sequential method for the determination of uranium, thorium and 226ra by liquid scintillation alpha...
TRANSCRIPT
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: [email protected] (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