Acta Chim. Slov. 2001, 48, 199-213. 199 DETERMINATION OF 210Pb AND 2,0Po IN ENVIRONMENTAL SAMPLES Ljudmila Benedik, Polona Vreček Jožef Stefan Institute, Department of Environmental Sciences, Jamova 39, 1000 Ljubljana, Slovenia; e-mail: liudmila.benedik(a),iis.si Received 30-09-2000 Abstract The naturally-occurring radionuclides Pb and Po (22.3 y and 138.4 d half-lives, respectively), are important because of their contributions to the natural radiation dose and release into the environment from technologically-enhanced sources. Methods for determination of 210Pb and of 2l0Po are described. Both radionuclides were determined in waters and in air filter samples. The procedure for determination of 210Pb includes dithizone extraction for the separation of lead from the matrix and precipitation of PbCr04 with K2Cr207 from weak acetic acid media. After waiting for radiochemical equilibrium to be re-established between Pb and its daughter ' Bi (at least 20 days), the activity of Bi was measured on a gas flow proportional beta counter. The method for determination of 2l0Po is based on spontaneous deposition of polonium radioisotopes on a copper planchet and ¦ino ~) 1 ~) measurement by alpha spectrometry. Both methods were optimised using Po and ' Pb tracers. A procedure for preparation of 2l2Pb tracer that involves purging of thorium nitrate solution with nitrogen was developed. Introduction 910 910 910 zn'Pb and ",uPo are members of the uranium-radium decay chain. Pb has a half- 9 I 0 life of 22.3 years and is a beta emitter. Its daughter Bi is also a beta emitter. Its grand- 9 I 0 daughter "lwPo is an alpha emitter, with a half-life of 138.4 days which decays further to stable 206Pb. 210Pb (yß") -> 2l0Bi (ß") -> 2l0Po (a) -> 206Pb (stable). 910 910 In areas where uranium is mined and extracted, Pb and Po may accumulate in tailings and run-off, resulting in higher concentrations in the biosphere. Besides this, some anthropogenic sources exist; associated with phosphate fertilisers, fossil fuel burning power plants, lead production, cement manufacture and fallout from nuclear 9 I 0 weapon tests. High levels of Po are also known to occur naturally in deep-sea fish, shell fish and reindeer or caribou meat. 910 910 910 The natural nuclides Pb, Bi and Po, which originate in the emanation of the 999 999 rare-gas nuclide Rn from land surfaces, are of great importance. Rn is a noble gas, diffusing through fissures to aquifer layers, where its long-lived products can be dissolved. These radionuclides are important components of the natural radiation L. Benedik, P. Vreček: Determination of Pb and Po in environmental samples 200 Acta Chim. Slov. 2001, 48, 199-213. exposure. According to dose factors given by ICRP Publication 67 , very low limits for derived activity concentrations in drinking water would be obtained, if all these 9 I 0 radionuclides were taken into account. The absorbed doses from the Pb sub-series 9 I 0 depend mainly on the highly energetic alpha particles of Po, as the contribution from the beta emissions of Pb and Bi amounts to about 10% of the total. The annual 910 910 910 effective dose equivalent arising from the total intake of Pb, Bi and Po would be about 130 |u,Sv. This fact demands reliable analytical techniques for analyses of biospheric samples in potentially exposed areas. 9 1 0 Procedures which involve the spontaneous deposition of Po from acidic media onto a less noble metal such as Cu, Ag or Ni are commonly used for the isolation of both trace and macrolevels. Radiochemical determination of Pb is usually accomplished by 9 10 measuring the beta activity of its Bi daughter (Epmax= 1.17 MeV) or the alpha activity 910 910 of its Po granddaughter (Ea = 5.32 MeV).This is because direct determination of Pb is difficult due to its low gamma energy (EY = 0.047 MeV of low abundance (4.05 %)) and the low energy of its beta emission (Epmax= 0.06 MeV). Since it is the daughter or granddaughter radionuclide that is isolated and quantitatively measured, it is necessary 9 1 0 to know the degree of equilibrium that exists between the progeny and its parent Pb. 9 1 0 For Bi (half-life 5.01 days) an equilibrium of greater then 98% is established in 30 9 I 0 days. For Po (half-life 138.4 days) a period of over 4.5 months is required to reach 50% and 18 months to reach 88% ingrowth. Despite the fact that alpha spectroscopic 9 I O 90S measurement of the in-grown Po, often using Po as a tracer, can achieve an ultra-low detection limit, the long ingrowth period required to achieve this makes it an impractical method when rapid low level measurements are required. In these 9 I 0 circumstances a procedure for the isolation and measurement of the Bi daughter is 910 919 preferred. In separation of Pb, the naturally occurring radiotracer Pb (half-life 10 h) 919 is often used to determine the chemical yield. Preparation of pure tracer Pb is very important, because impure tracer could contaminate the sample with beta emitters that 9 10 interfere in the measurement of Bi on a beta counter. A method for preparation of 9 19 Pb tracer that includes extraction of thorium nitrate solution with dithizone in chloroform is known. ' ' Small amounts of other radionuclides from the thorium decay L. Benedik, P. Vreček: Determination of Pb and Po in environmental samples Acta Chim. Slov. 2001, 48, 199-213. 201 9 1 9 chain are also extracted making the Pb tracer impure. This paper describes a 9 I 9 alternative procedure for Pb preparation. 910 910 Determination of Pb and Po activities in water samples in the vicinity of the former uranium mine at Zirovski vrh in Slovenia is important in monitoring environmental contamination and ensuring radioecological protection. Determination of both radionuclides in the local streams Todraščica and Brebovščica is a part of the 910 910 environmental monitoring programme. Measurement of the activities of Pb and Po in air filters from the Šoštanj thermal power plant is also presented. Experimental Samples 910 910 Water samples for Pb and Po analysis were collected from the local streams Todraščica and Brebovščica. The samples were collected every day and immediately after sampling subjected to filtration (0.45 |um) and acidified by addition of 3 mL nitric acid per litre. We carried out analysis of Pb in one-month composite samples and 9 I 0 Po in three-month composite samples. Air filters were collected from the chimneys 4 and 5 of the Šoštanj thermal power plant. Determination of 210Pb 1. Radiochemical separation 9 I 0 To determine very low concentrations of Pb in water samples the radionuclide 9+ should be first concentrated by evaporation. 6 L of water sample was taken, 0.2 mg Pb (lead carrier) was added, the sample evaporated to dryness and the residue dissolved in 50-100 mL 6 M hydrochloric acid. Two analytical steps in the procedure for the 9 1 0 determination of Pb were then used: a) dithizone extraction for the separation of lead, b) precipitation of PbCrC>4 with K^C^Ch from weak acetic acid media. 5 mL of 1 M solution of sodium citrate that prevents the precipitation of hydroxides and 1 mL of a 10% solution of potassium cyanide that masks II and III L. Benedik, P. Vreček: Determination of Pb and Po in environmental samples 202 Acta Chim. Slov. 2001, 48, 199-213. valent ions were added. pH was adjusted to 8.5-10 with ammonia solution. After that extraction of lead with 10-15 mL of a 0.005% solution of dithizone in chloroform was performed. The organic phase was then evaporated and the residue diluted in 60 mL of 9+ distilled water. Then 20 mg of Pb and 1 mL of concentrated acetic acid were added and the pH adjusted to 4-5 with ammonia solution. The solution was heated, and PbCrC>4 precipitated by addition of 1 mL of IM K^C^Ch. The precipitate was then plated on a measuring planchet by centrifugation. After waiting for radiochemical equilibrium to be re-established between Pb and Bi (at least 20 days), the activity of Bi beta emission was measured on a gas flow proportional beta counter (Berthold MULTI-LOGGER LB 5310). The activity of 210Pb was calculated from Equation 1. APb =--------—^------------- Eq. 1 Apb activity of Pb in sample in Bq m" ABi activity of 2'°Bi in Bq Abi 1.605xl0"6s"' ? 1 0 t time of the Bi regrowth (s) r) overall recovery Sdet. counting efficiency Vs sample volume in m 2. Preparation of Pb tracer To determine the overall recovery (r\) and also to perform experiments to optimise adiochemical procedure the ra< preparation is presented in Figure 1. 9 I 9 the radiochemical procedure the radioactive tracer Pb was used. Equipment for its L. Benedik, P. Vreček: Determination of Pb and Po in environmental samples Acta Chim. Slov. 2001, 48, 199-213. 203 nitrogen n iZZt thorium nitrate so Mio n gl nitrogen Rn-220 î liquid nitrogen 212T Figure 1 : Equipment for Pb tracer preparation. The right hand part of the equipment contained a thorium nitrate solution (100 g Th(N03)4 in 100 mL). This solution was purged by nitrogen for 10 hours at a flow rate I 990 of about 20 mL min" . Rn that was removed from the solution is a short-lived (half- 2I6T life of 55.6 s) member of the thorium decay chain and decays via its daughter Po further to Pb. The left hand part of equipment was cooled with liquid nitrogen to trap 990 919 volatile ^'Rn. The decay product "'"Pb was collected on the glass walls from which it 917 was washed with a 0.5 M solution of HCl. Pb prepared in this way was observed to be very pure. Its purity was checked using gamma spectrometry and beta counting. A filter to trap any possible carry-over of spray containing Th(N03)4 was found to be 919 unnecessary. After preparation of Pb tracer its initial gamma activity (EYi = 238.6 keV (43.6 %), EY2 = 300.0 keV (3.3 %)) was measured on a gamma spectrometer. 3. Optimisation of experimental parameters As mentioned above some parameters required in the procedure for determination 910 919 oP,uPb were determined in a series of experiments and optimised using Pb tracer. We determined the optimal time of extraction, the overall recovery and the experimental stoichiometric factor. L. Benedik, P. Vreček: Determination of Pb and Po in environmental samples 204 Acta Chim. Slov. 2001, 48, 199-213. The gravimetric yield of precipitation, the experimental stoichiometric factor and the overall recovery were calculated from Equations 2. The gravimetric yield of precipitation is determined by weighing the lead Chromate precipitate. We presumed that lead precipitates only in lead Chromate form (but see below). 9 I 9 The chemical yield of precipitation is determined using Pb tracer. A known amount of Pb tracer was added to deionised water to perform just the precipitation 9 1 9 step. The gamma activity of Pb in lead Chromate form was measured by gamma spectrometry. The chemical yield of precipitation was then calculated as the ratio 919 919 between the activity of Pb added and activity of Pb found in lead Chromate. The overall recovery was calculated as the product of the chemical yield of extraction and the chemical yield of precipitation. The stoichiometric factor is the ratio between the mass of Pb in the precipitate and the mass of the precipitate. If lead would precipitate only in lead Chromate form, the stoichiometric factor would be 0.6411. An experimental stoichiometric factor was determined because the precipitate of lead Chromate has a nonstoichiometric composition, since it also includes Chromate ions and at pH lower than 5 lead does not precipitate only in the form of lead Chromate but also in the form of lead dichromate. Non-stoichiometry is due to the presence of Chromate ions and lead dichromate. m -0.6411 . / = K.0>6411 . ^x^ Eq.2 20 mg Kgr r)gr gravimetric yield of precipitation mpb mass of lead Chromate precipitate 0.6411 stoichiometric factor for lead Chromate precipitate f experimental stoichiometric factor r)p chemical yield of precipitation r) overall recovery r)e chemical yield of extraction L. Benedik, P. Vreček: Determination of Pb and Po in environmental samples Acta Chim. Slov. 2001, 48, 199-213. 205 4. Efficiency of the gas flow proportional counter 210 The counting efficiency of Bi on the beta counter was determined with a known amount of solid KCl (0.0117 % 40K, Ep = 1505 keV) and with 210Pb standard 9 1 0 solution evaporated on a lead Chromate precipitate ( Bi, Ep = 1161 keV). Determination of 210Po 1. Radiochemical separation The treatment of a water sample is very similar to the treatment of the sample for determination of Pb. The difference is in the amount of sample (15 L) that was taken and in the addition of Po tracer in the first stage. To separate polonium, the technique of spontaneous deposition of polonium radioisotopes on a copper planchet for four hours at 50°C and pH 1 was performed. The equipment for spontaneous deposition of polonium radioisotopes is presented in Fig.2. The sample solution after evaporation to small volume, adjusted to pH 1 with hydrochloric acid, was put into the beaker together with the holder and copper planchet. The sample solution at 50°C must be stirred continuously during spontaneous deposition.8 Then polonium radioisotopes were measured by alpha spectrometry (Tennelec TC 9 1 0 256 alpha spectrometer) and the activity of Po was calculated from Equation 3. . AreaPo Ap° = 7.------- .v Eq-3 l ' Khem. ' Kdet. ' 's Apo activity of Po in water sample in Bq m" 9 1 0 Areapo peak area for"luPo(decay/s) T time of measurement (s) TAO T|chem yield of Po tracer r)det counting efficiency 3\ Vs sample volume (m ) L. Benedik, P. Vreček: Determination of Pb and Po in environmental samples 206 Acta Chim. Slov. 2001, 48, 199-213. 7 L »h .VI Figure 2: Equipment for spontaneous deposition of polonium radioisotopes. 2. Optimisation of plating time 210T For optimisation of the procedure for the determination of Po the radioactive 208 tracer Po was used and the plating efficiency determined as a function of time. 3. Determination of efficiency of alpha detector The counting efficiency of the a-detector was determined with a calibrated source of 230Th (E„=4687 keV (76.3 %)). Determination of 210Pb and 210Po in air filters Samples of air filters were leached for 24 hours in a solution of 6 M hydrochloric 208 acid together with lead carrier and Po tracer. The solution prepared in this way was 210 divided into two equal parts. In one part Pb was determined and in the second part 210 Po was determined, using the procedures described above. L. Benedik, P. Vreček: Determination of Pb and Po in environmental samples Acta Chim. Slov. 2001, 48, 199-213. 207 Results and discussion Determination of 210Pb Table 1 shows results obtained for the yield of the first and second extraction of 9 I 9 lead by dithizone by Pb tracer experiments. In the separation of Pb by dithizone extraction, it can be observed from Table 1 that in 1 minute around 95 % of lead is extracted. Even if the time of extraction is extended, chemical yields are not higher. A second extraction is thus not necessary, because only around an additional 2 % of lead is then extracted. Table 1: Yield of the first and the second extraction as a function of extraction time. Time of extraction Yield of first extraction (%) (n=4) Yield of second extraction (%) (n=4) 1 min 94.4+1.7 2.4 + 0.6 2 min 95.8 + 3.1 1.9 + 0.4 3 min 95.0 + 4.9 2.0 + 0.7 4 min 94.9+1.1 1.4 + 0.1 Table 2 shows the overall recovery and the experimental stoichiometric factor found for lead Chromate precipitation. The overall recovery was determined as 91.9 % and the experimental stoichiometric factor was determined as 0.524. Table 2: Determination of overall recovery and experimental stoichiometric factor. Chemical yield of extraction (%) r\e (n=18) 95.4 + 4.2 Chemical yield of precipitation (%) r\v (n=18) 96.5 + 5.2 Gravimetric yield of precipitation (%) %r. (n=18) 118 + 2 Overall recovery (%) r\ (n=18) 91.9 r3.0 Experimental stoichiometric factor f (n=18) 0.524 r 0.027 This procedure was then verified by the standard addition method. Tap water samples with different amounts of Pb standard addition were prepared. The activity 9 1 0 of z,uPb in the sample was found to be 103 mBq, while the standard addition method L. Benedik, P. Vreček: Determination of Pb and Po in environmental samples 208 Acta Chim. Slov. 2001, 48, 199-213. gave a value of 95.4 mBq. This agreement shows that the method for determination of 2l0Pb is reliable. The counting efficiency of the gas flow proportional counter determined with solid KCl (from 40K) and by 2l0Pb standard solution was 43.1 % and 43 % , respectively. 9 I 0 The minimum detectable activity (MDA) of Pb was determined using the Currie equation9: im,,„ 3n 4.65-5A+2.71 MDA (Bq m~3) =------------------- Currie HAet. 'Kehern. '* 'V equation Sb the standard deviation of a series of counts of an appropriate blank sample Sdet counting efficiency T|chem chemical recovery t counting time for the sample (s) V sample volume (m ) 9 I 0 In the case of determination of Pb in water samples we made a series of 5 experiments with blank samples and determined the MDA as 1.8 + 0.2 Bq m" when 6 L of water sample was used. 9 1 0 Tables 3 and 4 show results for the activity of Pb in water samples from the 9 10 Todraščica and Brebovščica streams (year 1998) and activities of Pb in water samples from wells in the vicinity of the former Zirovski vrh uranium mine. L. Benedik, P. Vreček: Determination of Pb and Po in environmental samples Acta Chim. Slov. 2001, 48, 199-213. 209 Table 3: Activities of Pb in samples of Todraščica and Brebovščica streams, 1998. Sample Todraščica 210pb (Bq m"3) Brebovščica 2IOpb (Bq m"3) 1 14 + 1 15+2 2 85 + 9 13 + 1 3 63 + 6 23+2 4 68 + 7 17 + 2 5 22 + 2 38 + 4 6 16 + 2 21+2 7 23+2 6 + 1 8 12 + 1 2.0 + 0.2 9 16 + 2 4.9 + 0.5 10 13 + 1 3.4 + 0.3 11 9 + 1 13 + 1 12 4.2 + 0.4 2.8 + 0.2 100 "? 80 s er 60 m o 40 ^ 20 * 0 123456789 10 11 12 Sample Figure 3: Diagram showing activities of Pb for monthly samples of Todraščica and Brebovščica streams, 1998. Table 4: Activities of Pb in wells from the vicinity of the former uranium mine at Zirovski vrh. Sample of water from well 210Pb(Bqm"3) BS-26 5.6 + 0.6 BS-30 12 + 1 Drmota 8.4 + 0.8 - D Todraščica ¦ Brebovščica L. Benedik, P. Vreček: Determination of Pb and Po in environmental samples 210 Acta Chim. Slov. 2001, 48, 199-213. Determination of 210Po The results of the experiments to determine the plating efficiency as a function of plating time are shown in Fig.4. It can be observed that the curve reaches a plateau at around 240 min. At that time around 90 % of polonium radioisotopes are deposited. 100 90 L 80 oo 0 70 ^ L 50 CD 1 40 tu M 30 S 20 u- 10 o Figure 4: Plating efficiency as a function of plating time in spontaneous deposition of Po radioisotopes on a Cu planchet at 50°C, pH=l. The counting efficiency of the a-detector was found to be 30.9 %. 9 10 For determination of Po the MDA was also determined using the Currie equation. The average of 5 experiments with blank samples gave a value of 0.040 ± 0.003 Bq m" if 15 L of water sample were used. 910 Table 5 shows results for the activity of Po in water samples of the Todrašččica and Brebovščica streams (year 1998). 91 0 Table 5: Activities of Po in samples of Todraščica and Brebovščica streams, 1998. Sample Todraščica (Bq m"3) Brebovščica (Bq m"3) 1st quarter 3.4 + 0.2 2.8 + 0.2 2" quarter 2.3 + 0.1 1.8 + 0.1 rti 3 quarter 2.3 + 0.1 1.7 + 0.1 4* quarter 0.99 + 0.06 0.86 + 0.05 50 100 150 200 250 300 350 400 Plating time (min) L. Benedik, P. Vreček: Determination of Pb and Po in environmental samples Acta Chim. Slov. 2001, 48, 199-213. 211 o cš 4 3.5 3 2.5 2 1.5 1 0.5 0 .-----. ? Todraščica ^^ ¦ Brebovščica Juin 1st quarter 2nd quarter 3rd quarter 4th quarter 910 Figure 5: Diagram showing activities of Po as a function of season for samples from the Todraščica and Brebovščica streams, 1998. Fig.6 shows an alpha spectrum of polonium radioisotopes separated from a water 210T sample. The spectrum is radiochemically pure; the peaks of Po at 5.3 MeV and of 208 Po tracer at 5.1 MeV can be observed. 2000 3280 5200 6500 7200 Energy (keV) 7370 Figure 6: Alpha spectrum of 210Po in sample from the Brebovščica, 3rd quarter, 1998, with 208Po tracer. Determination of 210Pb and 210Po in air filters Both radionuclides were also determined in air filters from two chimneys of the Šoštanj thermal power plant. In Table 6 the results are presented. The coal used by the thermal power plant contains relatively high activities of naturally-occurring radionuclides, particularly uranium (~ 100 Bq kg"1). While uranium and radium remain in the ash, polonium and lead, because they are volatile, are released with exhaust gases 910 and contaminate the surroundings. It can be observed that the activities of Pb and L. Benedik, P. Vreček: Determination of Pb and Po in environmental samples 212 Acta Chim. Slov. 2001, 48, 199-213. Po in air filters from chimney 4 are much lower than the activities of these radionuclides determined in air filters from chimney 5. That is because in chimney 4 desulphurisation plant to clean the exhaust gases is installed. In this procedure dust is also removed and environmental contamination by volatile radionuclides is successfully reduced. 910 910 Table 6: Activities of Pb and Po in samples of air filters from chimneys 4 and 5 of the Šoštanj thermal power plant. Sample Air volume filtered (L) Dust amount (mg) 2iopb (Bqkg1) 210pb (mBq m" ) 2iopo (Bqkg1) 2iopo (mBq m" ) 4 Filterl7 3150 21.4 8685 ± 869 59 + 6 2949+177 20 + 1 Filter 18 3417 59.6 2757 ± 276 48 + 5 1091+65 19 + 1 5 Filter 13 602 38.8 2451+245 158 + 16 2296 + 138 148 + 9 Filter 15 681 26.1 3575 + 358 137+14 3705 + 222 142 + 9 Conclusions 910 910 Determination of Pb and Po in the Todraščica and Brebovščica streams in the vicinity of the uranium mine is part of the regular environmental monitoring programme. Variation of activities of both radionuclides is a consequence of the weather (rainfalls) in this region. Even if activities in some monthly samples are sometimes a little higher, 9 I 0 these levels are still under the maximal permitted activity for Pb in waters, which is Q 910 ^ 400 Bq m" . The maximal permitted activity for Po in waters is 2000 Bq m" . From 9 I 0 Table 5 can be observed that the activities of Pb in underground waters are very similar to the activities in surface waters. 910 The method developed for determination of Pb is selective and accurate. The 9 I 9 overall recovery measured using Pb is more than 90 %. The minimum detectable activity (MDA) for a sample size of 6 litres is 1.8 Bq m°. 9 1 9 A new method was developed for Pb tracer preparation. This method is simple and quick and tracer prepared in this way was observed to be pure enough for its use in determination of the yield with routine samples. L. Benedik, P. Vreček: Determination of Pb and Po in environmental samples Acta Chim. Slov. 2001, 48, 199-213. 213 9 I 0 The method for determination of Po is very reliable and precise. The plating efficiency was about 90 % after four hours plating at 50°C. The minimum detectable activity (MDA) for a sample size of 15 litres is 0.040 Bq m" . Both optimised methods were shown to be suitable for routine measurements of water samples and air filters in monitoring programmes around the former Zirovski vrh uranium mine and a coal-fired electricity generating plant, respectively. Acknowledgements This work was financially supported by Ministry of Science and Technology of Slovenia (Project No. Jl-1194) 9 I 9 We thank Dr.A.R.Byrne for his suggestion concerning preparation of Pb tracer. References 1. Radioactivity in Food and the Environment 1998 -4, MA FF & S EPA Report, London, 1999. 2. ICRP Publications No 67, Annals oflCRP, 1993, 23(3/4). 3. UNSCEAR Report, Ionizing Radiation: Sources and Biological Effects, UN 1982. 4. J. R. W. Woittiez, J. E. Sloof, K. J. Kroon,./. Radioanal. Nucl. Chem., Articles, 1995,194, 11,319-329. 5. D. A. Wickenden, J. Toole, Sci. Tot. Environ.,, 1995,173/174, 117-123. 6. R. F. Clayton, E. J. Bradley, Sci.Tot. Environ.,, 1995,173/174, 23-28. 7. B. Smodiš, Z. Planinšek, P. Stegnar, Radiochemical determination of2U'Pb, XIV* Regional Congress ofIRPA, Dubrovnik 1987. 8. K. Južnič, I. Kobal,./. Radioanal. Nucl. Chem., Articles, , 1986, 102, II, 493-498. 9. L. A. Currie, Anal. Chem., 1968, 40, 586-593. Povzetek Razvili smo metodi za določanje 210Pb in 210Po v vzorcih iz okolja. Oba radionuklida smo določali v vzorcih vode in zračnih filtrov. Svinec smo separirali z ekstrakcijo z ditizonom in obarjanjem v obliki svinčevega kromata. Po vzpostavitvi radiokemičnega ravnotežja med 2l0Pb in njegovim potomcem 210Bi smo beta aktivnost 210Bi pomerili na beta proporcionalnem števcu. Polonijeva radioizotopa smo separirali s spontano depozicijo na bakrovo ploščico in le-to pomerili na alfa spektrometru. Obe metodi sta bili optimizirani z uporabo sledilcev Po in Pb. Uporabili smo postopek za pripravo sledilca Pb, ki temelji na prepihavanju raztopine torijevega nitrata z dušikom. L. Benedik, P. Vreček: Determination of Pb and Po in environmental samples