Determination of 222Rn and 226Ra in aqueous samples using a low-level liquid scintillation counter

Appl. Radiat. Isot. Vol. 47, No. 9/10, pp. 861-867, 1996 Copyright © 1996 ElsevierScienceLtd Printed in Great Britain.All rights reserved PII: S0969-8043(96)00076-0 0969-8043/96 $15.00+ 0.00

Pergamon

Determination of 222Rn and 226Ra in Aqueous Samples Using a Low-level Liquid Scintillation Counter V. G O M E Z

E S C O B A R * ' , F. V E R A TOMI~ l, J.C. L O Z A N O 2 a n d A. M A R T I N S A N C H E Z 1

~Departamento de Fisica, Universidad de Extremadura, E-06071 Badajoz, Spain and ~-Laboratorio de Radiactividad Ambiental, Facultad de Ciencias, Universidad de Salamanca, 37008 Salamanca, Spain A method for measuring -nZRnand -'-~#Rain aqueous samples has been studied. The technique uses low-level liquid scintillation counting from vials containing an insoluble high efficiency mineral oil scintillation cocktail. A Wallac 1220T M Quantulus LS counter was used for all the experiments. The optimum pulse-shape discrimination value was evaluated by using the minimum detectable activity criterion for each condition of measurement. Experiments on the optimum volume and sample-cocktail ratio, the influence of shaking, type of vial and diffusion of radon from vials were carried out. The study was applied to the determination of very low levels of z2-'Rn and -'-'6Rafrom aqueous environmental samples. The results reached using this method were very satisfactory and even better than from other more laborious procedures. Copyright © 1996 Elsevier Science Ltd

Introduction Liquid scintillation counting (LSC) is an effective technique for the determination of radionuclides. This technique dates back to the end of the forties and beginning of the fifties (Reynolds et al., 1950). It was first developed for beta counting, being now the most important tool used for the assay of beta emitting radionuclides. It was later expanded (Horrocks, 1964) for the determination of alphaparticle emitting nuclides due to the advantages of high counting efficiency (nearly 100%), easy sample preparation, and easy automatization (a review is given by McDowell and McDowell, 1993). Compared with other techniques, LSC has, however, some handicaps such as the poor energy resolution [-300 keV with equipment designed esPecially for alpha spectrometry (McDoweU, 1992)]. There has lately been increasing interest in the study of-'22Rn and 2-'6Ra levels in water samples, due to their potential danger. The determination of 222Rn is usually done by mixing with a gel-type scintillation cocktail (Sch6nhofer and Henrich, 1987; Salonen, 1988), or by extracting with a selective scintillator (Sch6nhofer, 1992; McDowell, 1992). The determination of 226Ra is usually performed indirectly through the daughter -'-'-'Rn emissions, using the above methods. An appropriate technique for the assay of low-level -'2-'Rn and ~6Ra samples in short *To whom all correspondence should be addressed.

counting times would be very convenient. It should include an easy and fast procedure for the preparation of samples. In this present work, a method for measuring 22-'Rn and ~rRa in aqueous samples using an extractive selective cocktail has been studied.

Experimental Procedure Counting equipment For the measurement of the arRa and 222Rn concentrations in water samples, a low-level Wallac 1220 T M Quantulus LS spectrometer was used. It is designed to measure very low-level activities by its use of an active shield in anticoincidence and the materials selected for its construction. It is equipped with a pulse-shape analyser (PSA) which separates pulses caused by alpha or beta decays into different spectra. Sample preparation Alpha sources for measurement were prepared by adding different quantities of a standard solution of ~rRa provided by C I E M A T with nominal activity 181 _+ 2 Bq/mL to 10 mL of distilled water. Solutions so prepared were added (Pilchard and Gesell, 1983) to vials containing 10 mL of the insoluble high efficiency mineral oil scintillation cocktail (NEF-957G, N E N Research Products) (Barnett and McKlveen, 1992). The first measurement of the sample was immediately after preparation. After

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shaking, only 222Rn dissolved in the water is extracted into the organic phase (cocktail) producing scintillation. After radioactive equilibrium (1 month if no very high concentrations of 2-'2Rn are initially present in the sample), the same sample was measured again (Chereji, 1992). In this case ~'Rn in equilibrium with 2-'6Ra is measured and so the -'26Ra present in t h e sample can easily be determined. The studies were performed using low-diffusion PE-vials (Packard) and glass vials (Packard). Samples with different cocktail/sample ratios were studied. The volume ratio 10/10 was found to give the best compromise between background and overall

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alpha efficiency. The ratio 5/15 was more efficient but with relatively high background, whereas the ratio 15/5 presented a better background but lower efficiency. geasuremen t

The LSC alpha spectra are taken, transferred to a PC through an RS 232 C serial interface and stored on hard or floppy disks for subsequent analysis. Figure 1 shows two -'26Ra spectra measured 1 month after sample preparation. Resolution is better for the low diffusion PE-vial. No 226Ra can be extracted, which is demonstrated by the absence o f the 226Ra peak.

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Figure 2 shows the growth of activity in a spectrum from a standard sample measured for up to nearly 70 days following the preparation of the samples. The activity of the sample becomes practically constant (95% of the equilibrium value) after 17 days. Nevertheless, we decided to measure all the samples after a waiting period of 1 month. Results

shaking the sample before measurement to help the extraction of "'-'Rn from aqueous to organic phase. To verify this assumption, evolutions of two 2:6Ra standard sources were studied, one with shaking at the time of preparation and the other without. The results were similar in the two cases, showing that the diffusion of '-2:Rn in the organic phase is independent of shaking the vial during the preparation of samples.

Influence of shaking

Diffusion of radon

Some authors (Irlweck and WaNner, 1993), using a similar sample preparation procedure, recommend

In order to study the diffusion of radon out of the low-diffusion PE-vial, five standard sources were

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prepared. After 1 month, part of the organic phase was transferred to another vial, where more scintillator was added. If there is no diffusion of 222Rn, the activity must decay following the 222Rn half life. Figure 3 shows an example for the decay of the activity in one of the samples. For each sample four curves were fitted (two windows with and without PSA). From the 20 values, the half-life was calculated giving 3.73 + 0.15 days. The comparison with the value of 222Rn half-life (3.825 ___0.004 days) indicates that the diffusion from vials can be considered negligible. Similar results were reached for glass vials.

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Determination of 2Z~Rnand 226Rain aqueous samples where V is the volume (in litres), T is the background measurement time, E is the efficiency and Ca is the background count rate. Figure 4 shows the variation of MDA with the PSA value for the two kinds of vials used and for the two regions of interest considered. From these results, PSA values of 100 and 110 seem to be the best measurement conditions for low diffusion PE-vials and glass vials, respectively. For these values of PSA and for a measurement time of 200 min and 10 mL of sample volume, the average MDA for low diffusion PE-vials was 0.23 Bq/L for the total window and 0.10 Bq/L for the region of 2a4po, whereas for glass vials the values were 0.25 and 0.09 Bq/L respectively. For high PSA values, great variations in MDA were observed, probably due to the low counting time, although the optimun PSA value did not change.

diffusion PE-vials and glass vials, respectively. Under these conditions, samples with activities of about 0.10 Bq/L (PE-vials) and 0.09 Bq/L (glass vials) of 226Ra can be accurately measured using LSC without prior concentration. If the activity of samples is lower than the quoted MDA, the method can still be used for measuring 226Ra, after an initial concentration by evaporation avoiding precipitation of salts. For instance, a source from an environmental water sample was evaporated down from 1 L to 10 mL giving an activity of 69 + 4 mBq/L with 500 rain of counting. For comparison, this same sample was measured by alpha spectrometry with semiconductor detectors giving 71 _ 8 mBq/L, but in this case 5 L of sample and about 37,000 min were necessary in the determination. Acknowledgements--Thanks are due to ENRESA and

ENUSA (Project No. 0703401) for financial support.

Efficiency

With these chosen PSA values, the efficiency was evaluated by preparing several 226Ra standard sources with different activities. The calculated average efficiency was 37 + 2% for the low-diffusion PE-vial and 24 _ 3% for glass vials in the complete window, and 57 _ 3% and 43 _ 4%, respectively, in the 214po window. The variation of efficiency vs PSA showed that the efficiency was higher in the second region of interest for both kinds of vial (see Fig. 5). The difference between the efficiencies in the two regions became higher as the PSA increased, with the contrary behaviour at low PSA (due to the beta counts principally being registered in the first region). The efficiencies of the two windows in the alpha channel becomes zero at high PSA because all the counts are registered in the beta channel. Because the difference between the efficiencies in the two regions of interest indicates a different behaviour of PSA value with changing alpha particle energy.

Conclusions The method described in this work seems to be well-suited for measurements of 226Ra in aqueous samples. Measurements can be made after a waiting period of approx. 20 days, and diffusion of 222Rn from vials can be neglected. Moreover, shaking during the preparation of samples is not necessary. The PSA values of 100 and 110 can be chosen as optimal on the basis of the minimum detectable activity (MDA) reached for these values in low

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References Barnett J. M. and McKlveen J. W. (1992) The measurement of 222Rnin drinking water by low-levelliquid scintillation counting. J. Radioanal. Nucl. Chem. Art. 161, 357. Chereji I. (1992) 222Rn(226Ra) determinations in water by scintillation methods. J. Radioanal. Nucl. Chem. Lett. 165, 263. Currie L. (1968) Limits for qualitative detection and quantitative determination. Analyt. Chem. 40, 586. Horrocks D. L. (1964) Alpha particle energy resolution in liquid scintillators. Rev. Sci. lnstrum. 35, 334. Irlweck K. and Wallner G. (1993) Determination of 226Ra in lake sediments by liquid scintillation counting of cocktail-extracted 2~Rn. In Liquid Scintillation Spectrometry 1992 (Noakes J. E., Sch6nhofer F. and Polach H. A., Eds), p. 391. Radiocarbon, Arizona. McDowell W. J. (1992) Photon/electron-rejecting alpha liquid scintillation (PERALS~) spectrometry: a review. Radioact. Radiochem. 3, 26. McDowell W. J. and McDowell B. L. (1993) The growth of a radioanalytical method: alpha liquid scintillation spectrometry. In Liquid Scintillation Spectrometry 1992 (Noakes J. E., Schrnhofer F. and Polach H. A., Eds), p. 193. Radiocarbon, Arizona. Prichard H. and Gesell T. F. (1983) Radon-222 in municipal water supplies in the central United States. Health Phys. 45, 991. Reynolds G. T., Harrison F. B. and Salvine G. (1950). Liquid Scintillation Counters. Phys. Rev. 78, 488. Salonen L. (1988) Natural radionuclides in ground water in Finland. Radiat. Prot. Dosim. 24, 163. Sch6nhofer F. and Henrich E. (1987) Recent progress and application of low-level liquid scintillation counting. J. Radioanal. Nucl. Chem. Art. 115, 317. Sch6nhofer F. (1992) Measurement of Ra-226 in water and Rn-222 in water and air by liquid scintillation counting. Radiat. Protec. Dosim. 45, 123.