60 Acta Chim. Slov. 2005, 52, 60–66 Scientific Paper Sequential Determination of 241Am, 237Np, Pu Radioisotopes and 90Sr in Soil and Sediment Samples Polona Tavčar,* Rožle Jakopič, and Ljudmila Benedik Jožef Štefan Institute, Department of Environmental Sciences, Jamova 39, 1000 Ljubljana, Slovenia Received 05-10-2004 Abstract A sensitive and reliable method for the simultaneous determination of 241Am, 237Np, plutonium radioisotopes and 90Sr in soil and sediment samples is described. Analysis involves leaching of the samples in cone. HN03, follovved by radiochemical separation and purification. The radiochemical procedure begins with separation of 241Am/90Sr, plutonium radioisotopes and 237Np by anion exchange chromatography. 241Am and 90Sr from the combined efflu-ent are separated using TRU resin and Sr resin. Counting sources for alpha spectrometric measurements were prepared by the microcoprecipitation technique. Radiochemical yields were determined using 243Am, 239Np and 242Pu tracers. Strontium recovery was determined gravimetrically and Sr counting sources were analysed by liq-uid scintillation counting. The method was successfully tested by analysis of six reference materials and on two sediment samples with high activities of the selected radionuelides. The results were compared with reference and literature values. Key words: Actinides, strontium, extraction chromatography, microcoprecipitation technique, liquid scintillation counting. Introduction Radionuclide contamination resulting from global fallout and diseharges from nuclear facilities includes the important man-made radionuelides 237Np, 238Pu, 239/240Pu, 241Am and 90Sr, so in environmental control attention has to be paid to the abundance and the be-haviour of these radionuelides in various samples. The determination of eritical man-made radionuelides in environmental samples is obviouslv important in view of the inereasing energv produetion by nuclear reac-tors, the associated fuel cycle, reprocessing and waste disposal, the inereased potential for environmental contamination and public concern over the potential hazards. Their determination is an important topic in relation to health, nuclear waste management from nuclear reactors, reeveling and final storage of radioactive waste, control of illicit nuclear activities, etc. Methods for determination of radionuelides in a wide varietv of environmental samples include non-destructive gamma spectrometrv, alpha spectrometrv, beta counting and scintillation techniques, as well as mass spectrometrv and neutron aetivation analvsis for some nuelides.13 Evaluation of the contamination source (nuclear power plant accidents, reprocessing plant diseharges, nuclear weapon tests, etc.) requires accurate and precise iso-topic analvsis of actinides, especiallv uranium and plutonium. This can be achieved by alpha spectrometrv4-12 or by inductively coupled plasma mass spectrometry after chemical separation. Alpha spectrometry has been widely used in our laboratory for many yearsn~17 and since we have very positive experience with this technique, we decided to use it also in the present work. The radioactive characteristics of the radionuelides to be determined or were used as tracers are stated in Table 1. Table 1. Radioactive characteristics of radionuclides of interest.18 Isotope Alpha energya (keV) max. beta Gamma energya energy (keV) (keV) 239Pu 5155 (73.3%) 5143 (15.1%) 240pu 5168 (73.5 %) 5123 (26.4%) 238pu 5499 (71.6%) 5456 (28.3%) 242Pu 4900 (78.0%) 4856 (22.4%) 241Am 5486 (85.2%) 5443 (12.8%) 59.5 (36.3%) 243Am 5275 (87.9%) 5233 (10.6%) 74.7 (66%) 237Np 4788 (47.0%) 4771 (25.0%) 239Np - 277.6 (14.1%) 714 228.2 (10.7%) 90Sr - 546 ' two most abundant energies are stated for each isotope. Tavčar et al. Determination of Am, Np, Pu Radioisotopes and 90Sr Acta Chim. Slov. 2005, 52, 60–66 61 The aim of this study was to develop a rapid and sensitive analytical procedure for the sequential determination of critical man-made radioisotopes in environmental samples based on alpha spectrometry following radiochemical separation by ion exchange and extraction chromatography after leaching. Honvitz et al. described two extraction chromatographic materi-als commercially available as Sr and TRU resin from EIChroM Industries, USA. Sr resin is a strontium selec-tive resin composed of bis-t-butyl-cis-dicyclohexano-18-crown-6 in 1-octanol as stationary phase immobilized on an inert polymeric support. Strontium is adsorbed on the column from nitrate media. TRU resin is a transu-ranium specific resin composed of octylphenyl-Ar^V-di-isobutylcarbamoylmethylphosphine oxide (CMPO) in tri-n-butyl phosphate (TBP) immobilized on an inert porous polymeric support. Extraction chromatography can be used for a number of analytical purposes, in-cluding the separation of actinides as a group from the matrix, separation of Sr from the matrix and sequential separation of individual actinides and Sr.4111931 These procedures represent a significant improvement over classical radiochemical separation methods. In this publication we used as a basis the method developed by Moreno et al,31 which we further modified to fit our purpose. We used leaching with cone. nitric acid for sam-ple preparation instead of destruetion in a conventional microwave oven. We used 9M HC1 for Th removal from anion exchange column instead 10M HC1 and we found that we can use I" ions in the form of KI or NH4I and not just in a form of NH4I. Neptunium determination was also included in the separation scheme. The micro-coprecipitation sources were prepared differently from that described by Moreno et al.31 Our method for thin source preparation for alpha spectrometry measure-ments is described in detail below. The method was ap-plied to reference materials and to sediments from the Cumbrian Coastline and from the river Ribble, Great Britain, which are contaminated by nuclear facilities. Experimental Samples Reference materials IAEA-135 (Radionuclides in Sediment) and IAEA-368 (Pacific Ocean Sediment), SOIL 6 (Soil), IAEA-300 (Radionuclides in Baltic Sea Sediment), IAEA-375 (Soil), NIST-SRM 4350b (Co-lumbia River Sediment) and two sediments which were collected in the vicinity of Sellafield and Springfields. Ribble Sediment, a sediment from the tidal zone of the river Ribble, Lancashire, UK, is influenced both by tidal borne radionuclides originating from Sellafield and by manufacture of fuel elements at Springfields upstream. An intertidal Sediment from the Cumbrian Coastline was collected for an international characterisation in 1997 and the Jožef Štefan Institute participated in this comparative exercise.32 Tracers 243Am and 242Pu tracers: calibrated solutions, with activities of 0.746 Bq/g and 0.83 Bq/g, respectively were purchased from the National Physical Laboratory, Ted-dington Middlesex, United Kingdom. 239Np tracer: This was prepared by irradiation of |ag quantities of a solution of uranium in the Institute’s TRIGA Mark II research reactor. Sample preparation Up to 10 g of sample was leached with up to 200 mL of cone. HN03 on a hot plate at 50-80 °C overnight in the presence of 243Am, 242Pu and 239Np tracers and 30 mg of strontium carrier. Leaching was performed in a covered beaker on a hot plate with magnetic stir-ring. The leachant and the residue were separated by filtration. The solution was evaporated and the residue dissolved in 1M HN03. Analvtical procedure Oxidation state adjustment is very important for the separation of plutonium and neptunium radioisotopes. To assure as low losses of radionuclides as possible, plutonium must be present in solution for absorption on the anion exchange column as Pu(IV) and neptunium as Np(IV). During separation Pu is selectively reduced to Pu(III) with ammonium iodide, while Np remains as Np(IV). To the 1M HN03 solution about 1 g of hydroxylammine hydrochloride was added and the solution was warmed gently for about 15 min to ensure that plutonium was present as Pu(III) and neptunium as Np(III). After cooling the sample 2-3 g of sodium nitrite were added to ensure conversion of Pu(III) to Pu(IV) and Np(III) to Np(IV). The solution was then converted to 8M using cone. HN03 This 8M HN03 solution was passed through an anion exchange column (Dowex 1X8, mesh 100-200) and washed with 100 mL 8M HN03. The 8M HN03 effluent was used for analysis of americium and strontium radionuclides. The column was washed with 100 mL 9M HC1 to elute thorium. Pu was eluted with 100 mL of 0.1M L/9M HC1 solution. Iodide ions selec-tively reduce Pu(IV) to Pu(III). Pu(III) is not adsorbed on the column under these conditions and elutes. Iodide can be added in the form of KI or NH4I. Neptunium remain on the column in Np(IV) form and it was later stripped with 100 mL of 4M HC1 solution. Fe(III) that may be present in the samples in-terfere in separation of americium on the TRU resin Tavčar et al. Determination of Am, Np, Pu Radioisotopes and 90Sr 62 ________________ Acta Chim. Slov. 2005, 52, 60-66_________________ Soil or sediment sample (up to 10 g) + 243Am, 239Np and 242Pu tracers + 30 mg Sr carrier Leaching with cone. HN03, filtration, evaporation and dissolution inlM HNO, 1 g of hydroxylammine hydrochloride, gently warming, 2-3 g NaN02, converting of solution to 8M Loading on Dowex 1x8, 100-200 mesh in 8M HN03 ELUATE: Sr, Am COLUMN: Pu, Np Am and Sr co-precipitation on Ca oxalate, destruetion of the precipitate with cone. HN03, addition of deionised water to 2M Elution of Sr with 2M HN03, Elution of Am with 4M HC1, evaporation and dissolution in co-precipitation on NdF3 and 3M NCV alpha spectrometric i measurement Loading on a Sr resin column Rinsing of the column with 9M HC1 Elution of Pu with 0.1179M HC1, co-precipitation on NdF3 and alpha spectrometric measurement 1 Z Elutiorfof Np with 4M HC1, co-precipitation on NdF3 and alpha spectrometric measurement Elution of Sr with deionised water Sr oxalate precipitation, gravimetric yield determination LSC counting Figure 1. Analytical procedure for the simultaneous determination of plutonium, neptunium, americium, and strontium in soil and sediment samples. column. To the Am/Sr effluent about 10 g of oxalic acid were added and the pH was adjusted to 5.5-6.0 with am-monia solution. If no precipitate was formed, up to 500 mg of calcium nitrate was added. Both americium and strontium co-precipitated with calcium oxalate. Iron, both 2+ and 3+, forms soluble complexes and remains in the solution. The precipitate was centrifuged and washed with deionised water. The oxalate precipitate was destroyed with cone. HN03 and the residue was taken up in 2M HN03. Separation of americium was performed using a TRU resin column. 2M HN03 was tested for Fe(III) using NH4SCN solution. If the test was positive up to 300 mg of aseorbic acid was added to the solution to reduce Fe(III) to Fe(II). 3 g TRU resin was mixed with deionised water and used to fill a column 15 cm long and 1 cm in diameter. The sample was loaded onto the column and washed with 100 mL 2M HN03. Strontium was not retained on the column and elutes in the 2M HN03 fraetion. Americium was stripped with 100 mL 4M HC1 solution. The strontium fraetion was evaporated to dryness and the residue was dissolved in 3M N03". This can be done using 3M HN03 or 1M A1(N03)3. A1(N03)3 prevents destruetion of bis-t-butyl-cis-dicyclohexano-18-crown-6, which is the extractant of the Sr resin column. To use this column repeatedly, then A1(N03)3 should be used instead of 3M HN03. Column capacity decreases up to 5-times slower when using A1(N03)3 instead of nitric acid. However, both of these possible-loading solutions perform the separation in the same way with the same selectivity. The strontium fraetion was loaded onto a Sr resin column and washed with 100 mL 3M N03". If using A1(N03)3 then the washing of the column was followed by elution with 20 mL 3M HN03 which eliminates aluminium from the column (that can othenvise interfere with LSC counting). Strontium was eluted from the column with 100 mL deionised water. i. Tavčar et al. Determination of Am, Np, Pu Radioisotopes and 90Sr Acta Chim. Slov. 2005, 52, 60–66 63 : ' 1 1 1 : : 241 Am : : / : - 243 Am - ~ M / ~ : ~-^--~-—i^.,.J^r-J'-~"Y^'--r l, , : 4537.25 5609.75 5073.50 Energy (keV) Acquired: 21/06/0412:23:26 Real Time: 232441.02 s. Live Time: 232439.83 s. File: \\Radon\spektri\Polona\aktinidi\A2AmlAEA135, 3-zareza28.5.04.Chn Channels: 256 Detector: #11 RADON MCB 5 Input2 Figure 2. Alpha spectrum of americium radioisotopes isolated from IAEA-135 reference material. - iii _ — 239/240p,, ~ : 242pu / ; : \| ž ~- 238pu —_ -_ / -_ z 1 J \\ .A, , ~ 4365.50 7062.50 5714.00 Energy (keV) Acquired: 21/06/0413:00:23 Real Time: 60361.96 s. Live Time: 60361.64 s. File: \\Radon\spektri\Polona\aktinidi\A2 Pu Ribble sed., 3-zareza24.5.04.Chn Channels: 256 Detector: #11 RADON MCB 5 Input2 Figure 3. Alpha spectrum of plutonium radioisotopes isolated from Ribble sediment. About 300 mg of oxalic acid were added to the strontium effluent and strontium oxalate precipitated from pH 9 to 10, adjusting the pH with ammonia so-lution. The precipitate was centrifuged, plated on a measuring planchet, dried and weighed for recovery calculation. The precipitate was then dissolved in 2 mL 1M HN03 and mixed with 14 mL Ultima Gold XR scintillation cocktail. The microcoprecipitation method for thin source preparation for alpha spectrometry was performed for aH solutions of americium, plutonium, and neptunium radionuclides isolated from soil or sediment samples. Solutions of the isolated radionuclides were evaporated and dissolved in 1-2 mL 1M HC1. About 100 liL 0.5 mg mL4 of Nd carrier were added and shaken about one minute on a mini shaker. About 0.5 mL of cone. HF was added, the solutions shaken again and left on ice for half an hour. In the meantime a filter for microcoprecipitation (Supor-100 membrane filter, 25mm, O.lLtm) was prepared. Through this filter were first passed 5 mL 80% ethanol, then 5 mL solution containing 50 Lig of freshlv precipitated NdF3 and then the sample fraetion in the form of the NdF3 suspension. The filter was washed twice with 2 mL 0.58M HF, twice with deionized water and twice with 80% ethanol. The filter was then dried, glued on an Al planched and measured on an alpha spectrometer. Anah/tičal procedure is schematicallv presented in Figure 1. Tavčar et al. Determination of Am, Np, Pu Radioisotopes and 90Sr 64 Acta Chim. Slov. 2005, 52, 60–66 - J 1 237Np I - ~A MuU AAAA jA„A UuU,a ,Aa A .. h 1 .^ 4310.75 Acqiiired: 23/07/04 07:31:37 File: A:\A2, Np(1 7)Selafield.Chn Detector: #11 RADON MCB5 Input2 5640.50 Energy (keV) Real Time: 436219.8 . Live Time: 436218.13 s. Channels: 256 Figure 4. Alpha spectrum of 237Np isolated from Cumbrian sediment. Table 2. Activities of 239/240Pu, 238Pu, 241Am and 237Np in some reference materials and samples of sediments collected in the vicinity of Sellafield and Springfields. Table 3. Activities of 90Sr in sediment and soil samples obtained by liquid scintillation counting. Sample Nuclide IAEA 300 IAEA 375 IAEA 368 IAEA 135 SOIL-6 SRM-4350b Ribble sediment Cumbrian sediment 239/240p 238pu 241Am 237Np 239/240p 238pu 241Am 237Np 239/240p 238pu 241Am 237Np 239/240p 238pu 241Am 237Np 239/240p 238pu 241Am 237Np 239/240p 238pu 241Am 237Np 239/240p 238pu 241Am 237Np 239/240p 238pu 241Am 237Np Activity concentration (Bqkg-1) This work Ref. or lit. value 3.49 ± 0.23 < 0.2 1.21 ± 0.18 < 0.05 0.34 ± 0.06 0.08 ± 0.03 0.15 ± 0.03 < 0.2 31 ± 5 8.2 ± 1.0 1.35± 0.19 0.20 ± 0.08 217± 12 46 ± 4 313 ±14 0.80 ± 0.28 1.03 ± 0.16 < 0.2 0.39 ± 0.08 < 0.1 0.56 ± 0.07 < 0.02 < 0.2 < 0.2 187± 10 32 ± 3 288 ± 23 1.72± 0.14 960 ± 74 193 ± 19 1508 ± 99 6.45 ± 0.14 3.55 0.15 1.38 0.3 ± 0.04 0.071 ± 0.014 0.13 ± 0.02 31 ± 2 8.5 ± 0.9 1.3 0.25 12 213 43 318 0.87 12 1.04 ± 0.07 < 0.036 0.39-0.45 0.508 ± 0.030 0.013 ± 0.002 0.15 ± 0.03 161 ± 8" 33 ± 5" 320 ± 18" 1.66-1.83 12 995 ± 19 20 211 ± 4 20 1590 ± 13 20 6.33 ± 0.53 20 Sample 90Sr(Bq/kg) This work Ref. or lit. value IAEA 300 IAEA 375 IAEA 368 IAEA 135 SOIL-6 SRM-4350b Ribble sediment Cumbrian sediment 11.4±2.7 114.0 ±2.4 <6.5 58 ±41 28.1 ±1.4 <9.9 18± 10 194 ±97 10.8 108 ±6 1.8 64.5 30.3 ±3.6 5.3 22±3" 234±1220 Results and discussion The results obtained by this method for simultane-ous determination of americium, neptunium, plutonium and strontium radionuclides are shown in Tables 2-3. Spectra are presented in Figures 2-4. The results obtained with above-mentioned method are in good agreement with reference, informa-tion or literature values. Average chemical recoveries for plutonium, americium and neptunium were 61%, 66% and 40%, respectiveh/. Overall recoveries for neptunium were generalh/ lower than for the other radionuclides. This is due to loss of neptunium because of inappropriate oxidation state adjustment. Some of the neptunium remains in the Np(V or VI) state which are poorlv retained by the column and wash out with 8M HN03. In addition, some impurities of 242Pu at 4900 keV and 239/240Pu at approx. 5155 keV can be observed in the alpha spectrum, but they usually do not interfere with the 237Np peak. If however the tails of Pu radio- " Tavčar et al. Determination of Am, Np, Pu Radioisotopes and 90Sr Acta Chim. Slov. 2005, 52, 60–66 65 isotopes do interfere, further purification or repeated analysis is necessary. Average chemical recovery for 90Sr was 67%. Lower activities of 90Sr are difficult to measure because of the high background and the small amounts of sample taken for the analysis. This is especially seen in results for IAEA 135, Ribble and Cumbrian sediments where we took a very small amount of sample (0.5-lg) and due to high background the results have high measurement uncertainty. The average background of the beta coun-ter was 33 cpm. The limit of detection was calculated for each sample separately taking mass and recovery into consideration. Conclusions The radiochemical procedure described here was found to be reliable and accurate. In addition, the advantage of this method is the determination of the most critical alpha activation products and 90Sr simul-taneously thus saving time and the amount of sample needed for analysis. The main disadvantage is its higher detection limit for determination of 90Sr by liquid scin-tillation counting due to the high background of the counter. This problem can be avoid using more sensitive beta proportional counting but the time for the analysis is then extended due to time needed for 90Y ingrowth. A beta proportional counter has very low efficiency for low the beta energies of 90Sr (17%). References 1. L. Benedik, A. R. Byrne, /. Radioanal. Nucl. Chem. Articles 1995, 189, 325-331. 2. L. Benedik, A. R. Byrne, Acta Chim. Slov. 1994, 41, 1-13. 3. A. R. Byrne, A. Komosa, Sci. Total Environ. 1993, 130/131, 197-206. 4. J. Moreno, J. J. LaRosa, P. R. Danesi, K. Burns, P. DeRegge, N. Vajda, M. Sinojmeri, Radioactivity & Radiochemistry, 1998, 9, 35-44. 5. R. Pilviö, M. Bicke\, Applied Radiation and Isotopes 2000, 53, 273-277. 6. M. P. Rubio Montero, A. Martin Sánchez, M. T. Crespo Vázquez, J. L. Gascón Murillo, Applied Radiation and Isotopes 2000, 53, 259-264. 7. J. A. Sanchez-Cabeza, J. Molero, /. Environ. Radioactivity 2000, 51, 211-228. 8. P. E. Warwick, I. W. Croudace, J.-S. Oh, Anal. Chem. 2001, 73, 3410-3416. 9. M. P. Rubio Montero, A. Martin Sánchez, Applied Radiation and Isotopes 2001, 55, 97-102. 10. I. Vioque, G. Manjon, R. Garcfa-Tenorio, F. El-Daoushy, Analyst 2002, 127, 530-535. 11. H. Pintar, L. Benedik, Young Investigators Seminar on Analytical Chemistry - YISAC 2000, Proceedings, Graz, July 2-5, 2000, 53-58. 12. L. Benedik, U. Repinc, 9"* International Symposium on Environmental Radiochemical Analysis, 18-20 September, Maidstone, 2002. 13. L. Benedik, P. Tavčar, Acta Chim. Slov. 2001, 48, 199-213. 14. P. Tavčar, L. Benedik, Mine water environ. 2002, 21, 156-159. 15. P. Tavčar, L. Benedik, Czechoslovak Journal of Physics 2003, 53, 51-55. 16. P. Tavčar, T. Benedik, Appl. Radiat. Isotopes. 2004, 60, 717-723. 17. A. R. Byrne, T. Benedik, Anal. Chem. 1997, 69, 996-999. 18. E. Browne, R. B. Firestone, Table of Radioactive Isotopes, John Wiley & Sons, 1986. 19. E. P. Honvitz, R. Chiarizia, M. T. Dietz, H. Diamond, Anal. Chim. Acta 1993, 281, 361-372. 20. N. Vajda, A. Ghods-Esphahani, E. Cooper, P. R. Danesi, /. Radioanal. Nucl. Chem, Articles 1992, 162, No. 2, 307-323. 21. M. Rodriguez, J. A. Suarez, A. G. Espartero, Nucl. Instr. Meth. Phys. Res. A 1996, 169, 348-352. 22. A. Alvarez. N. Navarro,^p/. Radiat. Isot. 1996, 47, No. 9/10, 869-873. 23. E. P. Honvitz, M. Dietz, R. Chiarizia, H. Diamond, S. T. Maxwell, M. Nelson, Anal. Chim. Acta 1995, 310, 63-78. 24. E. P. Honvitz, R. Chiarizia, M. T. Dietz, Solvent Extr. Ion Exch. 1992, 10(2), 313-336. 25. E. P. Honvitz, M. T. Dietz, R. Chiarizia, /. Radioanal. Nucl. Chem. 1992, 161, 575-583. 26. E. P. Honvitz, M. T. Dietz, D. E. Fisher, Anal. Chem. 1991, 63, 522-525. 27. J. Mellado, M. Tlaurado, G. Rauret, Anal. Chim, Acta 2002, 458, 361-31 A. 28. J. E. Roane, T. A. DeVol, J. D. Teyba, R. A. Fjeld, /. Envirin. Rad. 2003, 66, 227-245. 29. G. Kim, W. C. Burnett, E. P. Honvitz, Anal. Chem. 2000, 72, 4882^1-887. 30. J. W. Grate, O. B. Egorov, Anal. Chem. 1998, 70, 3920-3929. 31. J. Moreno, N. Vajda, P. R. Danesi, J. J. Ta Rosa, E. Zeiller, M. Sinojmeri, /. Radioanal. Nucl. Chem. 1997, 226, 279-284. 32. I. Adsley, D. Andrevv, D. Arnold, R. Bojanovvski, Y. Bourlat, A. R. Byrne, M-T Crespo, J. Desmond, P. de Felice, A.Fazio, J. T. Gascon, R. S. Grieve, A. S. Holmes, S. M. Jerome, M. Korun, M. Magnoni, K. J. Odeli, D.S. Popplevvell, I. Poupaki, G. Sutton, J. Toole, M. W. Wakerley, H. Wershofen, M. J. Woods, M. J. Youngman, App. Radiat. Isot. 1998, 49, 1295-1300. Tavčar et al. Determination of Am, Np, Pu Radioisotopes and 90Sr 66 Acta Chim. Slov. 2005, 52, 60–66 Povzetek Razvili smo zanesljivo in občutljivo metodo za istočasno določanje 241Am, 237Np, Pu radioizotopov in 90Sr v vzorcih sedimentov in prsti. Ločitev radionuklidov sem izvedla z anionsko izmenjevalno kolono, kjer sta se selektivno ločila Pu in Np. Za ločitev Am in Sr sem uporabila ekstrakcijsko kromatografijo z uporabo TRU in Sr kolon. Tanke merilne vire za alfa spektrometrično detekcijo smo pripravili z mikroobarjanjem in mikrofiltracijo. Izkoristke radiokemičnega postopka smo določili z uporabo radioaktivnih sledilcev 243Am, 239Np in 242Pu. Za stroncij smo izkoristek določili gravimetrično s tehtanjem oborine stroncijevega oksalata, merilni vir pa smo izmerili s tekočinsko-scintilacijskim števcem. Metodo smo testirali z analizo referenčnih vzorcev sedimentov ter zemelj. Rezultate smo primerjali z referenčnimi ali literaturnimi vrednostmi in ugotovili dobro ujemanje. Tavčar et al. Determination of Am, Np, Pu Radioisotopes and 90Sr