Comparison and Improvement of theDeterminations of Actinide Low Activities UsingSeveral r Liquid Scintillation Spectrometers

Nicolas Dacheux,*,† Jean Aupiais,‡ Olivier Courson,† and Cedric Aubert‡

Groupe de Radiochimie, Institut de Physique Nucleaire, Bat. 100, Universite Paris-Sud, 91406 Orsay Cedex, France, andDepartement Analyse Surveillance Environnement, Service Radioanalyses Chimie Environnement, CEA,BP 12, 91680 Bruyeres le Chatel, France

We applied three procedures using two r liquid scintil-lation spectrometers (PERALS and TRI-CARB) and twoscintillation cocktails (Alphaex and Ultima Gold LLT) forthe determination of r-emitter low activities. For eachprocedure, the limit of detection, the resolution, theseparation factor, and the Fischer coefficient were deter-mined in order to perform 232U-234U-238U isotopicmeasurements. The deconvolution usually performed isclean when the PERALS spectrometer is used. This is notpossible for the TRI-CARB spectrometer using the UltimaGold LLT scintillation cocktail. This problem was solvedby combining the advantages of both techniques using theAlphaex scintillation cocktail in the TRI-CARB spectrom-eter. Under these conditions, the limit of detection wasimproved, the resolution decreased from 500-800 to420-590 keV, and the separation factor increased from0.9 to 1.1-1.2. This third procedure was applied withsuccess for 232U-234U-238U isotopic experiments.

The determination of very low activities of nuclides in thebiosphere has become very important.1-16 Particular attention hasbeen paid, on one hand, to 241Am (t1/2 ) 433 years)17 and 237Np(t1/2 ) 2 × 106 years),18 which are decay products of 241Pu (t1/2 )14.4 years), and, on the other hand, to uranium and plutoniumisotopes. All these nuclides have been released via atmosphericweapon testing, nuclear power plants, and spent-fuel reprocessing.Among the nuclear techniques usually used to determine verylow activities, such as R and mass spectrometries, R liquidscintillation with reduction of the â-γ background has becomeimportant in the past several years.9,19-23 Among them, the photonelectron rejecting R liquid scintillation (PERALS) apparatus

combines in the same step the chemical extraction of actinidesby liquid-liquid extraction into the organic phase and themeasurement of the R activity by R liquid scintillation. Thistechnique allows the total rejection of the â emission (99.95%) bymeans of a pulse-shaped discrimination (PSD). PSD reduces thebackground induced by photoelectrons produced by ambient γ-rayactivity and also eliminates interference from â emitters coex-tracted with R emitters or produced by decay in the extractive-scintillating cocktails.

In our previous papers, we reported a general scheme forseparation of actinides that can be applied to solutions containingsimultaneously thorium, uranium, plutonium, americium, andcurium.12 We also reported the optimal conditions for extraction(pH, ions present in the solution, volume ratio between organicand aqueous solutions) to obtain the measurement of severalactinides such as polonium,24 radium,15 thorium,12 uranium,12

neptunium,25 plutonium,12,16 americium, and curium isotopes.12,14

To obtain a comparison of the PERALS spectrometer with morecommon conventional R-â scintillation instruments, we alsoperformed studies using the TRI-CARB spectrometer for whichthe activity measurement is rather different (in this case, a water-organic miscible cocktail is used).

The aim of this study was to evaluate the performances of bothmethods by determining the limits of detection, the resolutions,and the separation factors. Direct comparison of both techniquesis difficult because of the differences in the measurements andthe procedures required for the sample preparation. Nevertheless,we try to present, in this paper, the main advantages and

* Corresponding author. Phone: +33-1-69157342. Fax: +33-1-69157150.E-mail: dacheux@ipno.in2p3.fr.

† Universite Paris-Sud.‡ CEA.

(1) Cadieux, J. R.; Reboul, S. H. Radioact. Radiochem. 1996, 7, 30-34.(2) Cadieux, J. R.; Clark, S.; Fjeld, R. A.; Reboul, S.; Sowder, A. Nucl. Instrum.

Methods Phys. Res., Sect. A 1994, 353, 534-538.(3) Cadieux, J. R. Presented at the 7th Symposium on Radiation Measurements

and Applications, University of Michigan, Ann Arbor, MI, May 21-24, 1990.(4) Brown, D. D.; Fjeld, R. A.; Cadieux, J. R. Presented at the 39th Annual

Conference on Bioassay, Colorado Springs, CO, 1993.(5) McDowell, W. J. Report NAS-NS 3116; Nuclear Science Series on Radio-

chemical Techniques; U.S. Government Printing Office: Washington, DC,1986; pp 88-89.

(6) McDowell, W. J. Report NAS-NS 3116; Technical Information Center, U.S.Department of Energy: Oak Ridge, TN, 1986.

(7) Leyba, J. D.; Vollmar, H. S.; Fjeld, R. A.; Devol, T. A.; Brown, D. D.; Cadieux,J. R. J. Radioanal. Nucl. Chem. 1995, 194, 337-344.

(8) Bourlat, Y.; Millies-Lacroix, J. C.; Nazard, R. J. Radioanal. Nucl. Chem. 1995,197, 387-408.

(9) Zhu, Y. J.; Yang, D. Z. J. Radioanal. Nucl. Chem. 1995, 194, 173-175.(10) Yang, D. Z.; Gou, Y. F.; Zhu, Y. J. J. Radioanal. Nucl. Chem. 1995, 194,

177-183.(11) Rao, R. R.; Cooper, E. L. J. Radioanal. Nucl. Chem. 1995, 197, 133-148.(12) Dacheux, N.; Aupiais, J. Anal. Chem. 1997, 69, 2275-2282.(13) Boatner, L. A.; Sales, B. C. In Radioactive waste forms for the future; Lutze,

W., Ewing, R. C., Eds.; Elsevier Science Publishing: New York, 1988; pp556-558.

(14) Dacheux, N.; Aupiais, J. Anal. Chim. Acta 1998, 363, 279-294.(15) Aupiais, J.; Fayolle, C.; Gilbert, P.; Dacheux, N. Anal. Chem. 1998, 70, 2353-

2359.(16) Aupiais, J. J. Radioanal. Nucl. Chem. 1997, 218, 201-207.

Anal. Chem. 2000, 72, 3150-3157

3150 Analytical Chemistry, Vol. 72, No. 14, July 15, 2000 10.1021/ac9914216 CCC: $19.00 © 2000 American Chemical SocietyPublished on Web 05/27/2000

drawbacks of each system.It is possible to take advantage of combining the scintillating-

extractive cocktails usually used to perform the measurement bythe PERALS spectrometer with the activity determination usingthe TRI-CARB spectrometer. This allows a significant improve-ment in the limit of detection, the resolution, and the separationfactor. Under these conditions, the peak separation is better, andthis makes the deconvolution of peaks from several actinidespossible. The main results of this study are presented in this paper.

THEORYDetermination of the Limit of Detection. The determination

of the limit of detection is usually required to evaluate theperformance of each scintillation technique. Thus, we calculatedthe values obtained for the PERALS and TRI-CARB spectrometersfor several counting times. According to the literature, and takinginto account a Poisson distribution for the background, the limitof detection γd can be defined for R and â statistical risks equalto 2.5% as follows:26,27

where (∆c)s is the threshold of detection, Xbkgd is the backgroundcount number, and t is the counting time, which must be identicalfor the background and for the sample.

Determination of the Separation Factor RS. The perfor-mance of each technique was also evaluated by determining theseparation factor, RS, which represents the capability of separatingthe peaks. It can be calculated using the equation28

where Xi is the location of peak i (in energy) and fwhmi is thefull width at half-maximum of peak i (in energy).

This factor, which is dimensionless, must be distinguishedfrom the resolution, R (equal to the fwhm), which is homogeneousin energy and usually expressed in keV. The higher the valueobtained for RS, the better is the separation of peaks (it is theopposite for the resolution, R, which corresponds to the fwhm).It is usually observed that the separation of two peaks which arealmost the same in energy is good when the separation factor,RS, is higher than 1. For lower values, the separation of both peaksbecomes very difficult and the deconvolution process leads toinaccurate results.

Fitting Function and Determination of Peak Asymmetry.For all of the nuclides, the peaks obtained by PERALS or TRI-CARB spectrometry cannot be fitted using a pure Gaussianfunction because of the asymmetry observed. We assumed duringthe deconvolution process a Gaussian distribution for low-activitymeasurements because, under these conditions, the statisticalerror is higher than the fitting one. For higher activities, a bi-Gaussian peak can be considered. This function is based on twoGaussian functions with different standard deviations on the leftand on the right of the peak center position (xc). Its expressioncan be formulated as

where H is the peak height, xc is the peak location at maximumheight, σ1 is the width of the peak on the left side of the maximum(at 0.882H), σ2 is the width of the peak on the right side of themaximum (at 0.882H).

The peak asymmetry (Fischer’s coefficient) can be calculatedby the equation27

where m3 corresponds to the third-order moment (symmetryfactor related to the Gaussian peak) and σ is the width of thepeak at 0.882H (σ is equal to fwhm/2.354 and to 1/2(σ1 + σ2)).

The third central moment can be calculated by27

where f(x) is the function described in eq 3.

EXPERIMENTAL SECTIONSample Preparation. All nuclide solutions were stored in 4

M HNO3 at room temperature and were from our own laboratorysource. The specific activity was determined for each solution byR spectrometry.

PERALS Procedure. The photon electron rejecting R liquidscintillation spectrometer (model 8100) is an R liquid scintillationapparatus produced by Ordela Inc. (Oak Ridge, TN). It is basedon the use of liquid-liquid extraction by a scintillating-extractivecocktail that contains a scintillating and an extractive molecule.1,6

Among the extractive molecules usually used are bis(2-ethylhexy-l)phosphoric acid (HDEHP), amines such as (1-nonyldecyl)amineand tri-n-octylamine (TNOA), and organophosphorus compoundssuch as tri-n-octylphosphine oxide (TOPO). All can be obtainedfrom Etrac (Oak Ridge, TN). For each actinide and each extractivemolecule, it is necessary to determine a distribution ratio, D, whichrepresents the performance of each extraction step. These values,as well as the equations concerning the theory of liquid-liquidextraction,wereextensivelyreportedinourpreviouspapers.12,14-16,24,25

We earlier discussed the pH extraction range, the volume ratios

(17) Warwick, P. E.; Croudace, I. W.; Carpenter, R. Appl. Radiat. Isot. 1996,47, 627-642.

(18) McCubbin, K. S.; Leonard. Radiochim. Acta 1995, 69, 97-102(19) Salonen, L. Liquid scintillation spectrometry. Radiocarbon 1993, 35, 361.(20) Case, G. N.; McDowell, W. J. Talanta 1982, 29, 845-848.(21) Ong, C. G.; Prasad, A.; Leckie, J. O. Anal. Chem. 1995, 67, 3893-3896.(22) Case, G. N.; McDowell, W. J. J. Radioact. Radiochem. 1990, 1, 58-69.(23) Burnett, W. C.; Tai, W. Anal. Chem. 1992, 64, 1691-1697.(24) Veronneau, C.; Aupiais, J.; Dacheux, N. Anal. Chim. Acta 2000, 415, 229-

238.(25) Aupiais, J.; Dacheux, N.; Thomas, A. C.; Matton, S. Anal. Chim. Acta 1999,

398, 205-218.(26) Limite de detection d’un signal dans le bruit de fond. Application aux mesures

de radioactivite par comptage; Report CEM R-5201; CEN: Grenoble, France,1983.

(27) Neuilly, M. Statistique appliquee a l’exploitation des mesures; Masson: Paris,1986; pp 53, 113-117.

γd ) 2(∆c)s ) 4t(1 + x1 + 2Xbkgd) (1)

RS )X2 - X1

1/2(fwhm1 + fwhm2)(2)

f(x) ) H exp[-(x - xc)2/2σ1

2] for 0 < x < xc (3a)

f(x) ) H exp[-(x - xc)2/2σ2

2] for xc < x < ∞ (3b)

γ1 )m3

σ3 )m3

1/8(σ1 + σ2)3 )

m3

(fwhm/2.354)3 (4)

m3 ) ∫-∞

∞(x - xc)

3 f(x) dx (5)

Analytical Chemistry, Vol. 72, No. 14, July 15, 2000 3151

required between aqueous and organic phases, and the effect ofthe ions present in solution. On the basis of this experience, theextraction steps were performed in 1 M HNO3, 10-1 M HNO3,and 10-3 M HNO3, respectively, to obtain optimal recoveryconditions for the determination of thorium, uranium, and cu-rium-americium activities by a PERALS spectrometer.12,14 Forplutonium, a redox process is required to obtain quantitativerecovery during the liquid-liquid extraction step. It is based onthe reduction of plutonium to its trivalent state using ascorbic acid(1 g/L), followed by the oxidation of Pu(III) to Pu(IV) using asodium nitrite solution (1 g/L).12 In this work, we only presentthe results obtained using the Alphaex cocktail, which allows goodenergy resolution. This cocktail consists of a mixture containingHDEHP (64 g/L, i.e., 0.2 M), as the extractive molecule (purifiedby precipitation/recrystallization), naphthalene (180 g/L), andPBBO (4 g/L) in toluene.29-31

Each extraction step was performed according to the followingprocedure: A 50 µL-1 mL quantity of the spike solution wasintroduced into the culture tube, depending on the specific activity.The acidity and/or the acid concentration was adjusted byconsidering the optimal extraction conditions for each element.The final volume of aqueous solution was always equal to 5 mL.A 1.2 mL portion of the extractive-scintillation cocktail was addedto the tube (volume ratio between organic and aqueous solutions0.24). Both phases were equilibrated by shaking for 5 min andthen were separated by centrifugation for 5-10 min at 2000 rpm.A 1 mL aliquot of the organic phase was removed and deoxygen-ated by sparging with dry-toluene-saturated oxygen-free argon for5 min to improve the PERALS resolution. The culture tube wassealed, and its content was then measured with the PERALSspectrometer.

Under these conditions, the concentration factors of theactinides in aqueous solution can reach 170 (250 mL of aqueoussolution/1.5 mL of organic phase).12

TRI-CARB Procedure Using Ultima Gold LLT. The TRI-CARB spectrometer is an R-â liquid scintillation spectrometersupplied by Packard (Canberra Co., Meriden, CT) which usestime-resolved pulse decay analysis (TR-PDA) technology toseparate the R spectrum from the â spectrum.32 For this measure-ment, no liquid-liquid extraction step is required. The nuclidesolution is directly added to the scintillation cocktail, providedthat a homogeneous mixture between the solution analyzed(which is usually an aqueous solution) and the scintillation cocktail(Ultima Gold LLT for our experiments) is obtained. When thefinal mixture is homogeneous, the counting efficiency allows 4πgeometry. The use of liquid scintillation counting for R emitterswith the TRI-CARB spectrometer seems to be very attractivebecause it combines a quantitative measurement of the R activityand rapid and simple sample preparation. It requires particularattention for the rejection of the pulse shape parameter, which

must be adjusted carefully for each type of solution. We will seein the following sections that it does not give satisfactory resolutionleading to an optimum limit of detection because of the presenceof water molecules in the mixture.31

For this study, all experiments were performed using 3 mL ofanalyzed solution and 2 mL of Ultima Gold LLT to obtain a totalvolume of solution analyzed equal to 5 mL. This cocktail wasespecially tailored for low-level tritium (LLT) activity determinationand is usually used for low actinide activities. It is characterizedby a high capacity for water from a variety of different sources(distilled, deionized, tap, rain, river, or sea water), a very lowbackground contribution, a long-term stability, and subambienttemperature stability, and it is compatible with the main mineralacid species usually encountered in R/â counting applications. Ithas the following composition: diisopropylnaphthalene (DIN)(57-67%), as the solvent base which enhances R/â resolution inLSC,32 2,5-diphenyloxazole (PPO) (0-1%), and 1,4-bis(2-methyl-styryl)benzene (BMSB) (0-1%). It also contains surfactant mol-ecules, such as ethoxylated alkylphenols (20-30%), diethyleneglycol butyl ether (5-8%), and ethoxylated alcohols (2-4%).

TRI-CARB Procedure Using Alphaex. Since the presenceof water molecules in the mixture obtained for the measurementof R activity using the TRI-CARB spectrometer can degrade theresolution, the limit of detection, and the separation factor, RS,we tested a third way of measurement by using a nonaqueouscocktail (like Alphaex) in the TRI-CARB spectrometer. Thismethod combined several advantages of the PERALS spectrometer(performances of the scintillating cocktails and selectivity of theliquid-liquid extraction steps, which also allowed the separationof several ions from the actinides) and of the TRI-CARB spec-trometer (automatic sample changing). The third procedure wasbased on the same steps that already have been described forthe PERALS procedure. The measurement was then performedin the TRI-CARB spectrometer with the Alphaex cocktail. Forthese experiments, the nuclide was first extracted into the organicphase. A 1 mL quantity of the organic phase was transferred to atube and then diluted with toluene to obtain a total volume ofsolution equal to 5 mL (to keep the same geometry as that forthe previous procedure).

RESULTS AND DISCUSSIONDetermination of the Limit of Detection. The limit of

detection values were determined for both systems by consideringeach procedure described earlier and using eq 1 for a countingtime equal to 16 h. In our published works, we reported that therecovery of actinides in the organic phase remains good (recoverybetween 90 and 100%), even for large volumes of aqueous solutions(up to 250 mL), after one or two liquid-liquid extraction steps.12,14

Nevertheless, we routinely performed the experiments using 50mL of aqueous solution to avoid the second extraction steprequired. For this reason, the limits of detection were calculatedby considering volumes of organic and aqueous phases equal to1.5 and 50 mL, respectively, for PERALS spectrometry and forTRI-CARB spectrometry using the Alphaex scintillating-extractivemixtures. For TRI-CARB spectrometry employing Ultima GoldLLT, the values were calculated for a mixture containing 8 mL ofthe solution analyzed and 12 mL of Ultima Gold LLT. Indeed, forhigher volume ratios between the aqueous solution and the UltimaGold LLT cocktail, it was difficult to determine the actinide

(28) Marhol, M. In Ion exchangers in analytical chemistry. Their properties anduse in inorganic chemistry; Svehla, G., Ed.; Elsevier Scientific Publishing:New York, 1982; pp 66-67.

(29) Duffey, J. M.; Case, F. I.; Metzger, R. L.; Jessop B. J.; Scweitzer, G. K. J.Radioanal. Nucl. Chem. 1997, 221, 1-2, 115-122.

(30) McKlveen, J. W.; McDowell, W. J. Nucl. Instrum. Methods Phys. Res. 1984,223, 372-376.

(31) McDowell, W. J.; McDowell, B. L. In Liquid Scintillation Alpha Spectrometry;CRC Press: Boca Raton, FL, 1994; pp 25, 39, 67-70.

(32) Passo, C. J., Jr.; Cook, G. T. Handbook of Environmental Liquid ScintillationSpectrometry; Packard Instrument Co.: Meriden, CT, 1994.

3152 Analytical Chemistry, Vol. 72, No. 14, July 15, 2000

activities with a good accuracy because of the observation of anemulsion between the two phases. Moreover, this volume ratiohad already been found to be optimal for 3H efficiency. All of thecalculated limit of detection values are given in Table 1. The resultsobtained show, without any ambiguity, that the limit of detectiondetermined for the PERALS spectrometer is lower than thatdetermined for the TRI-CARB spectrometer. This great differenceis mainly due to the very low level of background from â-γemitters in PERALS spectrometry. Indeed, because of the incom-plete separation of â/γ and R pulses, the R efficiency depends onthe percentage of â pulses rejected.33 Thus, the â/γ rejectioncannot exceed 99.5% (to be compared to 99.95% obtained for thePERALS spectrometer). A higher rejection value is possible butleads to R efficiency lower than 100%.

After 16 h of counting time, the background obtained fromambient γ-rays and â emitters in the R-energy range correspondsto about 4 × 10-3 Bq for the TRI-CARB spectrometer, while itcorresponds to 5 × 10-4 Bq for the PERALS spectrometer. Themain conclusion of this study is that the PERALS apparatus allowsthe determination of R-emitting nuclides at activities lower thanthose determined by the TRI-CARB spectrometer.

Nevertheless, we mentioned earlier that it should be possibleto improve the limit of detection obtained for the TRI-CARBspectrometer by using a liquid-liquid extraction step withAlphaex. This third procedure should also improve the TRI-CARB performance because of the deoxygenation of the scintil-lating cocktail by sparging with dry-toluene-saturated oxygen-freeargon. For these experiments, the volume of the solution analyzedwas adjusted to 5 mL to keep the same detection geometry. Asindicated in Table 1, the limit of detection is about 8 times lowerthan that obtained using Ultima Gold LLT, which shows thatAlphaex can be used to great advantage for the measurement ofactinides by the TRI-CARB spectrometer.

The results reported in Table 1 can be compared with thoseobtained by other methods such as inductively coupled plasmamass spectrometry (ICP-MS) (γd ) 2 × 10-13-10-15 M).33 Thiscomparison shows that both R liquid scintillation spectrometers(using the Alphaex cocktail) could be used with advantage forthe determination of nuclides with half-lives shorter than about

105 years (such as the main plutonium, americium, and curiumisotopes).

Determination of the resolution R (Fwhm). (a) Compari-son of the Three Procedures. Although the limit of detectionis very important for the determination of low actinide activities,it was also necessary to study the resolution, R, in energy for bothtechniques even though we reported that the separation factor,RS, was preferred to quantify the capability of peak separation.Thus, we determined experimentally the resolution for the threeprocedures previously described. The R values are reported inTable 2, while their variations in terms of the R energy are givenin Figure 1. For the PERALS spectrometer, several authorspreviously obtained resolutions between 150 and 200 keV. We onlyobtained such values for 3000-4000 keV R emitters such as 148Gd,232Th, and 238U. For all of the other nuclides studied, resolutionwas usually between about 190 keV for 232Th (ER ) 4011 keV)and 300 keV for 244Cm (ER ) 5806 keV).

The comparison of the values obtained for the three proceduresshowed that the resolution obtained using the PERALS (186-297 keV) spectrometer is better than the TRI-CARB resolution(500-800 keV) for each nuclide studied. This is due to the

(33) Chiappini, R.; Taillade, J. M.; Brebion, S. J. Anal. At. Spectrom. 1996, 11,497-503.

Table 1. Limits of Detection Obtained Using thePERALS and TRI-CARB Spectrometers

LoD (M)

nuclidePERALS-Alphaexa

TRI-CARB-Ultima Gold LLTb

TRI-CARB-Alphaexa

232Th 2 × 10-8 8 × 10-7 2 × 10-7

234U 2 × 10-13 1 × 10-11 2 × 10-12

238U 5 × 10-9 3 × 10-7 4 × 10-8

237Np 2 × 10-12 1 × 10-10 2 × 10-11

238Pu 1 × 10-16 5 × 10-15 1 × 10-15

239Pu 2 × 10-14 1 × 10-12 2 × 10-13

241Am 4 × 10-16 3 × 10-14 4 × 10-15

244Cm 2 × 10-17 1 × 10-15 2 × 10-16

a Obtained from 50 mL of aqueous phase and 1.5 mL of Alphaex.b Obtained from a mixture containing 8 mL of aqueous phase and 12mL of Ultima Gold LLT.

Table 2. Resolutions, R, Obtained Using the PERALSand TRI-CARB Spectrometers

R (keV)

nuclide ER (keV)PERALS-Alphaex

TRI-CARB-Ultima Gold LLT

TRI-CARB-Alphaex

148Gd 3183 115 ND ND232Th 4011 186 500 420238U 4196 200 640 425230Th 4688 228 ND ND234U 4776 233 670 460233U 4825 232 660 470239Pu 5155 256 680 490243Am 5277 263 ND ND232U 5321 266 700 500228Th 5340 ND 720 515241Am 5486 276 740 530238Pu 5499 276 ND ND236Pu 5768 293 ND ND244Cm 5806 297 800 550

a ND ) not determined.

Figure 1. Resolution determined for each procedure as a functionof the R energy: ([) PERALS-Alphaex; (∆) TRI-CARB-Alphaex;(1) TRI-CARB-Ultima Gold LLT).

Analytical Chemistry, Vol. 72, No. 14, July 15, 2000 3153

optimization of the PERALS spectrometer for high-resolutioncounting while the TRI-CARB spectrometer was designed forcounting large numbers of samples. This is also probably due tothe presence of water, oxygen, or high ion concentrations in theTRI-CARB scintillation mixture, which can degrade the scintillationprocess. For this reason, we used Alphaex instead of Ultima GoldLLT in the TRI-CARB spectrometer to improve the resolution.Indeed, water is not present in the scintillation cocktail Alphaexafter the liquid-liquid extraction step. The ions initially presentin the aqueous solution are usually partly extracted. Moreover,the Alphaex cocktail was also deoxygenated by sparging with dry-toluene-saturated oxygen-free argon, as already discussed in theExperimental Section all of these factors reduce the quenching.This third procedure led to resolution values between 420 and550 keV, a range which is 1.3-1.4 times lower than that obtainedusing Ultima Gold LLT. Even though the resolution values arenot as good as those obtained using the PERALS spectrometer,this procedure allows the separation of activities of severalactinides, which was not possible using Ultima Gold LLT. Thisimprovement is probably due to the presence of the PBBOscintillating molecule in the Alphaex cocktail. This scintillatingmolecule is usually considered fast (lifetime τ equal to about 1ns), which is not the case for the DIN molecule used in UltimaGold LLT (lifetime τ equal to about 10 ns).

(b) Variation of the Resolution with the R Energy. For eachprocedure, we observed a linear increase of the resolution withthe R energy. This is a purely statistical effect (Poisson’s law).The relative resolution (e.g., fwhm/ER) is almost constant andusually equal to about 4-7% for the PERALS system. The linearregression values obtained for the PERALS system using Alphaex(RP,A) and for the TRI-CARB spectrometer using Alphaex (RTC,A)and Ultima Gold LLT (RTC,LLT) scintillating cocktails led to thefollowing results:

The values of the slope (which represents the degradation ofthe resolution when the energy increases) are almost the samewhen the Alphaex cocktail is used with both spectrometers (eqs6 and 7). Thus, this seems to indicate that the slope is onlycharacteristic of the scintillation cocktail. In an attempt to verifythis point, several experiments were carried out using anothercocktail (Radaex) which contains the same components for thescintillation process but different molecules for the extractionprocess: 2-methyl-2-heptylnonanoic acid (HMHN) and dicyclo-hexano-21-crown-7 (Cy221C7). The experimental results gaveslopes similar to those obtained with Alphaex. The slopes are alsodependent on the photon-multiplicator quality. Indeed, we verifiedexperimentally, using several PERALS spectrometers, that theperformances varied by about 10% (they are characterized byrelative resolutions from about 5.8% to 6.6% at ER ≈ 8000 keV forthe Radaex cocktail).

We also verified that the use of Alphaex in the TRI-CARBspectrometer instead of Ultima Gold LLT improves the resolutionand its variation with the R energy (the slope obtained is 2 times

better when Alphaex is used, as shown in eqs 7 and 8). The higherslope value obtained in eq 8 is mainly due to the presence of water,oxygen, and high ion concentrations in Ultima Gold LLT, asreported earlier.

(c) Variation of the Resolution with the Volume Analyzed.We were also interested in the study of the resolution withincreased volume of analyzed solution. The detection processesare almost the same for both spectrometers, while the samplepreparations are completely different (concentration of the activityin the scintillation mixture for Alphaex and direct mixing forUltima Gold LLT). Thus, the volume of the scintillation cocktailshould be important in comparing both techniques. We studiedthe degradation of the resolution when the volume of the organicsolution used for the measurement was increased. For the TRI-CARB spectrometer, the total volume can routinely reach 20 mL,while for PERALS measurements, 1-1.2 mL of scintillationcocktail is recommended. We measured the resolution at ER )5155 keV (239Pu) for 0.3-2 mL of Alphaex scintillation cocktailusing the PERALS (Figure 2) and the TRI-CARB (Figure 3)spectrometers. In the latter case, the experiments were conductedwith both scintillation cocktails. For PERALS measurements, theresolution was not altered when between 0.3 and 1.6 mL quantitiesof Alphaex were used in the spectrometer. The resolutionincreased significantly for volumes higher than 1.7 mL, which wasprobably due to geometrical effects. The scintillation spectraobtained for 1, 1.7, and 2 mL of analyzed volumes are presented

Figure 2. Resolution obtained at ER ) 5155 keV (239Pu) for thePERALS spectrometer as a function of the volume analyzed.

Figure 3. Resolution obtained at ER ) 5155 keV (239Pu) for theTRI-CARB spectrometer as a function of the volume ratio, r (Vanal/Vtot), using Ultima Gold LLT (O) and Alphaex ([).

RP,A (keV) ) (0.069 ( 0.002) E (keV) - (101 ( 8) (6)

RTC,A (keV) ) (0.074 ( 0.005) E (keV) + (117 ( 24) (7)

RTC,LLT (keV) ) (0.147 ( 0.016) E (keV) - (65 ( 75) (8)

3154 Analytical Chemistry, Vol. 72, No. 14, July 15, 2000

in Figure 4. Simultaneously, we were interested in the determi-nation of Fischer’s coefficient (asymmetry coefficient) defined ineq 4 and described in our previous paper (Figure 5).34 We alsoreported the relative variation of the widths of the peak on theleft (σ1) and on the right (σ2) of the maximum. For analyzedvolumes between 0.3 and 1.6 mL, the average value of Fischer’scoefficient is equal to 0.17 ( 0.06, which is in very good agreementwith the value reported for 239Pu in our published work (0.15 (0.04).34 For these operating volumes, the resolution and the ratio(σ2 - σ1)/σ2 are about 260 ( 10 keV at 5155 keV and 17 ( 4%,respectively, which are quite acceptable considering this nuclide.On the other hand, for larger analyzed volumes (V > 1.7 mL),the resolution increases up to 310 keV and Fischer’s coefficientbecomes strongly negative (down to -0.56 ( 0.08), whichcorresponds to peak deformation on the left side (lower energy).This is confirmed by the strong decrease in the (σ2 - σ1)/σ2 ratio,which also becomes negative (e.g., peak broadening on the leftside, since σ2 < σ1). Under these conditions, the greater theanalyzed volume, the greater are the broadening (Figure 4: V )2.0 mL), the increase in energy resolution (Figure 2), the decreasein Fischer’s coefficient (Figure 5) and the decrease in the (σ2 -σ1)/σ2 ratio (Figure 6). We also observed a left shift in the peaklocation (2.7%), which is clearly visible in Figure 4, and we verifiedthat beyond 2.0 mL of analyzed solution, equality between theactivity expected and the activity measured by the PERALSspectrometer is not achieved.

In conclusion, we found that the PERALS resolution remainedconstant provided that the volume of analyzed solution was keptbetween 0.3 and 1.6 mL. For larger volumes, the measurementbecame nonquantitative because of geometrical effects. This isassociated with loss of R events (due to geometrical effects) andwith peak broadening, which could lead to inaccurate results forisotopic measurements.

For the TRI-CARB spectrometer, no significant variation of theresolution nor of the peak location was observed using UltimaGold LLT when the volume of solution analyzed, Vanal, and thetotal volume of solution, Vtot (including the scintillation cocktail),were increased and the volume ratio, r ) Vanal/Vtot, was keptconstant. We were interested in investigating the resolutiondegradation with an increase in the volume ratio, r. We foundthat the resolution was constant between r ) 0.033 (0.2 mL/6mL) and r ) 0.208 (1.25 mL/6 mL). On the other hand, for higherr values, the energy resolution increased with r. The degradationwas maximum when the emulsion between the solution andUltima Gold LLT appeared (R reached 1400 keV for r ) 0.667,i.e., 4 mL/6 mL). This increase was attributed to the presence ofwater molecules in the solution. With the third procedure, usingAlphaex in the TRI-CARB spectrometer, the resolution remainedconstant even for a volume of analyzed solution equal to 4 mL(for this procedure, it could be increased up to 6 mL consideringour geometrical conditions). For all experiments conducted usingthe third procedure, the resolution obtained was about 490 keVat ER ) 5155 keV (239Pu), regardless of the r values used (Figure3). Two opposing phenomena seem to be responsible for thisobservation. When Alphaex is diluted with toluene, both PBBOand HDEHP concentrations decrease. We already observed thata decrease in the PBBO concentration degrades the resolution,while dilution of HDEHP (contained in the Alphaex cocktail)improves the resolution (due to the HDEHP quenching, which isequal to about 10%).31

Determination of the Separation Factor RS. For a completecomparison of all procedures studied, the separation factor, RS,defined in eq 2, was determined using a solution containing themain natural uranium isotopes 238U (ER ) 4196 keV) and 234U (ER

) 4776 keV) and spiked with 232U (ER ) 5321 keV). The 232Uactivity was approximately equal to those of 238U and 234U. Thespectra obtained using the three procedures described (PERALS-Alphaex, TRI-CARB-Alphaex, and TRI-CARB-Ultima Gold LLT)are shown in Figures 7- Figure 9, respectively. The correspond-(34) Aupiais, J.; Dacheux, N. Radiochim. Acta, in press.

Figure 4. R liquid scintillation spectra of a 239Pu solution (PERALS).Analyzed volumes: 1, 1.7, and 2 mL.

Figure 5. Variation of Fischer’s coefficient for the 239Pu peak as afunction of the volume analyzed (PERALS).

Figure 6. Relative deformation of the 239Pu peak as a function ofthe volume analyzed (PERALS).

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ing separation factors, calculated between the 238U and 234U peaks,on one hand, and between the 234U and 232U peaks, on the otherhand, are summarized in Table 3.

Comparison of the three spectra clearly shows that peakdeconvolution is rather simple for the PERALS spectrometer usingAlphaex (Figure 7, Table 3: RS ) 1.8). It is possible using theTRI-CARB spectrometer and Alphaex (Figure 8, Table 3: RS )1.2) but seems impossible using Ultima Gold LLT (Figure 9, Table3: RS ≈ 0.9). These results are in good agreement with eq 3 inthe theoretical section, where we mentioned that accurate decon-volution of two peaks was possible provided that RS > 1 and that

the peak areas were almost the same. Thus, the Alphaex cocktailcan be used to advantage in the TRI-CARB spectrometer toachieve a clean separation of uranium isotopes. To verify theaccuracy of the deconvolution process, we also compared theisotopic ratios 234U/238U and 232U/238U obtained using Alphaex inthe PERALS and TRI-CARB spectrometers. The measured isotopicratios are given Table 4. For both procedures, the values are closeand are in good agreement with the values expected.

The second part of this study consisted of investigating thevariation of the separation factor, RS , as a function of the volumeratio, r (Figure 10). We found that, when the resolution is keptconstant using Ultima Gold LLT (i.e., 0.033 < r < 0.208, asdescribed in the previous section), the RS value is almost constanttoo (and equal to 0.80-0.95). Unfortunately, it remains too low toallow the separation of the three peaks. For higher r values, theRS value slightly decreases down to 0.5 for r ) 0.667, whichbecomes insignificant. It appears that the second proceduredescribed does not allow the determination of the ratios in the238U-234U-232U isotopic system whatever the operating conditions.For this reason, we also studied the variation of the RS value usingAlphaex in the TRI-CARB spectrometer. The results presentedin Figure 10 show that the RS value is almost constant (RS between1.0 and 1.1). For uranium isotopic measurements, this value isjust good enough to perform the peak deconvolution under goodconditions for the entire r range studied, if the peak areas aresimilar.

Figure 7. R liquid scintillation spectrum of a 238U-234U solutionspiked with 232U (PERALS-Alphaex).

Figure 8. R liquid scintillation spectrum of a 238U-234U solutionspiked with 232U (TRI-CARB-Alphaex).

Figure 9. R liquid scintillation spectrum of a 238U-234U solutionspiked with 232U (TRI-CARB-Ultima Gold LLT).

Table 3. Separation Factors, RS, Determined for a238U-234U Solution Spiked with 232U Using the ThreeProcedures

spectrometer scintillating cocktail RS(238U/234U) RS(234U/232U)

PERALS Alphaex 1.8 1.7TRI-CARB Alphaex 1.2 1.1TRI-CARB Ultima Gold LLT ∼0.9 ∼0.9

Table 4. Isotopic Ratios Obtained by the PERALS andTRI-CARB Spectrometers for a 238U-234U SolutionSpiked with 232U Using Alphaex

isotopicratio

referencevalue

PERALSvalue

dev(%)

TRI-CARBvalue

dev(%)

234U/238U 1.29 ( 0.02 1.31 ( 0.04 1.6% 1.37 ( 0.04 -6.2%232U/238U 1.11 ( 0.02 1.09 ( 0.03 1.8% 1.06 ( 0.03 -4.5%

Figure 10. Separation factor obtained at ER ) 5155 keV (239Pu)for the TRI-CARB spectrometer as a function the volume ratio, r (Vanal/Vtot), using Ultima Gold LLT (O) and Alphaex ([).

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CONCLUSIONThe aim of this study was to compare PERALS and TRI-CARB

spectrometers for low-activity actinide measurements. We foundthat the limits of detection calculated for the PERALS spectrometerare always lower than the TRI-CARB limits when the Ultima GoldLLT scintillating cocktail is used. Moreover, the resolutions,separation factors, and Fischer’s coefficients obtained for thePERALS spectrometer allow clean peak deconvolution for mostcases (for instance, for 238U-234U-232U isotopic measurements),which is not the case for the TRI-CARB spectrometer using UltimaGold LLT. This is probably due to the presence of water, oxygen,or high ion concentrations in the mixture finally obtained toperform the activity measurements.

We were able to improve the performance of the TRI-CARBspectrometer by combining the advantages of both techniques.This was achieved by using the Alphaex scintillation cocktail inthe TRI-CARB spectrometer. Under these conditions, water andoxygen are usually absent from the solution analyzed, while thehigh ion concentrations are only partly extracted during theliquid-liquid extraction step. This liquid-liquid extraction step

also improves the limit of detection. The γd values obtained usingthis procedure are 10 times poorer than those for the PERALSspectrometer but 4-8 times better than the values achieved usingthe Ultima Gold LLT scintillating cocktail. At the same time, theresolution decreases from 500-800 to 420-590 keV, while theseparation factor RS increases from 0.9 to 1.1-1.2. For isotopicmeasurements or determination of low activities using spiking,these values allow the deconvolution required to give accurateresults. This is the case for 238U-234U-232U experiments reportedin this paper and also for 244Cm measurements performed byspiking with 248Cm. This improvement is probably due to thepresence of the PBBO scintillating molecule (lifetime τ equal toabout 1 ns) in the Alphaex cocktail instead of the DIN moleculeused in the Ultima Gold LLT (lifetime τ equal to about 10 ns).

Received for review December 10, 1999. Accepted March30, 2000.

AC9914216

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