- Email: [email protected]
63Ni activity measurements a bilateral intercomparison
Applied Radiation and Isotopes 148 (2019) 60–63
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
63
Ni activity measurements a bilateral intercomparison a,∗
b
a
a
D.B. Kulkarni , Y. Sato , R. Anuradha , Leena Joseph , M.S. Kulkarni a b
T a
Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai, 400 085, India National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology, Umezono 1-1-1, Tsukuba, Ibaraki, 305-8568, Japan
H I GH L IG H T S
intercomparison organised between BARC and NMIJ. • Bilateral method used at BARC, India. • CIEMAT/NIST method used at NMIJ, Japan. • TDCR • The E score and degrees of equivalence indicates excellent agreement in results. n
A B S T R A C T
To assess the accuracy and capabilities of BARC, for standardization of 63Ni before participating in larger scale trial exercise to implement and test the methods for the extension of the SIR to β emitters, a bilateral intercomparison was organised with National Metrology Institute of Japan (NMIJ), Japan. Standardization of 63Ni was carried out and the results were compared with those obtained from NMIJ to assess the accuracy and capabilities of the laboratories. The CIEMAT/NIST efficiency tracing technique based on 3H standard was used for measurement of 63Ni activity concentration at BARC, India whereas Triple to Double Coincidence Ratio (TDCR) method was used at NMIJ, Japan. The procedures adopted for the standardization of 63Ni by CIEMAT/NIST method at BARC and TDCR method at NMIJ are presented. The percentage deviation in activity concentration of 63Ni between BARC, India and NMIJ, Japan is 0.27%. To evaluate the performance of techniques used at both the laboratories, En score (k = 2) and degrees of equivalence was calculated. The En score of −0.12 and degrees of equivalence −0.06 kBq g−1 clearly indicates that the activity concentration of 63Ni measured at BARC, India and NMIJ, Japan are in excellent agreement and comparable within uncertainty limits and demonstrates the degrees of equivalence of the standards maintained at BARC, India and NMIJ Japan.
1. Introduction 63
Ni is extensively used as a standard for calibration of contamination monitors because of its long half life and low beta energy. 63 Ni is also found as neutron-activation product in nuclear-power reactor and its environment. It is also one of the radioactive and chemically corrosive contaminants in high-level liquid-waste at nuclear fuel storage and reprocessing facilities. 63Ni is a beta emitter with a half life of 98.7 ± 0.24 years (Lee, 2003). It decays directly by beta emission to the ground state of 63Cu with maximum energy 66.945 ± 0.002 keV (100%) (Hetherington et al., 1987; Kawakami et al., 1992; Martin, 1995). 63Ni being a pure beta emitter, CIEMAT/NIST and Triple to Double Coincidence Ratio (TDCR) techniques are used for its primary standardization. International intercomparison of activity measurements using primary standards helps in establishing equivalence between various National Metrology Institutes (NMIs). The Extension of Système International de Référence (ESIR) Working Group (ESWG(II)) of the
∗
Consultative Committee for Ionizing Radiation (CCRI(II)) of Bureau international des poids et measures (BIPM) has proposed liquid scintillation counting (LSC) technique for extension of SIR to β emitters. A larger scale trial exercise to implement and test the methods for the ESIR to β emitters, using low-energy, medium-energy and electron capture radionuclides such as 3H, 63Ni, 14C and 55Fe, was agreed, at the CCRI(II) meeting held in 2013 at BIPM. Bhabha Atomic Research Centre (BARC) as designated institute for ionizing radiations in India consented for participation in intercomparison along with 19 National Metrology Institutes (NMIs)/Designated Institutes (DIs). To assess the accuracy and capabilities of BARC for standardization of 63Ni, before participating in larger scale trial exercise to implement and test the methods for ESIR, bilateral intercomparison was organised with National Metrology Institute of Japan (NMIJ), Japan. The procedure used for the standardization of 63Ni by CIEMAT/ NIST method at BARC and TDCR method at NMIJ is presented in this paper.
Corresponding author. E-mail address: [email protected] (D.B. Kulkarni).
https://doi.org/10.1016/j.apradiso.2019.03.025 Received 27 August 2018; Received in revised form 15 March 2019; Accepted 17 March 2019 Available online 20 March 2019 0969-8043/ © 2019 Published by Elsevier Ltd.
Applied Radiation and Isotopes 148 (2019) 60–63
D.B. Kulkarni, et al.
Fig. 1. The sources preparation scheme for 63Ni sources and 3H standards for measurements by CIEMAT/NIST method at BARC, India and by TDCR method at NMIJ, Japan.
indicating parameter i.e. tSIE using a133Ba radioactive source brought to the reproducible position above the vial during tSIE measurements. 63 Ni samples were counted in the spectrometer along with a set of 3 H quenching standards. The counting efficiencies of 3H versus figure of merit (FOM) and 63Ni versus FOM were calculated using the freeware program CN 2003 (Günther, 2002), developed by Günther of the Physikalisch-Technische Bundesanstalt (PTB), Germany, which is available on the ICRM Liquid Scintillation Counting Working Group website hosted by the Laboratoire National Henri Becquerel (LNHB) (http:// www.nucleide.org/ICRM_LSC_WG/icrmsoftware.htm). The beta emission to the ground state of 63Cu with maximum energy 66.945 ± 0.002 keV (100%) (Kawakami et al., 1992) was used for calculation of the detection efficiency in the program. The half-life used for the calculation was taken from DDEP and was obtained from the website hosted by LNHB (Lee, 2003). The plot of experimental 3H efficiencies versus quench indication parameter tSIE is shown in Fig. 2. The plot of calculated 63Ni efficiencies versus 3H efficiencies obtained
2. Experimental 2.1. Source preparation The radioactive solution of 63Ni in the form of nickel chloride was procured from Board of Radiation and Isotope Technology (BRIT), India. The original solution was diluted with dilution factor of 9.96 using inactive nickel chloride solution (50 μg/mL in 0.1 N HCl). The diluted solution was then transferred to NIST type ampoule (2.017623 g), flame sealed and dispatched to NMIJ for standardization at their end. From the same diluted solution eight 63Ni samples were prepared in 20 mL low potassium liquid scintillation vials with 15 mL Ultima Gold Scintillator and 1 mL water for measurements by CIEMAT/ NIST efficiency tracing technique as shown in Fig. 1. One blank sample was also prepared in 20 mL low potassium liquid scintillation vial with 15 mL Ultima Gold Scintillator and 1 mL water. As shown in Fig. 1, a set of 3H quenched standards were prepared from the available 3H solution provided by Laboratoire National Henry Becquerel (LNHB), France during international intercomparison of activity measurements of 3H (CCRI(II)-K2.H3, 2009)(Ratel G. and C. Michotte, 2009). Incremental amount of nitromethane was added to 3H standards and 63Ni samples for variation of counting efficiencies by chemical quenching. For TDCR measurements at NMIJ, Japan one blank sample and three 63Ni samples of about 0.05 mL were prepared in 20 mL low potassium liquid scintillation vials with 15 mL Ultima Gold Scintillator and 0.95 mL water as shown in Fig. 1. All the sources were prepared by gravimetric method using pycnometer. 2.2. Measurements at BARC, India Standardization was carried out with CIEMAT/NIST efficiency tracing technique at BARC. The CIEMAT/NIST efficiency tracing technique (Gracia-Torano et al., 1991; Grau Malonda and Garcia-Torano, 1982) is suitable for determining activity of pure beta, beta-gamma, pure electron capture (EC) and EC-gamma decaying radionuclides using commercial liquid scintillation spectrometer using 3H standard as tracer. Packard TRI-CARB 2900 TR liquid scintillation analyzer with two photomultipliers operated in coincidence was used for the measurements. The spectrometer calculates the external standard quench
Fig. 2. The plot of experimental 3H efficiencies versus Quench indication parameter tSIE and the fitting function. 61
Applied Radiation and Isotopes 148 (2019) 60–63
D.B. Kulkarni, et al.
Table 2 The detailed uncertainty budget for activity concentration measurement at NMIJ, Japan using TDCR method. Uncertainty components, in % of the activity concentration, due to: Factor
Relative standard uncertainty (%)
Evaluation type (A or B)
Counting statistics Weighing Dead time Background Half-life Input parameters and statistical model
0.10 0.03 0.08 0.05 0.01 1.1
A B B B B B
Combined standard uncertainty (%) Combined standard uncertainty (kBq g−1)
Fig. 3. The plot of calculated and the fitting function.
63
1.1 0.25
1) The 63Ni samples were counted in the liquid scintillation spectrometer along with the set of 3H quenching standards to record their count rates, quench indicating parameters i.e. tSIE and date and time of measurement. 2) The 3H efficiencies corresponding to experimentally measured tSIE values of 63Ni samples were calculated using the plot of experimental 3H efficiency versus quench indication parameter tSIE (Fig. 2). 3) 63Ni counting efficiencies (εβ) of the samples were calculated using the plot of calculated 63Ni efficiency versus 3H efficiency (Fig. 3) and the 3H efficiencies corresponding to experimentally measured tSIE values of 63Ni samples (3H efficiencies are calculated in step 2). 4) The activity concentration of the 63Ni solution was determined from the decay corrected count rate, calculated counting efficiency and weight of the sample.
Ni efficiencies versus calculated 3H efficiencies
The 63Ni quenching samples counting efficiencies (εβ) were varied in the range of 38%–67% by addition of incremental amount of nitromethane. The counting time for each sample was 5 min. A set of eight 63Ni samples and seven 3H standards were counted 6 times within a period of 17 days. Fig. 4. Double and triple coincidences versus the TDCR parameter value for 63 Ni.
2.3. Measurements at NMIJ, Japan The TDCR method is an absolute method for activity measurement and especially useful for determination of activity of pure beta and pure EC-emitters (Broda et al., 1988; Cassette and Bouchard, 2003). It involves detection efficiency calculation from a physical and statistical model of the photon distribution emitted by the scintillating source, models for energy deposition and quenching. The TDCR detector system consists of three photo multiplier tubes (PMT) placed at an angle of 1200 from each other facing the liquid scintillation vial in the counting chamber. The discrimination level for the PMT is set in the valley, between the electronic noise and single electron peak. To minimize the counts due to thermal noise, the coincidence method is used. There is not enough information in a 2-photodetectors system to determine the experimental detection efficiency without an additional reference source and this is why the TDCR method uses a 3-photomultipliers detector, allowing the observation of 3 kind of double coincidences (2-photodetectors) and triple coincidence (3-photodetectors) (Broda et al., 1988; Cassette and Bouchard, 2003). The TDCR parameter is calculated using the count rates of logical sum of double coincidences and triple coincidences. The experimental detection efficiencies of samples were calculated using the measured TDCR parameters and efficiency calculations model of TDCR method. The TDCR equipment and the fitting program developed by NMIJ were used for the measurements (Sato et al., 2012). High voltage of 2200 V was supplied to the PMTs. Resolution time of fast coincidence
Table 1 The detailed uncertainty budget for activity concentration measurement at BARC, India using CIEMAT/NIST method. Uncertainty components, in % of the activity concentration, due to: Factor
Relative standard uncertainty (%)
Evaluation type (A or B)
Counting statistics + Background Weighing Dead time Half-life Uncertainty due to tracer activity Quench indicating parameter
0.44 0.03 0.01 0.01 0.96 0.01
A B B B B B
Combined standard uncertainty (%) Combined standard uncertainty (kBq g−1)
1.06 0.24
from CN2003 program is shown in Fig. 3. The value of ionization quench parameter ‘kB’ used in the calculation program was 0.0075 cm/ MeV as it gives maximum stability towards measurements of activity concentrations. The activity concentrations of 63Ni samples were calculated using CIEMAT/NIST efficiency tracing technique as follows.
62
Applied Radiation and Isotopes 148 (2019) 60–63
D.B. Kulkarni, et al.
modules and paralysable dead time was 50 ns and 20 μs respectively. Measurement time for each sample was 500 s and the measurements were repeated three times within a period of 4 days. For variation of TDCR values of 63Ni samples, optical filters as scintillation light absorber were placed around the vials. The variation of counting efficiency depends on the amount of scintillation light absorbed by the optical filter. The beta minus decay to the ground state of 63Cu with maximum energy 66.98 ± 0.015 keV and probability of 100% (Lee, 2003) was used for the fitting calculation. The TDCR parameter values of 63Ni were in the range of 0.73–0.79. The plots of triple and double coincidence efficiencies versus TDCR is shown in Fig. 4 and was used for calculation of double coincidence or triple coincidence efficiencies corresponding to measured TDCR parameter values.
BARC, India and NMIJ, Japan are in excellent agreement and comparable within uncertainty limits. Acknowledgement The authors would like to express their thanks and gratitude to Dr. K.S. Pradeepkumar, Associate Director, Health Safety and Environment Group, and Head Radiation Safety Systems Division, Bhabha Atomic Research Centre, Trombay, Mumbai and Dr. A. Yunoki, Group leader, radioactivity and neutron standards group, National Metrology Institute of Japan, Tsukuba, Ibaraki for their constant encouragement and support. Authors would like to thank Mrs. P. J. Reddy for providing LSC counter for CIEMAT/NIST measurements. Appendix A. Supplementary data
3. Results and discussion Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apradiso.2019.03.025.
The activity concentration of the 63Ni solution and the ‘kB’ factor derived by the TDCR fitting as reported to BARC, India were 22.54 ± 0.25 kBq g−1 and 0.0126 cm MeV−1 as on reference date of 1st January 2017, 00:00 UTC. The 63Ni activity concentration obtained by CIEMAT/NIST method, reported to NMIJ, Japan is 22.48 ± 0.24 kBq g−1 on the reference date. The detailed uncertainty budget for activity concentration measurement at BARC, India using CIEMAT/ NIST method and that using TDCR method at NMIJ, Japan is shown in Table 1 and Table 2 respectively. To evaluate the performance of BARC with respect to NMIJ, En score (k = 2) was calculated using equation (1).
En =
References Broda, R., Pochwalski, K., Radoszewski, T., 1988. Calculation of Liquid-scintillation detector efficiency. Int. J. Appl. Radiat. Isot. part A 39, 159–164. Cassette, P., Bouchard, J., 2003. The design of a liquid scintillation counter based on the triple to double coincidence ratio method. Nucl. Instrum. Methods Phys. Res., Sect. A 505 (1–2), 72–75. Garcia-Toraño, E., Martin Casallo, M.T., Rodriguez, L., Grau, A., Los Arcos, J.M., 1991. On the standardization of beta-gamma emitting nuclides by liquid scintillation counting. In: Ross, H., Noakes, J.E., Spaulding, J.D. (Eds.), Liquid Scintillation Counting and Organic Scintillators. Lewis Publishers, Chelsea MI, pp. 307–316. Grau Malonda, A., Garcia-Toraño, E., 1982. Evaluation of counting efficiency in liquid scintillation counting of pure beta-ray emitters. Int. J. Appl. Radiat. Isot. 33, 249–253. Günther, E.W., 2002. What can we expect from the CIEMAT/NIST method? Appl. Radiat. Isot. 56 (1–2), 357–360. Hetherington, D.W., Graham, R.L., Lone, M.A., Geiger, J.S., Lee-Whiting, G.E., 1987. Upper limits on the mixing of heavy neutrinos in the beta decay of 63Ni. Phys. Rev. C36, 1504–1513. ISO 13528, International Organization for Standardization, 2015. Statistical Methods for Use in Proficiency Testing by Interlaboratory Comparison. Kawakami, H., Kato, S., Ohshima, T., Rosenfeld, C., Sakamoto, H., Sato, T., Shibata, S., Shirai, J., Sugaya, Y., Suzuki, T., Takahashi, K., Tsukamoto, T., Ueno, K., Ukai, K., Wilson, S., Yonezawa, Y., 1992. High sensitivity search for a 17 keV neutrino. Negative indication with an upper limit of 0. 095 %. Phys. Lett. B287, 45–50. Lee, K.B., 2003. Table de Radionucleides. Monographie BIPM- 5 (3), 29–31. Martin, M., August, 1995. (Oak Ridge National Laboratory), Evaluated Nuclear Structure Data File (ENSDF), private communication. Ratel, G., 2005. Evaluation of the uncertainty of the degree of equivalence. Metrologia 42, 140–144. Ratel, G., Michotte, C., 2009. Draft B report, International comparison of activity measurements of a solution of tritiated water. Sato, Y., Yamada, T., Matsumoto, M., Wakitani, Y., Hasegawa, T., Yoshimura, T., Murayama, H., Oda, K., Sato, T., Unno, Y., Yunoki, A., 2012. Efficiency fitting for TDCR measurement data using polynomial approximation and the Newton–Raphson method. Appl. Radiat. Isot. 70, 2184–2187. Willink, R., 2003. On the interpretation and analysis of degree-of-equivalence. Metrologia 40, 9–17.
(x i − x j ) 2ui2 + 2 uj2
(1)
Where, xi, ui are the activity concentration and its standard uncertainty respectively, obtained for measurements at BARC, India. xj, uj are the activity concentration and its standard uncertainty respectively, obtained for measurements at NMIJ, Japan. The En score value (k = 2) calculated using the activity concentrations and their associated uncertainties is −0.12. The results xi and xj agree if the absolute value of En score (k = 2) is lower than one (ISO 13528:2015). The En score value of −0.12 clearly indicates the excellent agreement between the activity concentration obtained by CIEMAT/NIST efficiency tracing method at India and that obtained by TDCR method at NMIJ, Japan. The degree of equivalence of the participants (BARC and NMIJ), Dij and its expanded uncertainty Uij was calculated using equation (2) (Willink, 2003; Ratel G., 2005).
Dij = x i − x j
and Uij = 2
ui2 + uj2
(2)
The Dij and Uij for activity concentration of Ni are −0.06 kBq g−1 and 0.69 kBq g−1 respectively. The En score and degrees of equivalence clearly indicates that the activity concentration of 63Ni measured at 63
63
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
63
Ni activity measurements a bilateral intercomparison a,∗
b
a
a
D.B. Kulkarni , Y. Sato , R. Anuradha , Leena Joseph , M.S. Kulkarni a b
T a
Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai, 400 085, India National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology, Umezono 1-1-1, Tsukuba, Ibaraki, 305-8568, Japan
H I GH L IG H T S
intercomparison organised between BARC and NMIJ. • Bilateral method used at BARC, India. • CIEMAT/NIST method used at NMIJ, Japan. • TDCR • The E score and degrees of equivalence indicates excellent agreement in results. n
A B S T R A C T
To assess the accuracy and capabilities of BARC, for standardization of 63Ni before participating in larger scale trial exercise to implement and test the methods for the extension of the SIR to β emitters, a bilateral intercomparison was organised with National Metrology Institute of Japan (NMIJ), Japan. Standardization of 63Ni was carried out and the results were compared with those obtained from NMIJ to assess the accuracy and capabilities of the laboratories. The CIEMAT/NIST efficiency tracing technique based on 3H standard was used for measurement of 63Ni activity concentration at BARC, India whereas Triple to Double Coincidence Ratio (TDCR) method was used at NMIJ, Japan. The procedures adopted for the standardization of 63Ni by CIEMAT/NIST method at BARC and TDCR method at NMIJ are presented. The percentage deviation in activity concentration of 63Ni between BARC, India and NMIJ, Japan is 0.27%. To evaluate the performance of techniques used at both the laboratories, En score (k = 2) and degrees of equivalence was calculated. The En score of −0.12 and degrees of equivalence −0.06 kBq g−1 clearly indicates that the activity concentration of 63Ni measured at BARC, India and NMIJ, Japan are in excellent agreement and comparable within uncertainty limits and demonstrates the degrees of equivalence of the standards maintained at BARC, India and NMIJ Japan.
1. Introduction 63
Ni is extensively used as a standard for calibration of contamination monitors because of its long half life and low beta energy. 63 Ni is also found as neutron-activation product in nuclear-power reactor and its environment. It is also one of the radioactive and chemically corrosive contaminants in high-level liquid-waste at nuclear fuel storage and reprocessing facilities. 63Ni is a beta emitter with a half life of 98.7 ± 0.24 years (Lee, 2003). It decays directly by beta emission to the ground state of 63Cu with maximum energy 66.945 ± 0.002 keV (100%) (Hetherington et al., 1987; Kawakami et al., 1992; Martin, 1995). 63Ni being a pure beta emitter, CIEMAT/NIST and Triple to Double Coincidence Ratio (TDCR) techniques are used for its primary standardization. International intercomparison of activity measurements using primary standards helps in establishing equivalence between various National Metrology Institutes (NMIs). The Extension of Système International de Référence (ESIR) Working Group (ESWG(II)) of the
∗
Consultative Committee for Ionizing Radiation (CCRI(II)) of Bureau international des poids et measures (BIPM) has proposed liquid scintillation counting (LSC) technique for extension of SIR to β emitters. A larger scale trial exercise to implement and test the methods for the ESIR to β emitters, using low-energy, medium-energy and electron capture radionuclides such as 3H, 63Ni, 14C and 55Fe, was agreed, at the CCRI(II) meeting held in 2013 at BIPM. Bhabha Atomic Research Centre (BARC) as designated institute for ionizing radiations in India consented for participation in intercomparison along with 19 National Metrology Institutes (NMIs)/Designated Institutes (DIs). To assess the accuracy and capabilities of BARC for standardization of 63Ni, before participating in larger scale trial exercise to implement and test the methods for ESIR, bilateral intercomparison was organised with National Metrology Institute of Japan (NMIJ), Japan. The procedure used for the standardization of 63Ni by CIEMAT/ NIST method at BARC and TDCR method at NMIJ is presented in this paper.
Corresponding author. E-mail address: [email protected] (D.B. Kulkarni).
https://doi.org/10.1016/j.apradiso.2019.03.025 Received 27 August 2018; Received in revised form 15 March 2019; Accepted 17 March 2019 Available online 20 March 2019 0969-8043/ © 2019 Published by Elsevier Ltd.
Applied Radiation and Isotopes 148 (2019) 60–63
D.B. Kulkarni, et al.
Fig. 1. The sources preparation scheme for 63Ni sources and 3H standards for measurements by CIEMAT/NIST method at BARC, India and by TDCR method at NMIJ, Japan.
indicating parameter i.e. tSIE using a133Ba radioactive source brought to the reproducible position above the vial during tSIE measurements. 63 Ni samples were counted in the spectrometer along with a set of 3 H quenching standards. The counting efficiencies of 3H versus figure of merit (FOM) and 63Ni versus FOM were calculated using the freeware program CN 2003 (Günther, 2002), developed by Günther of the Physikalisch-Technische Bundesanstalt (PTB), Germany, which is available on the ICRM Liquid Scintillation Counting Working Group website hosted by the Laboratoire National Henri Becquerel (LNHB) (http:// www.nucleide.org/ICRM_LSC_WG/icrmsoftware.htm). The beta emission to the ground state of 63Cu with maximum energy 66.945 ± 0.002 keV (100%) (Kawakami et al., 1992) was used for calculation of the detection efficiency in the program. The half-life used for the calculation was taken from DDEP and was obtained from the website hosted by LNHB (Lee, 2003). The plot of experimental 3H efficiencies versus quench indication parameter tSIE is shown in Fig. 2. The plot of calculated 63Ni efficiencies versus 3H efficiencies obtained
2. Experimental 2.1. Source preparation The radioactive solution of 63Ni in the form of nickel chloride was procured from Board of Radiation and Isotope Technology (BRIT), India. The original solution was diluted with dilution factor of 9.96 using inactive nickel chloride solution (50 μg/mL in 0.1 N HCl). The diluted solution was then transferred to NIST type ampoule (2.017623 g), flame sealed and dispatched to NMIJ for standardization at their end. From the same diluted solution eight 63Ni samples were prepared in 20 mL low potassium liquid scintillation vials with 15 mL Ultima Gold Scintillator and 1 mL water for measurements by CIEMAT/ NIST efficiency tracing technique as shown in Fig. 1. One blank sample was also prepared in 20 mL low potassium liquid scintillation vial with 15 mL Ultima Gold Scintillator and 1 mL water. As shown in Fig. 1, a set of 3H quenched standards were prepared from the available 3H solution provided by Laboratoire National Henry Becquerel (LNHB), France during international intercomparison of activity measurements of 3H (CCRI(II)-K2.H3, 2009)(Ratel G. and C. Michotte, 2009). Incremental amount of nitromethane was added to 3H standards and 63Ni samples for variation of counting efficiencies by chemical quenching. For TDCR measurements at NMIJ, Japan one blank sample and three 63Ni samples of about 0.05 mL were prepared in 20 mL low potassium liquid scintillation vials with 15 mL Ultima Gold Scintillator and 0.95 mL water as shown in Fig. 1. All the sources were prepared by gravimetric method using pycnometer. 2.2. Measurements at BARC, India Standardization was carried out with CIEMAT/NIST efficiency tracing technique at BARC. The CIEMAT/NIST efficiency tracing technique (Gracia-Torano et al., 1991; Grau Malonda and Garcia-Torano, 1982) is suitable for determining activity of pure beta, beta-gamma, pure electron capture (EC) and EC-gamma decaying radionuclides using commercial liquid scintillation spectrometer using 3H standard as tracer. Packard TRI-CARB 2900 TR liquid scintillation analyzer with two photomultipliers operated in coincidence was used for the measurements. The spectrometer calculates the external standard quench
Fig. 2. The plot of experimental 3H efficiencies versus Quench indication parameter tSIE and the fitting function. 61
Applied Radiation and Isotopes 148 (2019) 60–63
D.B. Kulkarni, et al.
Table 2 The detailed uncertainty budget for activity concentration measurement at NMIJ, Japan using TDCR method. Uncertainty components, in % of the activity concentration, due to: Factor
Relative standard uncertainty (%)
Evaluation type (A or B)
Counting statistics Weighing Dead time Background Half-life Input parameters and statistical model
0.10 0.03 0.08 0.05 0.01 1.1
A B B B B B
Combined standard uncertainty (%) Combined standard uncertainty (kBq g−1)
Fig. 3. The plot of calculated and the fitting function.
63
1.1 0.25
1) The 63Ni samples were counted in the liquid scintillation spectrometer along with the set of 3H quenching standards to record their count rates, quench indicating parameters i.e. tSIE and date and time of measurement. 2) The 3H efficiencies corresponding to experimentally measured tSIE values of 63Ni samples were calculated using the plot of experimental 3H efficiency versus quench indication parameter tSIE (Fig. 2). 3) 63Ni counting efficiencies (εβ) of the samples were calculated using the plot of calculated 63Ni efficiency versus 3H efficiency (Fig. 3) and the 3H efficiencies corresponding to experimentally measured tSIE values of 63Ni samples (3H efficiencies are calculated in step 2). 4) The activity concentration of the 63Ni solution was determined from the decay corrected count rate, calculated counting efficiency and weight of the sample.
Ni efficiencies versus calculated 3H efficiencies
The 63Ni quenching samples counting efficiencies (εβ) were varied in the range of 38%–67% by addition of incremental amount of nitromethane. The counting time for each sample was 5 min. A set of eight 63Ni samples and seven 3H standards were counted 6 times within a period of 17 days. Fig. 4. Double and triple coincidences versus the TDCR parameter value for 63 Ni.
2.3. Measurements at NMIJ, Japan The TDCR method is an absolute method for activity measurement and especially useful for determination of activity of pure beta and pure EC-emitters (Broda et al., 1988; Cassette and Bouchard, 2003). It involves detection efficiency calculation from a physical and statistical model of the photon distribution emitted by the scintillating source, models for energy deposition and quenching. The TDCR detector system consists of three photo multiplier tubes (PMT) placed at an angle of 1200 from each other facing the liquid scintillation vial in the counting chamber. The discrimination level for the PMT is set in the valley, between the electronic noise and single electron peak. To minimize the counts due to thermal noise, the coincidence method is used. There is not enough information in a 2-photodetectors system to determine the experimental detection efficiency without an additional reference source and this is why the TDCR method uses a 3-photomultipliers detector, allowing the observation of 3 kind of double coincidences (2-photodetectors) and triple coincidence (3-photodetectors) (Broda et al., 1988; Cassette and Bouchard, 2003). The TDCR parameter is calculated using the count rates of logical sum of double coincidences and triple coincidences. The experimental detection efficiencies of samples were calculated using the measured TDCR parameters and efficiency calculations model of TDCR method. The TDCR equipment and the fitting program developed by NMIJ were used for the measurements (Sato et al., 2012). High voltage of 2200 V was supplied to the PMTs. Resolution time of fast coincidence
Table 1 The detailed uncertainty budget for activity concentration measurement at BARC, India using CIEMAT/NIST method. Uncertainty components, in % of the activity concentration, due to: Factor
Relative standard uncertainty (%)
Evaluation type (A or B)
Counting statistics + Background Weighing Dead time Half-life Uncertainty due to tracer activity Quench indicating parameter
0.44 0.03 0.01 0.01 0.96 0.01
A B B B B B
Combined standard uncertainty (%) Combined standard uncertainty (kBq g−1)
1.06 0.24
from CN2003 program is shown in Fig. 3. The value of ionization quench parameter ‘kB’ used in the calculation program was 0.0075 cm/ MeV as it gives maximum stability towards measurements of activity concentrations. The activity concentrations of 63Ni samples were calculated using CIEMAT/NIST efficiency tracing technique as follows.
62
Applied Radiation and Isotopes 148 (2019) 60–63
D.B. Kulkarni, et al.
modules and paralysable dead time was 50 ns and 20 μs respectively. Measurement time for each sample was 500 s and the measurements were repeated three times within a period of 4 days. For variation of TDCR values of 63Ni samples, optical filters as scintillation light absorber were placed around the vials. The variation of counting efficiency depends on the amount of scintillation light absorbed by the optical filter. The beta minus decay to the ground state of 63Cu with maximum energy 66.98 ± 0.015 keV and probability of 100% (Lee, 2003) was used for the fitting calculation. The TDCR parameter values of 63Ni were in the range of 0.73–0.79. The plots of triple and double coincidence efficiencies versus TDCR is shown in Fig. 4 and was used for calculation of double coincidence or triple coincidence efficiencies corresponding to measured TDCR parameter values.
BARC, India and NMIJ, Japan are in excellent agreement and comparable within uncertainty limits. Acknowledgement The authors would like to express their thanks and gratitude to Dr. K.S. Pradeepkumar, Associate Director, Health Safety and Environment Group, and Head Radiation Safety Systems Division, Bhabha Atomic Research Centre, Trombay, Mumbai and Dr. A. Yunoki, Group leader, radioactivity and neutron standards group, National Metrology Institute of Japan, Tsukuba, Ibaraki for their constant encouragement and support. Authors would like to thank Mrs. P. J. Reddy for providing LSC counter for CIEMAT/NIST measurements. Appendix A. Supplementary data
3. Results and discussion Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apradiso.2019.03.025.
The activity concentration of the 63Ni solution and the ‘kB’ factor derived by the TDCR fitting as reported to BARC, India were 22.54 ± 0.25 kBq g−1 and 0.0126 cm MeV−1 as on reference date of 1st January 2017, 00:00 UTC. The 63Ni activity concentration obtained by CIEMAT/NIST method, reported to NMIJ, Japan is 22.48 ± 0.24 kBq g−1 on the reference date. The detailed uncertainty budget for activity concentration measurement at BARC, India using CIEMAT/ NIST method and that using TDCR method at NMIJ, Japan is shown in Table 1 and Table 2 respectively. To evaluate the performance of BARC with respect to NMIJ, En score (k = 2) was calculated using equation (1).
En =
References Broda, R., Pochwalski, K., Radoszewski, T., 1988. Calculation of Liquid-scintillation detector efficiency. Int. J. Appl. Radiat. Isot. part A 39, 159–164. Cassette, P., Bouchard, J., 2003. The design of a liquid scintillation counter based on the triple to double coincidence ratio method. Nucl. Instrum. Methods Phys. Res., Sect. A 505 (1–2), 72–75. Garcia-Toraño, E., Martin Casallo, M.T., Rodriguez, L., Grau, A., Los Arcos, J.M., 1991. On the standardization of beta-gamma emitting nuclides by liquid scintillation counting. In: Ross, H., Noakes, J.E., Spaulding, J.D. (Eds.), Liquid Scintillation Counting and Organic Scintillators. Lewis Publishers, Chelsea MI, pp. 307–316. Grau Malonda, A., Garcia-Toraño, E., 1982. Evaluation of counting efficiency in liquid scintillation counting of pure beta-ray emitters. Int. J. Appl. Radiat. Isot. 33, 249–253. Günther, E.W., 2002. What can we expect from the CIEMAT/NIST method? Appl. Radiat. Isot. 56 (1–2), 357–360. Hetherington, D.W., Graham, R.L., Lone, M.A., Geiger, J.S., Lee-Whiting, G.E., 1987. Upper limits on the mixing of heavy neutrinos in the beta decay of 63Ni. Phys. Rev. C36, 1504–1513. ISO 13528, International Organization for Standardization, 2015. Statistical Methods for Use in Proficiency Testing by Interlaboratory Comparison. Kawakami, H., Kato, S., Ohshima, T., Rosenfeld, C., Sakamoto, H., Sato, T., Shibata, S., Shirai, J., Sugaya, Y., Suzuki, T., Takahashi, K., Tsukamoto, T., Ueno, K., Ukai, K., Wilson, S., Yonezawa, Y., 1992. High sensitivity search for a 17 keV neutrino. Negative indication with an upper limit of 0. 095 %. Phys. Lett. B287, 45–50. Lee, K.B., 2003. Table de Radionucleides. Monographie BIPM- 5 (3), 29–31. Martin, M., August, 1995. (Oak Ridge National Laboratory), Evaluated Nuclear Structure Data File (ENSDF), private communication. Ratel, G., 2005. Evaluation of the uncertainty of the degree of equivalence. Metrologia 42, 140–144. Ratel, G., Michotte, C., 2009. Draft B report, International comparison of activity measurements of a solution of tritiated water. Sato, Y., Yamada, T., Matsumoto, M., Wakitani, Y., Hasegawa, T., Yoshimura, T., Murayama, H., Oda, K., Sato, T., Unno, Y., Yunoki, A., 2012. Efficiency fitting for TDCR measurement data using polynomial approximation and the Newton–Raphson method. Appl. Radiat. Isot. 70, 2184–2187. Willink, R., 2003. On the interpretation and analysis of degree-of-equivalence. Metrologia 40, 9–17.
(x i − x j ) 2ui2 + 2 uj2
(1)
Where, xi, ui are the activity concentration and its standard uncertainty respectively, obtained for measurements at BARC, India. xj, uj are the activity concentration and its standard uncertainty respectively, obtained for measurements at NMIJ, Japan. The En score value (k = 2) calculated using the activity concentrations and their associated uncertainties is −0.12. The results xi and xj agree if the absolute value of En score (k = 2) is lower than one (ISO 13528:2015). The En score value of −0.12 clearly indicates the excellent agreement between the activity concentration obtained by CIEMAT/NIST efficiency tracing method at India and that obtained by TDCR method at NMIJ, Japan. The degree of equivalence of the participants (BARC and NMIJ), Dij and its expanded uncertainty Uij was calculated using equation (2) (Willink, 2003; Ratel G., 2005).
Dij = x i − x j
and Uij = 2
ui2 + uj2
(2)
The Dij and Uij for activity concentration of Ni are −0.06 kBq g−1 and 0.69 kBq g−1 respectively. The En score and degrees of equivalence clearly indicates that the activity concentration of 63Ni measured at 63
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