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0Applied Radiation and Isotopes I (I
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Applied Radiation and Isotopes
journal homepage: www.elsevier.com/locate/apradiso
Characterisation of the IAEA-375 Soil Reference Material for radioactivity
T. Altzitzoglou *, A. Bohnstedt
European Commission, Joint Research Centre, Institute for Reference Materials and Measurements, Retieseweg 111, B-2440 Geel, Belgium
HIGHLIGHTS
• Upgrade of the IAEA-375 Soil Reference Material (RM).
• Massic activity values traceable to the SI units.
• 40K, 134Cs, 137Cs, 212Pb, 212Bi, 214Pb and 214Bi measured by y-ray spectrometry.
• Chemical separation of Sr by extraction chromatography.
• 90Sr assessed by liquid scintillation counting.
• The RM was used for the 2010 EC Interlaboratory Comparison on Radionuclides in Soil.
ARTICLE INFO ABSTRACT
Article history: The Joint Research Centre Institute for Reference Materials and Measurements (JRC-IRMM) participated
Received 6 April 2015 in a research project initiated by the International Atomic Energy Agency (IAEA) to upgrade some of its
Accepted 20 November 2015 existing reference materials (RMs). The aim of the work described in this article was to determine the
--activity concentration of a series of radionuclides in the IAEA-375 soil RM with values traceable to the SI
Keywords: units. The radionuclides 40K, 134Cs, 137Cs, 212Pb, 212Bi, 214Pb and 214Bi were measured by y-ray spectro-
Chemical separations metry after drying the sample and placing it in a suitable container. The 90Sr was assessed by liquid
Gamma-ray ^edrometny scintillation counting after dissolution of the soil by wet digestion and chemical separation of Sr by
Llquld scintillation counang extraction chromatography. This soil RM was used later as basis for the 2010 EC Interlaboratory Com-
RNaudciloeaacrtivity parison on Radionuclides in Soil.
t ■ , © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND
Certified reference materials J r
S] units license (http://creativecommons.org/Iicenses/by-nc-nd/4.0/). Traceability
1. Introduction
Certified reference materials (CRM) are useful for evaluating and comparing radiochemical separation and measurement procedures and for calibrating instruments. In addition, reference materials are needed for the quality control and the comparability of measurement results among laboratories, especially those working in surveying and monitoring radioactive materials in the environment and the foodstuff.
Radioactivity levels in foodstuff and the environment are of particular concern; the dose to which humans are exposed depends directly on the level of the radioactivity in the environment they live in and the food they consume. Soil is a natural matrix important in environmental monitoring, as it affects humans both
* Corresponding author. E-mail address: timotheos.altzitzoglou@ec.europa.eu (T. Altzitzoglou).
directly and via the food chain. Few reference materials of environmental matrices, certified for their radioactivity contents, exist and even fewer with their values traceable to the SI units (BIPM, 1998).
The JRC-IRMM participated at an International Atomic Energy Agency (IAEA) - organised Coordinated Research Project (CRP) to upgrade existing reference materials and in this case the IAEA-375 Soil RM, in order to assign property values traceable to the International System of Units (SI). The material was collected for the Agency's Laboratories Seibersdorf AQCS (Analytical Quality Assurance Services) program from a field in the Brjansk region, Russia, in July 1990. The top soil to a depth of 20 cm was collected, dried, milled, sieved (0.3 mm) and shipped to the IAEA. The material was homogenised, bottled and sterilised by y-ray irradiation to a total dose of 25 kGy using a 60Co source (Strachnov et al., 1996; IAEA, 2000).
Randomly selected bottles, containing 250 g of soil each were sent to the CRP participating laboratories for assay. The JRC-IRMM
http://dx.doi.org/10.1016/j.apradiso.2015.11.053
0969-8043/© 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/).
2 T. Altzitzoglou, A. Bohnstedt / Applied Radiation and Isotopes I (l
received 6 bottles and participated at the CRP by determining the activity concentrations in the soil RM of the radionuclides 40K, 134Cs, 137Cs, 212Pb, 212Bi, 214Pb and 214Bi measured by y-ray spectrometry and that of 90Sr by liquid scintillation counting after chemical separation of Sr. The measurement of the activity concentration of 226Ra, 235U, 234U, 238U, 238Pu, 239+240Pu, 230Th and 232Th by a-particle spectrometry after chemical separation of the corresponding elements will be the subject of a future article. More details can be found by Altzitzoglou et al. (2006). The laboratories were encouraged to use their own routine procedures, so that a variety of analytical methodologies is used.
2. Measurements by high-resolution Y-ray spectrometry
One of the advantages of high-resolution y-ray spectrometry is that it can be used to assess y-ray emitting radionuclides in the samples without any prior chemical treatment. In that way, the source preparation is very rapid and simple and in the majority of cases non-destructive. The content of each of the 6 bottles of the soil received from the IAEA was shaken vigorously, using a 3D Turbula mixer (type T2C, Willy A. Bachofen AG Maschinenfabrik, Basel, Switzerland) and was dried in an oven for 48 h at 105 °C to constant weight. During this process, the soil samples lost between 1.7% and 1.9% of their weight.
After cooling to room temperature in a desiccator, the samples were weighed using an analytical balance, calibrated with a calibrated weight set traceable to the IRMM kilogram, which is directly traceable to the BIPM (Bureau International des Poids et Mesures, Sèvres, France) kilogram by regular direct comparisons. One sample was prepared in cylindrical polypropylene containers from each bottle supplied by the IAEA, with the sample mass ranging from 39.4 to 42.0 g. The containers were tapped and placed directly on top of the detector end-cap and measured for 412 days each. A 36% relative efficiency, high-purity germanium (HPGe) co-axial detector system (Model GC3518, Canberra Industries, Inc., Meriden, CT, USA) was used for the measurements. The detector was housed in a 10-cm thick Pb shield of circular cross-section, lined with 1 mm Cd and 1 mm Cu; the inner 2 cm of the Pb shield was made of high radiopurity Pb.
In the acquisition system a calibrated stable quartz oscillator with a frequency of 100 kHz provided the time base of the livetime clock gates. As time base, the legal time in Germany on the basis of Coordinated World Time (UTC), generated at the Physikalisch-Technische Bundesanstalt (PTB, Braunschweig, Germany) by atomic clocks and broadcasted through the LF transmitter DCF77, was used. More details can be found by Altzitzoglou et al. (2004).
The high-resolution y-ray spectrometry method is an indirect method, using the detection system as a comparator of the sample to a reference sample and therefore it must be calibrated, ideally with a calibration standard source traceable to the SI units and of the same geometry as that of the sample. Then, the measurement result is directly linked to the known value of the calibration standard.
The detector system employed in this work was calibrated for peak efficiency using single-nuclide point sources, as well as multi-nuclide liquid standards prepared in the same geometry as the actual samples. In addition, actual samples of soil were spiked with known amounts of standard 54Mn, 60Co, 65Zn, 137Cs and 241Am solutions, mixed thoroughly and measured, in order to obtain information on the matrix self-absorption. The standard radionuclide solutions used to prepare the calibration standards originated from standardisation campaigns - usually Key Comparisons organised by the BIPM Comité Consultatif pour les Rayonnements Ionisants (CCRI(II)).
Wherever a calibration transfer from one geometry to another was done, the calculation of the efficiency was performed using the Monte Carlo computer code GEOLEP (Solé, 1990; Lépy et al., 2001). The same code was used for the calculation of the total efficiency, necessary to calculate the coincidence summing corrections in the case of 134Cs and 60Co. The efficiency calculated using GEOLEP agreed within less than 2.2% with the experimentally measured efficiency for the standard point sources, liquid standards and the spiked soil.
The measured data either for the efficiency or the activity determination, were corrected for background, decay, decay during measurement and, where appropriate (i.e., 134Cs and 60Co), for coincidence summing. For the calculation of the 134Cs activity its most prominent peaks at 605 and 795 keV were used.
3. Measurements by liquid scintillation counting
The measurement of 90Sr requires Sr to be separated from the matrix and from other interfering radionuclides first. The material was first dried in an oven at 105 °C to constant weight. The mass of each sample used was determined gravimetrically, using an analytical balance (model AT21, Mettler-Toledo, Greifensee, Switzerland), calibrated with standard weights traceable to the IRMM kilogram which is traceable to the BIPM kilogram. Sample amounts of the order of 5 g were used and in total six samples, one from each received bottle, were analysed.
The soil was heated to 200 °C for one hour and combusted for at least 4 h at 550 °C to destroy organic compounds before the microwave digestion. The sample mass reduction after ashing was about 10%. After adding the tracer (85Sr) for the chemical recovery determination, wet digestion with concentrated nitric/hydrofluoric acids and hydrogen peroxide was performed with a Mars 5 Digestion System (CEM Corp., Matthews, NC, USA). Finally, this solution was treated with concentrated HCl and 65% HNO3 until it became visually clear and ready for the chemical separation. Details can be found in Hill et al., (2004).
The digested sample in 3M HNO3 was passed through an extraction chromatography column (2 mL pre-packed TRU-resin column, density 0.37 g mL_1, particle size 100-150 mm, Eichrom Technologies, Inc., Darien, IL, USA). The eluate of the TRU column containing Sr was then passed through a Sr-resin column (2 mL pre-packed, density 0.33 g mL_ 1, particle size 100-150 mm) conditioned with 8 M HNO3, to obtain a pure Sr fraction. The final Sr eluate was evaporated and the residue taken up by 6 mL 0.05M HNO3 was transferred to a 20-mL High-Performance Packard scintillation vial containing 14 mL of InstaGel Plus (PerkinElmer, Boston, MA, USA) LS cocktail.
The samples were measured immediately after separation of Sr and several times later, using a Wallac 1220 Quantulus (Perki-nElmer) ultra low-level liquid scintillation spectrometer. Blanks introduced before and after each sample measurement, were prepared by adding 6 mL 0.05 N HNO3 into 14 mL of Insta-Gel Plus LS cocktail. The data reduction and analysis included the background subtraction, decay correction, decay during measurement correction, correction for the contribution of the tracer (85Sr) and the ingrowth of 90Y.
Since the sample went through digestion and chemical separation, in order to isolate the strontium, a tracer for the chemical recovery calculation was used. We have opted for 85Sr, which we then measured by y-ray spectrometry and the chemical recovery was calculated as the ratio of the counts under the 514-keV y-ray peak of the sample to that of a reference source (in the same geometry).
For the counting efficiency calibration of the LSC for 90Sr, the CIEMAT/NIST 3H efficiency tracing method (Grau Malonda and
T. Altzitzoglou, A. Bohnstedt / Applied Radiation and Isotopes I (l
Table 1
Activity concentration results for radionuclides assessed in the IAEA-375 Soil RM. The reference date is 1991-12-31 0:00 UTC. The uncertainties are expanded uncertainties with k—2.
Nuclide Activity concentration Uncertainty Uncertainty (%)
(Bqkg-1) (Bqkg-1)
40K 4.1 E + 02 0.2E + 02 4.9
134Cs 6.0E + 02 0.3E+02 4.7
137Cs 5.4E + 03 0.2E+03 4.4
90Sr 116 8 6.9
212Pb 22.7 1.1 4.7
212Bi 24.0 2.9 12
214Pb 23.6 1.4 6.1
214Bi 25.2 1.4 5.5
Fig. 1. Plot of the activity concentration results of 137Cs (1a) and 90Sr (1b) in the IAEA-375 Soil. The activity concentration values are calculated for the reference date 1991-12-31 0:00 UTC. All uncertainties are combined uncertainties at the 1s level (k=1). The solid horizontal line indicates the weighted mean and the dashed lines the expanded uncertainty (k=2).
Garcia-Torano, 1982; Grau Malonda et al., 1985) was used, requiring 3H standards only for the instrument efficiency calibration. In addition, we standardised a 90Sr/90Y solution, in the same way the sample measurements were done and submitted part of the standardised solution to BIPM, together with the value obtained for the activity concentration, as input to the Extended SIR (Altzitzoglou et al., 2004).
4. Results and conclusions
The activity concentration mean results on dry weight basis and the uncertainties obtained for 40K, 134Cs, 137Cs, 90Sr assessed in the IAEA-375 soil RM are summarised in Table 1. In addition, the
Table 2
Uncertainty budget for 40K, 134Cs, 137Cs and 90Sr in the IAEA-375 Soil. The uncertainty budget shows the typical uncertainties for a single measurement of a sample at the 1s level. The combined uncertainty is the quadratic sum of all components (k=1).
Uncertainty component
Uncertainty (%)
40K 134Cs 137Cs 90Sr
Counting statistics (incl. background) 0.15 0.35 0.01 1.5
Weighing 0.02 0.02 0.02 0.2
Geometrical repeatability 0.2 0.2 0.2 -
Dead time 0.005 0.005 0.005 0.05
Detection efficiency 2.2 2.2 2.2 -
Efficiency (incl. quenching and interpolation from - - - 1.0
curve)
Gamma-ray emission prob. 1.0 0.06 0.235 -
Chemical recovery - - - 3.5
Ratio 90Y/90Sr - - - 0.1
Timing 0.005 0.005 0.005 0.05
Half-life - 0.11 0.14 0.11
Summing correction - 0.2 - -
Sample stability - - - 0.1
Combined uncertainty (quadratic sum) 2.4 2.2 2.2 3.9
40K 134Cs 137Cs 90Sr 212Pb 212Bi 214Pb Radionuclide
Fig. 2. A comparison of the activity concentration value of each radionuclide determined in this work with that recommended by IAEA (Strachnov et al., 1996; IAEA, 2000), given as the ratio of the two values. The uncertainties are expanded uncertainties with k—2 and they include both the uncertainties on the results of the present work and those on the IAEA recommended values.
activity concentrations of 212Pb, 212Bi, 214Pb and 214Bi are given as information values, as their assessment was not requested by the IAEA. The reference date is 1991-12-31 0:00 UTC; this makes the direct comparison with the IAEA recommended values (Strachnov et al., 1996; IAEA, 2000) easy. The uncertainties in Table 1 are expanded uncertainties (k—2). In Fig. 1(a) and (b), the individual results for 137Cs and 90Sr, respectively, are plotted and in Table 2 the uncertainty budget for a typical measurement is given for the main assessed radionuclides. The final combined uncertainty is calculated as the quadratic sum of the uncertainty components, all given with a coverage factor of k—1.
A comparison of the activity concentration values determined in this work with those recommended by IAEA (Strachnov et al., 1996; IAEA, 2000) is given in Fig. 2 and shows good agreement for most of the radionuclides assessed. It is worthwhile, however, to mention that the activity concentration of 134Cs at the measurement time was about 45 times lower, due to its natural decay, than that at the reference date; furthermore, the coincidence summing correction applied amounts to 30%.
Special care was taken to ensure traceability to the SI units, by means of the calibrated standard weights used, the standard calibration sources for y-ray spectrometry, the Extended SIR for liquid scintillation counting and the use of the PTB standard time. The reader is referred to Altzitzoglou et al. (2004) for a more
4 T. Altzitzoglou, A. Bohnstedt / Applied Radiation and Isotopes l (
detailed description of the methods used to ensure traceability to the SI units.
The same RM was used for the EC Interlaboratory Comparison (ILC) on Radionuclides in Soil, organised by the JRC IRMM in 2010 (Meresova et al., 2012a, 2012b). In that case, the material received from the IAEA, was reprocessed by drying, mixing for 2 h (Dyna Mix CM200 mixer) and was filled in units of approximately 250 g in amber glass bottles. This way, the origin of the material (IAEA-375) could be concealed from the participants of the ILC. The activity concentrations used as reference values for the ILC were those determined in this work.
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