- Email: [email protected]
Actinide measurements by AMS using fluoride matrices
Nuclear Instruments and Methods in Physics Research B 361 (2015) 317–321
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Actinide measurements by AMS using fluoride matrices R.J. Cornett a,b,⇑, Z.H. Kazi a,b, X.-L. Zhao a,c, M.G. Chartrand a,b, R.J. Charles a,c, W.E. Kieser a,c a
André E. Lalonde AMS Laboratory, University of Ottawa, 150 Louis Pasteur, Ottawa, ON K1N 6N5, Canada Department of Earth Sciences, University of Ottawa, 150 Louis Pasteur, Ottawa, ON K1N 6N5, Canada c Department of Physics, University of Ottawa, 150 Louis Pasteur, Ottawa, ON K1N 6N5, Canada b
a r t i c l e
i n f o
Article history: Received 20 November 2014 Received in revised form 11 February 2015 Accepted 15 February 2015 Available online 4 March 2015 Keywords: Actinide Plutonium Americium Fluoride AMS
a b s t r a c t Actinides can be measured by alpha spectroscopy (AS), mass spectroscopy or accelerator mass spectrometry (AMS). We tested a simple method to separate Pu and Am isotopes from the sample matrix using a single extraction chromatography column. The actinides in the column eluent were then measured by AS or AMS using a fluoride target matrix. Pu and Am were coprecipitated with NdF3. The strongest AMS beams of Pu and Am were produced when there was a large excess of fluoride donor atoms in the target and the NdF3 precipitates were diluted about 6–8 fold with PbF2. The measured concentrations of 239,240Pu and 241Am agreed with the concentrations in standards of known activity and with two IAEA certified reference materials. Measurements of 239,240Pu and 241Am made at A.E. Lalonde AMS Laboratory agree, within their statistical uncertainty, with independent measurements made using the IsoTrace AMS system. This work demonstrated that fluoride targets can produce reliable beams of actinide anions and that the measurement of actinides using fluorides agree with published values in certified reference materials. Ó 2015 Published by Elsevier B.V.
1. Introduction Actinide isotopes with masses higher than uranium (U) are produced in nuclear reactors and in nuclear weapons tests. These isotopes have been released into the environment from weapons testing fall out [1,2], reactor accidents [3] and emissions [4,5], and fuel reprocessing operations [1]. The most abundant anthropogenic actinide isotopes distributed globally in the environment are 239,240Pu and 241Am [2]. These isotopes emit alpha particles and they accumulate to higher concentrations in bone [6]. As a result 239,240Pu and 241Am have a higher dose per unit intake, they are often the most important radionuclides per unit activity in environmental assessments and they are measured to assess the risks to human and biotic exposures. 239,240 Pu and 241Am must be extracted and separated from the sample matrix before their concentrations can be measured by alpha or mass spectrometry. The separation and purification are important steps because several different actinide isotopes emit alpha particles with similar energies [7]. For example, the alpha energy spectra for 228Th and 241Am overlap as do the energies of 239 Pu and 240Pu. 241Am and 241Pu are isobaric, and some molecular
⇑ Corresponding author at: Department of Earth Sciences, University of Ottawa, 150 Louis Pasteur, Ottawa, ON K1N 6N5, Canada. E-mail address: [email protected] (R.J. Cornett). http://dx.doi.org/10.1016/j.nimb.2015.02.039 0168-583X/Ó 2015 Published by Elsevier B.V.
interferences can overlap in the mass spectrum so the separation is important for mass spectrometry measurements [8]. Many techniques have been developed to separate Am and Pu from matrix materials including solvent extraction [9,10], solid phase extraction [11], ion exchange [12], and extraction chromatography [7]. 241 Am that has been extracted in concentrated acid is difficult to separate from the sample matrix because this isotope has a low affinity for many chromatography systems in high concentrations of HCl and HNO3. Extraction chromatography using DGA resin is an efficient extraction technique for both Am and Pu from strongly acidic solutions [13]. DGA Resin, developed by Eichrom Technologies [14], has an N,N,N0 ,N0 -tetra-n-octyldiglycolamide active group (DGA resin, normal) which has a high affinity for 241Am in concentrated nitric or hydrochloric acid media [13,14]. Zhao et al. [15,16] and Fifield et al. [17] first reported measurements of actinides by AMS using targets of iron oxide where UO and PuO beams were extracted from Cs+ sputter ion source. Both actinides were stripped of their electrons to U+5 or Pu+5, and each was selected and accelerated to the dE/dx detector. Subsequently, several other facilities have performed actinide measurements using isotope dilution techniques, again with oxides beams extracted from their cesium sputter ion sources [18–20]. However, Zhao et al. [21] noted several potential advantages to extracting beams of fluoride anions from a Cs+ sputter-ion source. First, they found stronger MFn than MOn beams, suggesting that the
318
R.J. Cornett et al. / Nuclear Instruments and Methods in Physics Research B 361 (2015) 317–321
fluoride species might produce a more sensitive AMS measurement with a better lower limit of detection. Second, in a sputter target made with a highly fluorinated matrix, Th formed predominantly ThF 5 , U formed mostly UF5 with a small fraction of UF6 , and the PuFn ions were almost all PuF and PuF with roughly equal prob4 5 ability [21]. Measurements of AmFn were later found to form roughly 70% AmF 4 and 30% AmF5 . Following this trend the heavier actinides are expected to mostly form MF 4 , as their chemistry is similar to the lanthanides. The observation that U forms mostly UF 5 provides a significant advantage in Pu analyses since it reduces a potential interference from 238U tails on 239Pu measurements. In addition, fluorine is mono-isotopic, while oxygen has 3 stable isotopes so a beam of fluoride has less potential interferences than a beam of actinide oxide anions. This work builds upon these tests of actinides beams reported by Zhao et al. [21] and further explores the use of fluoride anions to produce beams of Pu and Am for analysis by isotope dilution using AMS. The specific objectives of this work were to: 1. Test further the actinide extraction techniques reported by Kazi et al. [22], 2. Test methods to increase the beam current of Pu and Am using mixtures of fluoride anions, 3. Examine the potential interferences from the fluoride matrix on Pu and Am measurements, 4. Compare measurements of Pu and Am isotopes performed previously at the IsoTrace 2.2 MV AMS laboratory with measurements made using the new 3 MV AMS facility at A.E. Lalonde Laboratory (AELL), University of Ottawa, and 5. Compare these measurements with the values of 239,240Pu and 241 Am in certified reference materials. 2. Methods The concentration of 239,240Pu and 241Am was measured by isotope dilution mass spectrometry using the A.E. Lalonde 3 MV AMS facility at the University of Ottawa [23]. Standard solutions of Pu and Am were obtained from Eckert and Ziegler, Valencia, California, USA. About 20 pg of 242Pu and 243Am were added to each sample and used as the reference isotopes of known concentration. Standards of 239Pu, 240Pu and 241Am also were used to test the AMS system performance. Some samples also were analyzed by alpha spectrometry using the methods described by Cornett and Chant [5] and Kazi et al. [22]. The IAEA certified reference materials from Irish Sea sediment (IAEA 385) and Fangataufa Lagoon sediment (IAEA 384) were used to test the methodology. Other samples were collected from sediment in the Ottawa River near Chalk River Laboratories [24]. The actinides were extracted from the environmental samples by digestion in aqua regia and the Pu and Am isotopes were separated from the sample matrix using column extraction chromatography. DGA resin [14] was used following the extraction protocol detailed by Kazi et al. [22]. In brief, all samples were loaded onto the DGA column in 8 M HNO3 after oxidation of Pu (III) to Pu (IV) using NaNO2. Transformation of all the Pu in the sample to Pu (IV) is essential for its separation from Am and other interfering cations by DGA resin because only Pu (IV) is retained on the DGA cartridge. The column was rinsed with 10 mL each of 8 M and 3 M HNO3, followed by 10 mL 0.1 M HNO3 to remove other cations such as Al and Ca that might interfere with the preparation of fluoride targets for AMS analysis. Then Am was eluted in 15 mL of 0.1 M HCl. Subsequently 15 mL 0.02 M TiCl3 in 0.1 M HCl was used for on-column reduction of Pu (IV) and its desorption from the column. The TiCl3 reduces the Pu (IV) to Pu (III), and the HCl elutes the Pu (III) from the column [22]. One mL of 48% HF and 6 mg Nd, were added to the eluent to coprecipitate the Am and
Table 1 Comparison of AMS operation conditions for the actinides using the same Th + PbF2 targets. Parameter
IsoTrace
Lalonde AMS
Cs+ sputter source
834 type (miniprobe) 10 kV 15 lA 17.5 keV <100 nA
860 type (hemispheric ionizer) 35 kV 15 lA 7 keV >500 nA
0.88 MV 8%
2.50 MV 15%
Total extraction voltage Cs+ flux Cs+ energy Typical 232ThF 5 currents from fresh targets Terminal voltage 232 232 ThF Th+3 transmission 5 ?
Pu from the solution using NdF3. The precipitate was washed, dried and ground to prepare a target for AMS measurement. The Pu or Am co-precipitate with NdF3 was mixed with PbF2 in a ratio of 1:7 (wt/wt). Each target contains 35 mg this sample mix. This separation and target preparation process always produced recoveries measured by alpha spectrometry of greater than 95% [22]. The sample separation used to prepare the samples for analysis by alpha spectrometry was identical to the procedure used for the preparation of AMS targets except the amount of precipitate used to prepare the source (or target) was increased from 60 lg to 8 mg. We believe that the efficiency of the AMS preparation is the same as the preparation used for alpha spectrometry. The A.E. Lalonde Laboratory AMS system was described by Kieser et al. [23]. Pu and Am isotopes were extracted at 35 keV ener gies from an SO-110 Cs+ sputter-ion source as PuF 4 and AmF4 +3 +3 anions and analyzed as Pu or Am using a terminal voltage of 2.5 MV and dual anode dE/dx gaseous ionization detector. The AMS operating parameters used for the actinide measurements are summarized in Table 1. The AMS measurements performed at IsoTrace Laboratories were reported by Kazi et al. [22] using the AMS system parameters reported by Zhao et al. [21]. 3. Results and discussion 3.1. Measuring actinides using fluoride beams 241,243 Am and 239,240,242Pu triply-charged ions were separated from other molecular fragments using a dual anode dE/dx gaseous ionization detector. Fig. 1 shows an example of 243Am+3 spectra which has one of the strongest presence of potentially interfering ions. For the detection of 239242Pu+3 and 241,243Am+3 the most probable interferences are 80Se+, 81Br+ and 159162Dy+2. The interferences result from trace levels of these elements that are present in sample and in the PbF2 and NdF3 that were used to prepare the targets. The count rates of these +1 and +2 ions with the same mass
Fig. 1. 243Am two dimensional spectrum in the gas ionization detector filled with 10 mBar isobutane gas and equipped with a 75 nm SiN entrance window.
319
R.J. Cornett et al. / Nuclear Instruments and Methods in Physics Research B 361 (2015) 317–321
1e5
Amount measured, fg
1e4
1e3
1e2
1e1 Fig. 2. The effect of PbF2 concentration on the counts per second (cps) or current of NdF 4 and PbF3 produced in the 834 ion source at IsoTrace. Each target contained 20 pg of 243Am and 242Pu yield tracer and 30 mg of PbF2/NdF3 in the weight ratio shown in the figure. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
to charge and energy to charge ratios were all low and did not overlap with any of the two dimensional windows set up for the isotopes of interest. The DGA separation reduced the concentration of these elements in the sample matrix so that the low count-rates of these contaminants did not affect the Am and Pu measurements. PbF2 is used with the NdF3 co-precipitated as a matrix in Am and Pu analysis. Various PbF2:NdF3 ratio (2, 4, 6, 9) mix were studied to find an optimum AMS analysis condition Am and Pu (Fig. 2). The PbF2:NdF3 mass ratio in targets containing a fixed 35 mg of total mixture was adjusted to test the influence of the target composition on the count rates of Pu and Am. The PbF 3 current was measured before the accelerator (after the low energy magnet), and it increased as more PbF2 was added to the target (green bars in Fig. 2). The current of NdF 4 , also measured after the low energy magnet, decreased as the mass ratio of PbF2:NdF3 was increased from 4 to 20 in the targets (red bars in Fig. 2). The count rate of both Am and Pu increased as the NdF3 precipitate was diluted with PbF2, until the PbF2:NdF3 ratio exceeded 8:1 (blue and purple bars in Fig. 2). The count rates of Am and Pu should have decreased when the sample target of NdF3 containing the actinide was diluted with PbF2. Based upon only dilution, a decrease of about 8 fold was predicted but the measured count rates increased about 2 fold. A similar observation using fluoride anions was reported during the measurement of 210Pb [25]. Sookdeo et al. [25] added additional fluoride as CsF and AgF to their AMS targets and concluded that an excess of fluorine donor molecules was essential to maximize the production of 210Pb anions. In this work, an excess of fluoride donor molecules was important since this analytical method used higher oxidation states (PuF 4 and AmF4 beams) that need additional fluorine atoms. The highest count rates of Am and Pu were both measured with mass ratios of PbF2:NdF3 of 6 and 8 (Fig. 2) so a mass ratio of 7 was used in all measurements. 3.2. Intercomparisons with standards and certified reference materials 241,243 Am and 239,240,242Pu standards with known activity, samples measured by alpha spectrometry and the 241Am and 239,240Pu extracted from two certified reference materials were all measured at A.E. Lalonde Laboratories to test this Am and Pu separation and measurement technique. Fig. 3 summarizes the measurements. All of the 241,243Am and 239,240,242Pu measurements agree with the expected values of the certified IAEA standards. The measured values fell on a 1:1 line within the statistical uncertainty of the
1e0 1e0
1e1
1e2
1e3
1e4
1e5
Amount added, fg Fig. 3. Comparison of 239,240Pu & 241Am concentrations in standards and certified references materials with the values measured at A.E. Lalonde AMS laboratory. (N 239 Pu, d 240Pu, 241Am; data point represents amount of isotope per AMS target).
measurements (slope = 0.985 ± 0.020, p > 0.999, R2 = 0.997) when they are compared to the expected quantity of each isotope in the target. The external error is shown by the error bars (±r) in Fig. 3 and it was usually less than the size of the symbol shown on the figure. The external error was slightly larger than the internal error (average 0.8–2.3%), due to the statistical fluctuations in counts collected in the detector (0.5–2%). The difference between the internal and external error reflects fluctuations in the current of PuF 4 and AmF4 from the ion source during the measurement. Each isotope was measured for 5 s at the final GIC counter using a slow bouncing routine of the injection magnet [23]. When the beam current from the target changes during this counting cycle, the variation in the isotope ratio will be larger than the statistical uncertainty in the counting rate. Methods to control the beam current from fluoride sources more precisely are currently being developed. The amount of Pu and Am in the targets ranged from a few fg in the samples to 1–6 pg in the standards. The quantity of the yield tracer that was measured was less than the amount added to the sample because only a fraction of each precipitate (about 12%) was used in each AMS target. The count rate of the 242Pu and 243 Am yield tracers ranged from 59 to 95 and 82 to 131 counts per second per picogram (cps/pg) of tracer, respectively. A subset of these standards and reference materials was also measured by AMS at IsoTrace, so an intercomparison between the two measurements is possible. A comparison between the two laboratories is useful because the same extraction and preparation technique was used for all the samples so the only variable is the measurement system. Fig. 4 compares the measurements performed on the IsoTrace AMS system with the same samples measured at A.E. Lalonde Laboratory. All of the 241,243Am and 239,240,242 Pu measurements agreed with the expected values and the values from the two measurement systems agreed well. The measured values fell on a 1:1 line within the statistical uncertainty of the measurements (slope = 0.970 ± 0.018, p > 0.999, R2 = 0.996). The external errors, shown by the error bars (Fig. 4), were larger than the internal errors due to the fluctuations in count rate measured using the IsoTrace AMS system. The new accelerator mass spectrometer at A.E. Lalonde was designed to address these items and it improved the resolution
320
R.J. Cornett et al. / Nuclear Instruments and Methods in Physics Research B 361 (2015) 317–321 Table 3 Comparison of AMS measurements of Pu and Am using MF 4 and MO ions.
1e6
Parameter
ANU [17]
LLNL [18]
VERA [19,26]
ANSTO [27]
ETH [28,29]
This study
Negatively charged ions 242 Pu sensitivity (cps/pg) 243 Am sensitivity (cps/pg) 239 Pu external precision (%) 240 Pu external precision (%) 241 Am external precision (%) 239 Pu detection limit (fg) 240 Pu detection limit (fg) 241 Am detection limit (fg) Charge state (M+)
Oxide
Oxide
Oxide
Oxide
Oxide
Fluoride
0.24 ± 0.14
–
13.5
–
170
78 ± 15
–
–
–
–
–
106 ± 21
7.5
4.6
3.2
3.4
2.4
0.82
9.2
4.6
3.6
3.1
2.1
1.3
–
–
–
–
–
2.3
0.2
0.2
–
3.2
0.4
1.3
0.5
0.3
–
0.5
0.4
0.4
–
–
–
–
0.08
1.8
7+
5+
5+
5+
3+
3+
AE Lalonde data, fg g-1
1e5
1e4
1e3
1e2
1e1
1e0 1e0
Where no error is cited, no error was reported in the cited publication.
1e1
1e2
1e3
1e4
1e5
1e6
IsoTrace data, fg g-1 Fig. 4. Intercomparison of A.E. Lalonde AMS systems.
239,240
Pu and
241
Am measurements using IsoTrace and
and stability of the magnets, stability of the electronics and high voltage control as well as improved detection capabilities [23]. Table 2 summarizes the performance of the Pu and Am measurements of the two systems. The A.E. Lalonde AMS has a better sensitivity, precision and lower process blank compared to the IsoTrace facility. The uncertainty in the measurements at AELL is less than at IsoTrace because of most notably stronger and more stable ion beam produced from the SO-110 Cs sputter source. The result is less scattering and better detection limits. The 241,243Am and 239,240,242Pu measurements made in this study using fluoride anions were compared with published data that used the oxide anion generation method (Table 3). The published reports that describe the oxide anion production technique all used co-precipitates of Pu with iron hydroxide that was calcinated and then mixed with either Al, Nb, Ag to prepare a target for AMS measurement. Then the PuO anions were stripped to produce Pu+ ions; usually to an oxidation state of five or higher. The higher oxidation states were selected because these oxidation states eliminate some potential interference [17–19,26,27]. To date, the highest count rates of 242Pu per picogram of isotope were generated at the ETH Zurich facility [29]. The count rates measured at A.E. Lalonde Laboratories using beams of fluoride anion beams are about 50% of the best measurements with the oxide anion beam. There are insufficient measurements of 241Am to provide a detailed comparison. The detection limit of 239,240Pu and 241Am Table 2 Summary of Pu and Am measurement performance. Parameter
IsoTrace
Lalonde
Mass of sample required (mg) Mass ratio of PbF2:NdF3 in target 242 Pu sensitivity (cps/pg) 243 Am sensitivity (cps/pg) 239 Pu external precision (%) 240 Pu external precision (%) 241 Am external precision (%) 239 Pu process blank (fg) 240 Pu process blank (fg) 241 Am process blank (fg)
35 7 2.1 ± 0.4 3.5 ± 0.6 3.0 4.0 7.3 4.8 4.3 4.7
8 7 78 ± 15 106 ± 21 0.82 1.3 2.3 0.03 0.03 0.02
at each laboratory are determined by trace amounts of these actinides in chemicals used in the preparation of the samples. All laboratories report detection limits that are a few femtograms or lower (Table 3). The fluoride method has a slightly better precision than the precision reported by laboratories that use the oxide anion technique. In summary, the production of actinide fluoride anions produced similar AMS results to the AMS measurements made using oxide anions. In the future, an evaluation of these two methods using the same accelerator system and sample preparation techniques would be useful to provide a direct comparison of the advantages of each method. 4. Conclusions 239,240 Pu and 241Am were successfully separated from environmental sample matrices and analyzed in fluoride matrix by AMS at A.E. Lalonde Laboratories using the extraction methods of Kazi et al. [22]. The dual anode GIC effectively separated Am and Pu in charge state +3 from potential interferences present in the fluoride targets. The ion count rates of Pu and Am isotopes were optimal when the targets were prepared with an excess of PbF2 in a ratio of PbF2:NdF3 of 7:1. Measurements of 239,240Pu and 241Am made at A.E. Lalonde Laboratory agreed, within their statistical uncertainty, with independent measurements made using the IsoTrace AMS system. The measured concentrations of 239,240Pu and 241Am agreed with the concentration in standards of known activity, with measurements of these isotopes by alpha spectroscopy and with two IAEA certified reference materials (IAEA384 and IAEA385). The sensitivity of the measurements was 78 ± 15 cps/pg for 242Pu and 106 ± 21 cps/pg for 243Am. The precision of the measurements ranged from 0.8% to 2.3% when the measurement was not limited by counting statistics. The precision was controlled by the stability of the molecular fluoride anion current from the ion source, and the detection limit was controlled by the presence of about 0.03 fg of these actinides in reagents that were present in our chemical preparation process. Work is currently underway that will address these items.
References [1] UNSCEAR, Sources and effects of ionizing radiation., 2008 (accessed 07.14). [2] E.P. Hardy, P.W. Krey, H.L. Volchok, Nature 241 (1973) 444.
R.J. Cornett et al. / Nuclear Instruments and Methods in Physics Research B 361 (2015) 317–321 [3] UNSCEAR, The Chernobyl accident. UNSCEAR’s assessments of the radiation effects., 2014 (accessed 07.14). [4] R.J. Cornett, T. Eve, A.E. Docherty, E.L. Cooper, Appl. Radiat. Isot. 11 (1995) 1239. [5] R.J. Cornett, L. Chant, Can. J. Fish. Aquat. Sci. 45 (1988) 407. [6] G.M. Herring, J. Vaughan, M. Williamson, Health Phys. 8 (1962) 430. [7] D. Lariviere, V.F. Taylor, R.D. Evans, R.J. Cornett, Spectrochim. Acta B 61 (2006) 877. [8] L.K. Fifield, Quat. Geochronol. 3 (2008) 276. [9] M. Ayranov, U. Kraehenbuhl, H. Sahli, S. Rollin, M. Burger, Radiochim. Acta 93 (2005) 249. [10] J.-S. Oh, P.E. Warwick, I.W. Croudace, S.-H. Lee, J. Radioanal. Nucl. Chem. 298 (2013) 353. [11] J.J. Charlton, M.J. Sepaniak, A.K. Sides, T.G. Schaaff, D.K. Mann, J.A. Bradshaw, J. Anal. At. Spectrom. 28 (2013) 711. [12] J. Jernstrom, J. Lehto, M. Betti, J. Radioanal. Nucl. Chem. 274 (2007) 95. [13] E.P. Horwitz, D.R. McAlister, A.H. Bond, R.E. Barrans Jr., Solvent Extr. Ion Exch. 23 (2005) 319. [14] Eichrom Industries Inc., , 2014 (accessed 03.14). [15] X.-L. Zhao, M.-J. Nadeau, L.R. Kilius, A.E. Litberland, Nucl. Instr. Meth. B 92 (1994) 249. [16] X.-L. Zhao, M.-J. Nadeau, M.A. Garwan, L.R. Kilius, A.E. Litherland, Nucl. Instr. Meth. B 92 (1994) 258.
321
[17] L.K. Fifield, R.G. Cresswell, M.L. di Tada, T.R. Ophel, J.P. Day, A.P. Clacher, S.J. King, N.D. Priest, Nucl. Instr. Meth. B 117 (1996) 295. [18] T.A. Brown, A.A. Marchetti, R.E. Martinelli, C.C. Cox, J.P. Knezovich, T.F. Hamilton, Nucl. Instr. Meth. B 223–224 (2004) 788. [19] E. Hrnecek, P. Steier, A. Wallner, Appl. Radiat. Isot. 63 (2005) 633. [20] E. Chamizo, S.M. Enamorado, M. García-León, M. Suter, L. Wacker, Nucl. Instr. Meth. Phys. Res. B 266 (2008) 4948. [21] X.-L. Zhao, W.E. Kieser, X. Dai, N.D. Priest, S. Kramer-Tremblay, J. Eliades, A.E. Litherland, Nucl. Instr. Meth. B 294 (2013) 356. [22] Z.H. Kazi, R.J. Cornett, X.-L. Zhao, W.E. Kieser, Anal. Chim. Acta 829 (2014) 75. [23] W.E. Kieser, I.D. Clark, R.J. Cornett, A.E. Litherland, X.L. Zhao, M. Klein, D. Mous, J.F. Alary, Nucl. Instr. Meth. B. (2014), (Submitted for publication). [24] V.F. Taylor, R.D. Evans, R.J. Cornett, J. Environ. Radioact. 99 (2008) 109. [25] A. Sookdeo, R.J. Cornett, X.L. Zhao, W.E. Kieser, Nucl. Instr. Meth. B. (Submitted for publication). [26] C. Vockenhuber, I. Ahmad, R. Golser, W. Kutschera, V. Liechtenstein, A. Priller, P. Steier, S. Winkler, J. Mass Spectrom. 223–224 (2003) 713. [27] M.A.C. Hotchkis, D.P. Child, B. Zorko, Nucl. Instr. Meth. B 268 (2010) 1257. [28] M. Christl, C. Vockenhuber, P.W. Kubik, L. Wacker, J. Lachner, V. Alfimov, H.-A. Synal, Nucl. Instr. Meth. B 294 (2013) 29. [29] X. Dai, M. Christl, S. Kramer-Tremblay, H.-A. Synal, J. Anal. At. Spectrom. 27 (2012) 126.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Actinide measurements by AMS using fluoride matrices R.J. Cornett a,b,⇑, Z.H. Kazi a,b, X.-L. Zhao a,c, M.G. Chartrand a,b, R.J. Charles a,c, W.E. Kieser a,c a
André E. Lalonde AMS Laboratory, University of Ottawa, 150 Louis Pasteur, Ottawa, ON K1N 6N5, Canada Department of Earth Sciences, University of Ottawa, 150 Louis Pasteur, Ottawa, ON K1N 6N5, Canada c Department of Physics, University of Ottawa, 150 Louis Pasteur, Ottawa, ON K1N 6N5, Canada b
a r t i c l e
i n f o
Article history: Received 20 November 2014 Received in revised form 11 February 2015 Accepted 15 February 2015 Available online 4 March 2015 Keywords: Actinide Plutonium Americium Fluoride AMS
a b s t r a c t Actinides can be measured by alpha spectroscopy (AS), mass spectroscopy or accelerator mass spectrometry (AMS). We tested a simple method to separate Pu and Am isotopes from the sample matrix using a single extraction chromatography column. The actinides in the column eluent were then measured by AS or AMS using a fluoride target matrix. Pu and Am were coprecipitated with NdF3. The strongest AMS beams of Pu and Am were produced when there was a large excess of fluoride donor atoms in the target and the NdF3 precipitates were diluted about 6–8 fold with PbF2. The measured concentrations of 239,240Pu and 241Am agreed with the concentrations in standards of known activity and with two IAEA certified reference materials. Measurements of 239,240Pu and 241Am made at A.E. Lalonde AMS Laboratory agree, within their statistical uncertainty, with independent measurements made using the IsoTrace AMS system. This work demonstrated that fluoride targets can produce reliable beams of actinide anions and that the measurement of actinides using fluorides agree with published values in certified reference materials. Ó 2015 Published by Elsevier B.V.
1. Introduction Actinide isotopes with masses higher than uranium (U) are produced in nuclear reactors and in nuclear weapons tests. These isotopes have been released into the environment from weapons testing fall out [1,2], reactor accidents [3] and emissions [4,5], and fuel reprocessing operations [1]. The most abundant anthropogenic actinide isotopes distributed globally in the environment are 239,240Pu and 241Am [2]. These isotopes emit alpha particles and they accumulate to higher concentrations in bone [6]. As a result 239,240Pu and 241Am have a higher dose per unit intake, they are often the most important radionuclides per unit activity in environmental assessments and they are measured to assess the risks to human and biotic exposures. 239,240 Pu and 241Am must be extracted and separated from the sample matrix before their concentrations can be measured by alpha or mass spectrometry. The separation and purification are important steps because several different actinide isotopes emit alpha particles with similar energies [7]. For example, the alpha energy spectra for 228Th and 241Am overlap as do the energies of 239 Pu and 240Pu. 241Am and 241Pu are isobaric, and some molecular
⇑ Corresponding author at: Department of Earth Sciences, University of Ottawa, 150 Louis Pasteur, Ottawa, ON K1N 6N5, Canada. E-mail address: [email protected] (R.J. Cornett). http://dx.doi.org/10.1016/j.nimb.2015.02.039 0168-583X/Ó 2015 Published by Elsevier B.V.
interferences can overlap in the mass spectrum so the separation is important for mass spectrometry measurements [8]. Many techniques have been developed to separate Am and Pu from matrix materials including solvent extraction [9,10], solid phase extraction [11], ion exchange [12], and extraction chromatography [7]. 241 Am that has been extracted in concentrated acid is difficult to separate from the sample matrix because this isotope has a low affinity for many chromatography systems in high concentrations of HCl and HNO3. Extraction chromatography using DGA resin is an efficient extraction technique for both Am and Pu from strongly acidic solutions [13]. DGA Resin, developed by Eichrom Technologies [14], has an N,N,N0 ,N0 -tetra-n-octyldiglycolamide active group (DGA resin, normal) which has a high affinity for 241Am in concentrated nitric or hydrochloric acid media [13,14]. Zhao et al. [15,16] and Fifield et al. [17] first reported measurements of actinides by AMS using targets of iron oxide where UO and PuO beams were extracted from Cs+ sputter ion source. Both actinides were stripped of their electrons to U+5 or Pu+5, and each was selected and accelerated to the dE/dx detector. Subsequently, several other facilities have performed actinide measurements using isotope dilution techniques, again with oxides beams extracted from their cesium sputter ion sources [18–20]. However, Zhao et al. [21] noted several potential advantages to extracting beams of fluoride anions from a Cs+ sputter-ion source. First, they found stronger MFn than MOn beams, suggesting that the
318
R.J. Cornett et al. / Nuclear Instruments and Methods in Physics Research B 361 (2015) 317–321
fluoride species might produce a more sensitive AMS measurement with a better lower limit of detection. Second, in a sputter target made with a highly fluorinated matrix, Th formed predominantly ThF 5 , U formed mostly UF5 with a small fraction of UF6 , and the PuFn ions were almost all PuF and PuF with roughly equal prob4 5 ability [21]. Measurements of AmFn were later found to form roughly 70% AmF 4 and 30% AmF5 . Following this trend the heavier actinides are expected to mostly form MF 4 , as their chemistry is similar to the lanthanides. The observation that U forms mostly UF 5 provides a significant advantage in Pu analyses since it reduces a potential interference from 238U tails on 239Pu measurements. In addition, fluorine is mono-isotopic, while oxygen has 3 stable isotopes so a beam of fluoride has less potential interferences than a beam of actinide oxide anions. This work builds upon these tests of actinides beams reported by Zhao et al. [21] and further explores the use of fluoride anions to produce beams of Pu and Am for analysis by isotope dilution using AMS. The specific objectives of this work were to: 1. Test further the actinide extraction techniques reported by Kazi et al. [22], 2. Test methods to increase the beam current of Pu and Am using mixtures of fluoride anions, 3. Examine the potential interferences from the fluoride matrix on Pu and Am measurements, 4. Compare measurements of Pu and Am isotopes performed previously at the IsoTrace 2.2 MV AMS laboratory with measurements made using the new 3 MV AMS facility at A.E. Lalonde Laboratory (AELL), University of Ottawa, and 5. Compare these measurements with the values of 239,240Pu and 241 Am in certified reference materials. 2. Methods The concentration of 239,240Pu and 241Am was measured by isotope dilution mass spectrometry using the A.E. Lalonde 3 MV AMS facility at the University of Ottawa [23]. Standard solutions of Pu and Am were obtained from Eckert and Ziegler, Valencia, California, USA. About 20 pg of 242Pu and 243Am were added to each sample and used as the reference isotopes of known concentration. Standards of 239Pu, 240Pu and 241Am also were used to test the AMS system performance. Some samples also were analyzed by alpha spectrometry using the methods described by Cornett and Chant [5] and Kazi et al. [22]. The IAEA certified reference materials from Irish Sea sediment (IAEA 385) and Fangataufa Lagoon sediment (IAEA 384) were used to test the methodology. Other samples were collected from sediment in the Ottawa River near Chalk River Laboratories [24]. The actinides were extracted from the environmental samples by digestion in aqua regia and the Pu and Am isotopes were separated from the sample matrix using column extraction chromatography. DGA resin [14] was used following the extraction protocol detailed by Kazi et al. [22]. In brief, all samples were loaded onto the DGA column in 8 M HNO3 after oxidation of Pu (III) to Pu (IV) using NaNO2. Transformation of all the Pu in the sample to Pu (IV) is essential for its separation from Am and other interfering cations by DGA resin because only Pu (IV) is retained on the DGA cartridge. The column was rinsed with 10 mL each of 8 M and 3 M HNO3, followed by 10 mL 0.1 M HNO3 to remove other cations such as Al and Ca that might interfere with the preparation of fluoride targets for AMS analysis. Then Am was eluted in 15 mL of 0.1 M HCl. Subsequently 15 mL 0.02 M TiCl3 in 0.1 M HCl was used for on-column reduction of Pu (IV) and its desorption from the column. The TiCl3 reduces the Pu (IV) to Pu (III), and the HCl elutes the Pu (III) from the column [22]. One mL of 48% HF and 6 mg Nd, were added to the eluent to coprecipitate the Am and
Table 1 Comparison of AMS operation conditions for the actinides using the same Th + PbF2 targets. Parameter
IsoTrace
Lalonde AMS
Cs+ sputter source
834 type (miniprobe) 10 kV 15 lA 17.5 keV <100 nA
860 type (hemispheric ionizer) 35 kV 15 lA 7 keV >500 nA
0.88 MV 8%
2.50 MV 15%
Total extraction voltage Cs+ flux Cs+ energy Typical 232ThF 5 currents from fresh targets Terminal voltage 232 232 ThF Th+3 transmission 5 ?
Pu from the solution using NdF3. The precipitate was washed, dried and ground to prepare a target for AMS measurement. The Pu or Am co-precipitate with NdF3 was mixed with PbF2 in a ratio of 1:7 (wt/wt). Each target contains 35 mg this sample mix. This separation and target preparation process always produced recoveries measured by alpha spectrometry of greater than 95% [22]. The sample separation used to prepare the samples for analysis by alpha spectrometry was identical to the procedure used for the preparation of AMS targets except the amount of precipitate used to prepare the source (or target) was increased from 60 lg to 8 mg. We believe that the efficiency of the AMS preparation is the same as the preparation used for alpha spectrometry. The A.E. Lalonde Laboratory AMS system was described by Kieser et al. [23]. Pu and Am isotopes were extracted at 35 keV ener gies from an SO-110 Cs+ sputter-ion source as PuF 4 and AmF4 +3 +3 anions and analyzed as Pu or Am using a terminal voltage of 2.5 MV and dual anode dE/dx gaseous ionization detector. The AMS operating parameters used for the actinide measurements are summarized in Table 1. The AMS measurements performed at IsoTrace Laboratories were reported by Kazi et al. [22] using the AMS system parameters reported by Zhao et al. [21]. 3. Results and discussion 3.1. Measuring actinides using fluoride beams 241,243 Am and 239,240,242Pu triply-charged ions were separated from other molecular fragments using a dual anode dE/dx gaseous ionization detector. Fig. 1 shows an example of 243Am+3 spectra which has one of the strongest presence of potentially interfering ions. For the detection of 239242Pu+3 and 241,243Am+3 the most probable interferences are 80Se+, 81Br+ and 159162Dy+2. The interferences result from trace levels of these elements that are present in sample and in the PbF2 and NdF3 that were used to prepare the targets. The count rates of these +1 and +2 ions with the same mass
Fig. 1. 243Am two dimensional spectrum in the gas ionization detector filled with 10 mBar isobutane gas and equipped with a 75 nm SiN entrance window.
319
R.J. Cornett et al. / Nuclear Instruments and Methods in Physics Research B 361 (2015) 317–321
1e5
Amount measured, fg
1e4
1e3
1e2
1e1 Fig. 2. The effect of PbF2 concentration on the counts per second (cps) or current of NdF 4 and PbF3 produced in the 834 ion source at IsoTrace. Each target contained 20 pg of 243Am and 242Pu yield tracer and 30 mg of PbF2/NdF3 in the weight ratio shown in the figure. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
to charge and energy to charge ratios were all low and did not overlap with any of the two dimensional windows set up for the isotopes of interest. The DGA separation reduced the concentration of these elements in the sample matrix so that the low count-rates of these contaminants did not affect the Am and Pu measurements. PbF2 is used with the NdF3 co-precipitated as a matrix in Am and Pu analysis. Various PbF2:NdF3 ratio (2, 4, 6, 9) mix were studied to find an optimum AMS analysis condition Am and Pu (Fig. 2). The PbF2:NdF3 mass ratio in targets containing a fixed 35 mg of total mixture was adjusted to test the influence of the target composition on the count rates of Pu and Am. The PbF 3 current was measured before the accelerator (after the low energy magnet), and it increased as more PbF2 was added to the target (green bars in Fig. 2). The current of NdF 4 , also measured after the low energy magnet, decreased as the mass ratio of PbF2:NdF3 was increased from 4 to 20 in the targets (red bars in Fig. 2). The count rate of both Am and Pu increased as the NdF3 precipitate was diluted with PbF2, until the PbF2:NdF3 ratio exceeded 8:1 (blue and purple bars in Fig. 2). The count rates of Am and Pu should have decreased when the sample target of NdF3 containing the actinide was diluted with PbF2. Based upon only dilution, a decrease of about 8 fold was predicted but the measured count rates increased about 2 fold. A similar observation using fluoride anions was reported during the measurement of 210Pb [25]. Sookdeo et al. [25] added additional fluoride as CsF and AgF to their AMS targets and concluded that an excess of fluorine donor molecules was essential to maximize the production of 210Pb anions. In this work, an excess of fluoride donor molecules was important since this analytical method used higher oxidation states (PuF 4 and AmF4 beams) that need additional fluorine atoms. The highest count rates of Am and Pu were both measured with mass ratios of PbF2:NdF3 of 6 and 8 (Fig. 2) so a mass ratio of 7 was used in all measurements. 3.2. Intercomparisons with standards and certified reference materials 241,243 Am and 239,240,242Pu standards with known activity, samples measured by alpha spectrometry and the 241Am and 239,240Pu extracted from two certified reference materials were all measured at A.E. Lalonde Laboratories to test this Am and Pu separation and measurement technique. Fig. 3 summarizes the measurements. All of the 241,243Am and 239,240,242Pu measurements agree with the expected values of the certified IAEA standards. The measured values fell on a 1:1 line within the statistical uncertainty of the
1e0 1e0
1e1
1e2
1e3
1e4
1e5
Amount added, fg Fig. 3. Comparison of 239,240Pu & 241Am concentrations in standards and certified references materials with the values measured at A.E. Lalonde AMS laboratory. (N 239 Pu, d 240Pu, 241Am; data point represents amount of isotope per AMS target).
measurements (slope = 0.985 ± 0.020, p > 0.999, R2 = 0.997) when they are compared to the expected quantity of each isotope in the target. The external error is shown by the error bars (±r) in Fig. 3 and it was usually less than the size of the symbol shown on the figure. The external error was slightly larger than the internal error (average 0.8–2.3%), due to the statistical fluctuations in counts collected in the detector (0.5–2%). The difference between the internal and external error reflects fluctuations in the current of PuF 4 and AmF4 from the ion source during the measurement. Each isotope was measured for 5 s at the final GIC counter using a slow bouncing routine of the injection magnet [23]. When the beam current from the target changes during this counting cycle, the variation in the isotope ratio will be larger than the statistical uncertainty in the counting rate. Methods to control the beam current from fluoride sources more precisely are currently being developed. The amount of Pu and Am in the targets ranged from a few fg in the samples to 1–6 pg in the standards. The quantity of the yield tracer that was measured was less than the amount added to the sample because only a fraction of each precipitate (about 12%) was used in each AMS target. The count rate of the 242Pu and 243 Am yield tracers ranged from 59 to 95 and 82 to 131 counts per second per picogram (cps/pg) of tracer, respectively. A subset of these standards and reference materials was also measured by AMS at IsoTrace, so an intercomparison between the two measurements is possible. A comparison between the two laboratories is useful because the same extraction and preparation technique was used for all the samples so the only variable is the measurement system. Fig. 4 compares the measurements performed on the IsoTrace AMS system with the same samples measured at A.E. Lalonde Laboratory. All of the 241,243Am and 239,240,242 Pu measurements agreed with the expected values and the values from the two measurement systems agreed well. The measured values fell on a 1:1 line within the statistical uncertainty of the measurements (slope = 0.970 ± 0.018, p > 0.999, R2 = 0.996). The external errors, shown by the error bars (Fig. 4), were larger than the internal errors due to the fluctuations in count rate measured using the IsoTrace AMS system. The new accelerator mass spectrometer at A.E. Lalonde was designed to address these items and it improved the resolution
320
R.J. Cornett et al. / Nuclear Instruments and Methods in Physics Research B 361 (2015) 317–321 Table 3 Comparison of AMS measurements of Pu and Am using MF 4 and MO ions.
1e6
Parameter
ANU [17]
LLNL [18]
VERA [19,26]
ANSTO [27]
ETH [28,29]
This study
Negatively charged ions 242 Pu sensitivity (cps/pg) 243 Am sensitivity (cps/pg) 239 Pu external precision (%) 240 Pu external precision (%) 241 Am external precision (%) 239 Pu detection limit (fg) 240 Pu detection limit (fg) 241 Am detection limit (fg) Charge state (M+)
Oxide
Oxide
Oxide
Oxide
Oxide
Fluoride
0.24 ± 0.14
–
13.5
–
170
78 ± 15
–
–
–
–
–
106 ± 21
7.5
4.6
3.2
3.4
2.4
0.82
9.2
4.6
3.6
3.1
2.1
1.3
–
–
–
–
–
2.3
0.2
0.2
–
3.2
0.4
1.3
0.5
0.3
–
0.5
0.4
0.4
–
–
–
–
0.08
1.8
7+
5+
5+
5+
3+
3+
AE Lalonde data, fg g-1
1e5
1e4
1e3
1e2
1e1
1e0 1e0
Where no error is cited, no error was reported in the cited publication.
1e1
1e2
1e3
1e4
1e5
1e6
IsoTrace data, fg g-1 Fig. 4. Intercomparison of A.E. Lalonde AMS systems.
239,240
Pu and
241
Am measurements using IsoTrace and
and stability of the magnets, stability of the electronics and high voltage control as well as improved detection capabilities [23]. Table 2 summarizes the performance of the Pu and Am measurements of the two systems. The A.E. Lalonde AMS has a better sensitivity, precision and lower process blank compared to the IsoTrace facility. The uncertainty in the measurements at AELL is less than at IsoTrace because of most notably stronger and more stable ion beam produced from the SO-110 Cs sputter source. The result is less scattering and better detection limits. The 241,243Am and 239,240,242Pu measurements made in this study using fluoride anions were compared with published data that used the oxide anion generation method (Table 3). The published reports that describe the oxide anion production technique all used co-precipitates of Pu with iron hydroxide that was calcinated and then mixed with either Al, Nb, Ag to prepare a target for AMS measurement. Then the PuO anions were stripped to produce Pu+ ions; usually to an oxidation state of five or higher. The higher oxidation states were selected because these oxidation states eliminate some potential interference [17–19,26,27]. To date, the highest count rates of 242Pu per picogram of isotope were generated at the ETH Zurich facility [29]. The count rates measured at A.E. Lalonde Laboratories using beams of fluoride anion beams are about 50% of the best measurements with the oxide anion beam. There are insufficient measurements of 241Am to provide a detailed comparison. The detection limit of 239,240Pu and 241Am Table 2 Summary of Pu and Am measurement performance. Parameter
IsoTrace
Lalonde
Mass of sample required (mg) Mass ratio of PbF2:NdF3 in target 242 Pu sensitivity (cps/pg) 243 Am sensitivity (cps/pg) 239 Pu external precision (%) 240 Pu external precision (%) 241 Am external precision (%) 239 Pu process blank (fg) 240 Pu process blank (fg) 241 Am process blank (fg)
35 7 2.1 ± 0.4 3.5 ± 0.6 3.0 4.0 7.3 4.8 4.3 4.7
8 7 78 ± 15 106 ± 21 0.82 1.3 2.3 0.03 0.03 0.02
at each laboratory are determined by trace amounts of these actinides in chemicals used in the preparation of the samples. All laboratories report detection limits that are a few femtograms or lower (Table 3). The fluoride method has a slightly better precision than the precision reported by laboratories that use the oxide anion technique. In summary, the production of actinide fluoride anions produced similar AMS results to the AMS measurements made using oxide anions. In the future, an evaluation of these two methods using the same accelerator system and sample preparation techniques would be useful to provide a direct comparison of the advantages of each method. 4. Conclusions 239,240 Pu and 241Am were successfully separated from environmental sample matrices and analyzed in fluoride matrix by AMS at A.E. Lalonde Laboratories using the extraction methods of Kazi et al. [22]. The dual anode GIC effectively separated Am and Pu in charge state +3 from potential interferences present in the fluoride targets. The ion count rates of Pu and Am isotopes were optimal when the targets were prepared with an excess of PbF2 in a ratio of PbF2:NdF3 of 7:1. Measurements of 239,240Pu and 241Am made at A.E. Lalonde Laboratory agreed, within their statistical uncertainty, with independent measurements made using the IsoTrace AMS system. The measured concentrations of 239,240Pu and 241Am agreed with the concentration in standards of known activity, with measurements of these isotopes by alpha spectroscopy and with two IAEA certified reference materials (IAEA384 and IAEA385). The sensitivity of the measurements was 78 ± 15 cps/pg for 242Pu and 106 ± 21 cps/pg for 243Am. The precision of the measurements ranged from 0.8% to 2.3% when the measurement was not limited by counting statistics. The precision was controlled by the stability of the molecular fluoride anion current from the ion source, and the detection limit was controlled by the presence of about 0.03 fg of these actinides in reagents that were present in our chemical preparation process. Work is currently underway that will address these items.
References [1] UNSCEAR, Sources and effects of ionizing radiation.
R.J. Cornett et al. / Nuclear Instruments and Methods in Physics Research B 361 (2015) 317–321 [3] UNSCEAR, The Chernobyl accident. UNSCEAR’s assessments of the radiation effects.
321
[17] L.K. Fifield, R.G. Cresswell, M.L. di Tada, T.R. Ophel, J.P. Day, A.P. Clacher, S.J. King, N.D. Priest, Nucl. Instr. Meth. B 117 (1996) 295. [18] T.A. Brown, A.A. Marchetti, R.E. Martinelli, C.C. Cox, J.P. Knezovich, T.F. Hamilton, Nucl. Instr. Meth. B 223–224 (2004) 788. [19] E. Hrnecek, P. Steier, A. Wallner, Appl. Radiat. Isot. 63 (2005) 633. [20] E. Chamizo, S.M. Enamorado, M. García-León, M. Suter, L. Wacker, Nucl. Instr. Meth. Phys. Res. B 266 (2008) 4948. [21] X.-L. Zhao, W.E. Kieser, X. Dai, N.D. Priest, S. Kramer-Tremblay, J. Eliades, A.E. Litherland, Nucl. Instr. Meth. B 294 (2013) 356. [22] Z.H. Kazi, R.J. Cornett, X.-L. Zhao, W.E. Kieser, Anal. Chim. Acta 829 (2014) 75. [23] W.E. Kieser, I.D. Clark, R.J. Cornett, A.E. Litherland, X.L. Zhao, M. Klein, D. Mous, J.F. Alary, Nucl. Instr. Meth. B. (2014), (Submitted for publication). [24] V.F. Taylor, R.D. Evans, R.J. Cornett, J. Environ. Radioact. 99 (2008) 109. [25] A. Sookdeo, R.J. Cornett, X.L. Zhao, W.E. Kieser, Nucl. Instr. Meth. B. (Submitted for publication). [26] C. Vockenhuber, I. Ahmad, R. Golser, W. Kutschera, V. Liechtenstein, A. Priller, P. Steier, S. Winkler, J. Mass Spectrom. 223–224 (2003) 713. [27] M.A.C. Hotchkis, D.P. Child, B. Zorko, Nucl. Instr. Meth. B 268 (2010) 1257. [28] M. Christl, C. Vockenhuber, P.W. Kubik, L. Wacker, J. Lachner, V. Alfimov, H.-A. Synal, Nucl. Instr. Meth. B 294 (2013) 29. [29] X. Dai, M. Christl, S. Kramer-Tremblay, H.-A. Synal, J. Anal. At. Spectrom. 27 (2012) 126.