- Email: [email protected]
Preparation of spiked grass for use as an environmental radioactivity reference material
Applied Radiation and Isotopes 87 (2014) 456–460
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Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Preparation of spiked grass for use as an environmental radioactivity reference material V. Lourenço n, L. Ferreux, D. Lacour, I. Le Garrérès, S. Morelli CEA, LIST, Laboratoire National Henri Becquerel (LNE-LNHB), 91191 Gif-sur-Yvette Cedex, France
H I G H L I G H T S
Preparation of a low density treated grass matrix spiked with mixed gamma emitters. Spiking yield close to 100% regardless of the radionuclide considered. Characterization of the matrix in terms of density, particle–size, moisture content. Reference values and results of homogeneity tests are given. Results of a proficiency test organized with this matrix are presented.
art ic l e i nf o
a b s t r a c t
Available online 25 November 2013
Measurement of radionuclides from environmental samples includes a wide variety of matrix compositions and densities. To improve the traceability of environmental monitoring, LNE-LNHB intends to produce mixed γ-ray reference materials with a known mass activity and a composition as representative as possible of real environmental samples. This paper describes the preparation and characterization of a low density treated grass matrix spiked with mixed γ-emitters. This material was used in a proficiency test exercise whose results are presented. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Spiked grass Mixed gamma emitter reference material Environment
1. Introduction The monitoring of environmental radioactivity is important for public health protection. In France, environmental radioactivity is specifically monitored by a network of certified laboratories. Indeed the Nuclear Safety Authority (ASN) delivers three to five year governmental agreements to each laboratory provided it succeeds in proficiency tests (PTs) organized by the Institute for Radiological Protection and Nuclear Safety (IRSN). To ensure a direct traceability chain in radioactivity measurements, the Laboratoire National Henri Becquerel (LNE-LNHB), as the French primary laboratory in the field of radionuclide metrology, has been organizing national PTs for more than 40 years. LNE-LNHB also regularly realizes specific PTs to train the laboratories to the regulatory tests of IRSN. Most tests are based on aqueous solutions but there is a growing demand for tests on solid matrices to be measured by γ-spectrometry. A wide variety of samples exists either homogenized raw or treated materials (i.e. dried, sieved or ashed samples). Thus the composition and density of solid environmental monitoring samples can be very different from
n
Corresponding author. Tel.: þ 33 1 6908 3951; fax: þ 33 1 6908 2619. E-mail address: [email protected] (V. Lourenço).
0969-8043/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apradiso.2013.11.034
one sample to another which can significantly influence γspectrometry measurements. There are mainly two kinds of reference materials: either characterized real contaminated samples (Nour et al., 2008; Shakhashiro et al., 2012; Merešová et al., 2012) or simplified matrices spiked with known activities (de Sanoit, 1994; de Felice et al., 1998; Harms and Gilligan, 2012). Reference values of the first type result from measurements by national metrology institutes or comparison results between them and the radionuclidic content is imposed. The traceability of the second type is more direct, but the matrix is often different from real samples (density, composition). The use of natural matrix reference materials is not only expensive (due to timeconsuming characterization steps), but also limited as far as PTs are concerned, since the reference materials can easily be “recognized” afterwards. To keep the chemical and physical forms of the radionuclides present into the natural matrix, the IRSN characterizes real environmental samples. Nevertheless, the suitability of using spiked materials has been demonstrated too. For instance the IAEA has recently produced two reference materials using this approach on uncontaminated soils (Shakhashiro et al., 2006, 2007); a lot of work has also been invested in spiking natural materials as far back as the 1970s in the USA (Sill and Hindman, 1974; Olson and Bernabee, 1988). LNE-LNHB is working on the production of suitable calibration reference materials to improve the traceability of environmental
V. Lourenço et al. / Applied Radiation and Isotopes 87 (2014) 456–460
2. Experimental 2.1. Processing and initial characterization The matrix chosen is grass because it is routinely measured to monitor environmental radioactivity; it is easy to collect and of low density compared to the usual liquid calibration standards. About 60 kg of grass were collected and stored in the laboratory to start the drying procedure as soon as possible in order to avoid fermentation. The material was air dried at 60 1C for 48 h in a temperature test chamber with forced convection. 6 kg of the resulting dried material were milled to a grain size of less than 250 mm using a knife grinder. To prevent clogging of the sieve, the grinder is equipped with a cyclonic aspiration system which also speeds up the process. The bulk homogeneity of the blank matrix was tested by measuring the particle–size distribution after each homogenization step. Each homogenization step consisted in dividing all the material into quarters two of which were mixed before being again quartered and repeating the operation until a homogeneous batch was obtained. The homogeneity was tested through the particle–size distribution of the blank material. Two laboratory sieving machines were used equipped with five sieves: 250 mm, 180 mm, 150 mm, 106 mm and 75 mm. Fig. 1 shows the particle–size distribution of the blank material after milling and sieving. The diagram shows two maximum distributions: below 75 mm (28%) and between 75 and 180 mm (64%). All six test samples have an equivalent particle–size distribution to within 73% of the mean, indicating repeatability is acceptable and the milling and sieving process is uniform. The natural mass activity of the raw material was measured. Among man-made radionuclides, only 137Cs was present at (2.570.2) Bq kg 1 dry mass. The material was thus considered appropriate as a blank material to be spiked. To test the ability of environmental radioactivity monitoring laboratories, the suitable level of spiking corresponds to two or three times the activity of the radionuclides of interest present in the blank material. Nevertheless, since the purpose of this study was to prove the feasibility of the procedure, the spiking level chosen was 100–1000 Bq kg 1 dry mass.
35
% of material in the sieve
radioactivity measurements in France. To address this issue, LNELNHB intends to produce mixed γ-ray reference materials with a known mass activity and a composition as representative as possible of real environmental samples. The use of such materials will also improve the calibration of γ-spectrometry measurement systems due to a more accurate determination of the selfattenuation correction by measuring a known sample whose composition is close to the real one. This paper describes the preparation and characterization of a low density matrix made of treated grass, spiked with various γ-ray emitters. The composition of the spiking cocktail includes low-energy γ-ray emitters, to account for self-attenuation in the matrix, and high-energy γ-ray emitters to ensure a wide energy range is covered. By comparing the measured reference values with the radioactive amount of tracers added, it is also intended to assess if the spiking procedure is quantitative. A PT exercise has been organized with the resulting matrix in order to estimate the analytical performance of the laboratories for the determination of six radionuclides in a plant matrix. The results of the participants according to two performance criteria are presented. Throughout the paper, all stated uncertainties are given with a coverage factor equal to one (k ¼1).
457
Test sample 1 Test sample 2
30
Test sample 3 25
Test sample 4 Test sample 5
20
Test sample 6 Mean value
15 10 5 0
< 75 µm
< 110 µm
< 150 µm
< 180 µm
< 250 µm
Particle-size distribution Fig. 1. Particle–size distribution of the blank grass after milling and sieving at 250 mm. Uncertainty bars are shown at one standard deviation.
also been included to test the ability to estimate coincidence corrections. The preparation of the matrix was done by homogeneously incorporating aliquots of traceable solutions of radionuclides into this natural matrix. Standard stock solutions of 22Na, 60 Co, 109Cd, 134Cs and 137Cs with mass activity ranging from 0.09 to 0.6 MBq g 1 were used and were gravimetrically diluted with suitable amounts of carriers to prepare the spiking solution. This solution was measured by γ-spectrometry to confirm the mass activity value of each radionuclide to better than 1.2%. This solution was then used to spike the matrix with 1-mL aliquots, using a calibrated micropipette (the repeatability of the mass of liquid delivered was assessed resulting in an uncertainty of 0.2%). Two spiking procedures were tested. The first one consisted of mixing an aliquot of the spiking cocktail with methanol and adding it to a weighed portion of grass. The slurry formed was thoroughly mixed in a Turbulas mixer and the sample was left to dry in an oven with forced convection at 60 1C overnight. This procedure was not suitable since it favored the production of hot spots in the matrix. Another spiking procedure was tested using a rotary evaporator (de Sanoit, 1994). This second procedure involved mixing a known amount of weighed grass and analytical grade acetone in an evaporation flask. Around 1 g of radioactive aqueous standard solution was added to the slurry using the calibrated micropipette and the flask was left rotating for 1 h to homogenize. Then the acetone was evaporated and the dried spiked grass was recovered. To produce 10 L of the spiked matrix, it was necessary to repeat this step 20 times. Thus a further homogenization step was needed: the bulk of the spiked material was recombined and homogenized manually by quartering several times the matrix in four 10 L drums, as described before. The material was then bottled in 0.5 L plastic containers sealed with security caps with 140 g in each unit and labeled with the participant code. This step was carried out in a glove box to prevent contamination of the operators because the material was very dusty and electrostatic. It is also the reason why it was decided to avoid a repackaging step by the test participants, to prevent them from contaminating their laboratories. The average moisture content of the sample after bottling was determined by drying several aliquots in a test chamber with forced convection at 60 1C for 48 h and was found to be (2.96 70.02) %.
2.2. Spiking procedure
3. γ-Spectrometry measurements
The energy range covered by the radionuclides present in the matrix is 88 keV to 1461 keV. Multi-γ-emitting radionuclides have
γ-Spectrometry measurements were performed with a Canberra low background hyperpure germanium detector (n-type;
V. Lourenço et al. / Applied Radiation and Isotopes 87 (2014) 456–460
220 cm3 Ge crystal) equipped with active anti-cosmic shielding, consisting of plastic scintillators in anti-coincidence mode. The full width at half maximum is 0.99 keV at 122 keV and 2.05 keV at 1.332 MeV.
3.1. Efficiency calibration Aqueous standard solutions in the same measurement geometry were used to determine the efficiency curve of the detector. Since the spiked grass matrix has a much lower density, it was necessary to apply a suitable self-attenuation correction to take into account the matrix difference between the calibration solution and the sample. This complex step, described below, could be avoided if the laboratories could directly use the present matrix to calibrate their detectors.
Environmental samples cover a wide range of densities from 0.1 to 2 g cm 3. Mass attenuation coefficients differ from one sample to another mainly due to their density rather than their composition. The self-attenuation correction is obtained as the ratio of the self-attenuation for both materials according to Eq. (1): C corr ¼
1e
μGrass ðEÞ ρGrass ρGrass xGrass
1e
μSol ðEÞ ρSol ρSol xSol
ð1Þ
where the linear attenuation coefficient as a function of the energy E is m(E) in cm 1, ρ is the density, and x the thickness of the material. The subscripts “Grass” and “Sol” refer to the spiked matrix and the calibration solution respectively. mSol(E) was derived from the XCOM database of the NIST for hydrochloric acid (Berger et al., 2005). mGrass(E) was estimated using the Beer– Lambert's law, using external radioactive point sources (152Eu and 133Ba, from 53 keV to 1.41 MeV), a collimating system and successively an empty container and a similar container filled with the grass matrix. The measurement of mGrass(E) is then derived from Eq. (2): mGrass ðEÞ ¼
1 I 0 ðE Þ ln xGrass I ðEÞ
Energy (keV) Fig. 2. Self-attenuation correction curve implemented into Interwinner Software to correct for the differences between the self-attenuation of the spiked grass and the calibration solution. Uncertainty bars are shown with a coverage factor equal to one (k ¼ 1).
Table 1 Values of the ratios s2B =s2W for each radionuclide to
3.2. Self-attenuation correction
μSol ðEÞ ρSol μGrass ðEÞ ρGrass
Self-attenuation correction
458
ð2Þ
where I 0 is the intensity of the parallel beam of photons transmitted through the empty container, I is the intensity of the beam of photons transmitted through the container filled with thickness xGrass of material. This resulted in a self-attenuation correction curve which was a logarithmic equation of the fourth degree. This curve (Fig. 2), together with its associated uncertainties, was implemented into the data treatment software (Interwinner Software) to derive the mass activity of the radionuclides from the γ-spectrometry measurements. The correction is especially important at low energy, below 500 keV, before becoming flat above 1.5 MeV. As stated before, the spiked matrix was difficult to handle due to it being highly electrostatic property and a settling of the grass was noticed during the sampling steps. The spiked grass matrix exhibits a final density of (0.37 70.02) g cm 3. The filling height of the containers was slightly different from one sample to another. To account for this variation, an efficiency transfer was realized, i.e. calculation of the efficiency of the detector under measurement conditions different from those of calibration. The Efficiency Transfer for Nuclide Activity measurements (ETNA) software developed at LNE-LNHB was used (Piton et al., 2000). It includes databases making it possible to record characteristics of different measurement geometries.
be compared to F 5% ð4;10Þ ¼3.48. Nuclide
F-ratio
Present in the spiking cocktail 22 Na 1.84 60 Co 1.46 and 0.77 109 Cd 1.36 134 Cs 1.82 and 0.83 137 Cs 3.25 Naturally present in the matrix 40 K 1.50 For 60Co and 134Cs the results obtained with the two main transitions are shown.
3.3. Homogeneity assessment After spiking, the material was tested for homogeneity by measuring 10 sub-samples of 14 g each (10% of one shipped sample). The sub-samples were measured for 48 h by γ-spectrometry. The aim was to check the homogeneity between similar sub-samples, not to derive the mass activity from these measurements. Thus, after correction for decay and sample mass, the uncertainties on these measurements were only those due to counting statistics. The homogeneity test was performed by one-way analysis of variance (ANOVA) according to the ISO-Guide 35 (ISO, 2006). The point of this analysis is to test whether all the sub-samples come from the same population and, hence, have the same variance. ANOVA consists in splitting the total variation in the data into two components, namely the within-group and between-group variances. The within-group variance (s2W ) should be compared with the between-group variance (s2B ) using the Fisher distribution (Ftest). For n measurements of m sub-samples, s2W is given by the average of the within-group variances and s2B is calculated by multiplying the variance of the m between-group means by the number of measurements n. If the ratio s2B =s2W is lower than the critical value of F for a one-tailed test (with an alpha risk of 5%), the null hypothesis assuming that the measurements are independent of the sampling cannot be rejected at this significance level (thus is considered true). Table 1 shows the values of the ratios s2B =s2W for each radionuclide as well as the critical value of F. The results will be commented in the discussion section of this paper. 3.4. Reference values and uncertainty budget Among the twenty 140 g samples prepared, seven samples were measured for 72 h to assess the reference values. The
V. Lourenço et al. / Applied Radiation and Isotopes 87 (2014) 456–460
Table 2 Measured reference values (Ar meas.), calculated target values from the reference activities of the spiking cocktail (Acalc.) and subsequent spiking yield obtained with the spiking procedure. Uncertainties mentioned are calculated with a coverage factor equal to one (k ¼ 1) and the reference date is 17 September 2012 12 h UTC. Nuclide Ar meas. mBq g 1
ur meas. mBq g 1
Acalc. mBq g 1
Present in the spiking cocktail 22 Na 165.0 3.9 169.8 60 Co 176.6 4.4 175.3 109 Cd 894 29 906 134 Cs 113.3 3.0 115.2 137 Csa 171.9 4.3 174.3 Naturally present in the real matrix 40 K 1213 33 / a
γ-Ray of
137m
Ba in equilibrium with
137
ucalc. mBq g 1
Spiking yield (%)
usp. (%)
1.2 1.1 11 1.3 1.3
97.2 100.7 98.6 98.4 98.6
2.5 2.6 3.4 2.8 2.6
/
/
/
Cs.
combined uncertainty of the measured reference values is composed of: counting uncertainties (0.8% for 109Cd, otherwise below 0.5%), efficiency correction uncertainties (2.7% for 109Cd, otherwise below 1.5%), self-attenuation correction uncertainty (1.7%) and variability due to imperfect homogenization (always below 1.3%, 1.1% for 40K). To assess the yield of the spiking procedure, the target mass activity values were calculated from the measured spiking solution, taking into account the mass of spiking solution and the amount of matrix being spiked. These calculated values are compared to the measured reference values from γspectrometry measurements, together with their respective uncertainties, in Table 2. The combined uncertainty for the calculated target values includes one major component which is the uncertainty of the activity of the spiking solution (from 0.6% to 1.2% depending on the radionuclide) and weighing uncertainties (0.2% due to the micropipette and 0.2% for the mass of spiked grass). The spiking yield is the ratio between measured reference values and calculated target values; its uncertainty is the combined uncertainty of the measured and calculated values. In the framework of the use of quantitative spiking to assess the reference values of a material, the uncertainty on the spiking yield would be the dominant uncertainty component. Two additional contributions would have to be determined as well: the homogeneity uncertainty and the stability uncertainty.
4. Proficiency test Proficiency testing is widely used by testing laboratories to demonstrate to their clients, or accreditation bodies, the validity of their data. Also, participation in this kind of external qualityassurance programme is highly recommended according to the ISO/IEC standard 17025 (ISO, 2005a). The test samples were sent to the participants at the beginning of October 2012 and their measurement results were expected by the end of November 2012. As expected by the participants in this test, their results were evaluated according to the same rules applied by IRSN for regulatory PTs and can be consulted here: http://www.mesure-radioactivite.fr/public/IMG/pdf/5_criteres_ techniques.pdf. An individual report was issued and sent to each participant. This report showed the results of the participant compared to the results of the others with blind codes used according to the two following evaluation parameters: The relative difference between the participant's value (Ap) and the reference value (Ar) for each radionuclide present, expressed as
459
a percentage (Eq. (3)): Relative dif f erence ¼ ep ¼ 100%
Ap Ar Ar
ð3Þ
The En number uses the expanded measurement uncertainties (k ¼2) of the participant's result (Up) and the reference value (Ur) and is defined below (Eq. (4)): Ar Ap En;p ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi ð4Þ ðU 2p þ U 2r Þ The uncertainties of the reference values can have an effect on a laboratory's ability to meet the En,p requirement, especially if they are “too low”, the requirement then becomes harder to meet. Since the aim of the PT is to check the ability of the participant to perform the measurements correctly, the reference value is intentionally measured by a “similar measurement technique” than the one used by the participant. Similarly if the uncertainty provided by a participant is too large, it helps the laboratory to pass the criterion. That is why in some cases the value of the expanded uncertainty provided is limited to twice the standard deviation of all submitted values for the radionuclide, which is representative of the measurement difficulty. The results are classified as follows: ep o 15% or En;p r1 indicates an acceptable result stating
that the laboratory value is compatible with the reference value; 15% o ep o 20% or 1 o En;p o1.3 means a questionable result. It is a first ‘‘warning signal’' that the laboratory values differ significantly from the reference value, sources of deviation should be investigated and corrected; level of critical value is defined by ep 4 20% or a second En;p Z 1.3 indicating a discrepant result. This is an ‘‘action signal’', there is urgent need to investigate and find the source of the large deviation.
The classification of the results proposed here for ep and En numbers is specific to the French regulatory certification process and is somewhat different from the classification expected by the application of ISO 13528:2005 (ISO, 2005b) through the suggested values of z-scores (namely 2 and 3). The consequence is that the French range for questionable results (for ep and En numbers) is smaller than expected in the ISO standard. Similarly, discrepant results are obtained for lower values of the criteria. The data evaluation in this PT is based on reference values independent of the results of the participants which allow an evaluation not only of the deviation but also of the accuracy of the reported results. Relative differences calculated from the participants' results and associated means and standard deviations are presented in Table 3 and will be discussed in the next section.
5. Discussion According to Table 2, we can conclude that the spiking procedure resulted in a spiking yield close to 100% regardless of the radionuclide considered. To check for possible losses when preparing the matrix, the recovered acetone and the preparation flasks were measured on a suitable γ-spectrometer and no activity was observed which supports the good spiking yields. The homogeneity of the batch was verified by suitable analysis on sub-samples representing only one tenth (14 g) of the size of the dispatched sample. According to the ANOVA test, to state the homogeneity of the batch, we should check that s2B and s2W are not significantly different from one another. The values of the ratios s2B =s2W for each radionuclide (Table 1) are lower than the critical
460
V. Lourenço et al. / Applied Radiation and Isotopes 87 (2014) 456–460
Table 3 Relative differences (%) calculated from the participants' results and associated means and standard deviations (k ¼ 1). Blind code
22
200 201 202 203 oei 4 s(ei) o|ei| 4
0.7 6.7 4.9 1.3 0.6 5.8 3.4
Na
40
K
5.0 1.7 2.7 – 2.0 3.1 3.1
60
109
5.2 5.1 5.4 4.0 0.4 5.8 4.9
5.5 3.1 6.5 10.4 0.4 8.5 6.4
Co
Cd
134
137
o ep 4
s(ep)
o|ep|4
s(|ep|)
2.6 4.1 5.8 0.7 0.9 5.0 3.3
8.1 2.2 7.1 2.1 2.7 5.4 4.9
4.5 3.8 5.4 3.7
2.6 1.9 1.5 4.0
4.5 3.8 5.4 3.7
2.6 1.9 1.5 4.0
Cs
Cs
The subscripts i and p refer to the ith radionuclide identified in the matrix and participant respectively. o ei 4, oep 4 and o |ep| 4 are mean values. s(ei), s(ep) and s(|ep|) are standard deviations. o|ei| 4 and o|ep|4 are means of absolute relative differences.
value of F for a one-tailed test. This means that s2B is not significantly greater than s2W : To check if s2W is not significantly lower than s2B either, we have to test if s2W =s2B is lower than F(10,4) ¼5.96. This is a simple way to perform a two-tailed ANOVA test. According to the results, we can conclude that, at a 5% significance level, for all the radionuclide considered the material can be considered to be sufficiently homogeneous within the range of mass used. The participants' results are equally distributed above and below the reference values, indeed the mean value of the 23 relative differences (Table 3) is 0.8%. The mean values of the relative differences per radionuclide are between 0.6% (for 22Na) and þ2.7% (for 137Cs). Nevertheless the standard deviation of the 109 Cd results from the participants is the highest one: 8.5%, as expected due to its low-emission energy and low emission probability. All En numbers calculated were found acceptable, even though in some cases the value of the expanded uncertainty provided had to be limited. Hence, some laboratories may be over-estimating their uncertainty budget. 6. Conclusion The spiking procedure aimed to produce a reference material with a quantifiable, but still low, overall mass activity. The specific activity of the spiked material is about 100 to 1000 Bq kg 1 dry mass depending on the radionuclide. The spiking procedure resulted in a spiking yield close to 100% regardless of the radionuclide considered. The homogeneity of the batch was verified by suitable analysis on sub-samples. The analysis of the results according to two evaluation parameters showed that all the results were acceptable, that is to say in agreement with the reference values, even though in some cases (En numbers) the value of the provided expanded uncertainty had to be limited. Even if the number of participants to this feasibility test, organized with this first sample of a low density solid matrix, was limited, the results of the participants are encouraging since they are as good as those for solution samples, commonly organized. The LNE-LNHB is monitoring the long-term stability of the material and the participants will be informed in the event of any observed change.
References Berger, M.J., Hubbell, J.H., Seltzer, S.M., Chang, J., Coursey, J.S., Sukumar, R., Zucker, D.S., 2005. XCOM: Photon Cross Section Database (version 1.3). National Institute of Standards and Technology, Gaithersburg, MD, USA http://physics.nist.gov/xcom. (Online available). de Felice, P., Iskandar, D., Fazio, A., Biagini, R, 1998. The quality control in the production of spiked reference materials for gamma-ray spectrometry. Appl. Radiat. Isot. 49 (9–11), 1421–1427. de Sanoit, J., 1994. Silica spiked with mixed γ-ray standards for use as environmental reference material. Radiochim. Acta 65, 249–257. Harms, A., Gilligan, C., 2012. Development of synthetic environmental radioactivity reference materials. Appl. Radiat. Isot. 70, 1940–1943. Interwinner Software, version 5.05.1044 from ORTEC (〈http://www.interwinner. com〉). ISO, 2005a. ISO/IEC 17025:2005 General Requirements for the Competence of Testing and Calibration Laboratories. International Organization for Standardization, Geneva, Switzerland. ISO, 2005b. ISO 13528:2005 Statistical Methods for use in Proficiency Testing by Interlaboratory Comparisons. International Organization for Standardization, Geneva, Switzerland. ISO, 2006. ISO Guide 35:2006 Reference materials—General and Statistical Principles for Certification. International Organization for Standardization, Geneva, Switzerland. Merešová, J., Wätjen, U., Altzitzoglou, T., 2012. Determination of natural and anthropogenic radionuclides in soil—results of an European Union comparison. Appl. Radiat. Isot. 70, 1836–1842. Nour, S., Inn, K., Filliben, J., 2008. Development of the NIST rocky flats soil standard. J. Radioanal. Nucl. Chem. 277, 161–168. Olson, D.G., Bernabee, R.P., 1988. Preparation and analysis of high-quality spiked soil standards. Health Phys. 54 (4), 451–459. Piton, F., Lépy, M.-C, Bé, M.-M., Plagnard, J., 2000. Efficiency transfer and coincidence summing corrections for gamma-ray spectrometry. Appl. Radiat. Isot. 52, 791–795. Shakhashiro, A., AzeredoGondinDaFoseca, A.M., Sansone, U., Kim, C.-K., Kis-Benedek, G., Trinkl, A., Benesch, T., Schorn, R., 2006. Report on the IAEA-CU-2006-02 Poficiency Test on the Determination of 137Cs and 210Pb in Spiked Soil. Report IAEA/AL/166, International Atomic Energy Agency, Vienna, Austria. Shakhashiro, A., Azeredo Gondin Da Foseca, A.M., Sansone, U., Fajgelj, A., 2007. Matrix materials for proficiency testing: optimization of a procedure for spiking soil with gamma-emitting radionuclides. Anal. Bioanal. Chem. 387, 2509–2515. Shakhashiro, A., Tarjan, S., Ceccatelli, A., Kis-Benedek, G., Betti, M., 2012. IAEA-447: a new certified reference material for environmental radioactivity measurements. Appl. Radiat. Isot. 70, 1632–1643. Sill, C.W., Hindman, F.D., 1974. Preparation and testing of standard soils containing known quantities of radionuclides. Anal. Chem. 46 (1), 113–118.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Preparation of spiked grass for use as an environmental radioactivity reference material V. Lourenço n, L. Ferreux, D. Lacour, I. Le Garrérès, S. Morelli CEA, LIST, Laboratoire National Henri Becquerel (LNE-LNHB), 91191 Gif-sur-Yvette Cedex, France
H I G H L I G H T S
Preparation of a low density treated grass matrix spiked with mixed gamma emitters. Spiking yield close to 100% regardless of the radionuclide considered. Characterization of the matrix in terms of density, particle–size, moisture content. Reference values and results of homogeneity tests are given. Results of a proficiency test organized with this matrix are presented.
art ic l e i nf o
a b s t r a c t
Available online 25 November 2013
Measurement of radionuclides from environmental samples includes a wide variety of matrix compositions and densities. To improve the traceability of environmental monitoring, LNE-LNHB intends to produce mixed γ-ray reference materials with a known mass activity and a composition as representative as possible of real environmental samples. This paper describes the preparation and characterization of a low density treated grass matrix spiked with mixed γ-emitters. This material was used in a proficiency test exercise whose results are presented. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Spiked grass Mixed gamma emitter reference material Environment
1. Introduction The monitoring of environmental radioactivity is important for public health protection. In France, environmental radioactivity is specifically monitored by a network of certified laboratories. Indeed the Nuclear Safety Authority (ASN) delivers three to five year governmental agreements to each laboratory provided it succeeds in proficiency tests (PTs) organized by the Institute for Radiological Protection and Nuclear Safety (IRSN). To ensure a direct traceability chain in radioactivity measurements, the Laboratoire National Henri Becquerel (LNE-LNHB), as the French primary laboratory in the field of radionuclide metrology, has been organizing national PTs for more than 40 years. LNE-LNHB also regularly realizes specific PTs to train the laboratories to the regulatory tests of IRSN. Most tests are based on aqueous solutions but there is a growing demand for tests on solid matrices to be measured by γ-spectrometry. A wide variety of samples exists either homogenized raw or treated materials (i.e. dried, sieved or ashed samples). Thus the composition and density of solid environmental monitoring samples can be very different from
n
Corresponding author. Tel.: þ 33 1 6908 3951; fax: þ 33 1 6908 2619. E-mail address: [email protected] (V. Lourenço).
0969-8043/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apradiso.2013.11.034
one sample to another which can significantly influence γspectrometry measurements. There are mainly two kinds of reference materials: either characterized real contaminated samples (Nour et al., 2008; Shakhashiro et al., 2012; Merešová et al., 2012) or simplified matrices spiked with known activities (de Sanoit, 1994; de Felice et al., 1998; Harms and Gilligan, 2012). Reference values of the first type result from measurements by national metrology institutes or comparison results between them and the radionuclidic content is imposed. The traceability of the second type is more direct, but the matrix is often different from real samples (density, composition). The use of natural matrix reference materials is not only expensive (due to timeconsuming characterization steps), but also limited as far as PTs are concerned, since the reference materials can easily be “recognized” afterwards. To keep the chemical and physical forms of the radionuclides present into the natural matrix, the IRSN characterizes real environmental samples. Nevertheless, the suitability of using spiked materials has been demonstrated too. For instance the IAEA has recently produced two reference materials using this approach on uncontaminated soils (Shakhashiro et al., 2006, 2007); a lot of work has also been invested in spiking natural materials as far back as the 1970s in the USA (Sill and Hindman, 1974; Olson and Bernabee, 1988). LNE-LNHB is working on the production of suitable calibration reference materials to improve the traceability of environmental
V. Lourenço et al. / Applied Radiation and Isotopes 87 (2014) 456–460
2. Experimental 2.1. Processing and initial characterization The matrix chosen is grass because it is routinely measured to monitor environmental radioactivity; it is easy to collect and of low density compared to the usual liquid calibration standards. About 60 kg of grass were collected and stored in the laboratory to start the drying procedure as soon as possible in order to avoid fermentation. The material was air dried at 60 1C for 48 h in a temperature test chamber with forced convection. 6 kg of the resulting dried material were milled to a grain size of less than 250 mm using a knife grinder. To prevent clogging of the sieve, the grinder is equipped with a cyclonic aspiration system which also speeds up the process. The bulk homogeneity of the blank matrix was tested by measuring the particle–size distribution after each homogenization step. Each homogenization step consisted in dividing all the material into quarters two of which were mixed before being again quartered and repeating the operation until a homogeneous batch was obtained. The homogeneity was tested through the particle–size distribution of the blank material. Two laboratory sieving machines were used equipped with five sieves: 250 mm, 180 mm, 150 mm, 106 mm and 75 mm. Fig. 1 shows the particle–size distribution of the blank material after milling and sieving. The diagram shows two maximum distributions: below 75 mm (28%) and between 75 and 180 mm (64%). All six test samples have an equivalent particle–size distribution to within 73% of the mean, indicating repeatability is acceptable and the milling and sieving process is uniform. The natural mass activity of the raw material was measured. Among man-made radionuclides, only 137Cs was present at (2.570.2) Bq kg 1 dry mass. The material was thus considered appropriate as a blank material to be spiked. To test the ability of environmental radioactivity monitoring laboratories, the suitable level of spiking corresponds to two or three times the activity of the radionuclides of interest present in the blank material. Nevertheless, since the purpose of this study was to prove the feasibility of the procedure, the spiking level chosen was 100–1000 Bq kg 1 dry mass.
35
% of material in the sieve
radioactivity measurements in France. To address this issue, LNELNHB intends to produce mixed γ-ray reference materials with a known mass activity and a composition as representative as possible of real environmental samples. The use of such materials will also improve the calibration of γ-spectrometry measurement systems due to a more accurate determination of the selfattenuation correction by measuring a known sample whose composition is close to the real one. This paper describes the preparation and characterization of a low density matrix made of treated grass, spiked with various γ-ray emitters. The composition of the spiking cocktail includes low-energy γ-ray emitters, to account for self-attenuation in the matrix, and high-energy γ-ray emitters to ensure a wide energy range is covered. By comparing the measured reference values with the radioactive amount of tracers added, it is also intended to assess if the spiking procedure is quantitative. A PT exercise has been organized with the resulting matrix in order to estimate the analytical performance of the laboratories for the determination of six radionuclides in a plant matrix. The results of the participants according to two performance criteria are presented. Throughout the paper, all stated uncertainties are given with a coverage factor equal to one (k ¼1).
457
Test sample 1 Test sample 2
30
Test sample 3 25
Test sample 4 Test sample 5
20
Test sample 6 Mean value
15 10 5 0
< 75 µm
< 110 µm
< 150 µm
< 180 µm
< 250 µm
Particle-size distribution Fig. 1. Particle–size distribution of the blank grass after milling and sieving at 250 mm. Uncertainty bars are shown at one standard deviation.
also been included to test the ability to estimate coincidence corrections. The preparation of the matrix was done by homogeneously incorporating aliquots of traceable solutions of radionuclides into this natural matrix. Standard stock solutions of 22Na, 60 Co, 109Cd, 134Cs and 137Cs with mass activity ranging from 0.09 to 0.6 MBq g 1 were used and were gravimetrically diluted with suitable amounts of carriers to prepare the spiking solution. This solution was measured by γ-spectrometry to confirm the mass activity value of each radionuclide to better than 1.2%. This solution was then used to spike the matrix with 1-mL aliquots, using a calibrated micropipette (the repeatability of the mass of liquid delivered was assessed resulting in an uncertainty of 0.2%). Two spiking procedures were tested. The first one consisted of mixing an aliquot of the spiking cocktail with methanol and adding it to a weighed portion of grass. The slurry formed was thoroughly mixed in a Turbulas mixer and the sample was left to dry in an oven with forced convection at 60 1C overnight. This procedure was not suitable since it favored the production of hot spots in the matrix. Another spiking procedure was tested using a rotary evaporator (de Sanoit, 1994). This second procedure involved mixing a known amount of weighed grass and analytical grade acetone in an evaporation flask. Around 1 g of radioactive aqueous standard solution was added to the slurry using the calibrated micropipette and the flask was left rotating for 1 h to homogenize. Then the acetone was evaporated and the dried spiked grass was recovered. To produce 10 L of the spiked matrix, it was necessary to repeat this step 20 times. Thus a further homogenization step was needed: the bulk of the spiked material was recombined and homogenized manually by quartering several times the matrix in four 10 L drums, as described before. The material was then bottled in 0.5 L plastic containers sealed with security caps with 140 g in each unit and labeled with the participant code. This step was carried out in a glove box to prevent contamination of the operators because the material was very dusty and electrostatic. It is also the reason why it was decided to avoid a repackaging step by the test participants, to prevent them from contaminating their laboratories. The average moisture content of the sample after bottling was determined by drying several aliquots in a test chamber with forced convection at 60 1C for 48 h and was found to be (2.96 70.02) %.
2.2. Spiking procedure
3. γ-Spectrometry measurements
The energy range covered by the radionuclides present in the matrix is 88 keV to 1461 keV. Multi-γ-emitting radionuclides have
γ-Spectrometry measurements were performed with a Canberra low background hyperpure germanium detector (n-type;
V. Lourenço et al. / Applied Radiation and Isotopes 87 (2014) 456–460
220 cm3 Ge crystal) equipped with active anti-cosmic shielding, consisting of plastic scintillators in anti-coincidence mode. The full width at half maximum is 0.99 keV at 122 keV and 2.05 keV at 1.332 MeV.
3.1. Efficiency calibration Aqueous standard solutions in the same measurement geometry were used to determine the efficiency curve of the detector. Since the spiked grass matrix has a much lower density, it was necessary to apply a suitable self-attenuation correction to take into account the matrix difference between the calibration solution and the sample. This complex step, described below, could be avoided if the laboratories could directly use the present matrix to calibrate their detectors.
Environmental samples cover a wide range of densities from 0.1 to 2 g cm 3. Mass attenuation coefficients differ from one sample to another mainly due to their density rather than their composition. The self-attenuation correction is obtained as the ratio of the self-attenuation for both materials according to Eq. (1): C corr ¼
1e
μGrass ðEÞ ρGrass ρGrass xGrass
1e
μSol ðEÞ ρSol ρSol xSol
ð1Þ
where the linear attenuation coefficient as a function of the energy E is m(E) in cm 1, ρ is the density, and x the thickness of the material. The subscripts “Grass” and “Sol” refer to the spiked matrix and the calibration solution respectively. mSol(E) was derived from the XCOM database of the NIST for hydrochloric acid (Berger et al., 2005). mGrass(E) was estimated using the Beer– Lambert's law, using external radioactive point sources (152Eu and 133Ba, from 53 keV to 1.41 MeV), a collimating system and successively an empty container and a similar container filled with the grass matrix. The measurement of mGrass(E) is then derived from Eq. (2): mGrass ðEÞ ¼
1 I 0 ðE Þ ln xGrass I ðEÞ
Energy (keV) Fig. 2. Self-attenuation correction curve implemented into Interwinner Software to correct for the differences between the self-attenuation of the spiked grass and the calibration solution. Uncertainty bars are shown with a coverage factor equal to one (k ¼ 1).
Table 1 Values of the ratios s2B =s2W for each radionuclide to
3.2. Self-attenuation correction
μSol ðEÞ ρSol μGrass ðEÞ ρGrass
Self-attenuation correction
458
ð2Þ
where I 0 is the intensity of the parallel beam of photons transmitted through the empty container, I is the intensity of the beam of photons transmitted through the container filled with thickness xGrass of material. This resulted in a self-attenuation correction curve which was a logarithmic equation of the fourth degree. This curve (Fig. 2), together with its associated uncertainties, was implemented into the data treatment software (Interwinner Software) to derive the mass activity of the radionuclides from the γ-spectrometry measurements. The correction is especially important at low energy, below 500 keV, before becoming flat above 1.5 MeV. As stated before, the spiked matrix was difficult to handle due to it being highly electrostatic property and a settling of the grass was noticed during the sampling steps. The spiked grass matrix exhibits a final density of (0.37 70.02) g cm 3. The filling height of the containers was slightly different from one sample to another. To account for this variation, an efficiency transfer was realized, i.e. calculation of the efficiency of the detector under measurement conditions different from those of calibration. The Efficiency Transfer for Nuclide Activity measurements (ETNA) software developed at LNE-LNHB was used (Piton et al., 2000). It includes databases making it possible to record characteristics of different measurement geometries.
be compared to F 5% ð4;10Þ ¼3.48. Nuclide
F-ratio
Present in the spiking cocktail 22 Na 1.84 60 Co 1.46 and 0.77 109 Cd 1.36 134 Cs 1.82 and 0.83 137 Cs 3.25 Naturally present in the matrix 40 K 1.50 For 60Co and 134Cs the results obtained with the two main transitions are shown.
3.3. Homogeneity assessment After spiking, the material was tested for homogeneity by measuring 10 sub-samples of 14 g each (10% of one shipped sample). The sub-samples were measured for 48 h by γ-spectrometry. The aim was to check the homogeneity between similar sub-samples, not to derive the mass activity from these measurements. Thus, after correction for decay and sample mass, the uncertainties on these measurements were only those due to counting statistics. The homogeneity test was performed by one-way analysis of variance (ANOVA) according to the ISO-Guide 35 (ISO, 2006). The point of this analysis is to test whether all the sub-samples come from the same population and, hence, have the same variance. ANOVA consists in splitting the total variation in the data into two components, namely the within-group and between-group variances. The within-group variance (s2W ) should be compared with the between-group variance (s2B ) using the Fisher distribution (Ftest). For n measurements of m sub-samples, s2W is given by the average of the within-group variances and s2B is calculated by multiplying the variance of the m between-group means by the number of measurements n. If the ratio s2B =s2W is lower than the critical value of F for a one-tailed test (with an alpha risk of 5%), the null hypothesis assuming that the measurements are independent of the sampling cannot be rejected at this significance level (thus is considered true). Table 1 shows the values of the ratios s2B =s2W for each radionuclide as well as the critical value of F. The results will be commented in the discussion section of this paper. 3.4. Reference values and uncertainty budget Among the twenty 140 g samples prepared, seven samples were measured for 72 h to assess the reference values. The
V. Lourenço et al. / Applied Radiation and Isotopes 87 (2014) 456–460
Table 2 Measured reference values (Ar meas.), calculated target values from the reference activities of the spiking cocktail (Acalc.) and subsequent spiking yield obtained with the spiking procedure. Uncertainties mentioned are calculated with a coverage factor equal to one (k ¼ 1) and the reference date is 17 September 2012 12 h UTC. Nuclide Ar meas. mBq g 1
ur meas. mBq g 1
Acalc. mBq g 1
Present in the spiking cocktail 22 Na 165.0 3.9 169.8 60 Co 176.6 4.4 175.3 109 Cd 894 29 906 134 Cs 113.3 3.0 115.2 137 Csa 171.9 4.3 174.3 Naturally present in the real matrix 40 K 1213 33 / a
γ-Ray of
137m
Ba in equilibrium with
137
ucalc. mBq g 1
Spiking yield (%)
usp. (%)
1.2 1.1 11 1.3 1.3
97.2 100.7 98.6 98.4 98.6
2.5 2.6 3.4 2.8 2.6
/
/
/
Cs.
combined uncertainty of the measured reference values is composed of: counting uncertainties (0.8% for 109Cd, otherwise below 0.5%), efficiency correction uncertainties (2.7% for 109Cd, otherwise below 1.5%), self-attenuation correction uncertainty (1.7%) and variability due to imperfect homogenization (always below 1.3%, 1.1% for 40K). To assess the yield of the spiking procedure, the target mass activity values were calculated from the measured spiking solution, taking into account the mass of spiking solution and the amount of matrix being spiked. These calculated values are compared to the measured reference values from γspectrometry measurements, together with their respective uncertainties, in Table 2. The combined uncertainty for the calculated target values includes one major component which is the uncertainty of the activity of the spiking solution (from 0.6% to 1.2% depending on the radionuclide) and weighing uncertainties (0.2% due to the micropipette and 0.2% for the mass of spiked grass). The spiking yield is the ratio between measured reference values and calculated target values; its uncertainty is the combined uncertainty of the measured and calculated values. In the framework of the use of quantitative spiking to assess the reference values of a material, the uncertainty on the spiking yield would be the dominant uncertainty component. Two additional contributions would have to be determined as well: the homogeneity uncertainty and the stability uncertainty.
4. Proficiency test Proficiency testing is widely used by testing laboratories to demonstrate to their clients, or accreditation bodies, the validity of their data. Also, participation in this kind of external qualityassurance programme is highly recommended according to the ISO/IEC standard 17025 (ISO, 2005a). The test samples were sent to the participants at the beginning of October 2012 and their measurement results were expected by the end of November 2012. As expected by the participants in this test, their results were evaluated according to the same rules applied by IRSN for regulatory PTs and can be consulted here: http://www.mesure-radioactivite.fr/public/IMG/pdf/5_criteres_ techniques.pdf. An individual report was issued and sent to each participant. This report showed the results of the participant compared to the results of the others with blind codes used according to the two following evaluation parameters: The relative difference between the participant's value (Ap) and the reference value (Ar) for each radionuclide present, expressed as
459
a percentage (Eq. (3)): Relative dif f erence ¼ ep ¼ 100%
Ap Ar Ar
ð3Þ
The En number uses the expanded measurement uncertainties (k ¼2) of the participant's result (Up) and the reference value (Ur) and is defined below (Eq. (4)): Ar Ap En;p ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi ð4Þ ðU 2p þ U 2r Þ The uncertainties of the reference values can have an effect on a laboratory's ability to meet the En,p requirement, especially if they are “too low”, the requirement then becomes harder to meet. Since the aim of the PT is to check the ability of the participant to perform the measurements correctly, the reference value is intentionally measured by a “similar measurement technique” than the one used by the participant. Similarly if the uncertainty provided by a participant is too large, it helps the laboratory to pass the criterion. That is why in some cases the value of the expanded uncertainty provided is limited to twice the standard deviation of all submitted values for the radionuclide, which is representative of the measurement difficulty. The results are classified as follows: ep o 15% or En;p r1 indicates an acceptable result stating
that the laboratory value is compatible with the reference value; 15% o ep o 20% or 1 o En;p o1.3 means a questionable result. It is a first ‘‘warning signal’' that the laboratory values differ significantly from the reference value, sources of deviation should be investigated and corrected; level of critical value is defined by ep 4 20% or a second En;p Z 1.3 indicating a discrepant result. This is an ‘‘action signal’', there is urgent need to investigate and find the source of the large deviation.
The classification of the results proposed here for ep and En numbers is specific to the French regulatory certification process and is somewhat different from the classification expected by the application of ISO 13528:2005 (ISO, 2005b) through the suggested values of z-scores (namely 2 and 3). The consequence is that the French range for questionable results (for ep and En numbers) is smaller than expected in the ISO standard. Similarly, discrepant results are obtained for lower values of the criteria. The data evaluation in this PT is based on reference values independent of the results of the participants which allow an evaluation not only of the deviation but also of the accuracy of the reported results. Relative differences calculated from the participants' results and associated means and standard deviations are presented in Table 3 and will be discussed in the next section.
5. Discussion According to Table 2, we can conclude that the spiking procedure resulted in a spiking yield close to 100% regardless of the radionuclide considered. To check for possible losses when preparing the matrix, the recovered acetone and the preparation flasks were measured on a suitable γ-spectrometer and no activity was observed which supports the good spiking yields. The homogeneity of the batch was verified by suitable analysis on sub-samples representing only one tenth (14 g) of the size of the dispatched sample. According to the ANOVA test, to state the homogeneity of the batch, we should check that s2B and s2W are not significantly different from one another. The values of the ratios s2B =s2W for each radionuclide (Table 1) are lower than the critical
460
V. Lourenço et al. / Applied Radiation and Isotopes 87 (2014) 456–460
Table 3 Relative differences (%) calculated from the participants' results and associated means and standard deviations (k ¼ 1). Blind code
22
200 201 202 203 oei 4 s(ei) o|ei| 4
0.7 6.7 4.9 1.3 0.6 5.8 3.4
Na
40
K
5.0 1.7 2.7 – 2.0 3.1 3.1
60
109
5.2 5.1 5.4 4.0 0.4 5.8 4.9
5.5 3.1 6.5 10.4 0.4 8.5 6.4
Co
Cd
134
137
o ep 4
s(ep)
o|ep|4
s(|ep|)
2.6 4.1 5.8 0.7 0.9 5.0 3.3
8.1 2.2 7.1 2.1 2.7 5.4 4.9
4.5 3.8 5.4 3.7
2.6 1.9 1.5 4.0
4.5 3.8 5.4 3.7
2.6 1.9 1.5 4.0
Cs
Cs
The subscripts i and p refer to the ith radionuclide identified in the matrix and participant respectively. o ei 4, oep 4 and o |ep| 4 are mean values. s(ei), s(ep) and s(|ep|) are standard deviations. o|ei| 4 and o|ep|4 are means of absolute relative differences.
value of F for a one-tailed test. This means that s2B is not significantly greater than s2W : To check if s2W is not significantly lower than s2B either, we have to test if s2W =s2B is lower than F(10,4) ¼5.96. This is a simple way to perform a two-tailed ANOVA test. According to the results, we can conclude that, at a 5% significance level, for all the radionuclide considered the material can be considered to be sufficiently homogeneous within the range of mass used. The participants' results are equally distributed above and below the reference values, indeed the mean value of the 23 relative differences (Table 3) is 0.8%. The mean values of the relative differences per radionuclide are between 0.6% (for 22Na) and þ2.7% (for 137Cs). Nevertheless the standard deviation of the 109 Cd results from the participants is the highest one: 8.5%, as expected due to its low-emission energy and low emission probability. All En numbers calculated were found acceptable, even though in some cases the value of the expanded uncertainty provided had to be limited. Hence, some laboratories may be over-estimating their uncertainty budget. 6. Conclusion The spiking procedure aimed to produce a reference material with a quantifiable, but still low, overall mass activity. The specific activity of the spiked material is about 100 to 1000 Bq kg 1 dry mass depending on the radionuclide. The spiking procedure resulted in a spiking yield close to 100% regardless of the radionuclide considered. The homogeneity of the batch was verified by suitable analysis on sub-samples. The analysis of the results according to two evaluation parameters showed that all the results were acceptable, that is to say in agreement with the reference values, even though in some cases (En numbers) the value of the provided expanded uncertainty had to be limited. Even if the number of participants to this feasibility test, organized with this first sample of a low density solid matrix, was limited, the results of the participants are encouraging since they are as good as those for solution samples, commonly organized. The LNE-LNHB is monitoring the long-term stability of the material and the participants will be informed in the event of any observed change.
References Berger, M.J., Hubbell, J.H., Seltzer, S.M., Chang, J., Coursey, J.S., Sukumar, R., Zucker, D.S., 2005. XCOM: Photon Cross Section Database (version 1.3). National Institute of Standards and Technology, Gaithersburg, MD, USA http://physics.nist.gov/xcom. (Online available). de Felice, P., Iskandar, D., Fazio, A., Biagini, R, 1998. The quality control in the production of spiked reference materials for gamma-ray spectrometry. Appl. Radiat. Isot. 49 (9–11), 1421–1427. de Sanoit, J., 1994. Silica spiked with mixed γ-ray standards for use as environmental reference material. Radiochim. Acta 65, 249–257. Harms, A., Gilligan, C., 2012. Development of synthetic environmental radioactivity reference materials. Appl. Radiat. Isot. 70, 1940–1943. Interwinner Software, version 5.05.1044 from ORTEC (〈http://www.interwinner. com〉). ISO, 2005a. ISO/IEC 17025:2005 General Requirements for the Competence of Testing and Calibration Laboratories. International Organization for Standardization, Geneva, Switzerland. ISO, 2005b. ISO 13528:2005 Statistical Methods for use in Proficiency Testing by Interlaboratory Comparisons. International Organization for Standardization, Geneva, Switzerland. ISO, 2006. ISO Guide 35:2006 Reference materials—General and Statistical Principles for Certification. International Organization for Standardization, Geneva, Switzerland. Merešová, J., Wätjen, U., Altzitzoglou, T., 2012. Determination of natural and anthropogenic radionuclides in soil—results of an European Union comparison. Appl. Radiat. Isot. 70, 1836–1842. Nour, S., Inn, K., Filliben, J., 2008. Development of the NIST rocky flats soil standard. J. Radioanal. Nucl. Chem. 277, 161–168. Olson, D.G., Bernabee, R.P., 1988. Preparation and analysis of high-quality spiked soil standards. Health Phys. 54 (4), 451–459. Piton, F., Lépy, M.-C, Bé, M.-M., Plagnard, J., 2000. Efficiency transfer and coincidence summing corrections for gamma-ray spectrometry. Appl. Radiat. Isot. 52, 791–795. Shakhashiro, A., AzeredoGondinDaFoseca, A.M., Sansone, U., Kim, C.-K., Kis-Benedek, G., Trinkl, A., Benesch, T., Schorn, R., 2006. Report on the IAEA-CU-2006-02 Poficiency Test on the Determination of 137Cs and 210Pb in Spiked Soil. Report IAEA/AL/166, International Atomic Energy Agency, Vienna, Austria. Shakhashiro, A., Azeredo Gondin Da Foseca, A.M., Sansone, U., Fajgelj, A., 2007. Matrix materials for proficiency testing: optimization of a procedure for spiking soil with gamma-emitting radionuclides. Anal. Bioanal. Chem. 387, 2509–2515. Shakhashiro, A., Tarjan, S., Ceccatelli, A., Kis-Benedek, G., Betti, M., 2012. IAEA-447: a new certified reference material for environmental radioactivity measurements. Appl. Radiat. Isot. 70, 1632–1643. Sill, C.W., Hindman, F.D., 1974. Preparation and testing of standard soils containing known quantities of radionuclides. Anal. Chem. 46 (1), 113–118.