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Determination of 226Ra and 224Ra in drinking waters by liquid scintillation counting
Appl. Radiat. lsot. Vol.48, No. 4, pp. 535-540, 1997 © 1997ElsevierScienceLtd Printed in Great Britain.All rights reserved PII: S0969-8043(96)00297-7 0969-8043/97 $17.00+ 0.00
Pergamon
Determination of 226Ra and 224Ra in Drinking Waters by Liquid Scintillation Counting G. M A N J O N *~, I. V I O Q U E ~, H. M O R E N O ' , R. G A R C I A - T E N O R I O ~ a n d M. G A R C [ A - L E O N 2 ~E.T.S. Arquitectura, Universidad de Sevilla, Departamento de Fisica Aplicada, Avenida Reina Mercedes 2, 41012 Sevilla, Spain and 2Facultad de Fisica, Universidad de Sevilla, Departamento de Fisica At6mica, Molecular y Nuclear, Avenida Reina Mercedes s/n, Apartado 1065, 41080 Sevilla, Spain (Received 17 July 1996)
A method for the determination of Ra-isotopes in water samples has been developed. Ra is coprecipitated with Ba as sulphate. The precipitate is then dissolved with EDTA and counted with a liquid scintillation system after mixing with a scintillation cocktail. The study of the temporal evolution of the separated activity gives the isotopic composition of the sample, i.e. the 2-'4Raand 226Racontribution to the total activity. The method has been applied to some Spanish drinking waters. © 1997 Elsevier Science Ltd. All rights reserved
Introduction The determination of Ra-isotopes in environmental samples is very important from the radiological point of view. Due to their radiotoxicity, especially that of -'26Ra, the contamination hazard is potentially dangerous to human beings even at low concentration levels. Thus, easy, rapid and of course, reliable Ra determination methods are needed and there is an important scientific literature on this topic (Sch6nhofer, 1994). In this paper, we present a method for Ra-isotope determination in environmental waters. Ra-isotopes are coprecipitated with Ba and measured by liquid scintillation counting. Study of the time evolution of the activity, together with the use of Bateman's equations, gives the isotopic composition of the extracted Ra fraction. The method presented clearly competes with alphaand gamma-spectrometric methods due to its simplicity and sensitivity.
Radiochemical Method Ra is extracted from waters as Ra-BaSO4 as described previously (Mor6n et al., 1986). 5 mg of Ba 2+, from a BaCI2 (1 mg/mL Ba2+) solution, and some 20 mL of 1 M H2SO4 were added to 0.5 L of water, which had been previously neutralized with *To whom all correspondence should be addressed.
NH4OH. The solution was heated while stirring for 1 h. The sulphate precipitate was recovered by filtration through a Millipore filter (0.45 #m pore size). The precipitate was then dissolved with some few mL of 2 M EDTA at pH = 8-9 and the final solution evaporated to a volume of 5 mL. This solution was transferred into a low potassium content glass vial and mixed with 15 mL of scintillation cocktail (Pharmacia Wallac Optiphase Hisafe I1) for the measurement.
Ra-isotopes Determinations The detector
Ra-isotopes activities were measured with a low-background liquid scintillation spectrometer (Wallac Quantulus 1220TM). As is well known, this counter gives a very good response to a and/3 radiations. This was a drawback in our case, since some Ra-isotope descendants are /3-emitters, which could interfere with the Ra activity determination. Fortunately, the detector can distinguish between a and /3 particles by pulse-shape analysis (PSA). Indeed, it is well known that the delayed component of the fluorescence produced by a-particles is more important than that produced by/3-particles. Thus, an estimation of the relative importance of the delayed component area in a given electronic pulse can indicate if the pulse was a- or/3-produced. This can be done by using a software-controlled parameter, the PSA thereshold level, which varies
535
536
G. Manj6n et al.
from 1 to 256. According to such a parameter, :t- and fl-pulses are addressed to two different spectra: the :t-spectrum and the//-spectrum. Nevertheless, the discrimination is not perfect. There are always interferences and cross-over effects. The PSA thereshold level (PSA hereafter) should be chosen so as to minimize z, the total interference, which is given by: = z~ + z~
(1)
T, being the fraction of :t-particles observed in the //-spectrum and za the fraction of//-particles observed in the :t-spectrum (Sdmchez-Cabeza et al., 1993). For our counting conditions, the best PSA was 120 as shown in Fig. 1. z was measured by using calibrated samples of 239pu (:t-emitter) and 4°K (//-emitter). For it, some 5 mL from two aqueous solutions with well known acitivites of both radionuclides, respectively, were mixed with 15 mL of scintillation cocktail (Pharmacia Wallac Optiphase Hisafe II) in the vials. It can be seen that, within a certain range, the selection of the optimum PSA is not critical. Indeed, a similar ~ can be achieved for PSA values ranging from 110 to 130. The scintillation spectrometer is protected by a guard detector. The whole system is surrounded by a passive low-background shielding (Pb). The integral background of the :t-spectrum under these conditions was 0.45 _+ 0.05 cpm. Ra-isotope determinations
Four Ra-isotopes occur in nature. Three, 226Ra (T,/2 = 1600 yr), 224Ra (T~.2= 3.66 days) and 223Ra
0,81
(T~/2 = 11.434 days) are :t-emitters, while the fourth, 228Ra (T,,.2 = 5.75 yr) is a fl-emitter. Due to the relative scarcity of 235U in nature, the presence of 223Ra in the Ra-BaSOa precipitate is not significant, unless a clearly contaminated sample is analysed. On the other hand, a very small fraction of 228Ra could contribute to the :t-spectrum in the less favourable case. 228Ra decays into 228Th (T~,2 = 1.9 yr, :t-emitter) after producing 22SAc (Ttn= 6.13 h, 3-emitter), furthermore, 22STh decays into 224Ra. Nevertheless, the 22SRa half-life makes its presence unimportant, at least during the first 2 months after separation of Ra (Mor6n et al., 1986). Consequently, it seems clear that the relevant contributions to the :t-spectrum will come from 226Ra and 224Ra. Due to its half-life, 224Ra and descendants will be counted only during the first few days after Ra separation, 226Ra and its family will remain in the sample for a very long time. Thus, two measurements are enough to completely determine the 224Ra- and 226Ra-activities. One inmediately after the separation, when the 224Ra family is still active, and the other some 4 weeks later, when the only active family is that from 226Ra. By using the Bateman's equations (Mor6n et al., 1986) both contributions, those from 224Ra and 226Ra, to the gross integral of the :t-spectrum can be discerned. Alternatively, the 226Ra activities have been determined through the :t-peak from 2'4P0 (T,2 = 164.3/~s) which reaches secular-equilibrium with 22~Ra after some 4 weeks. This method is, in fact, the most used (Salonen, 1990; Buheitel, 1993). Due to the energy of the :t-particles emitted, its contribution to
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60 65 70 75 80 85 90 95 100 105 110 1151_0125130135140145150155160165170 PSA Fig. 1. z, + z~ interference vs PSA level. Optimization of the pulse-shape ana]yser of the QuantuJus 1220.
Best results, less interferences, correspond to PSA level 120.
Determination of 226Raand 224Rain drinking waters
537
226-Ra and daughters Alpha spectra 70
60 50 40 0
0
30
20
10
400
450
500
550
600
650 700 Channel ~
A
--
750
800
850
900
950
B
Fig. 2. ct Spectra of a 226Ra standard solution. The spectrum A was acquired inmediately after chemical separation, while the spectrum B was acquired some 4 weeks later. the :t-spectrum can be clearly separated. Figure 2 depicts two 226Ra ~t-spectra, one acquired after the separation and the other after 4 weeks. The 2'4po or-peak can be unambigously identified.
speaking, for large t, when secular equilibrium is reached, 6226(0 ~ 4. =6Ra can also be determined by counting 2"Po as explained above. In this case, an ('Ra can be defined as:
Calibration
The integral of the u-spectrum, or gross count rate, N, is converted into activity through the total efficiency, ~Ra. It is defined as: ~R~ = Y x G
(2)
where Y is the radiochemical yield and E,, the counting efficiency. The total efficiency, eu~, has been calculated by using a standard 226Ra solution of accurately known activity. Thus, several 0,5 L samples of distilled water were spiked with known 226Ra activities, d=6. After separation, according to the method described in Section 2, the final solution was measured with our system, either immediately or some weeks later when the a6Ra descendants reach secular equilibrium. If the total count rate obtained in the integral spectrum is N,, with the optimum PSA, ERa, becomes: N~
(3)
ERa - - (~226(/) X A226
~226(/) being the correction to N~ due to the contribution from the u-emitter descendants to the count rate. ~226(l) is calculated by using the Bateman's equations and its value will depend on the time, t, when the measurement is performed. Roughly ARI 48,'4~D
Npo
~'R~ = ~
(4)
where Npo is the integral of the 2t4po peak calculated when secular equilibrium is reached. Table 1 presents the results obtained for ¢R~,,with measurements made immediately after separation or some 4 weeks later, and E'R~. Lowest total efficiency data were obtained in the first essay. The increasing of these values was produced by more careful analysis. Surely, the diferences are due to a non-dissolution with EDTA. We also give some interesting additional data from the experiments performed. The agreement among the ca, values obtained with both methods is clear. This means that 6226(t) is correctly estimated, as is confirmed in Fig. 3, where the theoretical and experimental activity growth of one of the 226Ra standard sample is depicted. Such agreement also confirms that no losses of -'22Rnfrom the vials occur, which is very important as the reproducibility of the method is concerned. It is also clear that, E'R~< E~. This result is a not surprising as it has already been reported in the open literature (Salonen, 1990), Effects such as ionization quenching due to the high energy or-particles emitted by 214p0, and guard counter switching off by 2'4Bi
538
G. Manj6n et al.
Table 1. -~-'6Ratotal efficiency(Q and background (B) for the different methods used for activity determination. Lower limits of detection (LLD), when counting time (T) is 600 min and the sample volume is 500 mL are also given. LLD = 2x//2"~ tz (dpm) Counting window Immediately measured Measured some 4 weeks after separation Total ~-spectrum Total ~t-spectrum 214po~t-peak Background 0.45 _ 0.05 cpm 0.45 _+0.05 cpm 0.04 + 0.02 Total efficiency 55 _+3% 56 _+3% 40 __.2% 69+4 67+3 48+2 69_+4 66+3 41 _+7 68_+4 67+3 47+3 Mean value Mean value Mean value 65_+2 64_+2 44+2 Lower limit of detection 0.05 dpm 0.01 dpm 0.03 dpm 1.8 mBq/L 0.5 mBq/L 0.9 mBq/L
7-radiation could be the main reasons for such a low E'Ra value. Furthermore, as seen in Fig. 2, part of the counts produced by 2~4p0 can be lost when integrating the a-peak due to the low resolution of the spectrometer. A final comment on the above results is that 2~°Pb is also coprecipitated with Ba and dissolved with E D T A . In fact, we have found in some cases a clear contribution from some low energy fl-emitters in the fl-spectrum. This fact should be taken into account if the Ra sample is going to be measured 2 or 3 months after the chemical separation, since significant amounts of 2~°Po could have been produced during this time.
A Study Case: Ra-isotopes in Mineral Waters As an application of the Ra determination method just described, we analysed a set of mineral waters, commercially available in Spain, plus some samples
obtained from other drinking water sources. The geographical origin of the samples covers practically the whole map of Spain. Some 0.5 L of water was used for each analysis. The final samples were measured during two counting time cycles each, of 600 min. Table 2 gives the 226Ra and Z24Ra activity concentrations in m B q / L for the samples analysed. Errors were quoted considering 2 SD. It is clear, and was expected, that the 224Ra activities are lower than those of 226Ra and, in some cases, less than the limit of determination for our counting system. The highest 2Z4Ra and 2Z6Ra activity concentrations were found in the same sample, Mondariz-2, a mineral water coming from the Northwest of Spain. A daily consumption of 2.2 L of these waters by a standard person would produce an annual ingestion from 0.8 to 8.8 Bq, for 224Ra, and from 0.8 to 214 Bq, for 226Ra. This is well below the annual intake limits for 224Ra and 2Z6Ra, 3 × l& Bq and 7 x 103 Bq
26
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15-
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15 Time (days)
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20
25
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30
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Exp.alpha
Thaor. alpha
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Fig. 3. 226Rabuild up theoretical curve and experimental data. No losses of radon from the vial can be inferred from this experiment.
Determination of 226Raand 224Rain drinking waters Table 2. -'24Raand -'26Raactivityconcentration (mBq/L) obtained with the presentedmethodin a set of commercialand environmental water samples Samples ::'Ra (mBq/L) 226Ra(mBq/L) Lanjaron-2 Fontemilla-2 Fontvella-I Fontvella-2 Viladrau-I Viladrau-2 Vallecardoo2 Liviana-I Sanchis-I Fonxesta-2 Mondariz-2 Fontcelta-I Regas-I Emasesa-I Gelves-I Osuna-I Osuna-2
< I 2 _+ I 2.3 + 0.7 < 1 < 0.5 1.3 + 0.7 < I < 0.5 < 1.2 < 0.7 11 + 2 < 0.8 4.2 + 0•8 < 1•4 < 0.7 1.0 + 0.7 < 1
3+ 1 2.7 +_ 0.7 8 + I 9 _+ 1 6.3 + 0.7 5+ 1 5.0 + 0.7 < 0.7 11.3 + 0.7 < 0.7 267 + 3 4 + 1 19 + 1 1.3 + 0.7 4.7 + I 5.3 + 0.7 5+ 1
for 224Ra and 226Ra respectively, according to the Spanish regulations (Boletin Oficial del Estado, 1988). The last three samples in Table 2 correspond to underground waters consumed directly by the population. Their radiological significance as far as Ra is concerned is negligible. The sample EMASESA-I represents the waters that supply the town of Seville. Its 226Ra level, 1.3 mBq/L, should be compared with typical levels of some 20 mBq/L obtained, during this time period, in the river from which the drinking water is taken (Martinez-Aguirre et al., 1991). The efficiency of the water treatement system is apparent. Finally, the results presented in Fig. 4 clearly show the agreement between both methods of 226Ra
539
determination, when the secular equilibrium is reached, some 4 weeks after separation. There, we plot the 226Ra activity (dpm) of the 0.5 L water samples obtained by integration of the 2'4po or-peak and by integration of the whole spectrum. It is clear that both methods give compatible results. The only exception is that of sample REGAS-I which give 0.59 + 0.05 dpm with the first method and 0.92 + 0.03 dpm with the second. The difference could be explained by the presence of 2t°Po in the sample• Indeed, it is known that -'~°Pb coprecipitates as sulphate. This results in the appearance of its daughers in the sample after some time. Some of them, 2'°Bi, 2"Pb and 2t4Bi, in addition to 2'°Pb, are fl-emitters and their contribution to the fl-spectrum should be measurable. This occurred in the previously mentioned water sample, REGAS-I, where fl-activity was observed. This supports the hypothesis of 2~°pb precipitation and consequent ingrowth of its daughters, including 21°po.
Conclusions
In this work, a method is given for the measurement of Ra-isotopes in waters by liquid scintillation counting. Ra is recovered by co-precipitation with Ba as sulphate. The precipitate is dissolved with EDTA. The study of the time evolution of the total activity gives the isotopic composition of the extracted Ra. Thus, both 226Ra and 224Ra activities are determined in the water sample. The method has been applied to Spanish drinking
0,80,7 -
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0,3. 0,2~ 0,1
0-0,1
-0,1
0
0,1
0,2
0,3 0,4 0,5 0,6 Integral speclrum (dpm)
0,7
0,e
0,9
1
Fig. 4. 226Ra activities (dpm) calculated by integration of the 2'4Po or-peak vs 226Ra activities (dpm) calculated by the integration of the total spectrum. Results correspond to measurements made 4 weeks after separation.
G. Manj6n et al.
540
waters. N o special radiological significance of the studied waters has been found.
References Boletin Oficial del Estado (BOE) (1988) BOE No. 13/15-01-88. In Spanish. Buheitel, F. (1993) The determination of low-levels of radium isotopes and radon by a simple delayed-coincidence liquid scintillation spectroscopy method. In Liquid Scintillation Spectrometry, eds J. E. Noakes, F. Sch6nhoffer and H. A. Polach, pp. 83-88. RADIOCARBON, Department of Geosciences, The University of Arizona, Tucson, Arizona. Martinez-Aguirre, A., Mor6n, M. C. and Garcia-Le6n, M. (1991) Measurements of U- and Ra-isotopes in rainwater samples. J. Radioanalyt. Nucl, Chem. 152, 37-46. Mor6n, M. C., Garcia-Tenorio, R., Garcia-Montafio, E.,
Garcia-Le6n, M. and Madurga, G. (1986). Int. J. Rad. Appl. Instrum. Part A 37, 383. Salonen, L. (1990) A rapid method for monitoring of uranium and radium in drinking water. 6th International Symposium on Environmental Radiochemical Analysis, pp. 23-25. The Science of the Total Environment, Manchester. Sfinchez-Cabeza, J. A., Pujol, Ll., Merino, J., Le6n, L., Molero, J., Vidal-Quadras, A., Schell, W. R. and Mitchell, P. I. (1993) Optimization and calibration of a low background liquid scintillation counter for the simultaneous determination of alpha and beta emitters in acqueous samples. In Liquid Scintillation Spectrometry, eds J. E. Noakes, F. Sch6nhoffer and H. A. Polach, pp. 43-50. RADIOCARBON, Department of Geosciences, The University of Arizona, Tucson, Arizona. Sch6nhofer, F. (1994) The radon problem. In Low-Level Measurements of Radioactivity in the Environment, eds M. Garcia-Le6n and R. Gareia-Tenorio, pp. 227-240. World Scientific, Singapore.
Pergamon
Determination of 226Ra and 224Ra in Drinking Waters by Liquid Scintillation Counting G. M A N J O N *~, I. V I O Q U E ~, H. M O R E N O ' , R. G A R C I A - T E N O R I O ~ a n d M. G A R C [ A - L E O N 2 ~E.T.S. Arquitectura, Universidad de Sevilla, Departamento de Fisica Aplicada, Avenida Reina Mercedes 2, 41012 Sevilla, Spain and 2Facultad de Fisica, Universidad de Sevilla, Departamento de Fisica At6mica, Molecular y Nuclear, Avenida Reina Mercedes s/n, Apartado 1065, 41080 Sevilla, Spain (Received 17 July 1996)
A method for the determination of Ra-isotopes in water samples has been developed. Ra is coprecipitated with Ba as sulphate. The precipitate is then dissolved with EDTA and counted with a liquid scintillation system after mixing with a scintillation cocktail. The study of the temporal evolution of the separated activity gives the isotopic composition of the sample, i.e. the 2-'4Raand 226Racontribution to the total activity. The method has been applied to some Spanish drinking waters. © 1997 Elsevier Science Ltd. All rights reserved
Introduction The determination of Ra-isotopes in environmental samples is very important from the radiological point of view. Due to their radiotoxicity, especially that of -'26Ra, the contamination hazard is potentially dangerous to human beings even at low concentration levels. Thus, easy, rapid and of course, reliable Ra determination methods are needed and there is an important scientific literature on this topic (Sch6nhofer, 1994). In this paper, we present a method for Ra-isotope determination in environmental waters. Ra-isotopes are coprecipitated with Ba and measured by liquid scintillation counting. Study of the time evolution of the activity, together with the use of Bateman's equations, gives the isotopic composition of the extracted Ra fraction. The method presented clearly competes with alphaand gamma-spectrometric methods due to its simplicity and sensitivity.
Radiochemical Method Ra is extracted from waters as Ra-BaSO4 as described previously (Mor6n et al., 1986). 5 mg of Ba 2+, from a BaCI2 (1 mg/mL Ba2+) solution, and some 20 mL of 1 M H2SO4 were added to 0.5 L of water, which had been previously neutralized with *To whom all correspondence should be addressed.
NH4OH. The solution was heated while stirring for 1 h. The sulphate precipitate was recovered by filtration through a Millipore filter (0.45 #m pore size). The precipitate was then dissolved with some few mL of 2 M EDTA at pH = 8-9 and the final solution evaporated to a volume of 5 mL. This solution was transferred into a low potassium content glass vial and mixed with 15 mL of scintillation cocktail (Pharmacia Wallac Optiphase Hisafe I1) for the measurement.
Ra-isotopes Determinations The detector
Ra-isotopes activities were measured with a low-background liquid scintillation spectrometer (Wallac Quantulus 1220TM). As is well known, this counter gives a very good response to a and/3 radiations. This was a drawback in our case, since some Ra-isotope descendants are /3-emitters, which could interfere with the Ra activity determination. Fortunately, the detector can distinguish between a and /3 particles by pulse-shape analysis (PSA). Indeed, it is well known that the delayed component of the fluorescence produced by a-particles is more important than that produced by/3-particles. Thus, an estimation of the relative importance of the delayed component area in a given electronic pulse can indicate if the pulse was a- or/3-produced. This can be done by using a software-controlled parameter, the PSA thereshold level, which varies
535
536
G. Manj6n et al.
from 1 to 256. According to such a parameter, :t- and fl-pulses are addressed to two different spectra: the :t-spectrum and the//-spectrum. Nevertheless, the discrimination is not perfect. There are always interferences and cross-over effects. The PSA thereshold level (PSA hereafter) should be chosen so as to minimize z, the total interference, which is given by: = z~ + z~
(1)
T, being the fraction of :t-particles observed in the //-spectrum and za the fraction of//-particles observed in the :t-spectrum (Sdmchez-Cabeza et al., 1993). For our counting conditions, the best PSA was 120 as shown in Fig. 1. z was measured by using calibrated samples of 239pu (:t-emitter) and 4°K (//-emitter). For it, some 5 mL from two aqueous solutions with well known acitivites of both radionuclides, respectively, were mixed with 15 mL of scintillation cocktail (Pharmacia Wallac Optiphase Hisafe II) in the vials. It can be seen that, within a certain range, the selection of the optimum PSA is not critical. Indeed, a similar ~ can be achieved for PSA values ranging from 110 to 130. The scintillation spectrometer is protected by a guard detector. The whole system is surrounded by a passive low-background shielding (Pb). The integral background of the :t-spectrum under these conditions was 0.45 _+ 0.05 cpm. Ra-isotope determinations
Four Ra-isotopes occur in nature. Three, 226Ra (T,/2 = 1600 yr), 224Ra (T~.2= 3.66 days) and 223Ra
0,81
(T~/2 = 11.434 days) are :t-emitters, while the fourth, 228Ra (T,,.2 = 5.75 yr) is a fl-emitter. Due to the relative scarcity of 235U in nature, the presence of 223Ra in the Ra-BaSOa precipitate is not significant, unless a clearly contaminated sample is analysed. On the other hand, a very small fraction of 228Ra could contribute to the :t-spectrum in the less favourable case. 228Ra decays into 228Th (T~,2 = 1.9 yr, :t-emitter) after producing 22SAc (Ttn= 6.13 h, 3-emitter), furthermore, 22STh decays into 224Ra. Nevertheless, the 22SRa half-life makes its presence unimportant, at least during the first 2 months after separation of Ra (Mor6n et al., 1986). Consequently, it seems clear that the relevant contributions to the :t-spectrum will come from 226Ra and 224Ra. Due to its half-life, 224Ra and descendants will be counted only during the first few days after Ra separation, 226Ra and its family will remain in the sample for a very long time. Thus, two measurements are enough to completely determine the 224Ra- and 226Ra-activities. One inmediately after the separation, when the 224Ra family is still active, and the other some 4 weeks later, when the only active family is that from 226Ra. By using the Bateman's equations (Mor6n et al., 1986) both contributions, those from 224Ra and 226Ra, to the gross integral of the :t-spectrum can be discerned. Alternatively, the 226Ra activities have been determined through the :t-peak from 2'4P0 (T,2 = 164.3/~s) which reaches secular-equilibrium with 22~Ra after some 4 weeks. This method is, in fact, the most used (Salonen, 1990; Buheitel, 1993). Due to the energy of the :t-particles emitted, its contribution to
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60 65 70 75 80 85 90 95 100 105 110 1151_0125130135140145150155160165170 PSA Fig. 1. z, + z~ interference vs PSA level. Optimization of the pulse-shape ana]yser of the QuantuJus 1220.
Best results, less interferences, correspond to PSA level 120.
Determination of 226Raand 224Rain drinking waters
537
226-Ra and daughters Alpha spectra 70
60 50 40 0
0
30
20
10
400
450
500
550
600
650 700 Channel ~
A
--
750
800
850
900
950
B
Fig. 2. ct Spectra of a 226Ra standard solution. The spectrum A was acquired inmediately after chemical separation, while the spectrum B was acquired some 4 weeks later. the :t-spectrum can be clearly separated. Figure 2 depicts two 226Ra ~t-spectra, one acquired after the separation and the other after 4 weeks. The 2'4po or-peak can be unambigously identified.
speaking, for large t, when secular equilibrium is reached, 6226(0 ~ 4. =6Ra can also be determined by counting 2"Po as explained above. In this case, an ('Ra can be defined as:
Calibration
The integral of the u-spectrum, or gross count rate, N, is converted into activity through the total efficiency, ~Ra. It is defined as: ~R~ = Y x G
(2)
where Y is the radiochemical yield and E,, the counting efficiency. The total efficiency, eu~, has been calculated by using a standard 226Ra solution of accurately known activity. Thus, several 0,5 L samples of distilled water were spiked with known 226Ra activities, d=6. After separation, according to the method described in Section 2, the final solution was measured with our system, either immediately or some weeks later when the a6Ra descendants reach secular equilibrium. If the total count rate obtained in the integral spectrum is N,, with the optimum PSA, ERa, becomes: N~
(3)
ERa - - (~226(/) X A226
~226(/) being the correction to N~ due to the contribution from the u-emitter descendants to the count rate. ~226(l) is calculated by using the Bateman's equations and its value will depend on the time, t, when the measurement is performed. Roughly ARI 48,'4~D
Npo
~'R~ = ~
(4)
where Npo is the integral of the 2t4po peak calculated when secular equilibrium is reached. Table 1 presents the results obtained for ¢R~,,with measurements made immediately after separation or some 4 weeks later, and E'R~. Lowest total efficiency data were obtained in the first essay. The increasing of these values was produced by more careful analysis. Surely, the diferences are due to a non-dissolution with EDTA. We also give some interesting additional data from the experiments performed. The agreement among the ca, values obtained with both methods is clear. This means that 6226(t) is correctly estimated, as is confirmed in Fig. 3, where the theoretical and experimental activity growth of one of the 226Ra standard sample is depicted. Such agreement also confirms that no losses of -'22Rnfrom the vials occur, which is very important as the reproducibility of the method is concerned. It is also clear that, E'R~< E~. This result is a not surprising as it has already been reported in the open literature (Salonen, 1990), Effects such as ionization quenching due to the high energy or-particles emitted by 214p0, and guard counter switching off by 2'4Bi
538
G. Manj6n et al.
Table 1. -~-'6Ratotal efficiency(Q and background (B) for the different methods used for activity determination. Lower limits of detection (LLD), when counting time (T) is 600 min and the sample volume is 500 mL are also given. LLD = 2x//2"~ tz (dpm) Counting window Immediately measured Measured some 4 weeks after separation Total ~-spectrum Total ~t-spectrum 214po~t-peak Background 0.45 _ 0.05 cpm 0.45 _+0.05 cpm 0.04 + 0.02 Total efficiency 55 _+3% 56 _+3% 40 __.2% 69+4 67+3 48+2 69_+4 66+3 41 _+7 68_+4 67+3 47+3 Mean value Mean value Mean value 65_+2 64_+2 44+2 Lower limit of detection 0.05 dpm 0.01 dpm 0.03 dpm 1.8 mBq/L 0.5 mBq/L 0.9 mBq/L
7-radiation could be the main reasons for such a low E'Ra value. Furthermore, as seen in Fig. 2, part of the counts produced by 2~4p0 can be lost when integrating the a-peak due to the low resolution of the spectrometer. A final comment on the above results is that 2~°Pb is also coprecipitated with Ba and dissolved with E D T A . In fact, we have found in some cases a clear contribution from some low energy fl-emitters in the fl-spectrum. This fact should be taken into account if the Ra sample is going to be measured 2 or 3 months after the chemical separation, since significant amounts of 2~°Po could have been produced during this time.
A Study Case: Ra-isotopes in Mineral Waters As an application of the Ra determination method just described, we analysed a set of mineral waters, commercially available in Spain, plus some samples
obtained from other drinking water sources. The geographical origin of the samples covers practically the whole map of Spain. Some 0.5 L of water was used for each analysis. The final samples were measured during two counting time cycles each, of 600 min. Table 2 gives the 226Ra and Z24Ra activity concentrations in m B q / L for the samples analysed. Errors were quoted considering 2 SD. It is clear, and was expected, that the 224Ra activities are lower than those of 226Ra and, in some cases, less than the limit of determination for our counting system. The highest 2Z4Ra and 2Z6Ra activity concentrations were found in the same sample, Mondariz-2, a mineral water coming from the Northwest of Spain. A daily consumption of 2.2 L of these waters by a standard person would produce an annual ingestion from 0.8 to 8.8 Bq, for 224Ra, and from 0.8 to 214 Bq, for 226Ra. This is well below the annual intake limits for 224Ra and 2Z6Ra, 3 × l& Bq and 7 x 103 Bq
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Fig. 3. 226Rabuild up theoretical curve and experimental data. No losses of radon from the vial can be inferred from this experiment.
Determination of 226Raand 224Rain drinking waters Table 2. -'24Raand -'26Raactivityconcentration (mBq/L) obtained with the presentedmethodin a set of commercialand environmental water samples Samples ::'Ra (mBq/L) 226Ra(mBq/L) Lanjaron-2 Fontemilla-2 Fontvella-I Fontvella-2 Viladrau-I Viladrau-2 Vallecardoo2 Liviana-I Sanchis-I Fonxesta-2 Mondariz-2 Fontcelta-I Regas-I Emasesa-I Gelves-I Osuna-I Osuna-2
< I 2 _+ I 2.3 + 0.7 < 1 < 0.5 1.3 + 0.7 < I < 0.5 < 1.2 < 0.7 11 + 2 < 0.8 4.2 + 0•8 < 1•4 < 0.7 1.0 + 0.7 < 1
3+ 1 2.7 +_ 0.7 8 + I 9 _+ 1 6.3 + 0.7 5+ 1 5.0 + 0.7 < 0.7 11.3 + 0.7 < 0.7 267 + 3 4 + 1 19 + 1 1.3 + 0.7 4.7 + I 5.3 + 0.7 5+ 1
for 224Ra and 226Ra respectively, according to the Spanish regulations (Boletin Oficial del Estado, 1988). The last three samples in Table 2 correspond to underground waters consumed directly by the population. Their radiological significance as far as Ra is concerned is negligible. The sample EMASESA-I represents the waters that supply the town of Seville. Its 226Ra level, 1.3 mBq/L, should be compared with typical levels of some 20 mBq/L obtained, during this time period, in the river from which the drinking water is taken (Martinez-Aguirre et al., 1991). The efficiency of the water treatement system is apparent. Finally, the results presented in Fig. 4 clearly show the agreement between both methods of 226Ra
539
determination, when the secular equilibrium is reached, some 4 weeks after separation. There, we plot the 226Ra activity (dpm) of the 0.5 L water samples obtained by integration of the 2'4po or-peak and by integration of the whole spectrum. It is clear that both methods give compatible results. The only exception is that of sample REGAS-I which give 0.59 + 0.05 dpm with the first method and 0.92 + 0.03 dpm with the second. The difference could be explained by the presence of 2t°Po in the sample• Indeed, it is known that -'~°Pb coprecipitates as sulphate. This results in the appearance of its daughers in the sample after some time. Some of them, 2'°Bi, 2"Pb and 2t4Bi, in addition to 2'°Pb, are fl-emitters and their contribution to the fl-spectrum should be measurable. This occurred in the previously mentioned water sample, REGAS-I, where fl-activity was observed. This supports the hypothesis of 2~°pb precipitation and consequent ingrowth of its daughters, including 21°po.
Conclusions
In this work, a method is given for the measurement of Ra-isotopes in waters by liquid scintillation counting. Ra is recovered by co-precipitation with Ba as sulphate. The precipitate is dissolved with EDTA. The study of the time evolution of the total activity gives the isotopic composition of the extracted Ra. Thus, both 226Ra and 224Ra activities are determined in the water sample. The method has been applied to Spanish drinking
0,80,7 -
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0,3. 0,2~ 0,1
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0,1
0,2
0,3 0,4 0,5 0,6 Integral speclrum (dpm)
0,7
0,e
0,9
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Fig. 4. 226Ra activities (dpm) calculated by integration of the 2'4Po or-peak vs 226Ra activities (dpm) calculated by the integration of the total spectrum. Results correspond to measurements made 4 weeks after separation.
G. Manj6n et al.
540
waters. N o special radiological significance of the studied waters has been found.
References Boletin Oficial del Estado (BOE) (1988) BOE No. 13/15-01-88. In Spanish. Buheitel, F. (1993) The determination of low-levels of radium isotopes and radon by a simple delayed-coincidence liquid scintillation spectroscopy method. In Liquid Scintillation Spectrometry, eds J. E. Noakes, F. Sch6nhoffer and H. A. Polach, pp. 83-88. RADIOCARBON, Department of Geosciences, The University of Arizona, Tucson, Arizona. Martinez-Aguirre, A., Mor6n, M. C. and Garcia-Le6n, M. (1991) Measurements of U- and Ra-isotopes in rainwater samples. J. Radioanalyt. Nucl, Chem. 152, 37-46. Mor6n, M. C., Garcia-Tenorio, R., Garcia-Montafio, E.,
Garcia-Le6n, M. and Madurga, G. (1986). Int. J. Rad. Appl. Instrum. Part A 37, 383. Salonen, L. (1990) A rapid method for monitoring of uranium and radium in drinking water. 6th International Symposium on Environmental Radiochemical Analysis, pp. 23-25. The Science of the Total Environment, Manchester. Sfinchez-Cabeza, J. A., Pujol, Ll., Merino, J., Le6n, L., Molero, J., Vidal-Quadras, A., Schell, W. R. and Mitchell, P. I. (1993) Optimization and calibration of a low background liquid scintillation counter for the simultaneous determination of alpha and beta emitters in acqueous samples. In Liquid Scintillation Spectrometry, eds J. E. Noakes, F. Sch6nhoffer and H. A. Polach, pp. 43-50. RADIOCARBON, Department of Geosciences, The University of Arizona, Tucson, Arizona. Sch6nhofer, F. (1994) The radon problem. In Low-Level Measurements of Radioactivity in the Environment, eds M. Garcia-Le6n and R. Gareia-Tenorio, pp. 227-240. World Scientific, Singapore.