Comparison and Improvement of the Determinations of Actinide Low

May 27, 2000 - We applied three procedures using two α liquid scintillation spectrometers (PERALS and TRI-CARB) and two scintillation cocktails (Alph...
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Anal. Chem. 2000, 72, 3150-3157

Comparison and Improvement of the Determinations of Actinide Low Activities Using Several r Liquid Scintillation Spectrometers Nicolas Dacheux,*,† Jean Aupiais,‡ Olivier Courson,† and Ce´dric Aubert‡

Groupe de Radiochimie, Institut de Physique Nucle´ aire, Baˆ t. 100, Universite´ Paris-Sud, 91406 Orsay Cedex, France, and De´ partement Analyse Surveillance Environnement, Service Radioanalyses Chimie Environnement, CEA, BP 12, 91680 Bruye` res le Chaˆ tel, France We applied three procedures using two r liquid scintillation spectrometers (PERALS and TRI-CARB) and two scintillation cocktails (Alphaex and Ultima Gold LLT) for the determination of r-emitter low activities. For each procedure, the limit of detection, the resolution, the separation factor, and the Fischer coefficient were determined in order to perform 232U-234U-238U isotopic measurements. The deconvolution usually performed is clean when the PERALS spectrometer is used. This is not possible for the TRI-CARB spectrometer using the Ultima Gold LLT scintillation cocktail. This problem was solved by combining the advantages of both techniques using the Alphaex scintillation cocktail in the TRI-CARB spectrometer. Under these conditions, the limit of detection was improved, the resolution decreased from 500-800 to 420-590 keV, and the separation factor increased from 0.9 to 1.1-1.2. This third procedure was applied with success for 232U-234U-238U isotopic experiments. The determination of very low activities of nuclides in the biosphere has become very important.1-16 Particular attention has been 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 plutonium isotopes. All these nuclides have been released via atmospheric weapon testing, nuclear power plants, and spent-fuel reprocessing. Among the nuclear techniques usually used to determine very low activities, such as R and mass spectrometries, R liquid scintillation with reduction of the β-γ background has become important in the past several years.9,19-23 Among them, the photon electron rejecting R liquid scintillation (PERALS) apparatus * Corresponding author. Phone: +33-1-69157342. Fax: +33-1-69157150. E-mail: [email protected]. † 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 Radiochemical Techniques; U.S. Government Printing Office: Washington, DC, 1986; pp 88-89.

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combines in the same step the chemical extraction of actinides by liquid-liquid extraction into the organic phase and the measurement of the R activity by R liquid scintillation. This technique allows the total rejection of the β emission (99.95%) by means of a pulse-shaped discrimination (PSD). PSD reduces the background induced by photoelectrons produced by ambient γ-ray activity and also eliminates interference from β emitters coextracted with R emitters or produced by decay in the extractivescintillating cocktails. In our previous papers, we reported a general scheme for separation of actinides that can be applied to solutions containing simultaneously thorium, uranium, plutonium, americium, and curium.12 We also reported the optimal conditions for extraction (pH, ions present in the solution, volume ratio between organic and aqueous solutions) to obtain the measurement of several actinides 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 more common conventional R-β scintillation instruments, we also performed studies using the TRI-CARB spectrometer for which the activity measurement is rather different (in this case, a waterorganic miscible cocktail is used). The aim of this study was to evaluate the performances of both methods by determining the limits of detection, the resolutions, and the separation factors. Direct comparison of both techniques is difficult because of the differences in the measurements and the procedures required for the sample preparation. Nevertheless, we try to present, in this paper, the main advantages and (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; pp 556-558. (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, 23532359. (16) Aupiais, J. J. Radioanal. Nucl. Chem. 1997, 218, 201-207. 10.1021/ac9914216 CCC: $19.00

© 2000 American Chemical Society Published on Web 05/27/2000

drawbacks of each system. It is possible to take advantage of combining the scintillatingextractive cocktails usually used to perform the measurement by the PERALS spectrometer with the activity determination using the TRI-CARB spectrometer. This allows a significant improvement in the limit of detection, the resolution, and the separation factor. Under these conditions, the peak separation is better, and this makes the deconvolution of peaks from several actinides possible. The main results of this study are presented in this paper. THEORY Determination of the Limit of Detection. The determination of the limit of detection is usually required to evaluate the performance of each scintillation technique. Thus, we calculated the values obtained for the PERALS and TRI-CARB spectrometers for several counting times. According to the literature, and taking into account a Poisson distribution for the background, the limit of detection γd can be defined for R and β statistical risks equal to 2.5% as follows:26,27

4 γd ) 2(∆c)s ) (1 + x1 + 2Xbkgd) t

(1)

where (∆c)s is the threshold of detection, Xbkgd is the background count number, and t is the counting time, which must be identical for the background and for the sample. Determination of the Separation Factor RS. The performance of each technique was also evaluated by determining the separation factor, RS, which represents the capability of separating the peaks. It can be calculated using the equation28

X2 - X1

RS ) 1

/2(fwhm1 + fwhm2)

(2)

where Xi is the location of peak i (in energy) and fwhmi is the full width at half-maximum of peak i (in energy). This factor, which is dimensionless, must be distinguished from the resolution, R (equal to the fwhm), which is homogeneous in energy and usually expressed in keV. The higher the value obtained for RS, the better is the separation of peaks (it is the opposite for the resolution, R, which corresponds to the fwhm). It is usually observed that the separation of two peaks which are almost the same in energy is good when the separation factor, RS, is higher than 1. For lower values, the separation of both peaks becomes very difficult and the deconvolution process leads to inaccurate results. Fitting Function and Determination of Peak Asymmetry. For all of the nuclides, the peaks obtained by PERALS or TRICARB spectrometry cannot be fitted using a pure Gaussian function because of the asymmetry observed. We assumed during the deconvolution process a Gaussian distribution for low-activity measurements because, under these conditions, the statistical error is higher than the fitting one. For higher activities, a biGaussian peak can be considered. This function is based on two Gaussian functions with different standard deviations on the left and on the right of the peak center position (xc). Its expression can be formulated as

f(x) ) H exp[-(x - xc)2/2σ12] for 0 < x < xc

(3a)

f(x) ) H exp[-(x - xc)2/2σ22] for xc < x < ∞ (3b) where H is the peak height, xc is the peak location at maximum height, 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 the maximum (at 0.882H). The peak asymmetry (Fischer’s coefficient) can be calculated by the equation27

γ1 )

m3

m3 m3 )1 ) 3 σ /8(σ1 + σ2) (fwhm/2.354)3 3

(4)

where m3 corresponds to the third-order moment (symmetry factor related to the Gaussian peak) and σ is the width of the peak at 0.882H (σ is equal to fwhm/2.354 and to 1/2(σ1 + σ2)). The third central moment can be calculated by27

m3 )

∫

∞

-∞

(x - xc)3 f(x) dx

(5)

where f(x) is the function described in eq 3. EXPERIMENTAL SECTION Sample Preparation. All nuclide solutions were stored in 4 M HNO3 at room temperature and were from our own laboratory source. The specific activity was determined for each solution by R spectrometry. PERALS Procedure. The photon electron rejecting R liquid scintillation spectrometer (model 8100) is an R liquid scintillation apparatus produced by Ordela Inc. (Oak Ridge, TN). It is based on the use of liquid-liquid extraction by a scintillating-extractive cocktail that contains a scintillating and an extractive molecule.1,6 Among the extractive molecules usually used are bis(2-ethylhexyl)phosphoric acid (HDEHP), amines such as (1-nonyldecyl)amine and tri-n-octylamine (TNOA), and organophosphorus compounds such as tri-n-octylphosphine oxide (TOPO). All can be obtained from Etrac (Oak Ridge, TN). For each actinide and each extractive molecule, it is necessary to determine a distribution ratio, D, which represents the performance of each extraction step. These values, as well as the equations concerning the theory of liquid-liquid extraction,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) Ve´ronneau, C.; Aupiais, J.; Dacheux, N. Anal. Chim. Acta 2000, 415, 229238. (25) Aupiais, J.; Dacheux, N.; Thomas, A. C.; Matton, S. Anal. Chim. Acta 1999, 398, 205-218. (26) Limite de de´ tection 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 applique´ e a` l’exploitation des mesures; Masson: Paris, 1986; pp 53, 113-117.

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required between aqueous and organic phases, and the effect of the ions present in solution. On the basis of this experience, the extraction steps were performed in 1 M HNO3, 10-1 M HNO3, and 10-3 M HNO3, respectively, to obtain optimal recovery conditions for the determination of thorium, uranium, and curium-americium activities by a PERALS spectrometer.12,14 For plutonium, a redox process is required to obtain quantitative recovery during the liquid-liquid extraction step. It is based on the reduction of plutonium to its trivalent state using ascorbic acid (1 g/L), followed by the oxidation of Pu(III) to Pu(IV) using a sodium nitrite solution (1 g/L).12 In this work, we only present the results obtained using the Alphaex cocktail, which allows good energy resolution. This cocktail consists of a mixture containing HDEHP (64 g/L, i.e., 0.2 M), as the extractive molecule (purified by precipitation/recrystallization), naphthalene (180 g/L), and PBBO (4 g/L) in toluene.29-31 Each extraction step was performed according to the following procedure: A 50 µL-1 mL quantity of the spike solution was introduced into the culture tube, depending on the specific activity. The acidity and/or the acid concentration was adjusted by considering 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 added to the tube (volume ratio between organic and aqueous solutions 0.24). Both phases were equilibrated by shaking for 5 min and then were separated by centrifugation for 5-10 min at 2000 rpm. A 1 mL aliquot of the organic phase was removed and deoxygenated by sparging with dry-toluene-saturated oxygen-free argon for 5 min to improve the PERALS resolution. The culture tube was sealed, and its content was then measured with the PERALS spectrometer. Under these conditions, the concentration factors of the actinides in aqueous solution can reach 170 (250 mL of aqueous solution/1.5 mL of organic phase).12 TRI-CARB Procedure Using Ultima Gold LLT. The TRICARB spectrometer is an R-β liquid scintillation spectrometer supplied by Packard (Canberra Co., Meriden, CT) which uses time-resolved pulse decay analysis (TR-PDA) technology to separate the R spectrum from the β spectrum.32 For this measurement, no liquid-liquid extraction step is required. The nuclide solution is directly added to the scintillation cocktail, provided that 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 the final mixture is homogeneous, the counting efficiency allows 4π geometry. The use of liquid scintillation counting for R emitters with the TRI-CARB spectrometer seems to be very attractive because it combines a quantitative measurement of the R activity and rapid and simple sample preparation. It requires particular attention for the rejection of the pulse shape parameter, which (28) Marhol, M. In Ion exchangers in analytical chemistry. Their properties and use 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 Scintillation Spectrometry; Packard Instrument Co.: Meriden, CT, 1994.

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must be adjusted carefully for each type of solution. We will see in the following sections that it does not give satisfactory resolution leading to an optimum limit of detection because of the presence of water molecules in the mixture.31 For this study, all experiments were performed using 3 mL of analyzed solution and 2 mL of Ultima Gold LLT to obtain a total volume of solution analyzed equal to 5 mL. This cocktail was especially tailored for low-level tritium (LLT) activity determination and is usually used for low actinide activities. It is characterized by a high capacity for water from a variety of different sources (distilled, deionized, tap, rain, river, or sea water), a very low background contribution, a long-term stability, and subambient temperature stability, and it is compatible with the main mineral acid species usually encountered in R/β counting applications. It has the following composition: diisopropylnaphthalene (DIN) (57-67%), as the solvent base which enhances R/β resolution in LSC,32 2,5-diphenyloxazole (PPO) (0-1%), and 1,4-bis(2-methylstyryl)benzene (BMSB) (0-1%). It also contains surfactant molecules, such as ethoxylated alkylphenols (20-30%), diethylene glycol butyl ether (5-8%), and ethoxylated alcohols (2-4%). TRI-CARB Procedure Using Alphaex. Since the presence of water molecules in the mixture obtained for the measurement of R activity using the TRI-CARB spectrometer can degrade the resolution, the limit of detection, and the separation factor, RS, we tested a third way of measurement by using a nonaqueous cocktail (like Alphaex) in the TRI-CARB spectrometer. This method combined several advantages of the PERALS spectrometer (performances of the scintillating cocktails and selectivity of the liquid-liquid extraction steps, which also allowed the separation of several ions from the actinides) and of the TRI-CARB spectrometer (automatic sample changing). The third procedure was based on the same steps that already have been described for the PERALS procedure. The measurement was then performed in the TRI-CARB spectrometer with the Alphaex cocktail. For these experiments, the nuclide was first extracted into the organic phase. A 1 mL quantity of the organic phase was transferred to a tube and then diluted with toluene to obtain a total volume of solution equal to 5 mL (to keep the same geometry as that for the previous procedure). RESULTS AND DISCUSSION Determination of the Limit of Detection. The limit of detection values were determined for both systems by considering each procedure described earlier and using eq 1 for a counting time equal to 16 h. In our published works, we reported that the recovery of actinides in the organic phase remains good (recovery between 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 50 mL of aqueous solution to avoid the second extraction step required. For this reason, the limits of detection were calculated by considering volumes of organic and aqueous phases equal to 1.5 and 50 mL, respectively, for PERALS spectrometry and for TRI-CARB spectrometry using the Alphaex scintillating-extractive mixtures. For TRI-CARB spectrometry employing Ultima Gold LLT, the values were calculated for a mixture containing 8 mL of the solution analyzed and 12 mL of Ultima Gold LLT. Indeed, for higher volume ratios between the aqueous solution and the Ultima Gold LLT cocktail, it was difficult to determine the actinide

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

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

LoD (M) nuclide 232Th 234U 238U 237Np 238Pu 239Pu 241Am 244Cm

R (keV)

PERALSAlphaexa

TRI-CARBUltima Gold LLTb

TRI-CARBAlphaexa

2 × 10-8 2 × 10-13 5 × 10-9 2 × 10-12 1 × 10-16 2 × 10-14 4 × 10-16 2 × 10-17

8 × 10-7 1 × 10-11 3 × 10-7 1 × 10-10 5 × 10-15 1 × 10-12 3 × 10-14 1 × 10-15

2 × 10-7 2 × 10-12 4 × 10-8 2 × 10-11 1 × 10-15 2 × 10-13 4 × 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 12 mL of Ultima Gold LLT.

nuclide

ER (keV)

PERALSAlphaex

TRI-CARBUltima Gold LLT

TRI-CARBAlphaex

148Gd

3183 4011 4196 4688 4776 4825 5155 5277 5321 5340 5486 5499 5768 5806

115 186 200 228 233 232 256 263 266 ND 276 276 293 297

ND 500 640 ND 670 660 680 ND 700 720 740 ND ND 800

ND 420 425 ND 460 470 490 ND 500 515 530 ND ND 550

232Th 238U 230Th 234U 233U 239Pu 243Am 232U 228Th 241Am 238Pu 236Pu 244Cm a

activities with a good accuracy because of the observation of an emulsion between the two phases. Moreover, this volume ratio had already been found to be optimal for 3H efficiency. All of the calculated limit of detection values are given in Table 1. The results obtained show, without any ambiguity, that the limit of detection determined for the PERALS spectrometer is lower than that determined for the TRI-CARB spectrometer. This great difference is mainly due to the very low level of background from β-γ emitters in PERALS spectrometry. Indeed, because of the incomplete separation of β/γ and R pulses, the R efficiency depends on the percentage of β pulses rejected.33 Thus, the β/γ rejection cannot exceed 99.5% (to be compared to 99.95% obtained for the PERALS spectrometer). A higher rejection value is possible but leads to R efficiency lower than 100%. After 16 h of counting time, the background obtained from ambient γ-rays and β emitters in the R-energy range corresponds to about 4 × 10-3 Bq for the TRI-CARB spectrometer, while it corresponds to 5 × 10-4 Bq for the PERALS spectrometer. The main conclusion of this study is that the PERALS apparatus allows the determination of R-emitting nuclides at activities lower than those determined by the TRI-CARB spectrometer. Nevertheless, we mentioned earlier that it should be possible to improve the limit of detection obtained for the TRI-CARB spectrometer by using a liquid-liquid extraction step with Alphaex. This third procedure should also improve the TRICARB performance because of the deoxygenation of the scintillating cocktail by sparging with dry-toluene-saturated oxygen-free argon. For these experiments, the volume of the solution analyzed was adjusted to 5 mL to keep the same detection geometry. As indicated in Table 1, the limit of detection is about 8 times lower than that obtained using Ultima Gold LLT, which shows that Alphaex can be used to great advantage for the measurement of actinides by the TRI-CARB spectrometer. The results reported in Table 1 can be compared with those obtained by other methods such as inductively coupled plasma mass spectrometry (ICP-MS) (γd ) 2 × 10-13-10-15 M).33 This comparison shows that both R liquid scintillation spectrometers (using the Alphaex cocktail) could be used with advantage for the determination of nuclides with half-lives shorter than about (33) Chiappini, R.; Taillade, J. M.; Bre´bion, S. J. Anal. At. Spectrom. 1996, 11, 497-503.

ND ) not determined.

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

105 years (such as the main plutonium, americium, and curium isotopes). Determination of the resolution R (Fwhm). (a) Comparison of the Three Procedures. Although the limit of detection is very important for the determination of low actinide activities, it was also necessary to study the resolution, R, in energy for both techniques 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 three procedures previously described. The R values are reported in Table 2, while their variations in terms of the R energy are given in Figure 1. For the PERALS spectrometer, several authors previously obtained resolutions between 150 and 200 keV. We only obtained such values for 3000-4000 keV R emitters such as 148Gd, 232Th, and 238U. For all of the other nuclides studied, resolution was 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 procedures showed that the resolution obtained using the PERALS (186297 keV) spectrometer is better than the TRI-CARB resolution (500-800 keV) for each nuclide studied. This is due to the Analytical Chemistry, Vol. 72, No. 14, July 15, 2000

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optimization of the PERALS spectrometer for high-resolution counting while the TRI-CARB spectrometer was designed for counting large numbers of samples. This is also probably due to the presence of water, oxygen, or high ion concentrations in the TRI-CARB scintillation mixture, which can degrade the scintillation process. For this reason, we used Alphaex instead of Ultima Gold LLT in the TRI-CARB spectrometer to improve the resolution. Indeed, water is not present in the scintillation cocktail Alphaex after the liquid-liquid extraction step. The ions initially present in the aqueous solution are usually partly extracted. Moreover, the Alphaex cocktail was also deoxygenated by sparging with drytoluene-saturated oxygen-free argon, as already discussed in the Experimental Section all of these factors reduce the quenching. This third procedure led to resolution values between 420 and 550 keV, a range which is 1.3-1.4 times lower than that obtained using Ultima Gold LLT. Even though the resolution values are not as good as those obtained using the PERALS spectrometer, this procedure allows the separation of activities of several actinides, which was not possible using Ultima Gold LLT. This improvement is probably due to the presence of the PBBO scintillating molecule in the Alphaex cocktail. This scintillating molecule is usually considered fast (lifetime τ equal to about 1 ns), which is not the case for the DIN molecule used in Ultima Gold LLT (lifetime τ equal to about 10 ns). (b) Variation of the Resolution with the R Energy. For each procedure, we observed a linear increase of the resolution with the R energy. This is a purely statistical effect (Poisson’s law). The relative resolution (e.g., fwhm/ER) is almost constant and usually equal to about 4-7% for the PERALS system. The linear regression 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 the following results:

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)

The values of the slope (which represents the degradation of the resolution when the energy increases) are almost the same when the Alphaex cocktail is used with both spectrometers (eqs 6 and 7). Thus, this seems to indicate that the slope is only characteristic of the scintillation cocktail. In an attempt to verify this point, several experiments were carried out using another cocktail (Radaex) which contains the same components for the scintillation process but different molecules for the extraction process: 2-methyl-2-heptylnonanoic acid (HMHN) and dicyclohexano-21-crown-7 (Cy221C7). The experimental results gave slopes similar to those obtained with Alphaex. The slopes are also dependent on the photon-multiplicator quality. Indeed, we verified experimentally, using several PERALS spectrometers, that the performances varied by about 10% (they are characterized by relative resolutions from about 5.8% to 6.6% at ER ≈ 8000 keV for the Radaex cocktail). We also verified that the use of Alphaex in the TRI-CARB spectrometer instead of Ultima Gold LLT improves the resolution and its variation with the R energy (the slope obtained is 2 times 3154

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

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

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

better when Alphaex is used, as shown in eqs 7 and 8). The higher slope value obtained in eq 8 is mainly due to the presence of water, oxygen, and high ion concentrations in Ultima Gold LLT, as reported earlier. (c) Variation of the Resolution with the Volume Analyzed. We were also interested in the study of the resolution with increased volume of analyzed solution. The detection processes are almost the same for both spectrometers, while the sample preparations are completely different (concentration of the activity in the scintillation mixture for Alphaex and direct mixing for Ultima Gold LLT). Thus, the volume of the scintillation cocktail should be important in comparing both techniques. We studied the degradation of the resolution when the volume of the organic solution used for the measurement was increased. For the TRICARB spectrometer, the total volume can routinely reach 20 mL, while for PERALS measurements, 1-1.2 mL of scintillation cocktail is recommended. We measured the resolution at ER ) 5155 keV (239Pu) for 0.3-2 mL of Alphaex scintillation cocktail using the PERALS (Figure 2) and the TRI-CARB (Figure 3) spectrometers. In the latter case, the experiments were conducted with both scintillation cocktails. For PERALS measurements, the resolution was not altered when between 0.3 and 1.6 mL quantities of Alphaex were used in the spectrometer. The resolution increased significantly for volumes higher than 1.7 mL, which was probably due to geometrical effects. The scintillation spectra obtained for 1, 1.7, and 2 mL of analyzed volumes are presented

Figure 4. R liquid scintillation spectra of a Analyzed volumes: 1, 1.7, and 2 mL.

239Pu

solution (PERALS).

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

in Figure 4. Simultaneously, we were interested in the determination of Fischer’s coefficient (asymmetry coefficient) defined in eq 4 and described in our previous paper (Figure 5).34 We also reported the relative variation of the widths of the peak on the left (σ1) and on the right (σ2) of the maximum. For analyzed volumes between 0.3 and 1.6 mL, the average value of Fischer’s coefficient is equal to 0.17 ( 0.06, which is in very good agreement with 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 coefficient becomes strongly negative (down to -0.56 ( 0.08), which corresponds 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 left side, since σ2 < σ1). Under these conditions, the greater the analyzed volume, the greater are the broadening (Figure 4: V ) 2.0 mL), the increase in energy resolution (Figure 2), the decrease in Fischer’s coefficient (Figure 5) and the decrease in the (σ2 σ1)/σ2 ratio (Figure 6). We also observed a left shift in the peak location (2.7%), which is clearly visible in Figure 4, and we verified that beyond 2.0 mL of analyzed solution, equality between the activity expected and the activity measured by the PERALS spectrometer is not achieved. (34) Aupiais, J.; Dacheux, N. Radiochim. Acta, in press.

Figure 6. Relative deformation of the the volume analyzed (PERALS).

239Pu

peak as a function of

In conclusion, we found that the PERALS resolution remained constant provided that the volume of analyzed solution was kept between 0.3 and 1.6 mL. For larger volumes, the measurement became nonquantitative because of geometrical effects. This is associated with loss of R events (due to geometrical effects) and with peak broadening, which could lead to inaccurate results for isotopic measurements. For the TRI-CARB spectrometer, no significant variation of the resolution nor of the peak location was observed using Ultima Gold LLT when the volume of solution analyzed, Vanal, and the total volume of solution, Vtot (including the scintillation cocktail), were increased and the volume ratio, r ) Vanal/Vtot, was kept constant. We were interested in investigating the resolution degradation with an increase in the volume ratio, r. We found that the resolution was constant between r ) 0.033 (0.2 mL/6 mL) and r ) 0.208 (1.25 mL/6 mL). On the other hand, for higher r values, the energy resolution increased with r. The degradation was maximum when the emulsion between the solution and Ultima Gold LLT appeared (R reached 1400 keV for r ) 0.667, i.e., 4 mL/6 mL). This increase was attributed to the presence of water molecules in the solution. With the third procedure, using Alphaex in the TRI-CARB spectrometer, the resolution remained constant even for a volume of analyzed solution equal to 4 mL (for this procedure, it could be increased up to 6 mL considering our geometrical conditions). For all experiments conducted using the third procedure, the resolution obtained was about 490 keV at ER ) 5155 keV (239Pu), regardless of the r values used (Figure 3). Two opposing phenomena seem to be responsible for this observation. When Alphaex is diluted with toluene, both PBBO and HDEHP concentrations decrease. We already observed that a 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 is equal to about 10%).31 Determination of the Separation Factor RS. For a complete comparison of all procedures studied, the separation factor, RS, defined in eq 2, was determined using a solution containing the main natural uranium isotopes 238U (ER ) 4196 keV) and 234U (ER ) 4776 keV) and spiked with 232U (ER ) 5321 keV). The 232U activity was approximately equal to those of 238U and 234U. The spectra obtained using the three procedures described (PERALSAlphaex, TRI-CARB-Alphaex, and TRI-CARB-Ultima Gold LLT) are shown in Figures 7- Figure 9, respectively. The correspondAnalytical Chemistry, Vol. 72, No. 14, July 15, 2000

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Table 3. Separation Factors, RS, Determined for a 238U-234U Solution Spiked with 232U Using the Three Procedures spectrometer

scintillating cocktail

RS(238U/234U)

RS(234U/232U)

PERALS TRI-CARB TRI-CARB

Alphaex Alphaex Ultima Gold LLT

1.8 1.2 ∼0.9

1.7 1.1 ∼0.9

Table 4. Isotopic Ratios Obtained by the PERALS and TRI-CARB Spectrometers for a 238U-234U Solution Spiked with 232U Using Alphaex

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

238U-234U

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

238U-234U

solution

isotopic ratio

reference value

PERALS value

dev (%)

TRI-CARB value

dev (%)

234U/238U

1.29 ( 0.02 1.11 ( 0.02

1.31 ( 0.04 1.09 ( 0.03

1.6% 1.8%

1.37 ( 0.04 1.06 ( 0.03

-6.2% -4.5%

232U/238U

solution

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

ing separation factors, calculated between the 238U and 234U peaks, on one hand, and between the 234U and 232U peaks, on the other hand, are summarized in Table 3. Comparison of the three spectra clearly shows that peak deconvolution is rather simple for the PERALS spectrometer using Alphaex (Figure 7, Table 3: RS ) 1.8). It is possible using the TRI-CARB spectrometer and Alphaex (Figure 8, Table 3: RS ) 1.2) but seems impossible using Ultima Gold LLT (Figure 9, Table 3: RS ≈ 0.9). These results are in good agreement with eq 3 in the theoretical section, where we mentioned that accurate deconvolution of two peaks was possible provided that RS > 1 and that 3156 Analytical Chemistry, Vol. 72, No. 14, July 15, 2000

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 ([).

the peak areas were almost the same. Thus, the Alphaex cocktail can be used to advantage in the TRI-CARB spectrometer to achieve a clean separation of uranium isotopes. To verify the accuracy of the deconvolution process, we also compared the isotopic ratios 234U/238U and 232U/238U obtained using Alphaex in the PERALS and TRI-CARB spectrometers. The measured isotopic ratios are given Table 4. For both procedures, the values are close and are in good agreement with the values expected. The second part of this study consisted of investigating the variation of the separation factor, RS , as a function of the volume ratio, r (Figure 10). We found that, when the resolution is kept constant using Ultima Gold LLT (i.e., 0.033 < r < 0.208, as described in the previous section), the RS value is almost constant too (and equal to 0.80-0.95). Unfortunately, it remains too low to allow the separation of the three peaks. For higher r values, the RS value slightly decreases down to 0.5 for r ) 0.667, which becomes insignificant. It appears that the second procedure described does not allow the determination of the ratios in the 238U-234U-232U isotopic system whatever the operating conditions. For this reason, we also studied the variation of the RS value using Alphaex in the TRI-CARB spectrometer. The results presented in Figure 10 show that the RS value is almost constant (RS between 1.0 and 1.1). For uranium isotopic measurements, this value is just good enough to perform the peak deconvolution under good conditions for the entire r range studied, if the peak areas are similar.

CONCLUSION The aim of this study was to compare PERALS and TRI-CARB spectrometers for low-activity actinide measurements. We found that the limits of detection calculated for the PERALS spectrometer are always lower than the TRI-CARB limits when the Ultima Gold LLT scintillating cocktail is used. Moreover, the resolutions, separation factors, and Fischer’s coefficients obtained for the PERALS spectrometer allow clean peak deconvolution for most cases (for instance, for 238U-234U-232U isotopic measurements), which is not the case for the TRI-CARB spectrometer using Ultima Gold LLT. This is probably due to the presence of water, oxygen, or high ion concentrations in the mixture finally obtained to perform the activity measurements. We were able to improve the performance of the TRI-CARB spectrometer by combining the advantages of both techniques. This was achieved by using the Alphaex scintillation cocktail in the TRI-CARB spectrometer. Under these conditions, water and oxygen are usually absent from the solution analyzed, while the high ion concentrations are only partly extracted during the liquid-liquid extraction step. This liquid-liquid extraction step

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

Received for review December 10, 1999. Accepted March 30, 2000. AC9914216

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