- Email: [email protected]
Low-level radioactivity measurements in an ocean shellfish matrix
Applied Radiation and Isotopes 52 (2000) 539±544
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Low-level radioactivity measurements in an ocean shell®sh matrix T. Altzitzoglou* EC-JRC Institute for Reference Materials and Measurements, Retieseweg, B-2440 Geel, Belgium
Abstract Reference marine biological samples are necessary to test the performance of the analytical methods employed in surveying and monitoring radioactive materials in the sea. The measurement of arti®cial and natural radionuclide activity concentrations in ocean shell®sh material by nondestructive ultra low-level g-ray spectrometry in an underground laboratory is reported. The material analysed, a composite material made of Irish Sea and White Sea mussel and Japan Sea oyster, was prepared by the National Institute of Standards and Technology (NIST). # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Ultra low-level radioactivity measurement; Gamma-ray spectrometry; Marine biological samples
1. Introduction Reference materials can be used to evaluate and intercompare radiochemical methods, to develop new radiochemical procedures and to calibrate instruments. Reference marine biological samples are necessary to test the performance of the analytical methods employed in surveying and monitoring radioactive materials in the sea and assure comparability and reliability of data obtained from dierent laboratories. Few reference materials of that family, certi®ed for their radioactivity contents, exist. The National Institute of Standards and Technology (NIST), in cooperation with experienced international laboratories issues low-level radioactivity Standard Reference Materials (SRM) (Inn, 1987). It has prepared an ocean
* Tel.: +32-1457-1266; fax: +32-1458-4273. E-mail address: [email protected] Altzitzoglou).
(T.
shell®sh composite material containing 0.1% w/w Irish Sea mussel, 12% w/w White Sea mussel and 87.9% w/w Japan Sea oyster, to become the NIST SRM 4358 in the natural-matrix, environmental-level radioactive SRM series. The material has been freeze-dried, pulverised, blended, bottled and sterilised. Randomly selected bottles were sent to participating laboratories for assay. The laboratories were encouraged to employ their own routine procedures, so that a variety of analytical methodologies is used. After the successful completion of the certi®cation of the bone ash standard reference material (NIST SRM 4356) (PilvioÈ et al., 1999; Lin et al., 1998), the Institute for Reference Materials and Measurements (IRMM) participates in the analysis for the certi®cation of the ocean shell®sh future SRM, by measuring the radionuclide activity concentrations of 90Sr, 40K, 137 Cs, 210Pb, 226Ra, 228Ra, 228Th, 230Th, 232Th, 234U, 238 U, 238Pu, 239,240Pu and 241Am. In this article the assaying of the activity concentration of 40K, 137Cs, 210Pb, 226Ra, 228Ra, 228Th, 238U
0969-8043/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 4 3 ( 9 9 ) 0 0 2 0 7 - 9
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T. Altzitzoglou / Applied Radiation and Isotopes 52 (2000) 539±544
and 241Am by nondestructive ultra low-level g-ray spectrometry in an underground laboratory is described. A radiochemical separation will follow to isolate the actinides and 90Sr, which will be measured by a-particle spectrometry and liquid scintillation counting, respectively. 2. Equipment and method Environmental samples usually contain very low levels of radionuclides. Special techniques, mainly involving radiochemical separations, are employed in order to pre-concentrate the analytes and isolate the elements of interest for subsequent measurement by aparticle spectrometry. Ultra low level, high-resolution g-ray spectrometry, applied to the measurement of low event rates, can be perfectly adequate for the assessment of certain radionuclides in environmental samples. The obvious advantage is the simple sample preparation, involving mainly drying of the sample and placing it in an appropriate container, which guar-
antees the integrity of the original sample. The high resolution renders elemental separation unnecessary. To achieve low detection limits, necessary for the analysis of environmental samples, background reduction is often the only possible way. A systematic investigation of the background origin showed that, once the radon and its progenies are excluded from the detector surroundings, the major source of background at sea level is attributable to cosmic rays (Heusser, 1994; Wordel et al., 1996). To shield against cosmic rays, solid state photon detection systems based on HPGe detectors, have been placed in the underground research facility HADES (High Activity Disposal Experimental Site) at the Studiecentrum voor Kernenergie (SCK), Mol, Belgium (Wordel et al., 1994), at a depth of about 225 m (500 m water equivalent). Fig. 1 shows two background spectra obtained (a) with a 36% coaxial detector at sea level shielded by 100 mm of low-background Pb and (b) with the 60% coaxial detector in HADES. In the energy region from 50 to 1500 keV the integral background rate of the sys-
Fig. 1. Comparison of background spectra obtained (a) with a shielded HPGe detector at sea level and (b) with a low-background HPGe detector at the underground laboratory. Both spectra were acquired for 23 days and normalised to 1 kg Ge active mass. The 137Cs peak in spectrum (b) is due to a surface contamination of the Cu shield.
T. Altzitzoglou / Applied Radiation and Isotopes 52 (2000) 539±544
tem at sea level is 4750 counts per hour and kg of Ge, compared to 12 cph kgÿ1 of the underground detector. 3. Gamma-ray spectrometry measurements 3.1. Equipment and sample preparation Two independent low-background g-ray detection systems were used for the measurement of the ocean shell®sh samples in the underground laboratory. The ®rst was a HPGe coaxial diode with a sensitive volume of 233 cm3 and a 60% relative eciency. The second was a HPGe semi-planar detector, 20 mm thick with a 38 cm3 sensitive volume. The germanium crystals are mounted in U-type cryostats, a con®guration that allows to keep the dewar and preampli®er outside the lead shield. All construction materials of the detectors inside the shielding, including the detector end-caps and windows in particular, have been rigorously selected for lowest radioactivity contents. The coaxial detector is placed in a cylindrical shield made of low-background lead (<20 Bq kgÿ1 210Pb), with a wall thickness of 140 mm. The semi-planar detector is housed in a Pb shield with a 150 mm thick wall. The inner 100 mm of this shield is made of old low-background lead (<2 Bq kgÿ1 210Pb). In both cases, the cavity between the Pb shields and the detectors was almost ®lled with electrolytic copper, with the double function of shielding from radioactivity and excluding radon from occupying the space around the detectors. 3.2. Eciency calibration For the determination of the full-energy peak eciency calibration in the range from 40 to 1500 keV two multi-nuclide volume sources were used. The sources were prepared in our laboratory from standard radioactive solutions obtained from NPL, UK and CEA/DAMRI, France. The radioactive solutions were made to 32 ml in 0.1 M HCl and ®lled in Te¯on containers identical with those used for the samples themselves. The g-ray peaks of 241Am, 109Cd, 139Ce, 57Co, 137 Cs, 113Sn and 60Co were used for the eciency calibration. The Ba and La KX-rays were employed to extend the calibration down to about 30 keV. The eciency points were ®tted with a polynomial function. 3.3. Monte Carlo simulations In order to correct for geometry dierences between the eciency calibration sources and the actual samples, as well as for dierences in the matrix composition (i.e. dierences in self-absorption), the Monte Carlo computer code GEOLEP was used. This pro-
541
gram, an evolution of the older GEOMU (SoleÂ, 1990), calculates the detection eciency of a set-up, given the physical dimensions of the detector, the source and their relative position. In addition, the photon mass attenuation coecients and the apparent densities of the involved materials, i.e. sample components, sample container, detector end-cap and window and the detector itself, are required and they are computed using the program XCOM (Berger and Hubbell, 1987). Although the program calculates the detection eciency, its results are used as a correction to the experimental eciency, Eref, using the formula Ex Eref
Sx Sref
where Ex is the eciency for the sample and for a given energy and Sx and Sref are the GEOLEP outputs for the same energy for the sample and for the calibration standard. The code has been thoroughly tested by participating in a EUROMET exercise and the results obtained for source densities of 1 g cmÿ3 and below were within 5% from the experimental results and especially in the energy range from 100 to 1500 keV within 2%. In addition, the eciency values were compared with results obtained with the EGS4 code (Nelson et al., 1985) for a similar matrix (silica) and agreed within better than 10%. The overall uncertainty was estimated to be less than 6% and for the region below 80 keV about 8%.
3.4. Sample preparation and measurements Five bottles containing 150 g of the ocean shell®sh material each have been received and samples from each of them were prepared. Before opening, each bottle was vigorously shaked for at least 30 minutes using a three-dimensional-motion homogenisation mixer (Turbula type T2C, W. A. Bachofen Ð Maschinenfabriek, Basel, Switzerland). The material was dried in an oven for 48 hours at 658C and then placed for 48 hours in a desiccator under vacuum. During the drying process, the material lost from 3.6 to 4.1% (average 3.9%) of its weight. The samples were weighed and transferred to Te¯on containers of 100 cm3 internal volume. The container bottom is 0.5 mm thick, to reduce the attenuation of low-energy photons. The cover is gas-tight, by means of an O-ring, so that radon and its progenies can be kept in equilibrium, if necessary. The containers were tapped using a Dual Autotap Ð QuantaChrome model DA-1 (Ankersmit, Syosset, NY 11791, USA) to tightly pack the material. They could contain about 65 g of dry ocean shell®sh and the apparent density varied from 0.39 to 0.43 g cmÿ3. The samples were placed
542
T. Altzitzoglou / Applied Radiation and Isotopes 52 (2000) 539±544
directly on top of the detector end-cap and measured for 9 to 14 days each by both detectors.
4. Results and discussion The g-ray spectra were accumulated for one day each, checked individually for energy shift and excess noise and the problematic ones were discarded. The valid spectra were summed and the peak analysis was done manually by simple integration of the counts under the peak. A linear background was subtracted. Fig. 2 shows a spectrum obtained with the semi-planar low-background HPGe detector, while that from the coaxial HPGe detector was similar. The activity concentration A0 was obtained by dividing the observed count rate Ng by the full-energy peak eciency, Eg, the g-ray emission probability, pg, taken from the literature (ENSDF, 1991) and the mass, m, of the sample. A0 Ng =Eg pg m In case more than one g-ray peaks of a given radio-
nuclide were detected with sucient counts, all peaks were used to calculate the activity of that given nuclide. For example, for 228Ra three peaks of 228Ac were used at 338.3, 911.2 and 969 keV. Similarly, for the 228Th activity calculation the 238.6 keV g-rays of 212 Pb and the 583.2 keV g-rays of 208Tl were employed, assuming equilibrium. Additionally, in case the nuclide under investigation appears also in the background spectrum (e.g. 137Cs, 40K), or a g-ray peak of a dierent nuclide coincides in energy with that of the nuclide of interest, the corresponding background counts were subtracted. For peaks in the energy range from 40 to 100 keV, the spectra from the semi-planar detector, favoured by its thin dead layer, were used. For peaks in the energy range from 100 to 1500 keV, the data from both detectors were utilized. The results of the ®ve samples are given in Table 1; the value for each radionuclide is the weighted mean of all results obtained for that nuclide from dierent peaks and the two detectors. The results are based on the assumption that secular equilibrium had been reached (although this is not supported by the results for the 238U series). The last column is the weighted mean of the results of all ®ve samples. The
Fig. 2. g-ray spectrum of the ocean shell®sh future NIST SRM 4358 material, accumulated for approximately 14 days with the 8% semi-planar low-background HPGe detector at the underground laboratory. The g-ray peaks used for the activity calculations are indicated by the decaying nuclei.
T. Altzitzoglou / Applied Radiation and Isotopes 52 (2000) 539±544
543
Table 1 Measured activity concentrations for the ocean shell®sh future NIST SRM 4358. The uncertainties given correspond to one standard deviation. The values in the last column are the weighted mean of the results of all ®ve samples Nuclide
241
Am U 226 Ra 210 Pb 228 Ra 228 Th 137 Cs 40 K 238
Activity concentration (Bq kgÿ1) sample 1
sample 2
sample 3
sample 4
sample 5
weighted mean
0.1520.03 2.1520.15 0.5720.03 6.820.8 1.4820.08 1.3920.09 0.2620.03 17026
0.1520.03 2.2220.16 0.5620.03 6.520.7 1.3820.08 1.4020.10 0.3020.03 17327
0.1420.03 2.2420.19 0.5220.03 6.720.8 1.4520.08 1.4520.11 0.2920.03 17527
0.1420.03 2.4820.21 0.5620.03 6.220.7 1.2520.08 1.4020.10 0.2820.03 17327
0.1420.03 2.3520.19 0.5220.03 6.420.7 1.2920.09 1.4120.11 0.2720.03 17527
0.1520.02 2.2820.08 0.5520.03 6.520.4 1.3920.04 1.4120.05 0.2820.02 17323
uncertainties given are uncertainties of the mean, they correspond to one standard deviation and they include the uncertainties in the counting statistics, the background, the photon emission probabilities and the detection eciency. The results in Table 1 show that the activity level of the ocean shell®sh material was very low and for some radionuclides, e.g. 241Am, much lower than that of the bone ash SRM 4356 mentioned above. The analysis was based on peaks containing from 100 to 2000 counts, with the exception of the more abundant 40K, which explains the spurious results. However, all detected g-ray peaks attributed to a given nuclide and from both detectors indicate within the uncertainty the same activity level. In most cases, the standard deviation of the results of the ®ve samples analysed is comparable to the uncertainty of the individual results. For the more abundant radionuclides the distribution between the bottles received by IRMM was homogeneous with a standard deviation of less than 2%. The calculation for the 226Ra activity concentration is based on the 609.3 and 1764.5 keV peaks of 214Bi, assuming secular equilibrium. The calculation based on the 186.1 keV g-ray gave a value approximately 4 times higher, due to the contribution of the unresolved 185.7 keV 235U g-rays. The other prominent g-rays of 235 U were below detection limit. Once the amount of 235 U is known, its contribution can be subtracted and the 226Ra activity concentration can be calculated directly.
5. Conclusions The IRMM participates in the analysis for the certi®cation of the radionuclide activity concentrations in the ocean shell®sh future SRM. Some g-emitting radionuclides can be assessed by g-ray spectrometry without
the need for chemical treatment of the samples, with the advantage of very rapid and simple sample preparation. The use of ultra low-background detection systems in an underground laboratory oers the possibility of achieving extremely low detection limits. Consequently, the ocean shell®sh future NIST SRM 4358 samples were assessed by ultra low-level g-ray spectrometry at our underground laboratory for the activity concentrations of 40K, 137Cs, 210Pb, 226Ra, 228 Ra, 228Th, 238U and 241Am. Additionally, redundancy of data and methods guarantees the quality of the results. The use of two independent detection systems, two or more g-ray peaks (wherever possible) and other analysis methods, like a-particle spectrometry for the a-emitting nuclides (226Ra, 228Th, 238U, 241Am) that will follow, are all to assure the integrity of the ®nal results. In the particular case of the ocean shell®sh SRM and despite the use of ultra low-level detection systems, a larger sample and longer data acquisition times would improve the data statistics. The work will continue with the radiochemical separation of the actinides and Sr from the matrix. A radiochemical procedure based on extraction chromatography has been developed. The radionuclides will be measured, after the radiochemical treatment, by aparticle spectrometry and low-level liquid scintillation counting, respectively. The results of the destructive analysis and those of the nondestructive g-ray spectrometry will validate each other.
Acknowledgements The author would like to express his gratitude to Dr. M. KoÈhler, Dr. M. Hult and Mrs. M.-J. Martinez Canet of IRMM for their help with the measurements in the underground lab and Dr. V.A. Sole of ESRF, Grenoble, France, for providing the GEOLEP compu-
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T. Altzitzoglou / Applied Radiation and Isotopes 52 (2000) 539±544
ter code. He thanks also Dr. K. G. W. Inn and Dr. Zhichao Lin of NIST for supplying the ocean shell®sh material. References Berger, M.J., Hubbell, J.H., 1987. XCOM: Photon cross sections on a personal computer. National Bureau of Standards, Report NBSIR 87-3597, Gaithersburg, USA. ENSDF: Evaluated Nuclear Structure Data File, 1991. Nuclear Data Center, Brookhaven national Laboratory, Upton, New York. Heusser, G., 1994. Background in ionizing radiation detection Ð illustrated by Ge spectrometry. In: GarcõÂ a-LeoÂn, M., GarcõÂ a-Tenorio, R. (Eds.), Proceedings of the 3rd International Summer School on Low-level Measurements of Radioactivity in the Environment, Huelva, Spain, 1993. World Scienti®c, p. 69. Inn, K.G.W., 1987. The National Bureau of Standards fresh water lake sediment environmental-level radioactivity standard reference material. J. Radioanal. Nucl. Chem. 115, 91. Lin, Z., Inn, K.G.W., Altzitzoglou, T., Arnold, D., Cavadore,
D., Ham, G.J., Korun, M., Wershofen, H., Takata, Y., Young, A., 1998. Development of the NIST bone ash standard reference material for environmental radioactivity measurement. Appl. Radiat. Isot. 49, 1301. Nelson, W.R., Hirayama, H., Rogers, D.W.O., 1985. The EGS4 Code System. SLAC Report 265. PilvioÈ, R., LaRosa, J.J., Mouchel, D., Wordel, R., Bickel, M., Altzitzoglou, T., 1999. Measurement of low-level radioactivity in bone ash. J. Environ. Radioact. 43, 343. SoleÂ, V.A., 1990. A Monte Carlo program to calculate solid angle and transmission corrections in counting systems. IRMM Internal Report GE/R/RN/12/90. Wordel, R., Mouchel, D., Bonne, A., Meynendonckx, P., Vanmarcke, H., 1994. Low level gamma-ray measurements in a 225 m deep underground laboratory. In: GarcõÂ a-LeoÂn, M., GarcõÂ a-Tenorio, R. (Eds.), Proceedings of the 3rd International Summer School on Low-level Measurements of Radioactivity in the Environment, Huelva, Spain, 1993. World Scienti®c, p. 141. Wordel, R., Mouchel, D., Altzitzoglou, T., Heusser, G., Quintana Arnes, B., Meynendonckx, P., 1996. Study of neutron and muon background in low-level germanium gamma-ray spectrometry. Nucl. Inst. Methods A 369, 557.
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Low-level radioactivity measurements in an ocean shell®sh matrix T. Altzitzoglou* EC-JRC Institute for Reference Materials and Measurements, Retieseweg, B-2440 Geel, Belgium
Abstract Reference marine biological samples are necessary to test the performance of the analytical methods employed in surveying and monitoring radioactive materials in the sea. The measurement of arti®cial and natural radionuclide activity concentrations in ocean shell®sh material by nondestructive ultra low-level g-ray spectrometry in an underground laboratory is reported. The material analysed, a composite material made of Irish Sea and White Sea mussel and Japan Sea oyster, was prepared by the National Institute of Standards and Technology (NIST). # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Ultra low-level radioactivity measurement; Gamma-ray spectrometry; Marine biological samples
1. Introduction Reference materials can be used to evaluate and intercompare radiochemical methods, to develop new radiochemical procedures and to calibrate instruments. Reference marine biological samples are necessary to test the performance of the analytical methods employed in surveying and monitoring radioactive materials in the sea and assure comparability and reliability of data obtained from dierent laboratories. Few reference materials of that family, certi®ed for their radioactivity contents, exist. The National Institute of Standards and Technology (NIST), in cooperation with experienced international laboratories issues low-level radioactivity Standard Reference Materials (SRM) (Inn, 1987). It has prepared an ocean
* Tel.: +32-1457-1266; fax: +32-1458-4273. E-mail address: [email protected] Altzitzoglou).
(T.
shell®sh composite material containing 0.1% w/w Irish Sea mussel, 12% w/w White Sea mussel and 87.9% w/w Japan Sea oyster, to become the NIST SRM 4358 in the natural-matrix, environmental-level radioactive SRM series. The material has been freeze-dried, pulverised, blended, bottled and sterilised. Randomly selected bottles were sent to participating laboratories for assay. The laboratories were encouraged to employ their own routine procedures, so that a variety of analytical methodologies is used. After the successful completion of the certi®cation of the bone ash standard reference material (NIST SRM 4356) (PilvioÈ et al., 1999; Lin et al., 1998), the Institute for Reference Materials and Measurements (IRMM) participates in the analysis for the certi®cation of the ocean shell®sh future SRM, by measuring the radionuclide activity concentrations of 90Sr, 40K, 137 Cs, 210Pb, 226Ra, 228Ra, 228Th, 230Th, 232Th, 234U, 238 U, 238Pu, 239,240Pu and 241Am. In this article the assaying of the activity concentration of 40K, 137Cs, 210Pb, 226Ra, 228Ra, 228Th, 238U
0969-8043/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 4 3 ( 9 9 ) 0 0 2 0 7 - 9
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T. Altzitzoglou / Applied Radiation and Isotopes 52 (2000) 539±544
and 241Am by nondestructive ultra low-level g-ray spectrometry in an underground laboratory is described. A radiochemical separation will follow to isolate the actinides and 90Sr, which will be measured by a-particle spectrometry and liquid scintillation counting, respectively. 2. Equipment and method Environmental samples usually contain very low levels of radionuclides. Special techniques, mainly involving radiochemical separations, are employed in order to pre-concentrate the analytes and isolate the elements of interest for subsequent measurement by aparticle spectrometry. Ultra low level, high-resolution g-ray spectrometry, applied to the measurement of low event rates, can be perfectly adequate for the assessment of certain radionuclides in environmental samples. The obvious advantage is the simple sample preparation, involving mainly drying of the sample and placing it in an appropriate container, which guar-
antees the integrity of the original sample. The high resolution renders elemental separation unnecessary. To achieve low detection limits, necessary for the analysis of environmental samples, background reduction is often the only possible way. A systematic investigation of the background origin showed that, once the radon and its progenies are excluded from the detector surroundings, the major source of background at sea level is attributable to cosmic rays (Heusser, 1994; Wordel et al., 1996). To shield against cosmic rays, solid state photon detection systems based on HPGe detectors, have been placed in the underground research facility HADES (High Activity Disposal Experimental Site) at the Studiecentrum voor Kernenergie (SCK), Mol, Belgium (Wordel et al., 1994), at a depth of about 225 m (500 m water equivalent). Fig. 1 shows two background spectra obtained (a) with a 36% coaxial detector at sea level shielded by 100 mm of low-background Pb and (b) with the 60% coaxial detector in HADES. In the energy region from 50 to 1500 keV the integral background rate of the sys-
Fig. 1. Comparison of background spectra obtained (a) with a shielded HPGe detector at sea level and (b) with a low-background HPGe detector at the underground laboratory. Both spectra were acquired for 23 days and normalised to 1 kg Ge active mass. The 137Cs peak in spectrum (b) is due to a surface contamination of the Cu shield.
T. Altzitzoglou / Applied Radiation and Isotopes 52 (2000) 539±544
tem at sea level is 4750 counts per hour and kg of Ge, compared to 12 cph kgÿ1 of the underground detector. 3. Gamma-ray spectrometry measurements 3.1. Equipment and sample preparation Two independent low-background g-ray detection systems were used for the measurement of the ocean shell®sh samples in the underground laboratory. The ®rst was a HPGe coaxial diode with a sensitive volume of 233 cm3 and a 60% relative eciency. The second was a HPGe semi-planar detector, 20 mm thick with a 38 cm3 sensitive volume. The germanium crystals are mounted in U-type cryostats, a con®guration that allows to keep the dewar and preampli®er outside the lead shield. All construction materials of the detectors inside the shielding, including the detector end-caps and windows in particular, have been rigorously selected for lowest radioactivity contents. The coaxial detector is placed in a cylindrical shield made of low-background lead (<20 Bq kgÿ1 210Pb), with a wall thickness of 140 mm. The semi-planar detector is housed in a Pb shield with a 150 mm thick wall. The inner 100 mm of this shield is made of old low-background lead (<2 Bq kgÿ1 210Pb). In both cases, the cavity between the Pb shields and the detectors was almost ®lled with electrolytic copper, with the double function of shielding from radioactivity and excluding radon from occupying the space around the detectors. 3.2. Eciency calibration For the determination of the full-energy peak eciency calibration in the range from 40 to 1500 keV two multi-nuclide volume sources were used. The sources were prepared in our laboratory from standard radioactive solutions obtained from NPL, UK and CEA/DAMRI, France. The radioactive solutions were made to 32 ml in 0.1 M HCl and ®lled in Te¯on containers identical with those used for the samples themselves. The g-ray peaks of 241Am, 109Cd, 139Ce, 57Co, 137 Cs, 113Sn and 60Co were used for the eciency calibration. The Ba and La KX-rays were employed to extend the calibration down to about 30 keV. The eciency points were ®tted with a polynomial function. 3.3. Monte Carlo simulations In order to correct for geometry dierences between the eciency calibration sources and the actual samples, as well as for dierences in the matrix composition (i.e. dierences in self-absorption), the Monte Carlo computer code GEOLEP was used. This pro-
541
gram, an evolution of the older GEOMU (SoleÂ, 1990), calculates the detection eciency of a set-up, given the physical dimensions of the detector, the source and their relative position. In addition, the photon mass attenuation coecients and the apparent densities of the involved materials, i.e. sample components, sample container, detector end-cap and window and the detector itself, are required and they are computed using the program XCOM (Berger and Hubbell, 1987). Although the program calculates the detection eciency, its results are used as a correction to the experimental eciency, Eref, using the formula Ex Eref
Sx Sref
where Ex is the eciency for the sample and for a given energy and Sx and Sref are the GEOLEP outputs for the same energy for the sample and for the calibration standard. The code has been thoroughly tested by participating in a EUROMET exercise and the results obtained for source densities of 1 g cmÿ3 and below were within 5% from the experimental results and especially in the energy range from 100 to 1500 keV within 2%. In addition, the eciency values were compared with results obtained with the EGS4 code (Nelson et al., 1985) for a similar matrix (silica) and agreed within better than 10%. The overall uncertainty was estimated to be less than 6% and for the region below 80 keV about 8%.
3.4. Sample preparation and measurements Five bottles containing 150 g of the ocean shell®sh material each have been received and samples from each of them were prepared. Before opening, each bottle was vigorously shaked for at least 30 minutes using a three-dimensional-motion homogenisation mixer (Turbula type T2C, W. A. Bachofen Ð Maschinenfabriek, Basel, Switzerland). The material was dried in an oven for 48 hours at 658C and then placed for 48 hours in a desiccator under vacuum. During the drying process, the material lost from 3.6 to 4.1% (average 3.9%) of its weight. The samples were weighed and transferred to Te¯on containers of 100 cm3 internal volume. The container bottom is 0.5 mm thick, to reduce the attenuation of low-energy photons. The cover is gas-tight, by means of an O-ring, so that radon and its progenies can be kept in equilibrium, if necessary. The containers were tapped using a Dual Autotap Ð QuantaChrome model DA-1 (Ankersmit, Syosset, NY 11791, USA) to tightly pack the material. They could contain about 65 g of dry ocean shell®sh and the apparent density varied from 0.39 to 0.43 g cmÿ3. The samples were placed
542
T. Altzitzoglou / Applied Radiation and Isotopes 52 (2000) 539±544
directly on top of the detector end-cap and measured for 9 to 14 days each by both detectors.
4. Results and discussion The g-ray spectra were accumulated for one day each, checked individually for energy shift and excess noise and the problematic ones were discarded. The valid spectra were summed and the peak analysis was done manually by simple integration of the counts under the peak. A linear background was subtracted. Fig. 2 shows a spectrum obtained with the semi-planar low-background HPGe detector, while that from the coaxial HPGe detector was similar. The activity concentration A0 was obtained by dividing the observed count rate Ng by the full-energy peak eciency, Eg, the g-ray emission probability, pg, taken from the literature (ENSDF, 1991) and the mass, m, of the sample. A0 Ng =Eg pg m In case more than one g-ray peaks of a given radio-
nuclide were detected with sucient counts, all peaks were used to calculate the activity of that given nuclide. For example, for 228Ra three peaks of 228Ac were used at 338.3, 911.2 and 969 keV. Similarly, for the 228Th activity calculation the 238.6 keV g-rays of 212 Pb and the 583.2 keV g-rays of 208Tl were employed, assuming equilibrium. Additionally, in case the nuclide under investigation appears also in the background spectrum (e.g. 137Cs, 40K), or a g-ray peak of a dierent nuclide coincides in energy with that of the nuclide of interest, the corresponding background counts were subtracted. For peaks in the energy range from 40 to 100 keV, the spectra from the semi-planar detector, favoured by its thin dead layer, were used. For peaks in the energy range from 100 to 1500 keV, the data from both detectors were utilized. The results of the ®ve samples are given in Table 1; the value for each radionuclide is the weighted mean of all results obtained for that nuclide from dierent peaks and the two detectors. The results are based on the assumption that secular equilibrium had been reached (although this is not supported by the results for the 238U series). The last column is the weighted mean of the results of all ®ve samples. The
Fig. 2. g-ray spectrum of the ocean shell®sh future NIST SRM 4358 material, accumulated for approximately 14 days with the 8% semi-planar low-background HPGe detector at the underground laboratory. The g-ray peaks used for the activity calculations are indicated by the decaying nuclei.
T. Altzitzoglou / Applied Radiation and Isotopes 52 (2000) 539±544
543
Table 1 Measured activity concentrations for the ocean shell®sh future NIST SRM 4358. The uncertainties given correspond to one standard deviation. The values in the last column are the weighted mean of the results of all ®ve samples Nuclide
241
Am U 226 Ra 210 Pb 228 Ra 228 Th 137 Cs 40 K 238
Activity concentration (Bq kgÿ1) sample 1
sample 2
sample 3
sample 4
sample 5
weighted mean
0.1520.03 2.1520.15 0.5720.03 6.820.8 1.4820.08 1.3920.09 0.2620.03 17026
0.1520.03 2.2220.16 0.5620.03 6.520.7 1.3820.08 1.4020.10 0.3020.03 17327
0.1420.03 2.2420.19 0.5220.03 6.720.8 1.4520.08 1.4520.11 0.2920.03 17527
0.1420.03 2.4820.21 0.5620.03 6.220.7 1.2520.08 1.4020.10 0.2820.03 17327
0.1420.03 2.3520.19 0.5220.03 6.420.7 1.2920.09 1.4120.11 0.2720.03 17527
0.1520.02 2.2820.08 0.5520.03 6.520.4 1.3920.04 1.4120.05 0.2820.02 17323
uncertainties given are uncertainties of the mean, they correspond to one standard deviation and they include the uncertainties in the counting statistics, the background, the photon emission probabilities and the detection eciency. The results in Table 1 show that the activity level of the ocean shell®sh material was very low and for some radionuclides, e.g. 241Am, much lower than that of the bone ash SRM 4356 mentioned above. The analysis was based on peaks containing from 100 to 2000 counts, with the exception of the more abundant 40K, which explains the spurious results. However, all detected g-ray peaks attributed to a given nuclide and from both detectors indicate within the uncertainty the same activity level. In most cases, the standard deviation of the results of the ®ve samples analysed is comparable to the uncertainty of the individual results. For the more abundant radionuclides the distribution between the bottles received by IRMM was homogeneous with a standard deviation of less than 2%. The calculation for the 226Ra activity concentration is based on the 609.3 and 1764.5 keV peaks of 214Bi, assuming secular equilibrium. The calculation based on the 186.1 keV g-ray gave a value approximately 4 times higher, due to the contribution of the unresolved 185.7 keV 235U g-rays. The other prominent g-rays of 235 U were below detection limit. Once the amount of 235 U is known, its contribution can be subtracted and the 226Ra activity concentration can be calculated directly.
5. Conclusions The IRMM participates in the analysis for the certi®cation of the radionuclide activity concentrations in the ocean shell®sh future SRM. Some g-emitting radionuclides can be assessed by g-ray spectrometry without
the need for chemical treatment of the samples, with the advantage of very rapid and simple sample preparation. The use of ultra low-background detection systems in an underground laboratory oers the possibility of achieving extremely low detection limits. Consequently, the ocean shell®sh future NIST SRM 4358 samples were assessed by ultra low-level g-ray spectrometry at our underground laboratory for the activity concentrations of 40K, 137Cs, 210Pb, 226Ra, 228 Ra, 228Th, 238U and 241Am. Additionally, redundancy of data and methods guarantees the quality of the results. The use of two independent detection systems, two or more g-ray peaks (wherever possible) and other analysis methods, like a-particle spectrometry for the a-emitting nuclides (226Ra, 228Th, 238U, 241Am) that will follow, are all to assure the integrity of the ®nal results. In the particular case of the ocean shell®sh SRM and despite the use of ultra low-level detection systems, a larger sample and longer data acquisition times would improve the data statistics. The work will continue with the radiochemical separation of the actinides and Sr from the matrix. A radiochemical procedure based on extraction chromatography has been developed. The radionuclides will be measured, after the radiochemical treatment, by aparticle spectrometry and low-level liquid scintillation counting, respectively. The results of the destructive analysis and those of the nondestructive g-ray spectrometry will validate each other.
Acknowledgements The author would like to express his gratitude to Dr. M. KoÈhler, Dr. M. Hult and Mrs. M.-J. Martinez Canet of IRMM for their help with the measurements in the underground lab and Dr. V.A. Sole of ESRF, Grenoble, France, for providing the GEOLEP compu-
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