Journal of Radioanalytical and Nuclear Chemistry, Vol. 254, No. 1 (2002) 181–185

Development of a method for the determination of 226Raby liquid scintillation counting

U. Repinc,* L. BenedikJozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia

(Received March 18, 2002)

Liquid scintillation counting has not been widely applied to α-particle detection because of its poor energy resolution and variable background. Inthe present work, a time saving and reasonably accurate method for determination of 226Ra in water has been developed, using liquid scintillationspectrometry and pulse-shape analysis. The effect of three levels of chemical quench on the spillover of alpha interactions into the beta windowand vice versa was assessed. The advantages of liquid scintillation in comparison with other methods (radon emanation) for determination of 226Raare the high counting efficiency (~100%) and the easier sample preparation, with no need for sample preconcentration.

Introduction is a technique that offers a distinct improvement in allthese areas. Sample volumes can be small (1 liter orsmaller), easier separation procedures are possible,counting times are reduced because detector efficienciesare close to 100%. Further, it is possible to measuresimultaneously both α- and β-emitting radionuclideswith identification of pulses using pulse-shape analysis(PSA).7,8

Radium is a naturally occurring radioelement ofparticular interest in environmental samples. Among thefour natural isotopes of radium, the α-emitter 226Ra isthe most significant. Uranium and the long series of itsprogeny, among which 226Ra is of great importance, aredistributed in the earth’s crust and can occur in naturalwater bodies from the rocks and soil with which watercomes in contact. They enter the human body mainlythrough food and water, and can cause a significantincrease in the internal radiation dose.1 Many methodshave been developed for determination of 226Ra inwater. Some are direct ones, based on determination of226Ra, most of them are indirect ones, based ondetermination of its descendants. Sorption-emanation isan often used technique, based on determination of226Ra via 222Rn, after equilibrium has beenestablished.2,3 However, some undesirable aspects areassociated with this method. Large sample volumes arerequired when low activity samples are analyzed, theradon in-growth necessitates long waiting periods andthere is no internal control to assess the quality ofindividual results. This method is still widely applied formonitoring radioactivity in the environment. For low-level concentrations of 226Ra alpha-spectrometry isused.4–6 This technique requires preparation of acounting source of negligible thickness, usually byelectrodeposition. Such methods are direct, highlysensitive, and highly specific, and also decrease samplesize and reduce time compared to the sorption-emanationmethod. However, several problems also occur using α-spectrometry. One of them is isolation of radium fromchemically similar (Ca, Ba, Sr) elements and the other isseparation from thorium radioisotopes in order to avoidany interference in the energy spectrum. Direct methodscommonly use precipitation of radium with bariumsulphate. On the other hand, liquid scintillation counting

The greatest environmental risk in uranium mining isrepresented by rainwater, which seeps through the pileand leaches the constituents. Dissolved radioactivematerial can contaminate ground waters and local watersystems. From the radiological point of view it is for thisreason necessary to determine 226Ra in effluents frommine tailings and waste disposal sites. The purpose ofthe present study was to develop a time saving methodfor the determination of 226Ra in water samples for theenvironmental monitoring programme around formeruranium mine.

Experimental

Instrumentation

For the determination of 226Ra in aqueous samples aPackard Tri-Carb 2550TR/AB Liquid ScintillationAnalyzer was used. This instrument has no passive oractive shielding; the background is reduced by applyingPSA and BCC (Burst Counting Circuitry) features.9 Weoptimized the pulse discrimination capabilities of thedetector to achieve the best α/β separation, to reducebackground and potential interferences in α-MCA. Thus,in theory, the α events should be sent to onemultichannel analyzer (MCA) and the β-events to asecond, obviating the need for overlap calculations. Thebackground values, because of the lack of shielding,were quite high (~3–5 cpm, 226Ra window).

* E-mail: urska.repinc@ijs.si

0236–5731/2002/USD 17.00 Akadémiai Kiadó, Budapest© 2002 Akadémiai Kiadó, Budapest Kluwer Academic Publishers, Dordrecht

U. REPINC, L. BENEDIK: DEVELOPMENT OF A METHOD FOR THE DETERMINATION OF 226Ra

scintillator was added. Before direct measurement, thescintillation solutions were pre-count delayed (30minutes of adaptation in the dark and thermostatted inthe counting chamber). After conditioning the solutionswere counted immediately after separation, when onlythe photopeak due to 226Ra (4.78 MeV) was observed.The sensitivity of determination could be increased byre-counting after secular equilibrium of 226Ra with itsdescendants had been established, measuring the total α-activity of the scintillation solution.

After sampling, water was acidified and stored inpolyethylene bottles. To 1 liter of water carrier (10 mgof Ba) and tracer 133Ba (15 Bq) was added and theseparation procedure was performed. The detailedanalytical scheme is shown in Fig. 1. We preparedstandards with a known activity of 226Ra for empiricaldetermination of the counting efficiency using only thepeak from 226Ra. The same separation procedure as forsamples was performed and the counting efficiencydetermined for each sample from the standard set. Themean value of the counting efficiency was used in furthercalculations.12

Results and discusionFig. 1. Separation scheme for radium

PSA optimization13,14

The discrimination of α and β pulses is based on thewell-known difference between the delayed componentof their fluorescence decay. Although the most effectiveuse of PSA is for alpha-counting, the technique has alsobeen used for background discrimination. Theinstrument’s PSA feature compares the delayedcomponent of the pulse with its total area anddiscriminates the pulses using a software-controlledparameter that varies from 1 to 256. On Packard-TriCarb models with α/β discrimination, the optimumsetting is the setting where there is an equal andminimum spillover of alpha-pulses into the beta MCAand beta-pulses into the alpha MCA. Since theinstrument has no facility for background subtraction, wecompared the automatically determined PDD value withthe value we determined manually using different α andβ standards. The determination of the optimum PDDrequires two standards: a pure α and a pure β emitter,with energies similar to the radionuclide to bedetermined. First, we used an 241Am standard(α-emitter) and then a 90Sr/90Y standard (β-emitter).Second, as a check, we compared the determined PDDvalue with the value we obtained using a 234Th standardas the β-emitter. Standards were prepared by mixing1 ml of standard solution with 10 ml of scintillationcocktail. The actual activities of the standard are of noimportance in determination of the optimum PDD value.The results are presented in Fig. 2.

The 2550TR/AB instrument has the feature ofautomatically calculating the optimum pulse-decaydiscrimination (PDD) time setting, but has no facility forbackground subtraction in its misclassification routine.Since the background of our instrument was notnegligible, we calculated the optimum PDD valuemanually, taking into account the background values.

Reagents

Barium chloride carrier solution (10 mg/ml),potassium carbonate solution (50% w/w), 0.5M HNO3.133Ba tracer was purchased from Czech MetrologicalInstitute, 226Ra and 241Am standard solutions werepurchased from NIST. 234Th solution was prepared fromnatural uranium nitrate as described in the EMLProcedures Manual.10

Radium separation and counting

The analytical scheme was adapted from CASE andMCDOWELL.11 The procedure is based oncoprecipitation of radium with barium sulphate,conversion to more soluble carbonate and dissolution ofnitrate in water. 133Ba was added as yield tracer to allowthe determination of the chemical yield in theprecipitation and metathesis to carbonate steps, whichare not strictly quantitative. After the separation, the

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U. REPINC, L. BENEDIK: DEVELOPMENT OF A METHOD FOR THE DETERMINATION OF 226Ra

Fig. 2. α/β misclassification plot; 241Am, 90Sr/90Y, PDD = 141, automatically obtained (a), 241Am, 90Sr/90Y, PDD = 150,corrected for background (b), 241Am, 234Th, PDD = 133, automatically obtained (c), 241Am, 234Th, PDD = 145, corrected for background (d)

Fig. 3. Influence of quenching on misclassification of α and β events at different pulse-discriminator values

As an optimum value for the PDD parameter we usedthe value of PDD=149, when the misclassification of α/βevents was 6–8%.

the endpoint of the transformed external standardspectrum. It has a range of 0–1000, where 1000 indicatesno sample quenching. In most cases it has been foundthat quenching in our samples can vary by 50 tSIE units.Quenching can affect not only counting efficiencies butalso the α/β separation efficiency. We examined theeffect on α/β misclassification. We prepared 241Am and234Th standards using 1 ml of standard solution and10 ml of scintillation cocktail. The effect of quench wasstudied by adding small quantities of CCl4 to thescintillation solution. For each sample we measured the

Effect of quench on α/β misclassification9,15

Quenching in the counting solution alters the shapeand the position of the signals. In the Packard2550TR/AB, the tSIE parameter (transformed SpectralIndex of External Standard) is a parameter defining thedegree of sample quenching. The tSIE is a function of

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U. REPINC, L. BENEDIK: DEVELOPMENT OF A METHOD FOR THE DETERMINATION OF 226Ra

spectrum of an external 133Ba standard, which waspositioned below the sample and determined the tSIEparameter.

Table 4. Results of 226Ra activity in drinking and mineralwater samples

Water sample 226Ra, Bq.m–3

For fixed quenching we observed that an increase inthe pulse-shape discriminator value caused the αinterference to increase and β interference to decrease,as expected (Fig. 3).

Drinking water 9.9 ± 1.0Zala – natural mineral water 23.9 ± 2.5Radenska – Radin Spring 59.2 ± 2.5Radenska – Miral Spring 43.5 ± 2.5

Determination of chemical yield (Y) and countingefficiency (η) of the detector

For the yield determination of 226Ra we used 133Ba(Eγ = 356 keV, 62.2%) tracer. After the separation wemeasured the γ activity of the sample and standardsolution in similar geometry. The values of the chemicalyield found were 99±8%. For the determination ofcounting efficiency we prepared standard samples withknown concentrations of 226Ra from 10 to 58 Bq/l. Afterseparation we measured the chemical yield of eachsample, added scintillator and measured the scintillationsolution on the liquid scintillation detector. The resultsare presented in Table 1.

Fig. 4. Addition of 226Ra standard to real water sampleand determination by liquid scintillation counting

Table 1. Determination of the counting efficiency (η) of the detector;PDD = 149, t = 30 min

The minimal detectable activity was calculated foreach particular sample, using the count data of thatsample and the reagent blank sample, counted under thesame conditions. In order to calculate meaningfuldetection limits, we used CURRIE’s formula:16

Astd, Bq/l Ameas, Bq/l Y, % η, %

10.0 ± 0.5 8.5 ± 4.4 90.1 94.314.8 ± 0.7 13.2 ± 4.5 88.7 100.823.1 ± 1.2 20.2 ± 4.6 86.8 100.834.3 ± 1.7 29.0 ± 6.4 79.6 106.236.3 ± 1.8 31.0 ± 6.5 88.5 96.539.3 ± 2.0 37.4 ± 6.6 96.4 96.458.5 ± 2.9 60.0 ± 6.8 99.8 99.8

MDA =2.71+ 4.66 B t

Y t V

⋅⋅ ⋅ ⋅J

We considered another factor that influences theMDA, that is the counting time. Depending on thechemical yield, counting efficiency and backgroundcount rate, a longer count time results in a lower MDA.Results are presented in Table 2.Table 2. Minimal detectable activity (MDA), PDD = 149, <Y> = 90%,

<η> = 99%

Standard additiont, min tSIE B, cpm MDA, Bq/m3

1 259 557.2 6.5 14.02 345 580.7 3.9 9.43 509 548.7 2.8 6.64 720 552.9 4.6 7.05 900 514.6 2.5 4.6

Different amounts of 226Ra standard with an initialactivity of 1.3 Bq/g were added to an environmentalwater sample. Separation was performed following theseparation scheme in Fig. 1. Standards were immediatelycounted on the LSC together with an appropriate blank.The count rate of the blank was subtracted from thestandards. Neglecting the effect of quench on thespectra, we used a fixed counting window (150–300) and100% counting efficiency for the calculation of theresults presented in Fig. 4. The results show satisfactorycorrelation between the activity added and thatmeasured.

Table 3. Comparison of the results of 226Ra activity in water samplesnear the Z

∨irovski vrh uranium mine

Sample Liquid scintillation Sorption emanationcounting, Bq.m–3 of radon, Bq.m–3

1 13.3 ± 3.5 16.2 ± 1.62 10.2 ± 3.5 10.0 ± 1.23 10.1 ± 1.9 9.5 ± 1.24 14.1 ± 2.0 12.5 ± 1.35 11.7 ± 1.9 13.3 ± 1.36 8.6 ± 1.0 13.6 ± 1.6

We tested the method by comparison with thesorption-emanation method. Results for 226Ra activity inwater samples collected near the Z

∨irovski vrh uranium

mine are shown in Table 3.

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U. REPINC, L. BENEDIK: DEVELOPMENT OF A METHOD FOR THE DETERMINATION OF 226Ra

We also measured the activity of commerciallyavailable mineral waters and tap water from theLjubljana water supply by the LSC method. The resultsare presented in Table 4.

*

This work was financially supported by Ministry of Education,Science and Sport of Slovenia (Project Group P-0532-0106). TheDepartment of Organic and Physical Chemistry is gratefullyacknowledged for data obtained by the sorption emanation method.

Conclusions

ReferencesThis study was performed to evaluate the benefits of

liquid scintillation counting for determination of 226Racompared to the sorption-emanation method. The resultsin general show good agreement between these twomethods, taking into consideration the uncertaintiesattached to the analytical results. The only exception issample 6 where there is some disagreement. However,these values for radon are low and no systematicdifferences are apparent. In comparison with sorption-emanation, much smaller water volumes were used andthe time required to obtain the results was only 2 days.The variable background results in a rather highuncertainly of the results obtained with LSC. Certainly,factors that influence the background have a profoundeffect on the detection limit. With improvements tolower the background and diminish its variability, wewould increase the sensitivity of our instrument and themethod. Added features such as passive and activeshielding and background pulse discriminationelectronics would reduce the background and lower thedetection limit. For this reason, the method with thepresent instrument is most suitable for relatively highactivity samples, e.g., from contaminated areas, wheredeviation of the background does not have a significanteffect.

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