- Email: [email protected]
Standardization of the radionuclides 60Co and 59Fe by digital 4πβ(PC)-γ(NaI) coincidence counting
Applied Radiation and Isotopes 109 (2016) 341–344
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Standardization of the radionuclides 60Co and 59Fe by digital 4πβ(PC)-γ (NaI) coincidence counting Ming Zhang a,n, Shunhe Yao b, Junchen Liang a, Haoran Liu a a b
National Institute of Metrology, Beijing 100029, PR China China Institute of Atomic Energy, Beijing 102413, PR China
H I G H L I G H T S
The nuclides 60Co and 59Fe were standardized using digital 4πβ-γ coincidence counting for participation in international comparison exercises. High-detection efficiencies were achieved in the beta-channel when applying the extrapolation technique due to the high energy beta transition for both nuclides. The relative standard uncertainty obtained for 60Co and 59Fe is 0.26% and 0.34% respectively. Complementary measurements of the activity concentration were carried out by using a calibrated HP Ge γ-spectrometer.
art ic l e i nf o
a b s t r a c t
Article history: Received 28 February 2015 Accepted 26 November 2015 Available online 2 December 2015
The digital coincidence counting (DCC) technique has been developed at NIM, China to replace the classical analog coincidence units in the 4πβ-γ counting system. The detector system comprises two NaI (Tl) γ-ray detectors and a 4πβ proportional counter (PC) operated with a mixture of argon and methane at atmospheric pressure in a gas flow arrangement. To update the activity results of radionuclide 60Co in the KCDB and contribute to the APMP.RI(II)-K2.Fe-59 comparison, 60Co and 59Fe dry sources were prepared and measured using the digital 4πβ(PC)-γ(NaI) coincidence system by applying the efficiency extrapolation method. For 60Co nuclide, the activity concentration value equal to 290.6 kBq g 1 with a relative standard uncertainty of 0.26%, is consistent with the result given by the calibrated ionization chamber. For the nuclide 59Fe, the activity concentration value at the reference date was 471.7 kBq g 1 with a relative standard uncertainty of 0.34%. This value is in good agreement with the result obtained with the HPGe γ spectrometry, which was calibrated by using a series of standard point sources from PTB. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Radionuclide metrology Digital coincidence counting International comparison
1. Introduction In order to update the activity result of radionuclide 60Co in the BIPM key comparison database (KCDB) (Ratel and Michotte, 2003) and to participate in the APMP. RI(II)-K2.Fe-59 comparison piloted by National Metrology Institute of Japan (NMIJ), a set of dry sources were prepared on VYNS films for both radionuclides and measured using the digital 4πβ-γ coincidence system developed in our laboratory. The radionuclides 60Co and 59Fe are typical isotopes decaying mainly via β-γ cascade transitions. For 60Co nuclide, it decays via a dominant β-transition (317.3 keV, P ¼99.88%) followed by γ3,1 (1173.2 keV, P ¼99.85%) and γ1,0 (1332.5 keV, P ¼99.98%) to the ground state of 60Ni (Bé et al., 2006). For the n
Corresponding author. E-mail address: [email protected] (M. Zhang).
http://dx.doi.org/10.1016/j.apradiso.2015.11.104 0969-8043/& 2015 Elsevier Ltd. All rights reserved.
nuclide 59Fe, there are two β-γ branches with high intensity during the disintegration to the ground state of 59Co, i.e. β1 transition (273.6 keV, P ¼45.21%) followed by γ1,0 (1291.6 keV, P ¼43.21%) and β2 transition (465.9 keV, P ¼53.33%) followed by γ2,0 (1099.3 keV, P ¼56.59%) (Bé et al., 2004). Therefore, the activities can be determined with good accuracy by using the 4πβ-γ coincidence counting method, and small uncertainties are expected due to the high energy β transitions (Campion, 1959). 2. Digitized 4πβ-γ coincidence counting system The study of the digital coincidence counting (DCC) technique (Buckman and Ius, 1996; Keightley and Park, 2007) to replace the classical analog-based coincidence counting (ACC) in China started in 2001, and was realized in 2005. The current system includes the data acquisition hardware and coincidence calculation software
342
M. Zhang et al. / Applied Radiation and Isotopes 109 (2016) 341–344
for offline data analysis. The data acquisition device is designed to adapt to signals from a shaping amplifier with pulse widths from 50 ns to hundreds of microseconds and pulse heights from (0–10) V. The hardware consists of sampling ADCs, complex programmable logic device (CPLD), digital signal processor (DSP) and some control circuits. Two sampling ADCs with the sampling rate of 40 MSPS and the resolution of 13 bits were used to record the pulse waveform from beta and gamma channels, respectively. Then a CPLD chip and a DSP chip were utilized to real-time analysis of pulse waveform and get the height, width, rising time and time stamp of each pulse. All the above information of the pulse picked up by the CPLD and DSP are saved to the computer fixed disk via PCI bus. The software program for coincidence calculation was developed in the C þ þ language with a visual interface. It provides the functionalities for spectrum display, parameters setting, efficiency extrapolation, etc. The program implemented two dead-time modes (National Council on Radiation Protection and Measurements 1985, a hand book of Radioactivity Measurement Procedures, NCRP Report 58), and Smith formula is used to access accidental coincidences and correct dead-time for the non-extendable dead-time mode, and the Muller formula is used for the extendable dead-time mode (Smith, 1987). Also multi-gamma energy regions could be used simultaneously for coincidence calculation. Considering a complex decay of a radionuclide with β-γ cascade transition, the counting rates in beta and gamma channels can be expressed as the following equations:
⎡ ⎛ N ⎞⎤ Nβ=A ⎢ 1 − f ⎜ 1 − c ⎟ ⎥ ⎢⎣ Nγ ⎠ ⎥⎦ ⎝
Nβ N γ Nc
⎡ ⎛ 1− Nc ⎞ ⎤ ⎢ Nγ ⎟ ⎥ ⎜ =A ⎢ 1 − f ⎜ N ⎟ ⎥ c ⎟ ⎜ ⎢⎣ ⎝ Nγ ⎠ ⎥⎦
(1)
(2)
Where the function f is assumed to be a polynomial, usually of order lower than 2. While changing the β detection efficiency and
extrapolating Nc/Nγ to unity, the value of f becomes 0, so one can deduce the activity A from Eq. (1) or (2). More information of the design of the DCC hardware and software used in the present work can be found in Yuan et al. (2010). The block diagram of the digital 4πβ-γ primary standard currently used is shown in Fig. 1, which is composed of two 75 mm*75 mm NaI(Tl) scintillation detectors and a gas flow proportional counter forγ-ray and electron detection respectively. The counting gas is a mixture of 90% argon and 10% methane operating at 0.1 MPa pressure. The analog-based coincidence counting circuits are also retained for system check and adjustment.
3. Standardization of
60
Co
60 Co is considered as perhaps the simplest nuclide exhibiting βγ cascade decay, and can be standardized by means of 4πβ-γ co-
incidence method via recording the coincidence events between the β transition and the two γ-rays with different energies, namely 1173.2 keV and 1332.5 keV. In order to update the activity result of 60 Co in the KCDB submitted to the BIPM by the radioactivity group of NIM China in 1978 (Ratel and Michotte, 2003), a set of 60Co dry sources were prepared by depositing accurately weighed aliquots (from 10 mg to 30 mg) onto gold-coated VYNS films. The source solution of 60Co was in the chemical form of cobalt chloride (CoCl2), in a carrier solution containing 0.1 mol/L HCl. Meanwhile, about 3.6 g solution with the total activity approximately 1.0 MBq was prepared in an NBS-type ampoule supplied by the BIPM, and was submitted to the SIR after measured in the calibrated ionization chamber maintained at the NIM China. Ten sources, including one blank source, were measured by using the digital 4πβ(PC)-γ(NaI) coincidence system. Each source was counted for about 20 min to achieve suitable counting statistics. For 60Co nuclide, both γ-peaks of 1173.2 keV and 1332.5 keV were used for the determination of the β-efficiency, and the maximum achieved β-efficiency for each source is greater than 95% by varying the discrimination threshold of β-channel (Baerg, 1973). The fixed dead time and coincidence resolving time was set
Fig. 1. Block diagram of the digital 4πβ(PC)-γ(NaI) coincidence system.
M. Zhang et al. / Applied Radiation and Isotopes 109 (2016) 341–344
Table 1 Experimental results of
60
Co by digital 4πβ (PC)-γ (NaI) coincidence counting.
Source no.
Mass (mg)
Activity concentration (kBq g 1)
1# 2# 3# 4# 5# 6# 7# 8# 9#
19.28 18.43 12.82 29.81 21.78 21.86 28.48 21.84 26.73
290.5 290.6 291.3 289.9 290.4 290.9 290.6 290.7 290.1
Arithmetic mean: 290.6 kBq g 1 at the reference time 2014-06-07 0 h UTC.
Table 2 Uncertainty budget associated to the determination of the activity concentration of 60 Co solution by digital 4πβ-γ coincidence method. Component
Evaluation type Relative standard uncertainty (%)
Counting statistics Weighing Dead time Background Coincidence resolving time Extrapolation of efficiency curve Impurities Relative combined standard uncertainty
A B B B B B B
0.21 0.10 0.05 0.05 0.05 0.08 o 0.01 0.26
as 4.0 ms and 1.0 ms during the coincidence calculation, respectively. The measurement results of nine 60Co samples with the digital 4πβ-γ coincidence technique can be found in Table 1. By considering the recommended half-life value 5.2711(8) y of 60Co, an activity concentration value 290.6 kBq g 1 at the reference time 2014-06-07 0 h UTC was obtained with a relative standard uncertainty is 0.26% (details of the uncertainty budget was shown in Table 2). In the case of participation at BIPM.RI(II)-K1.Co-60 comparison to update the result of the NIM China, an ampoule containing 3.61128 g solution of 60Co in an NBS-type ampoule supplied by the BIPM was prepared and measured in a calibrated 4πγ ionization chamber IG11/N20, which was filled with nitrogen to a pressure of 2.0 MPa and a Radium 226 as the reference source. The ionization chamber was well calibrated with up to over 30 standard γ emission sources (including the 60Co radionuclide absolutely measured with 4πβ-γ coincidence counting before) covering a wide photon energy range. Each solution for calibration in the ionization chamber was sealed in 5 ml ampoule geometry with a mass of 3.6 70.2 g and fixed with a custom-built plastic holder. By combining the measured currents and the former calibration factor for 60Co, the evaluated activity concentration for the solution in the NBS-type ampoule was 291.0 kBq g 1, with a relative standard uncertainty of 0.52%, which showed a good agreement with the result obtained by 4πβ(PC)-γ(NaI) coincidence counting in the range of uncertainty.
4. Standardization of
59
343
β emission, the 4πβ-γ coincidence method is well-adapted to the standardization of 59Fe. In the frame of the APMP.RI(II)-K2.Fe-59 comparison organized by NMIJ, the standardization of 59Fe nuclide has been implemented in our laboratory. The solution was received in the chemical form of FeCl3 and the carrier concentration was 0.1 mg/g in 0.1 mol/L HCl. The solution was contained in a glass ampoule with the volume about 5 ml and the nominal activity concentration was approximately 500 kBq g 1 on June 1st, 2014. Before the preparation of sources, a preliminary estimate of the total activity of the original source in the glass ampoule was determined using the ionization chamber mentioned above. For coincidence measurements, an aliquot of the original solution of 59Fe was used to prepare a set of 11 active sources with masses between 60 and 90 mg deposited on goldcoated VYNS foils. In addition, seven point sources with the masses of approximately 80 mg were also prepared for impurity checks and activity measurement, using a calibrated HPGe spectrometer. To reduce the uncertainty of efficiency extrapolation, both γpeaks of 1099.3 keV and 1291.6 keV which related two beta decay branches were selected simultaneously as the gates to record the coincidence events. The dead time and coincidence resolving time were set using the same value as in the measurement of 60Co. Fig. 2 represents a typical result obtained with different types of βefficiency extrapolation. The maximum achieved β-efficiency is greater than 96% by varying the discrimination threshold of βchannel. Thus the uncertainty component due to the extrapolation of efficiency curve is relatively small. The value of this component is evaluated to 0.22% in the combined standard uncertainty. Measurement results of the 11 59Fe samples with 4πβ(PC)-γ (NaI) coincidence method are listed in Table 3. All the activity concentrations were corrected for the decay at the reference date 2014-06-01 0 h UTC by taking the half-life value equal to 44.495 (8) days. The average activity concentration value obtained is 471.7 kBq g 1, with a relative combined standard uncertainty of 0.34%. The uncertainty components evaluated in the determination of the massic activity for 59Fe solution by 4πβ-γ coincidence method was summarized in Table 4. Complementary measurements of the activity concentration of 59 Fe were performed with the calibrated HPGe spectrometer by using 1099.3 keV, 1291.6 keV, and 192.3 keV γ-rays. This measurement was also used to check the possible impurities of 55Fe and 60Co. A commercial 260 cm3 Extended Range Coaxial-type Ge detector combined with GENIE 2000 software for spectrum analysis was selected to carry out the measurements for its large
Fe
The isotope 59Fe (T1/2 ¼44.495(8) d) decays mainly via two strong β-branches to the ground state of 59Co, with the maximum β energies 465.9 keV (P ¼53.33%) and 273.6 keV (P ¼45.22%). The γ-rays related to each branch are 1099.3 keV and 1291.6 keV, with the emission probabilities equal to 43.22% and 56.60% respectively. Because high β-efficiencies can be achieved due to the high energy
Fig. 2. 4πβ-γ coincidence extrapolation curves obtained for the 1099.3 keV and 1291.6 keV γ-gates on the activity standardization of 59Fe.
344
Table 3 Experimental results of
M. Zhang et al. / Applied Radiation and Isotopes 109 (2016) 341–344
impurities were found in the sealed point sources. 59
Fe by digital 4πβ (PC)-γ (NaI) coincidence counting.
Source no.
Mass (mg)
Activity concentration (kBq g 1)
1# 2# 3# 4# 5# 6# 7# 8# 9# 10# 11#
62.79 93.33 93.69 80.23 63.59 91.18 70.66 76.29 81.50 77.81 81.63
472.4 471.0 470.8 471.3 472.1 471.6 471.4 471.4 472.4 472.7 471.7
5. Conclusions
Arithmetic mean: 471.7 kBq g 1 at the reference time 2014-06-01 0 h UTC. Table 4 Uncertainty budget associated to the determination of the activity concentration of 59 Fe solution by digital 4πβ-γ coincidence method.
In the frame of participation at the BIPM.RI(II)-K1.Co-60 and APMP.RI(II)-K2.Fe-59 comparisons, primary measurements of 60Co and 59Fe activity concentrations have been implemented in our laboratory by digital 4πβ-γ coincidence counting method. To provide confirmatory results, a solution containing 60Co radionuclide in the NBS-type ampoule and 59Fe sealed point sources were also measured in the calibrated 4πγ ionization chamber and the HPGe spectrometer, respectively. The results obtained from different methods for each radionuclide are in good agreement within the range of uncertainties. Both results have been submitted to the pilot laboratory.
Acknowledgments Component
Evaluation type Relative standard uncertainty (%)
Counting statistics Weighing Dead time Background Coincidence resolving time Adsorption Extrapolation of efficiency curve Half life Impurities Relative combined standard uncertainty
A B B B B B B B B
0.19 0.10 0.05 0.05 0.05 0.06 0.22 0.04 o 0.01 0.34
energy range detection. A series of standard radionuclide sources traceable to PTB, including 241Am, 137Cs, 60Co, 152Eu, 57Co and 133Ba were used to establish the efficiency calibration of the spectrometer. The obtained results showed that the massic activity values with 192.3 keV and 1291.6 keV γ-ray peaks were in good agreement with the results by 4πβ(PC)-γ(NaI) coincidence method, with a relative difference 0.08%, whereas the value obtained by analyzing 1099.3 keV γ-ray peak was found to be higher than the result by coincidence method with a relative deviation is 0.8%, which is still acceptable in the range of the uncertainty of the HPGe spectrometer. In addition, no photon-emitting radionuclides
The authors would like to express their sincere thanks to Prof. D.Q. Yuan (China Institute of Atomic Energy) for his guidance in this work.
References Bé, M.-M., et al., 2006. Table of radionuclide (Vol. 3-A ¼3-244) monographie BIPM5. BIPM, Sèvres, 23–28. Bé, M.-M., et al., 2004. Table of radionuclide (Vol. 1-A¼ 1-150) monographie BIPM5. BIPM, Sèvres, 99–103. Buckman, S.M., Ius, D., 1996. Digital coincidence counting. Nucl. Instrum. Methods A 369, 368–374. Baerg, A.P., 1973. The efficiency extrapolation method in coincidence counting. Nucl. Instrum. Methods 112, 143–150. Campion, P.J., 1959. The standardization of radioisotopes by the beta-gamma coincidence method using high efficiency detectors. Int. J. Appl. Radiat. Isot. 4, 232–248. Keightley, J., Park, T.S., 2007. Digital coincidence counting for radionuclide standardization. Metrologia 44, S32–S35. Ratel, G., Michotte, C., 2003. BIPM comparison BIPM.RI(II)-K1.Co-60 of the activity measurements of the radionuclide Co-60. Metrologia 40, 06007. Smith, D., 1987. Some developments in the Cox-Isham theory of coincidence corrections, including the extension to the computer discrimination method. Appl. Radiat. Isot. A 38, 813–821. Yuan, D.Q., et al., 2010. Digital coincidence system for beta-gamma coincidence counting. Nucl. Phys. A 834, 773c–776c.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Standardization of the radionuclides 60Co and 59Fe by digital 4πβ(PC)-γ (NaI) coincidence counting Ming Zhang a,n, Shunhe Yao b, Junchen Liang a, Haoran Liu a a b
National Institute of Metrology, Beijing 100029, PR China China Institute of Atomic Energy, Beijing 102413, PR China
H I G H L I G H T S
The nuclides 60Co and 59Fe were standardized using digital 4πβ-γ coincidence counting for participation in international comparison exercises. High-detection efficiencies were achieved in the beta-channel when applying the extrapolation technique due to the high energy beta transition for both nuclides. The relative standard uncertainty obtained for 60Co and 59Fe is 0.26% and 0.34% respectively. Complementary measurements of the activity concentration were carried out by using a calibrated HP Ge γ-spectrometer.
art ic l e i nf o
a b s t r a c t
Article history: Received 28 February 2015 Accepted 26 November 2015 Available online 2 December 2015
The digital coincidence counting (DCC) technique has been developed at NIM, China to replace the classical analog coincidence units in the 4πβ-γ counting system. The detector system comprises two NaI (Tl) γ-ray detectors and a 4πβ proportional counter (PC) operated with a mixture of argon and methane at atmospheric pressure in a gas flow arrangement. To update the activity results of radionuclide 60Co in the KCDB and contribute to the APMP.RI(II)-K2.Fe-59 comparison, 60Co and 59Fe dry sources were prepared and measured using the digital 4πβ(PC)-γ(NaI) coincidence system by applying the efficiency extrapolation method. For 60Co nuclide, the activity concentration value equal to 290.6 kBq g 1 with a relative standard uncertainty of 0.26%, is consistent with the result given by the calibrated ionization chamber. For the nuclide 59Fe, the activity concentration value at the reference date was 471.7 kBq g 1 with a relative standard uncertainty of 0.34%. This value is in good agreement with the result obtained with the HPGe γ spectrometry, which was calibrated by using a series of standard point sources from PTB. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Radionuclide metrology Digital coincidence counting International comparison
1. Introduction In order to update the activity result of radionuclide 60Co in the BIPM key comparison database (KCDB) (Ratel and Michotte, 2003) and to participate in the APMP. RI(II)-K2.Fe-59 comparison piloted by National Metrology Institute of Japan (NMIJ), a set of dry sources were prepared on VYNS films for both radionuclides and measured using the digital 4πβ-γ coincidence system developed in our laboratory. The radionuclides 60Co and 59Fe are typical isotopes decaying mainly via β-γ cascade transitions. For 60Co nuclide, it decays via a dominant β-transition (317.3 keV, P ¼99.88%) followed by γ3,1 (1173.2 keV, P ¼99.85%) and γ1,0 (1332.5 keV, P ¼99.98%) to the ground state of 60Ni (Bé et al., 2006). For the n
Corresponding author. E-mail address: [email protected] (M. Zhang).
http://dx.doi.org/10.1016/j.apradiso.2015.11.104 0969-8043/& 2015 Elsevier Ltd. All rights reserved.
nuclide 59Fe, there are two β-γ branches with high intensity during the disintegration to the ground state of 59Co, i.e. β1 transition (273.6 keV, P ¼45.21%) followed by γ1,0 (1291.6 keV, P ¼43.21%) and β2 transition (465.9 keV, P ¼53.33%) followed by γ2,0 (1099.3 keV, P ¼56.59%) (Bé et al., 2004). Therefore, the activities can be determined with good accuracy by using the 4πβ-γ coincidence counting method, and small uncertainties are expected due to the high energy β transitions (Campion, 1959). 2. Digitized 4πβ-γ coincidence counting system The study of the digital coincidence counting (DCC) technique (Buckman and Ius, 1996; Keightley and Park, 2007) to replace the classical analog-based coincidence counting (ACC) in China started in 2001, and was realized in 2005. The current system includes the data acquisition hardware and coincidence calculation software
342
M. Zhang et al. / Applied Radiation and Isotopes 109 (2016) 341–344
for offline data analysis. The data acquisition device is designed to adapt to signals from a shaping amplifier with pulse widths from 50 ns to hundreds of microseconds and pulse heights from (0–10) V. The hardware consists of sampling ADCs, complex programmable logic device (CPLD), digital signal processor (DSP) and some control circuits. Two sampling ADCs with the sampling rate of 40 MSPS and the resolution of 13 bits were used to record the pulse waveform from beta and gamma channels, respectively. Then a CPLD chip and a DSP chip were utilized to real-time analysis of pulse waveform and get the height, width, rising time and time stamp of each pulse. All the above information of the pulse picked up by the CPLD and DSP are saved to the computer fixed disk via PCI bus. The software program for coincidence calculation was developed in the C þ þ language with a visual interface. It provides the functionalities for spectrum display, parameters setting, efficiency extrapolation, etc. The program implemented two dead-time modes (National Council on Radiation Protection and Measurements 1985, a hand book of Radioactivity Measurement Procedures, NCRP Report 58), and Smith formula is used to access accidental coincidences and correct dead-time for the non-extendable dead-time mode, and the Muller formula is used for the extendable dead-time mode (Smith, 1987). Also multi-gamma energy regions could be used simultaneously for coincidence calculation. Considering a complex decay of a radionuclide with β-γ cascade transition, the counting rates in beta and gamma channels can be expressed as the following equations:
⎡ ⎛ N ⎞⎤ Nβ=A ⎢ 1 − f ⎜ 1 − c ⎟ ⎥ ⎢⎣ Nγ ⎠ ⎥⎦ ⎝
Nβ N γ Nc
⎡ ⎛ 1− Nc ⎞ ⎤ ⎢ Nγ ⎟ ⎥ ⎜ =A ⎢ 1 − f ⎜ N ⎟ ⎥ c ⎟ ⎜ ⎢⎣ ⎝ Nγ ⎠ ⎥⎦
(1)
(2)
Where the function f is assumed to be a polynomial, usually of order lower than 2. While changing the β detection efficiency and
extrapolating Nc/Nγ to unity, the value of f becomes 0, so one can deduce the activity A from Eq. (1) or (2). More information of the design of the DCC hardware and software used in the present work can be found in Yuan et al. (2010). The block diagram of the digital 4πβ-γ primary standard currently used is shown in Fig. 1, which is composed of two 75 mm*75 mm NaI(Tl) scintillation detectors and a gas flow proportional counter forγ-ray and electron detection respectively. The counting gas is a mixture of 90% argon and 10% methane operating at 0.1 MPa pressure. The analog-based coincidence counting circuits are also retained for system check and adjustment.
3. Standardization of
60
Co
60 Co is considered as perhaps the simplest nuclide exhibiting βγ cascade decay, and can be standardized by means of 4πβ-γ co-
incidence method via recording the coincidence events between the β transition and the two γ-rays with different energies, namely 1173.2 keV and 1332.5 keV. In order to update the activity result of 60 Co in the KCDB submitted to the BIPM by the radioactivity group of NIM China in 1978 (Ratel and Michotte, 2003), a set of 60Co dry sources were prepared by depositing accurately weighed aliquots (from 10 mg to 30 mg) onto gold-coated VYNS films. The source solution of 60Co was in the chemical form of cobalt chloride (CoCl2), in a carrier solution containing 0.1 mol/L HCl. Meanwhile, about 3.6 g solution with the total activity approximately 1.0 MBq was prepared in an NBS-type ampoule supplied by the BIPM, and was submitted to the SIR after measured in the calibrated ionization chamber maintained at the NIM China. Ten sources, including one blank source, were measured by using the digital 4πβ(PC)-γ(NaI) coincidence system. Each source was counted for about 20 min to achieve suitable counting statistics. For 60Co nuclide, both γ-peaks of 1173.2 keV and 1332.5 keV were used for the determination of the β-efficiency, and the maximum achieved β-efficiency for each source is greater than 95% by varying the discrimination threshold of β-channel (Baerg, 1973). The fixed dead time and coincidence resolving time was set
Fig. 1. Block diagram of the digital 4πβ(PC)-γ(NaI) coincidence system.
M. Zhang et al. / Applied Radiation and Isotopes 109 (2016) 341–344
Table 1 Experimental results of
60
Co by digital 4πβ (PC)-γ (NaI) coincidence counting.
Source no.
Mass (mg)
Activity concentration (kBq g 1)
1# 2# 3# 4# 5# 6# 7# 8# 9#
19.28 18.43 12.82 29.81 21.78 21.86 28.48 21.84 26.73
290.5 290.6 291.3 289.9 290.4 290.9 290.6 290.7 290.1
Arithmetic mean: 290.6 kBq g 1 at the reference time 2014-06-07 0 h UTC.
Table 2 Uncertainty budget associated to the determination of the activity concentration of 60 Co solution by digital 4πβ-γ coincidence method. Component
Evaluation type Relative standard uncertainty (%)
Counting statistics Weighing Dead time Background Coincidence resolving time Extrapolation of efficiency curve Impurities Relative combined standard uncertainty
A B B B B B B
0.21 0.10 0.05 0.05 0.05 0.08 o 0.01 0.26
as 4.0 ms and 1.0 ms during the coincidence calculation, respectively. The measurement results of nine 60Co samples with the digital 4πβ-γ coincidence technique can be found in Table 1. By considering the recommended half-life value 5.2711(8) y of 60Co, an activity concentration value 290.6 kBq g 1 at the reference time 2014-06-07 0 h UTC was obtained with a relative standard uncertainty is 0.26% (details of the uncertainty budget was shown in Table 2). In the case of participation at BIPM.RI(II)-K1.Co-60 comparison to update the result of the NIM China, an ampoule containing 3.61128 g solution of 60Co in an NBS-type ampoule supplied by the BIPM was prepared and measured in a calibrated 4πγ ionization chamber IG11/N20, which was filled with nitrogen to a pressure of 2.0 MPa and a Radium 226 as the reference source. The ionization chamber was well calibrated with up to over 30 standard γ emission sources (including the 60Co radionuclide absolutely measured with 4πβ-γ coincidence counting before) covering a wide photon energy range. Each solution for calibration in the ionization chamber was sealed in 5 ml ampoule geometry with a mass of 3.6 70.2 g and fixed with a custom-built plastic holder. By combining the measured currents and the former calibration factor for 60Co, the evaluated activity concentration for the solution in the NBS-type ampoule was 291.0 kBq g 1, with a relative standard uncertainty of 0.52%, which showed a good agreement with the result obtained by 4πβ(PC)-γ(NaI) coincidence counting in the range of uncertainty.
4. Standardization of
59
343
β emission, the 4πβ-γ coincidence method is well-adapted to the standardization of 59Fe. In the frame of the APMP.RI(II)-K2.Fe-59 comparison organized by NMIJ, the standardization of 59Fe nuclide has been implemented in our laboratory. The solution was received in the chemical form of FeCl3 and the carrier concentration was 0.1 mg/g in 0.1 mol/L HCl. The solution was contained in a glass ampoule with the volume about 5 ml and the nominal activity concentration was approximately 500 kBq g 1 on June 1st, 2014. Before the preparation of sources, a preliminary estimate of the total activity of the original source in the glass ampoule was determined using the ionization chamber mentioned above. For coincidence measurements, an aliquot of the original solution of 59Fe was used to prepare a set of 11 active sources with masses between 60 and 90 mg deposited on goldcoated VYNS foils. In addition, seven point sources with the masses of approximately 80 mg were also prepared for impurity checks and activity measurement, using a calibrated HPGe spectrometer. To reduce the uncertainty of efficiency extrapolation, both γpeaks of 1099.3 keV and 1291.6 keV which related two beta decay branches were selected simultaneously as the gates to record the coincidence events. The dead time and coincidence resolving time were set using the same value as in the measurement of 60Co. Fig. 2 represents a typical result obtained with different types of βefficiency extrapolation. The maximum achieved β-efficiency is greater than 96% by varying the discrimination threshold of βchannel. Thus the uncertainty component due to the extrapolation of efficiency curve is relatively small. The value of this component is evaluated to 0.22% in the combined standard uncertainty. Measurement results of the 11 59Fe samples with 4πβ(PC)-γ (NaI) coincidence method are listed in Table 3. All the activity concentrations were corrected for the decay at the reference date 2014-06-01 0 h UTC by taking the half-life value equal to 44.495 (8) days. The average activity concentration value obtained is 471.7 kBq g 1, with a relative combined standard uncertainty of 0.34%. The uncertainty components evaluated in the determination of the massic activity for 59Fe solution by 4πβ-γ coincidence method was summarized in Table 4. Complementary measurements of the activity concentration of 59 Fe were performed with the calibrated HPGe spectrometer by using 1099.3 keV, 1291.6 keV, and 192.3 keV γ-rays. This measurement was also used to check the possible impurities of 55Fe and 60Co. A commercial 260 cm3 Extended Range Coaxial-type Ge detector combined with GENIE 2000 software for spectrum analysis was selected to carry out the measurements for its large
Fe
The isotope 59Fe (T1/2 ¼44.495(8) d) decays mainly via two strong β-branches to the ground state of 59Co, with the maximum β energies 465.9 keV (P ¼53.33%) and 273.6 keV (P ¼45.22%). The γ-rays related to each branch are 1099.3 keV and 1291.6 keV, with the emission probabilities equal to 43.22% and 56.60% respectively. Because high β-efficiencies can be achieved due to the high energy
Fig. 2. 4πβ-γ coincidence extrapolation curves obtained for the 1099.3 keV and 1291.6 keV γ-gates on the activity standardization of 59Fe.
344
Table 3 Experimental results of
M. Zhang et al. / Applied Radiation and Isotopes 109 (2016) 341–344
impurities were found in the sealed point sources. 59
Fe by digital 4πβ (PC)-γ (NaI) coincidence counting.
Source no.
Mass (mg)
Activity concentration (kBq g 1)
1# 2# 3# 4# 5# 6# 7# 8# 9# 10# 11#
62.79 93.33 93.69 80.23 63.59 91.18 70.66 76.29 81.50 77.81 81.63
472.4 471.0 470.8 471.3 472.1 471.6 471.4 471.4 472.4 472.7 471.7
5. Conclusions
Arithmetic mean: 471.7 kBq g 1 at the reference time 2014-06-01 0 h UTC. Table 4 Uncertainty budget associated to the determination of the activity concentration of 59 Fe solution by digital 4πβ-γ coincidence method.
In the frame of participation at the BIPM.RI(II)-K1.Co-60 and APMP.RI(II)-K2.Fe-59 comparisons, primary measurements of 60Co and 59Fe activity concentrations have been implemented in our laboratory by digital 4πβ-γ coincidence counting method. To provide confirmatory results, a solution containing 60Co radionuclide in the NBS-type ampoule and 59Fe sealed point sources were also measured in the calibrated 4πγ ionization chamber and the HPGe spectrometer, respectively. The results obtained from different methods for each radionuclide are in good agreement within the range of uncertainties. Both results have been submitted to the pilot laboratory.
Acknowledgments Component
Evaluation type Relative standard uncertainty (%)
Counting statistics Weighing Dead time Background Coincidence resolving time Adsorption Extrapolation of efficiency curve Half life Impurities Relative combined standard uncertainty
A B B B B B B B B
0.19 0.10 0.05 0.05 0.05 0.06 0.22 0.04 o 0.01 0.34
energy range detection. A series of standard radionuclide sources traceable to PTB, including 241Am, 137Cs, 60Co, 152Eu, 57Co and 133Ba were used to establish the efficiency calibration of the spectrometer. The obtained results showed that the massic activity values with 192.3 keV and 1291.6 keV γ-ray peaks were in good agreement with the results by 4πβ(PC)-γ(NaI) coincidence method, with a relative difference 0.08%, whereas the value obtained by analyzing 1099.3 keV γ-ray peak was found to be higher than the result by coincidence method with a relative deviation is 0.8%, which is still acceptable in the range of the uncertainty of the HPGe spectrometer. In addition, no photon-emitting radionuclides
The authors would like to express their sincere thanks to Prof. D.Q. Yuan (China Institute of Atomic Energy) for his guidance in this work.
References Bé, M.-M., et al., 2006. Table of radionuclide (Vol. 3-A ¼3-244) monographie BIPM5. BIPM, Sèvres, 23–28. Bé, M.-M., et al., 2004. Table of radionuclide (Vol. 1-A¼ 1-150) monographie BIPM5. BIPM, Sèvres, 99–103. Buckman, S.M., Ius, D., 1996. Digital coincidence counting. Nucl. Instrum. Methods A 369, 368–374. Baerg, A.P., 1973. The efficiency extrapolation method in coincidence counting. Nucl. Instrum. Methods 112, 143–150. Campion, P.J., 1959. The standardization of radioisotopes by the beta-gamma coincidence method using high efficiency detectors. Int. J. Appl. Radiat. Isot. 4, 232–248. Keightley, J., Park, T.S., 2007. Digital coincidence counting for radionuclide standardization. Metrologia 44, S32–S35. Ratel, G., Michotte, C., 2003. BIPM comparison BIPM.RI(II)-K1.Co-60 of the activity measurements of the radionuclide Co-60. Metrologia 40, 06007. Smith, D., 1987. Some developments in the Cox-Isham theory of coincidence corrections, including the extension to the computer discrimination method. Appl. Radiat. Isot. A 38, 813–821. Yuan, D.Q., et al., 2010. Digital coincidence system for beta-gamma coincidence counting. Nucl. Phys. A 834, 773c–776c.