Sequential method for the determination of uranium,thorium and 226Ra by liquid scintillation alpha

spectrometry

M.P. Blanco Rodrõ gueza, F. Vera Tome a,*, J.C. Lozanob, V. Go mez Escobarc

aDepartamento de FõÂsica, Facultad de Ciencias, Universidad de Extremadura, Campus Univer. Avda. de Elvas s/n,

06071 Badajoz, SpainbLaboratorio de Radiactividad Ambiental, Facultad de Ciencias, Universidad de Salamanca, 37008 Salamanca, Spain

cDepartamento de FõÂsica, Escuela PoliteÂcnica, Universidad de Extremadura, 10071 CaÂceres, Spain

Accepted 24 June 1999

Abstract

A new procedure for the determination of uranium, thorium and 226Ra from the same aliquot of an aqueoussample using extractant scintillators and liquid scintillation alpha spectrometry is proposed. The procedure is

designed such that the same aqueous phase can be used in all the stages, with slight modi®cations. The procedure isthus very simple, requiring little manipulation of the sample. Testing of the procedure was performed obtainingsatisfactory results and high reproducibility. # 2000 Elsevier Science Ltd. All rights reserved.

1. Introduction

The behaviour of natural radionuclides in the en-

vironment is an important part of the general study of

radionuclide migration, and may even be taken as ana-

logous to the possible behaviour of radionuclides in

the case of an accident at a radioactive waste reposi-

tory or at a nuclear fuel plant. Some natural radio-

nuclides of concern in this study are uranium, thorium

and radium, since the concentration of these elements

at each site can lead to an understanding of the mobil-

ization mechanisms. Some examples of these studies

can be found in the recent literature (Greeman and

Rose, 1990; Sarin et al., 1990; Jurado Vargas et al.,

1995; Jurado Vargas et al., 1997).

The technique commonly used for the determination

of these radionuclides is alpha spectrometry with passi-

vated implanted planar silicon (PIPS) detectors

(Glover, 1984). The advantage of this technique is the

good energy resolution, but it has some drawbacks,

since it requires extensive chemical manipulation and

very long measurement times.

As an alternative, liquid scintillation counting (LSC)

may be used in that it has the advantage of providing

detection e�ciencies that approach 100%. Also, with

the use of selective extractant scintillators (URAEX1,

THOREX1, RADAEX1, etc.), the extent of chemical

manipulation can be greatly reduced. A review of the

situation has been published by McDowell and

McDowell (1994). In our laboratory these procedures

have been applied to uranium (Go mez Escobar et al.,

1998) and radium (Go mez Escobar et al., 1999a) deter-

minations, obtaining good agreement in the compari-

sons with other alternative procedures such as alpha

spectrometry with PIPS detectors.

Applied Radiation and Isotopes 52 (2000) 705±710

0969-8043/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.

PII: S0969-8043(99 )00233-X

www.elsevier.com/locate/apradiso

* Corresponding author. Tel.: +34-24-289-524; fax: +34-

24-275-428.

E-mail address: fvt@unex.es (F. Vera Tome ).

In this present work, we propose a new sequentialprocedure for the determination of 226Ra, natural

uranium and thorium from the same aliquot of asample by using of extractant scintillators and LSC.Our proposal covers the determination of chemical

recoveries and interference levels by using internalstandards. This is an important aspect because liquidscintillation counting o�ers poor energy resolution, so

that the interference levels are very di�cult to establishin the same spectrum. Also, interference levels varybetween samples due to the di�erent degrees of

quenching.The following sections describe a detailed study

aimed at optimizing the sequential procedure. For this,synthetic samples spiked with well known amounts of226Ra, natural uranium and 230Th were used. The mini-mum detectable activity was evaluated for two di�er-ent sample volumes. Tests of the procedure were

performed, and its application to 226Ra, naturaluranium and thorium activity determinations for sev-eral types of environmental samples showed it to be

suitable for the determination of these radionuclides indi�erent kinds of samples.

2. Description of the sequential procedure

Fig. 1 shows the di�erent stages of the proposed

sequential procedure.Prior to the treatment, the aqueous sample was

heated to 608C and stirred for approximately 1 h in

order to assure the elimination of radon (Go mezEscobar et al., 1998). Afterwards 30 g of Na2SO4 perlitre of aqueous sample and an appropriate amount of

10 M NaOH was added to achieve a known excess ofNaOH (Go mez Escobar et al., 1999a). The sample wasthen transferred to a separation funnel with 7 mL ofRADAEX1 cocktail and vigorously shaken. After

standing for approximately 1 h, the aqueous phase wasdecanted to another separation funnel for further treat-ment (when the phases were not su�ciently separated,

the organic phase was centrifuged), and then 5 mL ofthe organic phase were transferred by pipette into alow-di�usion polyethylene vial for 226Ra LSC determi-

nation.The aqueous phase was then adjusted to pH � 1

with H2SO4 18 M. Then 6 mL of URAEX1 cocktail

were added and the mixture was vigorously shaken.The sample was left to stand for approximately 1 hand the aqueous phase was transferred to another sep-aration funnel for the next stage. An aliquot of 5 mL

of the well-separated organic phase was moved to alow-di�usion polyethylene vial for natural uraniumLSC determination.

Following this, 18 M H2SO4 was added to the aqu-eous phase to adjust the H2SO4 concentration to 5 M.

After 6 mL of THOREX1 cocktail were added, themixture was thoroughly shaken. After approximately 1

h, the phases were su�ciently separated and the aqu-eous phase was discarded and 5 mL of the organicphase was transferred a low-di�usion polyethylene vial

for thorium LSC determination.

3. Counting equipment

A low-level LKB Quantulus 12202 spectrometer

was used for the measurements of samples by theliquid scintillation technique. This spectrometer isspeci®cally designed for the determination of very low-level activities, using both an anticoincidence active

and a passivated shield and low background construc-tion materials. It also includes a pulse shape analyser(PSA), which separates pulses produced by alpha and

beta radiations into di�erent spectra. In the PSAregion of 80±105, an almost constant value of theminimum detectable activity (MDA) was obtained. In

this interval, beta events were practically absent in thealpha spectrum, while the registered alpha pulses werenot signi®cantly a�ected. A PSA level of 100 was

therefore selected for convenience.

4. Experimental results and applications

4.1. Interference and e�ciency determinations

In order to evaluate the interferences and e�cienciesin the proposed method, several samples with knownactivities were prepared by adding di�erent aliquots of

standard solutions of 226Ra, natural uranium and230Th to deionized water.In a previous paper (Go mez Escobar et al., 1999a),

the extractive procedure for 226Ra determination using

RADAEX1 cocktail was performed in a nitrate med-ium. In order to utilize this procedure for 226Ra deter-mination with the sequential procedure for the

extraction of uranium with the use of the URAEX1

cocktail, we have now studied the e�ciencies and inter-ferences of RADAEX1 cocktail in sulfate medium

(unfortunately, the nitrate medium is not suitable forURAEX1 cocktail). The variation of the extractione�ciency for radium, uranium and thorium at di�erent

pH values is shown in Fig. 2. As can be seen, theradium e�ciency is near 100% for pH ` > 14' and pH`>>14', and decreases sharply for lower pH values.This behaviour is similar to that previously observed

in nitrate medium (Go mez Escobar et al., 1999a). Theuranium e�ciencies are high for pH values between 5and 10, but fall rapidly outside these limits, whereas

the thorium e�ciencies are lower over the whole pHrange. Therefore, the optimum conditions for radium

M.P. Blanco RodrõÂguez et al. / Applied Radiation and Isotopes 52 (2000) 705±710706

Fig. 1. Sequential procedure for the determination of 226Ra, natural uranium and thorium from the same aliquot of an aqueous

sample.

Fig. 2. Variation of the extraction e�ciency with RADAEX1 cocktail for 226Ra, natural uranium and thorium versus pH value.

The conditions pH `> 14' and pH `>>14' represent excesses measured of NaOH 10 M.

M.P. Blanco RodrõÂguez et al. / Applied Radiation and Isotopes 52 (2000) 705±710 707

extraction with no interferences from uranium and

thorium were found at the so-called pH `>>14'. Theseconditions were obtained by adding 30 g of Na2SO4

and 30 mL of 10 M NaOH to the aqueous phase at

pH � 7:After the radium extraction, the aqueous phase is al-

kaline, but the optimum condition for uranium extrac-

tion by URAEX1 cocktail requires a solution pH of

01 (Go mez Escobar et al., 1998). The necessary salt

concentration has been attained in the previous stage.

Thus we only need to add 18 M H2SO4 to reach the

desired pH. In this previous paper, we showed that in

these conditions radium was an interference in

uranium determinations. This interference can be easily

eliminated by adding a certain amount of NO3ÿ. The

desired NO3ÿ concentration strongly depends on the

volume of sample treated: for instance, 0.025 and

0.00175 M were found to be optimal for 16 mL and

1 L samples, respectively (Go mez Escobar et al., 1998).

Finally, it is necessary to study the radium, uranium

and thorium extraction e�ciencies in sulfate medium

using THOREX1 cocktail. The results showed that

the extraction e�ciencies for uranium and thorium

were near 100% over practically the whole pH range,

decreasing at lower and higher pH. In order to mini-

mize the uranium interference, several samples were

prepared with di�erent H2SO4 concentrations. Fig. 3

shows the results for 16 mL of aqueous sample and 4

mL of THOREX1 cocktail. As can be seen, theextraction e�ciency of uranium decreases as the

H2SO4 concentration increases faster than the thoriume�ciency. The uranium extraction e�ciency is low,around 5%, in 5 M H2SO4, whereas the thorium

extraction e�ciency remains near 90%. A 5M H2SO4

concentration was chosen for THOREX1 extractionconditions in the proposed procedure. As uranium

levels in environmental samples are usually higher thanthorium ones, this interference (around 5%) can stillbe considered severe. Therefore, it should be deter-

mined for each sample. At 5 M H2SO4 concentration,the radium interference level was negligible (<1%).The average e�ciencies obtained following the

sequential procedure proposed here for 200 mL aqu-

eous samples were: (44 2 1)%, (61 2 2)% and (53 24)% for radium, uranium and thorium, respectively.

4.2. Evaluation of minimum detectable activity

The MDA of the proposed method was evaluatedusing (Currie, 1968)

MDA �Bq=L� � Ld

VTE60,

with

Ld � 2:71� 4:65����������CBT

p,

Fig. 3. Variation of the extraction e�ciency with THOREX1 cocktail for 226Ra, natural uranium and thorium at di�erent sulfuric

acid concentrations.

M.P. Blanco RodrõÂguez et al. / Applied Radiation and Isotopes 52 (2000) 705±710708

where V is the volume of sample (L), T (min) thesample measurement time (which is the same as for thebackground), E the e�ciency, and CB the backgroundcount rate (counts per minute) using a radiochemical

blank.The MDA values obtained for two di�erent volumes

of aqueous samples and with T � 600 min are given in

Table 1. These MDA values should be taken only as areference since the sensitivity of the determination isa�ected by other factors including interferences and

quenching. When the procedure is applied to environ-mental samples, the MDA should be evaluated foreach sample taking into account these factors (Currie,1968).

4.3. Test of the procedure

In order to test the procedure, several samples werebeen prepared with known activities of 226Ra, naturaluranium and 230Th added to 200 mL of deionizedwater. The results are given in Table 2. As can be

seen, the calculated activities were very similar to theadded activities. The results also show the reproduci-bility of the procedure in each determination where

spiked samples are measured.

4.4. Application to environmental samples

We used the proposed procedure in the determi-nation of 226Ra, natural uranium and thorium in sev-

eral types of environmental samples (water, soil,vegetable, etc.) collected in a disused uranium mine.

Aqueous samples were ®ltered in order to removethe impurities and suspended matter. The pH wasadjusted to 1 with H2SO4 in order to avoid the growth

of organic matter in the sample and to minimizewater-container wall interactions during storage. Thesolid samples, once at the laboratory, were pretreated

by microwave digestion in order to completely dissolvethe sample. The samples were then diluted to 200 mLwith deionized water.

Due to the very di�erent compositions and charac-teristics of the environmental samples treated, a widerange of quenching was observed in the organic phasesto be measured. As quenching severely a�ects the e�-

ciency, one needs to evaluate this e�ciency for eachsample. Also, the presence of di�erent ions and theirconcentrations in the samples can also a�ect the

extraction recovery. The anomalies observed in theevaluation of detector quenching led us to propose aninner standard method for the correction of the total

e�ciency and interferences in each sample (Go mezEscobar et al., 1999b). Following this method, eachsample was divided into four aliquots and three of

them spiked with 226Ra, natural uranium and 230Th re-spectively. From those spiked aliquots the e�ciencyand interferences in each determination (radium,uranium and thorium fractions) could be evaluated

and then used to correct the counts obtained in eachfraction of the unspiked aliquot in order to determinethe 226Ra, natural uranium and thorium activity in the

sample.The results obtained from the application of the

sequential method to di�erent environmental samples

are given in Table 3.

5. Conclusions

In this work a sequential procedure for radium,uranium and thorium determination is presented usingthe extraction solvents RADAEX1, URAEX1 and

Table 1

MDA values for radium, uranium and thorium for two di�er-

ent volumes of aqueous sample

MDA (Bq/L)

(V � 200 mL) (V � 1000 mL)

Radium 0.019 0.0068

Uranium 0.010 0.0025

Thorium 0.012 0.0028

Table 2226Ra, natural uranium and 230Th activity determinations for four samples. Quoted uncertainties are 1s

Sample Added activity (Bq/L) Calculated activity (Bq/L)

226Ra natural uranium 230Th 226Ra natural uranium 230Th

1 0.09920.001 0.21520.002 0.10220.001 0.10220.007 0.2220.02 0.1220.02

2 0.09920.001 0.21520.002 0.10220.001 0.10720.009 0.2120.02 0.0920.01

3 0.09920.001 0.21520.002 0.10220.001 0.10020.007 0.2320.02 0.1120.02

4 0.09920.001 0.21520.002 0.10220.001 0.09920.008 0.2420.02 0.1220.02

M.P. Blanco RodrõÂguez et al. / Applied Radiation and Isotopes 52 (2000) 705±710 709

THOREX1. The e�ciencies and interferences wereevaluated for each case. The interference values mustbe determined for each sample, especially the uraniuminterference with THOREX1 cocktail. The minimum

detectable activity for the procedure was evaluated.We checked the procedure using several syntheticsamples spiked with a known activity of each isotope,

and obtained satisfactory results. We also applied themethod to di�erent types of environmental samples.Therefore, this method is suitable for the determi-

nation of radium, uranium and thorium in the environ-ment. The principal advantage that this procedurepresents is rapidity and simplicity. It also permits the

radium, uranium and thorium activities to be deter-mined from a single aliquot. This procedure will befurther developed by testing it with certi®cated naturalmatrix reference materials.

Acknowledgements

Thanks are due to the Centro de Investigaciones

Energe ticas, Medioambientales y Tecnolo gicas(CIEMAT) and to the Empresa Nacional de Resõ duosRadiactivos (ENRESA), the Spanish national agencyfor radioactive waste management, for ®nancial sup-

port. (Acuerdo Especõ ®co de colaboracio n CIEMAT/UEX). P.B.R., F.V.T. and J.C.L acknowledge ®nancialsupport from the Consejerõ a de Educacio n y Juventud,

Junta de Extremadura.

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Table 3226Ra, natural uranium and thorium activities determined by using the proposed sequential extractive method for several types of

environmental samples. Quoted uncertainties are 1s

Sample Volume (L) or mass (kg) Activity (Bq/L, Bq/kg)

226Ra uranium thorium

Ground water 1 0.2720.02 0.09320.005 0.02620.002

Soil 2.7� 10ÿ4 746278 349211 720277

Grass 2.4� 10ÿ4 232213 12126 207217

Sediments 6.9� 10ÿ5 11102100 23502150 570240

M.P. Blanco RodrõÂguez et al. / Applied Radiation and Isotopes 52 (2000) 705±710710