Research Project ENG08, entitled “MetroFission”. References Atmel, 2006. ATmega8515(L) Summary datasheet. 〈http://www.atmel.com/Images/ 2512S.pdf〉. Bergeron, D.E., Zimmerman, B.E., 2011. TDCR measurements on 241Pu at NIST. In: Cassette, P. (Ed.), LSC2010, Advances in Liquid Scintillation Spectrometry: Proceedings of the International Conference on Liquid Scintillation Spectrometry, 6–10 September 2010, Paris, France. Radiocarbon, The University of Arizona, Tucson, Arizona, USA, ISBN 978-0-9638314-7-7, pp. 171–179. Bobin, C., Thiam, C., Chauvenet, B., Bouchard, J., 2012. On the stochastic dependence between photomultipliers in the TDCR method. Appl. Radiat. Isot 70, 770–780. 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, 669–672.
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Broda, R., Cassette, P., Kossert, K., 2007. Radionuclide metrology using liquid scintillation counting. Metrologia 44, S36–S52. Capogni, M., Cassette, P., Johansson, L., Kossert, K., Nähle, O., Sephton, J., Wanke, C.., 2011. Protocol for the Design of a Portable Liquid Scintillation Counter Based on the Triple to Double Coincidence Ratio Method. EMRP WP6, Deliverable 6.1.1. P. Cassette, Private Communication. EMRP, 2009. Energy – Joint Research Project Summary – Metrology for New Generation Nuclear Power Plants MetroFission, 2009. See 〈http://www.eura met.org/index.php?id=a169jrps〉. Ivan, C., Wätjen, A.C., Cassette, P., Sahagia, M., Antohe, A., Grigorescu, E.L., 2010. Participation in the CCRI(II)-K2.H-3 comparison and study of the new TDCR-LS counter with 6 CPMs. Appl. Radiat. Isot 68, 1543–1545. Kossert, K., Jörg, G., Nähle, O., Lierse, v., Gostomski, C., 2009. High-precision measurement of the half-life of 147Sm. Appl. Radiat. Isot 67, 1702–1706. Nähle, O., Kossert, K., Brunzendorf, J., 2009. Study of light emission processes for the design of liquid scintillation counters. In: Eikenberg, J., Jäggi, M., Beer, H., Baehrle, H. (Eds.), LSC2008, Advances in Liquid Scintillation Spectrometry: Proceedings of the Liquid Scintillation Spectrometry Conference, 25–30 May 2008, Davos, Switzerland. Radiocarbon, The University of Arizona, Tucson, Arizona, USA, pp. 87–95. Nähle, O., Kossert, K., Cassette, P., 2010. Activity standardization of 3H with the new TDCR system at PTB. Appl. Radiat. Isot 68, 1534–1536. trenz, 2011. trenz electronic, Spartan-3 Micromodule. 〈http://www.trenz-electronic. de/de/produkte/fpga-boards/trenz-electronic/te0140-spartan-3-series.html〉. Wanke, C., Kossert, K., Nähle, O.J., 2012. Investigations on TDCR measurements with the HIDEX 300 SL using a free parameter model. Appl. Radiat. Isot 70, 2176–2183. Xilinx, 2012. Xilinx Inc., Spartan-3 FPGA Family Data Sheet. 〈http://www.xilinx. com/support/documentation/data_sheets/ds099.pdf〉.
