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Applied Radiation and Isotopes
journal homepage: www.elsevier.com/locate/apradiso
Standardisation of the 129I, 151 Sm and 166mHo activity concentration using the CIEMAT/NIST efficiency tracing method
Timotheos Altzitzoglou *, Andrej Rozkov
European Commission Joint Research Centre Institute for Reference Materials and Measurements, Retieseweg 111, 2440 Geel, Belgium
CrossMark
HIGHLIGHTS
• Standardisation of 129I, 151Sm and 166mHo by Liquid Scintillation Counting.
• Use of the CIEMAT/NIST efficiency tracing method and the CN2005 and MICELLE2 codes.
• Complete uncertainty budgets for each radionuclide standardisation.
• Work performed for the EMRP project "Metrology for Radioactive Waste Management".
ARTICLE INFO
ABSTRACT
Article history: Received 6 April 2015 Accepted 17 December 2015 Available online 19 December 2015
Keywords:
129j 166mi
Standardization
Activity concentration
CIEMAT/NIST efficiency tracing method
The 129I, 151 Sm and 166mHo standardisations using the CIEMAT/NIST efficiency tracing method, that have been carried out in the frame of the European Metrology Research Program project "Metrology for Radioactive Waste Management" are described. The radionuclide beta counting efficiencies were calculated using two computer codes CN2005 and MICELLE2. The sensitivity analysis of the code input parameters (ionization quenching factor, beta shape factor) on the calculated efficiencies was performed, and the results are discussed. The combined relative standard uncertainty of the standardisations of the
129I, 151Sm and 166mHo solutions were 0.4%, 0.5% and 0.4%, respectively. The stated precision obtained using the CIEMAT/NIST method is better than that previously reported in the literature obtained by the TDCR (129I), the 4ny-NaI (166mHo) counting or the CIEMAT/NIST method (151Sm).
© 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND
license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction
Accurate nuclear decay data are important input parameters in the development of technical solutions for a safe long-term disposal of nuclear waste. One of the aims of the European Metrology Research Program (EMRP) project "Metrology for Radioactive Waste Management (MetroRWM)", executed in 2011-2014, was to determine the half-lives of three long-lived radionuclides, 129I, 151 Sm and 166mHo, present in nuclear waste.
129I is one of the 235U fission products with high fission production yield of 0.7%. Due to its long half-life (T1/2 = 16.1 (7) • 106a, DDEP), chemical reactivity and mobility, it is considered as one of the most important dose contributors in the nuclear waste disposal storages in the future. The 129I half-life has been determined four times between 1951 and 1973, with values ranging from 15.6 (6) • 106 a (Katcoff et al., 1951) to 19.7 (14) • 106 a (Kuhry and Bontems, 1973) that were not consistent within their uncertainty limits.
* Corresponding author. E-mail address: timotheos.altzitzoglou@ec.europa.eu (T. Altzitzoglou).
151Sm is a relatively long-lived nuclide (T1/2=90 (6) a, DDEP) produced during the irradiation of uranium fuel rods in nuclear reactors. Its half-life was determined several times between 1950 and 1968. The last time it was measured in 2009 and the value of the half-life was determined to be 96.6 (2.4) a (He et al., 2009).
166mHo (T1/2 = 1200 (180) a, DDEP) is produced in nuclear fuel as the result of the neutron activation of erbium admixtures added to the fuel to improve the burn out of the uranium. Its half-life was determined once in 1965 (Faler, 1965). Recently, the 166mHo halflife has been measured again, resulting in a value with lower uncertainty (T1/2 = 1132.6 (3.9) a, Nedjadi, Bailat, Caffari et al., 2012).
The currently available data in the literature on the 129I, 151Sm and 166mHo half-lives are either scarce or inconsistent and new measurements are needed to obtain accurate values with reliable uncertainties. For such long-lived nuclides the half-life value is best obtained by independently measuring the activity concentration and the mass concentration of the given radionuclide in the same solution. In the course of the MetroRWM project the radionuclide activity concentrations were determined using a number of standardisation techniques at the highest level of
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accuracy and precision in collaboration with national metrology institutes. The 129I and 151 Sm standardisations were registered as the Bureau International des Poids et Mesures (BIPM) supplementary comparisons EURAMET.RI(II)-S6.I-129 and EURAMET.RI (II)-S7.Sm-151, respectively, and the 166mHo standardisation was registered as the BIPM Key Comparison EURAMET.RI(II)-K2.Ho-166m (Ratel et al., 2015). At the Joint Research Centre Institute for Reference Materials and Measurements (JRC-IRMM), the activity concentrations were measured both by the Triple-to-Double Coincidence Ratio (TDCR) Liquid Scintillation Counting (LSC) method and the CIEMAT/NIST efficiency tracing LSC method (Grau Mal-onda, Garcia-Torano, 1982), the latter being the subject of this article.
There are a few publications on the standardisation of the above mentioned radionuclides using the CIEMAT/NIST method. For example, there exists only one publication on the standardisation of the 129I solution, and this describes the determination of the radionuclide activity concentrations using the TDCR method and 4n(LS)-y coincidence counting (Cassette et al., 1994). The relative combined standard uncertainties of the standardisation of the 50 kBq g-1 solution using the TDCR method and the 4n(LS)-y counting were 0.5% and 0.3%, respectively.
There has been published so far one paper on the standardisation of the 151 Sm solution and this concerned the application of the CIEMAT/NIST efficiency tracing method, using 14C as tracer (He et al., 2009). The reported relative combined standard uncertainty of the standardisation solution was 2.5%.
166mHo has been standardized using a variety of standardisation techniques. In the BIPM publication (Michotte et al., 2009) the results of the international comparison of the 166mHo activity determination using four primary standardization techniques 4ny (NaI), 4nP(proportional counter)-y(NaI) coincidence, 4np(pres-surized proportional counter)-y(NaI) coincidence and 4nP(LS)-y (NaI) coincidence counting were presented. The relative combined standard uncertainty of the determination of the 166mHo activity concentration using the most precise 4nP(LS)-y(NaI) coincidence counting technique was 0.24% (Silva et al., 2012). In the papers (Nedjadi, Bailat, Caffari et al., 2012; Nedjadi, Bailat, Bochud, 2012; Nedjadi, Spring et al., 2008) the uncertainty of the standardisation using the 4ny(NaI) counting technique was 0.55% and 0.2-0.3% in case of other used techniques such as the 4nP(plastic scintillator)-y(NaI) coincidence, 4nP(proportional counter)-y(NaI) coincidence counting. The most recent publication is that of (Kossert, Cassette et al., 2014); the standardization of a 166mHo solutions was performed using liquid scintillation counting by both the TDCR and the the CIEMAT/NIST efficiency tracing methods to achieve an overall relative standard uncertainty of about 0.3%.
The aim of the work presented here was to standardize the 129I, 151 Sm and 166mHo solutions using the CIEMAT/NIST efficiency tracing method and with improved uncertainties of the radio-nuclide activity concentration determination compared with the uncertainties reported in the literature.
2. Method
2.1. Source preparation
The radioactive counting sources were prepared from the radionuclide solutions provided by the pilot laboratories according to the EMRP Joint Research Protocol. The 129I solution of an approximate activity concentration of 32 kBq • g- 1 was provided by CIEMAT, the 151Sm solution of about 80 kBq • g-1 was provided by the Laboratoire National de Métrologie et d'Essais - Laboratoire National Henri Becquerel (LNE-LNHB) and the 166mHo solution of an activity concentration of about 119 kBq • g-1 by the
Physikalisch-Technische Bundesanstalt (PTB).
After receiving the ampoules with the radioactive solutions at JRC-IRMM, the activities of the gamma-ray emitting impurities in the solutions were measured using high-resolution gamma-ray spectrometry employing HPGe detectors. There were no impurities detected in the 129I and 166mHo solutions. In the 151 Sm solution the 154Eu and 155Eu admixtures were measured and their relative activities were 0.03% and 0.01%, respectively, relative to the activity of 151 Sm. The amount of 151 Sm impurities was considered negligible and only a component was included in the uncertainty budget of the radionuclide standardisation to take it into account.
The radioactive sources were prepared using the pycnometer method (Sibbens and Altzitzoglou, 2007; Campion, 1975) by gravimetrically dispensing the radionuclide solution to the 20-mL low-potassium glass LSC vials containing the scintillation cocktail. One part of the sources was prepared in 15 mL of the Ultima Gold® (PerkinElmer, Boston, MA, USA) liquid scintillation (LS) cocktail, mixed with 1 mL of deionized water (hereafter Ultima Gold, for simplicity). The rest of the sources were prepared in 15 mL of In-staGel™ Plus (PerkinElmer) LS cocktail (hereafter InstaGel Plus). The aliquots dispensed into each LS source ranged from 10 to 50 mg. The traceability of the mass measurements was achieved via the calibration of the balance with certified mass standards.
The so prepared sources were shaken and left to dark adapt in the LSC instruments at a temperature between 12 and 14 °C. The sources were visually homogeneous and colourless. In total, the following radioactive sources were prepared:
- for 129I, 8 sources in Ultima Gold and 8 in InstaGel Plus with activities ranging from 0.3 to 1.7 kBq per sample,
- for 151Sm, 8 sources in Ultima Gold and 8 in InstaGel Plus with activities ranging from 0.8 to 3.2 kBq per sample, and
- for 166mHo 6 sources in Ultima Gold and 3 in InstaGel Plus with activities ranging from 1.3 to 3.8 kBq per sample.
2.2. Source measurement
The sources were counted using a Wallac 1220 Quantulus (PerkinElmer) and a Packard TriCarb 3100 TR/AB (PerkinElmer) LS counter. The quench parameter of each source was determined with the external standard method, using the external 152Eu and 137Cs radiation sources of the two LSC, respectively. Each of the 129I and 166mHo sources was measured for 30 min and each 151 Sm source for 20 min. All sources were measured repeatedly during a period of 4-6 weeks. Typical beta-particle energy spectra of the three radionuclides as measured with the Quantulus LSC are shown in Fig. 1. Blank sources were measured for the same time as the radioactive sources. Each source count rate was corrected for the background by subtracting the count rate of the blank, for dead time and for decay.
2.3. Radionuclide adsorption
The adsorption of the given radionuclide on the original glass ampoule walls was determined by rinsing the original emptied ampoule twice with 1 mL of 2 M HCl and once with 1 mL of deionized water. Then the ampoule was placed in an LSC vial, filled with 15 mL of Ultima Gold LS cocktail and counted. The remaining activity measured was less than 0.04% of the activity of the solution contained in the ampoule in the case of 129I and less than 0.01% for 151 Sm and 166mHo. In all cases, the amount of adsorption was considered negligible and it was not taken into account in the activity calculations, but only included in the uncertainty budget.
'"Sm \/ \ l29I
3. Results and discussion
3.1. Iodine-129
400 600
Channel
Fig. 1. Typical beta-particle energy spectra of 1291,151Sm and 166mHo, as measured with the Wallac 1220 Quantulus LSC (which employs logarithmic amplifiers).
2.4. Determination of the radionuclide beta counting efficiency
The radionuclide counting efficiencies were calculated using the CIEMAT/NIST efficiency tracing method (Grau Malonda and Garcia-Torano, 1982). The principle of the method is a combination of theoretical calculations of the radionuclide beta particle counting efficiency and an experimental determination of correction factors with the help of a tracer radionuclide, 3H in this case. The method requires the knowledge of the experimental counting efficiency of 3H for different degrees of chemical quench and the computation of a counting efficiency at the LS counter photocathode output for different values of a free parameter. The universal curve combines the free parameter as a function of the sample quenching. A model for the counting efficiency as a function of free parameter is being developed for the radionuclide of interest using a computer code, and that in combination with a universal curve enables to determine a radionuclide counting efficiency in a sample.
The computer codes CN2005 (Günther, 2002) and M1CELLE2 (Kossert and Grau Carles, 2010) were employed to calculate the radionuclide beta-particle counting efficiencies. 1n the CN2005 code, the calculation of the radionuclide counting efficiency is performed taking into account the radionuclide nuclear and atomic data, its decay scheme, and the interactions of produced ionizing particles in a defined counting geometry. M1CELLE2 is a more recent code, which improved the calculation of counting efficiencies of electron capture radionuclides. 1t comprises more realistic treatment of the photoelectrons and the subsequent rearrangement processes in the atomic shell and accounts for the energy deposits in scintillation cocktail micelles. Both codes use the same algorithm for the calculation of the ionization quenching correction factor (Los Arcos, Ortiz, 1997) and the XCOM library for the calculation of the interaction probability of the photons with the scintillator using the Monte Carlo method.
The calculations of the radionuclide beta counting efficiencies with CN2005 and M1CELLE2 were performed using the same nuclear and atomic input data derived from the Decay Data Evaluation Project (DDEP) recommended data. In order to test the performance of the codes, the sensitivity analysis of input parameters (ionization quenching factor, and in the case of 1291 the shape factor) on the calculated efficiencies was performed. Additionally, part of prepared radioactive sources were quenched with 20 and 40 mL of CH3NO2 to test the validity of the calculations at different levels of quenching. The results of the radionuclide beta-particle counting efficiency calculations are discussed in the Results and Discussion section.
The 1291 decays by beta-minus transition to the excited level of 129Xe at 39.6 keV. The transition from the 7/2 + state of the mother to the 3/2 + state of the daughter nuclide is a 2nd forbidden transition with a branching ratio of 99.5%, although the transition to the ground state of 129Xe has not been observed experimentally (DDEP).
The beta shape factor for a 2nd forbidden transition is given by p2 + q2, where p=(W2-1)1/2 and q = W0 - W are the momenta of the electron and neutrino, respectively. The endpoint energy W0, as well as the total energy W of the electron are given in units of mec2, with me being the electron rest mass. The shape factor of 1291 was first experimentally determined in 1954 (Mateosian and Wu, 1954) and found to be 0.1 p2 + q2. Then, in 1995 Grau Carles determined the shape factor to be 0.09p2+q2, very close to that of 1954 (Grau Carles, 1995) However, the shape factor 0.045p2 + q2 was used in the calculations (Kossert, 2014), as the recovered sample activities of quenched sources agree better with those of unquenched sources. Calculation with either code using the latter shape factor results in lower counting efficiency than with the former by 0.23% for unquenched sources and up to 0.6% for quenched sources.
The calculated 1291 counting efficiencies for sources in Ultima Gold and InstaGel Plus LS cocktails using the CN2005 and M1-CELLE2 codes are presented in Fig. 2. Using the same shape factor, the difference between the 1291 counting efficiency values calculated using the two codes CN2005 and M1CELLE2 was for un-quenched samples (3H efficiency of about 52%) of the same composition about 0.2% and for quenched samples (3H efficiency of about 39%) up to 0.7%. The ionization quenching parameter kB value 0.0075 cm MeV-1 was used in the modelling. Variation of the kB value between 0.0065 and 0.011 cm MeV-1 had a negligible effect of less than 0.05% on the calculated activities, and a corresponding component was included into the uncertainty budget.
The mean activity concentration of the 1291, calculated using the results of the measurements of all sources and the counting efficiency using the M1CELLE2 code and the shape factor of 0.045p2 + q2, was 33.0 (1) kBq g-1 (reference date 2013-07-01 0:00 UTC). The detailed uncertainty components of the 1291 standardisation are given in Table 1. The uncertainty of the decay data is based on the uncertainties of the DDEP nuclear data and on the uncertainty evaluation using the CN2005 code. The uncertainty of the calculation code dependence is the standard deviation in the activity concentration values calculated using either CN2005 or M1CELLE2.
Fig. 2. Counting efficiency of 1291 plotted against the counting efficiency of 3H calculated for sources in Ultima Gold and 1nstaGel Plus LS cocktails, using the CN2005 and M1CELLE2 codes.
Table 1
Uncertainty budget of the 129I, 151Sm and 166mHo standardisation. Typical uncertainties for a single measurement of a sample are given. The combined uncertainty is the quadratic sum of all components (k=1).
Uncertainty component Relative standard uncertainty of the activity concentration (%)
129i 151Sm 166mHo
Counting statistics 0.04 0.005 0.07
Source preparation 0.12 0.14 0.12
(Weighing)
Background 0.01 0.003 0.001
Dead time 0.1 0.1 0.1
Decay data 0.13 0.3 0.06
Quench parameter 0.1 0.2 0.02
Tracer (tritiated water) 0.05 0.2 0.04
Interpolation of the effi- 0.02 0.05 0.02
ciency curve
Decay correction (Half- 4.4 • 10 -8 8.3 • 10-3 5.10 • 10-4
Adsorption 0.04 0.01 0.01
Impurities - 0.05 -
kB (cm • MeV-1) 0.02 0.08 0.02
Sample stability 0.03 0.1 0.1
Shape factor dependence 0.15 - -
Calculation code 0.16 0.1 0.1
dependence
LS spectrometer 0.2 0.03 0.1
dependence
Source mass dependence - 0.1 0.3
Combined 0.38 0.49 0.40
3.2. Samarium-15i
The 151Sm decays by beta-minus primarily (99.09%) to the ground state of 151Eu, by a 1st forbidden transition, with a minor (0.91%) transition to the first excited state of 151 Eu at 21.54 keV. The typical beta counting efficiency for the unquenched 151 Sm sources was about 81%.
The ionization quenching parameter kB value 0.0075 cm MeV-1 was used in the modelling. Variation of the kB value between 0.0065 and 0.011 cm MeV-1 had an effect from 0.08 to 0.35%, slightly more pronounced than in the 129I case, and that was reflected in the corresponding component in the uncertainty budget. Efficiency calculations were performed with both the CN2005 and MICELLE2 codes; the latter gives slightly lower counting efficiency by 0.1% compared to CN2005.
The mean activity concentration of the 151 Sm, calculated using the results of the measurements of all sources and the averaged counting efficiency as obtained from the CN2005 and MICELLE2 codes, was 78.5 (4) kBq g-1 (reference date 2014-04-01 0:00 UTC). The uncertainty budget of the 151 Sm standardisation is given in Table 1.
3.3. Holmium-166m
The 166mHo decays by beta-minus transition with a complex decay scheme. The two major transitions are to the 1828 keV level of 166Er (17.2%) and the 1787 keV level of 166Er (74.8%). In total, 12 cascades involving seven transitions and 15 gamma-ray emissions were considered in the calculations with the CN2005 code and the counting efficiency quench curves agreed within 0.02% with those calculated using MICELLE2 at PTB (Kossert, 2014). The typical beta counting efficiency for the unquenched 166mHo sources was about 99%. The ionization quenching parameter kB value 0.0075 cm MeV"1 was used in the modelling. Variation of the kB value between 0.0065 and 0.011 cm MeV"1 had a negligible effect of less than 0.06% on the calculated activities and a component was included into the uncertainty budget.
The mean activity concentration of the 166mHo, calculated using the results of the measurements of all stable sources, was 119.3 (5) kBq • g-1 (reference date 2013-01-01 0:00 UTC). The uncertainty budget of the 166mHo standardisation is given in Table 1.
4. Conclusion
The 129I, 151Sm and 166mHo solutions have been standardised using the CIEMAT/NIST efficiency tracing method, and with 3H as tracer. Both codes CN2005 and MICELLE2 were used for the calculation of the counting efficiency of the radionuclides and they produced similar results, with MICELLE2 giving slightly lower by about 0.1 to 0.2% counting efficiency values for unquenched sources compared with CN2005. In the case of 129I two shape factors were tested and the one used gave more consistent results for quenched sources. The same sources prepared for this work were measured by a Triple-to-Double Coincidence Ratio (TDCR) LS spectrometer, designed and built at the JRC-IRMM. The results obtained by the CIEMAT/NIST method and the TDCR method were in excellent agreement, i.e. identical for 129I and 166mHo and for 151Sm less than 0.5% higher in the case of TDCR.
As mentioned, the standardisations of 129I, 151Sm and 166mHo were registered as CCRI(II) comparisons. The activity concentration values determined in this work are in excellent agreement with the proposed Comparison Reference Values (CRV), obtained as the power-moderated mean of all values reported by all participating laboratories to the BIPM. The degree of equivalence, D, defined as D=a - CRV, where a is the activity concentration result reported by a participating laboratory, is an indication of how close the result is to the CRV. The degrees of equivalence for the JRC-IRMM reported activity concentrations for 129I, 151Sm and 166mHo and their corresponding expanded uncertainties (k=2) are (-0.08 + 0.22) kBq g"1, (- 0.28 + 0.99) kBq g"1 and (0.02 + 0.20) kBqg-1 (Kossert, Altzit-zoglou et al. 2014), respectively.
The relative combined standard uncertainty of the 129I standardisation using the CIEMAT/NIST method of 0.4% is similar to the previously reported in the literature uncertainty of the standardisation using the TDCR method (0.5%) and the 4n(LS)-y coincidence method (0.3%) (Cassette et al., 1994). The uncertainty of the 166mHo standardisation using the CIEMAT/NIST method was 0.4%, slightly lower than the 0.55% uncertainty obtained by the 4ny(NaI) counting technique (Nedjadi, Bailat, Bochud, 2012) and slightly higher than the 0.3% relative standard uncertainty obtained at PTB using the same TDCR and CIEMAT/NIST efficiency tracing methods (Kossert, Cassette et al., 2014). Finally, the relative combined standard uncertainty of the 151 Sm standardisation using the CIEMAT/NIST method was 0.5% and that was five times lower than the uncertainty of the recent standardisation using CIEMAT/ NIST method with the 14C tracer (He et al., 2009).
These activity concentrations obtained from the CCRI(II) comparison exercises, combined with the mass spectrometric results on the same radioactive solutions will result in new and eventually more accurate values for the half-life of the three radionuclides. Currently, it is only the work on the 151 Sm half-life that has been published (Be et al., 2015).
Acknowledgment
This work was supported by the ENV09 MetroRWM project under the European Metrology Research Programme (EMRP). The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union.
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