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Determination of 135Cs by accelerator mass spectrometry
Nuclear Instruments and Methods in Physics Research B xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Determination of
135
Cs by accelerator mass spectrometry
C.M. MacDonald a,b,⇑, C.R.J. Charles a,b, X.-L. Zhao a,c, W.E. Kieser a,c, R.J. Cornett a,b, A.E. Litherland d a
Andre. 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 d IsoTrace Laboratory, University of Toronto, 60 St. George St., Toronto, ON M5S 1A7, Canada b
a r t i c l e
i n f o
Article history: Received 11 December 2014 Received in revised form 2 March 2015 Accepted 5 March 2015 Available online xxxx Keywords: AMS Caesium Isobar
a b s t r a c t The ratio of anthropogenic 135Cs and 137Cs isotopes is characteristic of a uranium fission source. This research evaluates the technique of isotope dilution (yield tracing) for the purpose of quantifying 135Cs by accelerator mass spectrometry with on-line isobar separation. Interferences from Ba, Zn2, and isotopes of equal mass to charge ratios were successfully suppressed. However, some sample crosstalk from source contamination remains. The transmission and di-fluoride ionization efficiencies of Cs isotopes were found to be 8 10 3 and 1.7 10 7 respectively. This quantification of 135Cs using yield tracing by accelerator mass spectrometry shows promise for future environmental sample analysis once the issues of sample crosstalk and low efficiency can be resolved. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Anthropogenic Cs isotopes have been released into the environment from nuclear weapons tests and reactor emissions. As a result, 137Cs and 134Cs have been used in many applications including sediment dating and nuclear forensics [1,2]. 135Cs, which is also produced, can provide additional information about the source of the Cs isotopes by observing the 135Cs:137Cs ratio. However, since the established techniques rely on radiometric measurement of the Cs isotopes, new techniques for the measurement of 135Cs must be developed because of its long half life (2.3 Ma) and pure beta emission. The first detection of 135Cs by accelerator mass spectrometry (AMS) was accomplished using an Isobar Separator for Anions (ISA) which eliminated the ubiquitous Ba isobar interference in Cs samples [3–5]. ISA-AMS has the potential to enhance nuclear contamination forensics that has been pioneered using inductively coupled mass spectrometry (ICPMS) and chronometric tracing techniques proposed by Lee et al., and oceanic tracing techniques which thus far have relied on the shorter lived 137Cs isotope. [2,6,7] For ISA-AMS to be used as an analytical method the ISA-AMS operating parameters must be optimized, its sample requirements and limitations must be determined, and a method to quantify the amount of 135Cs must be developed. A Cs anion beam must be produced in the ion source with a high efficiency to minimize the use ⇑ Corresponding author at: Andre. E. Lalonde AMS Laboratory, University of Ottawa, 150 Louis Pasteur, Ottawa, ON K1N 6N5, Canada.
of valuable sample material and analysis time. Many factors can affect the efficiency of an isotope measurement by AMS. These factors include: 1. Ionization efficiency: the production of the desired ion by the ion source. 2. Isobar suppressor efficiency: the transmission of analyte through the on-line separation of isobars. 3. System efficiency: the transmission of the ion beam through mass filters, electron stripping canal, and other system components. The objectives of this paper was to examine these factors and to determine the best operating conditions and techniques to measure 135Cs, to outline the limitations of the ISA-AMS system, and to develop a method to make an absolute measurement of 135Cs isotope concentration. 2. Methods These experiments were performed at the IsoTrace Laboratory (Toronto, Canada), the 3 MV AMS facility operated by the University of Ottawa (Ottawa, Canada) (Fig. 1). It was the very last use of this facility before its closure. A common AMS technique for quantifying an isotope is to monitor the rare isotope of interest and the abundant isotope of the same element with as little delay between measurements as possible. This can be done to get a relative value, the ratio of the
http://dx.doi.org/10.1016/j.nimb.2015.03.008 0168-583X/Ó 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: C.M. MacDonald et al., Determination of dx.doi.org/10.1016/j.nimb.2015.03.008
135
Cs by accelerator mass spectrometry, Nucl. Instr. Meth. B (2015), http://
2
C.M. MacDonald et al. / Nuclear Instruments and Methods in Physics Research B xxx (2015) xxx–xxx
Fig. 1. Top-down view of the IsoTrace ISA-AMS system.
isotopes, or an absolute value using isotope dilution. However, this technique could not be used for measuring 135Cs since the samples were sputtered with the abundant isotope, 133Cs. Therefore, to obtain the concentration of 135Cs, a known amount of 134Cs was added to the sample. The total yield of this isotope could be tracked through the sample preparation and measurement procedures and could then be used to calculate the absolute amount of 135Cs in an unknown sample. This process of yield tracing is effective as long as there is no significant isotope fractionation in the procedures. For the purposes of this experiment, the known isotope, analyte, and interferences needed to be monitored throughout the measurement process. Accordingly, masses 134, 135, 136, and 137 were observed throughout each experiment to monitor the radiogenic isotopes of Cs and also Ba interference (mass 136). Each mass was observed for 5 min per target, resulting in a total target analysis time of 20 min. Targets included analyte, spike, analyte–spike mixture, and process blanks so that contamination could be quantified, and corrected for if present. The ISA-AMS operating parameters for the full mass range are listed in Table 1. Samples of 135Cs and 134Cs were required to make this measurement. Furthermore, each sample needed to have low or no contamination of the other isotopes. Since there are no standards of 135Cs, and the amount of 135Cs in the available 134Cs standards was not quantified, the samples for these experiments had to be produced. A sample of 134Cs was produced for these experiments by exposing 80 mg of Caesium Carbonate (Aldrich AB8824CHM), 133Cs2CO3, to the neutron flux of the Slowpoke reactor at the Royal Military College of Canada (Kingston, Canada). By exposing this compound to the neutron flux from the reactor there is a probability,
Table 1 AMS system operating parameters for sequential measurements. The isotopes were analyzed as CsF2 ? Cs+3. B1 and B2 refer to the field generated by the injection and high energy analyzing magnets. Isotope
Mass 1 (amu)
B1 (T)
Terminal (MV)
Mass 2 (amu)
B2 (T)
Cs133 Cs134 Cs135 Ba136 Cs137
170.9022534 171.9035064 172.9026964 173.9013764 174.9038764
0.43390 0.43514 0.43642 0.43770 0.43900
1.3505 1.3500 1.3496 1.3491 1.3487
132.90544 133.90670 134.90589 135.90456 136.90707
0.98656500 0.99026279 0.99397331 0.99762948 1.00132165
Please cite this article in press as: C.M. MacDonald et al., Determination of dx.doi.org/10.1016/j.nimb.2015.03.008
represented by the constituents’ neutron cross section, that other heavier isotopes will be created by neutron capture. It was calculated that, given the reactor parameters, exposing the sample for 210 s would produce 18 kBq of 134Cs without significant creation of 135Cs (<0.01%) from a secondary neutron capture reaction. The sample of 135Cs was obtained by the decay of 135Xe. A section of Xe exhaust pipe was supplied by Chalk River Laboratories (Chalk River, Canada) from their Xe gas collection facility. The Xe gas collected after the dissolution of uranium oxide fuel to capture 99 Mo, the precursor to 99mTc which is used for diagnostic imaging. Due to the relatively short half-lives of 135Xe and 137Xe, and the stability of 134Xe, any radio-Cs that would have collected in this section of pipe by Xe decay would have been either 135Cs or 137 Cs. The pipe was rinsed with 5 ml of distilled water to collect any Cs that had built up in the pipe. This 5 ml rinse was measured radiometrically (beta- and hyper pure germanium gamma-spectroscopy) and the only radioactive Cs contaminant that was observed was 137Cs. Cs atoms do not readily form negative ions. Therefore, samples had to be prepared for measurement by AMS so that the ion source would produce a strong molecular anion of CsF2 [8]. To achieve this, the Cs samples were prepared as follows. PbF2 (Alfa Aesar Puratronic 99.999%) was added into pre-weighed clean 15 ml round-bottom Savillex PFA vials. Solutions containing known amounts of 134Cs, 135Cs and naturally abundant Ba isotopes were drawn into PTFE micro-tubing and weighed on a precision Mettler-Toledo microbalance before being expelled into the Savillex vials. 20 drops of 28.9 M HF were then added from a clean PFA Teflon dropper bottle directly into the Savillex vials containing the PbF2 + Cs or PbF2 + Ba. The mixture was capped tightly and fluxed inside an isolated, vented Plexiglas box at 90 °C on a hotplate in their same Savillex capsules with occasional mixing for at least 2 h. Samples were slowly evaporated to dryness for >5 h at 90 °C by removing the lids of the Savillex capsules; HF fumes were withdrawn under mild suction from inside the Plexiglas box. The final samples of 10 mg of dry material were pressed into 1.3 mm diameter stainless steel targets under 0.6 GPa of pressure. After sample preparation and target pressing, the targets were loaded into an SO-110 sputter source (High Voltage Engineering B.V., the Netherlands) for analysis. The SO-110 operating parameters for these experiments are in Table 2. The beam of ions goes from the source, through a low energy injection magnet, ISA, 135
Cs by accelerator mass spectrometry, Nucl. Instr. Meth. B (2015), http://
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C.M. MacDonald et al. / Nuclear Instruments and Methods in Physics Research B xxx (2015) xxx–xxx Table 2 SO-110 operation parameters for Cs measurements.
Table 3 Samples used in the Cs radioisotope experiments.
Parameter
Value
Sample
PbF2 (mg)
134
Target voltage Extraction voltage Cs boiler Ionizer current
7 kV 10.5 kV 90 °C 17 A
Blank Cs-only Cs-only 134 Cs and 135Cs
46.1 32.3 33.2 33.9
0 1.9 1012 0 1.8 1012
tandem accelerator, high energy bending magnet, electrostatic analyzer (ESA), and finally gas ionization chamber (GIC) detector (Fig. 1). Using these sample preparation and sequential injection procedures, Cs was measured on an ISA-AMS to perform the first assessments of the behavior of radioactive Cs beams and the efficiencies and limitations of different approaches for analysis. 3. Results and discussion 3.1. Caesium beam current and efficiency Cs does not produce a large atomic negative ion current due to its valence electron structure (6s1); this effect immediately diminishes the efficiency of Cs measurement by AMS unless a strong molecular beam can be obtained from the ion source. Initial tests to determine the optimal operating parameters for Cs were done using targets that had high concentrations of CsF in the PbF2 matrix, each target containing approximately (0.5 2) 1019 atoms of 133 Cs. Due to the hygroscopic nature of CsF, targets were only created up to a concentration of 30% CsF to PbF2 by mass. Concentrations higher than this quickly absorbed environmental moisture, compromising the integrity of the sample. Typical CsF2 currents measured at the Faraday cup located just after the injection magnet can be seen in Fig. 2. Concentrations of CsF, 7% by mass, produced around 75 nA. Mixing the samples by boiling in HF, as outlined in the methods, produced more stable currents due to the better homogenization of the target material. During experimentation with Cs radioisotopes, however, samples were produced at much lower Cs concentrations. Targets composed of 134Cs and/or 135Cs contained approximately 2 1012 and 5.5 1013 atoms respectively, a factor of 106 times less than stable Cs samples. Table 3 shows sample compositions for these
134 135
135
Cs atoms
Cs atoms
0 0 5.4 1013 5.6 1013
tests. Mass 134 and 135 measurements were taken at the final GIC detector. Here, count rates of 0.3 ± 0.2 and 4 ± 3 s 1 for 134Cs and 135Cs were recorded. All reported currents and count rates were normalized to the 133Cs current of the first measurement of an experiment to enable a direct comparison between experiments and to account for any variation in machine transmission. Two factors limit the detection of Cs: the transmission efficiency of the ISA-AMS system and the ionization efficiency of Cs. The efficiency of the components of the IsoTrace Laboratory ISAAMS system can be seen in Table 4. The magnet box listed in this table is the section of beam line between the ISA and the tandem that is occupied by the injection magnet of the second injection line. Ionization of Cs and formation of CsF2 has a much larger impact on efficiency. The low concentration radiocaesium targets produced a total measurement efficiency of 1.3 10 9 which, accounting for ISA-AMS transmission efficiencies, results in an ionization efficiency of 1.7 10 7. It should be noted however, that these efficiencies are derived from the sequential measurement method outlined in the methods. This process results in only 25% of the target measurement time being spent on a particular isotope so that the actual efficiency of an isotope is about 4 times higher. 3.2. Interferences and crosstalk All isobaric interferences need to be eliminated in order to measure the concentration of 135Cs. Many factors make Ba a significant interference for Cs analysis. Ba is naturally present at high abundances, the Cs isotopes of interest decay into their Ba isobar, and the efficiency of Ba in AMS is similar to Cs [9]. The ISA was used to suppress the isobaric interferences from 134/135Ba. Using 0.53 Pa of O2 gas in the reaction cell of the ISA was found to be sufficient for removing Ba while maintaining a usable Cs signal [3]. In addition to Ba, there was an unanticipated interference from Zn dimers, which span the mass range of interest (Fig. 3). The mechanism of interference can be seen in Table 5. These molecular interferences were eliminated by increasing the stripper pressure in the tandem accelerator until no counts were seen in mass 136. All dimer interferences were suppressed at a stripper gas pressure that resulted in a pressure of 2 10 5 Pa at the low and high energy ends of the tandem accelerator. This corresponds to a stripper gas thickness of 0.5 lg/cm2. Source memory effects introduce crosstalk between samples inside the source. This occurs when a target ejects some material into the source environment and the material remains in the source contaminating subsequent target measurements. This process was observed in the Cs measurements (Fig. 4).
Table 4 The measured efficiencies of the AMS system components.
Fig. 2. Average 133CsF2 currents over 20 min measured after the injection magnet. Samples marked with (j) were mechanically mixed using a glass stir rod and the samples marked with () were mixed by fluxing with HF. Error bars represent the stability of the beam over the 20 min analysis period.
Please cite this article in press as: C.M. MacDonald et al., Determination of dx.doi.org/10.1016/j.nimb.2015.03.008
135
Component
Efficiency
ISA @ 0.53 Pa of O2 2nd injection magnet box Tandem HE magnet + ESA Total
25% 57% 7% 80% 0.8%
Cs by accelerator mass spectrometry, Nucl. Instr. Meth. B (2015), http://
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C.M. MacDonald et al. / Nuclear Instruments and Methods in Physics Research B xxx (2015) xxx–xxx
Relative Abundance
133
134
135
136
137
138
1.0
1.0
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.0
0.0 133
134
135
136
137
138
Mass (amu)
3.3. Determining the concentration of
Injection magnet Tandem (1.3496 MV) HE magnet ESA
10
1
137 136
as
E/q
Form
mE/q2
E/q
CsF2
3.026
0.01750
Zn2[38]
3.025
0.01750
76.646
1.70278
76.646 76.646
1.70278 1.70278
Cs
76.679
1.70282
Cs+3 Cs+3
76.679 76.679
1.70282 1.70282
Zn+3 2 Zn+3 2
135 15 30 5 4 60 5 7 90 134 5 10 120 35 0 Energy 1 15 Loss (a .u.)
s
0
M
mE/q2
Zn+3 2
Counts
100
Zn dimer
Form
+3
Cs
The sequential analysis of the isotopes of interest, masses 134 through 137, produced an energy spectrum for each mass from a single target. The spectra for 134Cs and 135Cs can be seen in Fig. 5. From these spectra the appropriate region of interest was chosen and the counts in that region were recorded. The amount of 134Cs lost in the preparation and analysis procedures (i.e., atoms put in the sample target less the number of atoms measured in the detector) was used to determine the loss 135Cs. From this loss, the total amount of 135Cs in the target was calculated to be (7 ± 3) 1013 atoms. Currently, the factors limiting the precision of these measurements are the low efficiency of production of the Cs ions, the short life of the targets due to the expulsion of material, and the source memory effect.
Table 5 Comparison of Cs and Zn energy and mass characteristics as they would be analyzed by different AMS mass filters. A source voltage of 17.5 kV and tandem accelerator voltage of 1.3496 MV was used for these calculations. Values expressed in units of elementary charge, MeV, and atomic mass units. The initially accelerated molecule of Zn was not directly measured, therefore the molecule is represented as Zn2[38] . Cs 135
135
m u)
During analysis, a large emission of target material was observed after a few minutes of sputtering. This phenomenon was indicated by a drop in target voltage, due to the emission of ions and electrons, and a spike in target current, caused by the source attempting to maintain voltage. It is hypothesized that this event is due to the inefficient removal of heat from the sputtered surface, causing instability in the target material. Any un-ionized Cs ejected from the target during this emission event creates a partial pressure of sample Cs in the source. This partial pressure remains in the source during the analysis of subsequent targets. Cross contamination occurs when previously ejected Cs from the source environment becomes ionized by the source ionizer. The ion optics of the SO-110 source focuses the ionized Cs to the target surface, allowing the contaminant Cs to be reemitted. The expulsion of large amounts of material at the onset of sputtering is not unique to PbF2 targets; the process also occurs in iodine targets. For iodine, the expulsion is minimized by gradually increasing the sputtering current on the target, a process known as ‘‘conditioning’’. Doing so preserves the structural integrity of the sample material and improves the lifetime of the target. A similar procedure should be developed for the Cs + PbF2 targets. Additionally, copper target pieces, due to their superior thermal conductive properties, in lieu of stainless steel, should be tested on the sputter source since this should also reduce residual contamination.
Fig. 4. 135Cs signal in 3 consecutive samples. The first sample contained 135Cs and the following two contained none so the signals in samples 2 and 3 are from cross contamination. Each measurement was taken approximately 20 min apart.
(a
Fig. 3. Abundance of the interfering Zn dimers that can interfere with Cs isotope measurements. The dimer abundance is calculated by determining the probability that any two Zn isotopes will add to a given mass. These probabilities were then normalized to the most abundant Zn dimer, mass 134. For example, a dimer of mass 136 can be a combination of Zn isotopes of mass 66 (28% abundance) and 70 (6% abundance) or 68 and 68 (19% abundance).
Please cite this article in press as: C.M. MacDonald et al., Determination of dx.doi.org/10.1016/j.nimb.2015.03.008
Fig. 5. Spectrum of the 134 (red bars), 135 (blue bars), 136, and 137 mass spectra. The counts between channels 0 and 40 are 45Sc+1 ions. There is no interference from ions of equal mass to charge ratios in the higher energy channels used to monitor the Cs isotopes. The 136 and 137 spectra have zero counts indicating no Ba or 137Cs interference. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 135
Cs by accelerator mass spectrometry, Nucl. Instr. Meth. B (2015), http://
C.M. MacDonald et al. / Nuclear Instruments and Methods in Physics Research B xxx (2015) xxx–xxx
4. Conclusions These experiments outline a method for measuring 135Cs by AMS. Cs di-fluoride anion formation remains the limiting factor in Cs analysis with a measured efficiency of 10 7. Including the AMS system transmission efficiency of 0.8% the total efficiency of Cs measurement was found to be 1.3 10 9. Using the ISA, all Ba interferences were reduced to 0 counts in 20 min with 0.53 Pa of O2 in the reaction cell. Zn dimer interference was easily removed by the increasing of stripper pressure in the tandem accelerator. The remaining interference stems from sample crosstalk in the source environment. The successful first measurements of 135Cs are an important first step in the advancement of Cs analysis by AMS. References [1] J. Zheng, K. Tagami, W. Bu, S. Uchida, Y. Watanabe, Y. Kubota, S. Fuma, S. Ihara, 135 Cs/137Cs isotopic ratio as a new tracer of radiocesium released from the Fukushima nuclear accident, Environ. Sci. Technol. 10 (48) (2014) 5433–5438.
Please cite this article in press as: C.M. MacDonald et al., Determination of dx.doi.org/10.1016/j.nimb.2015.03.008
5
[2] V.F. Taylor, R.D. Evans, R.J. Cornett, Preliminary evaluation of 135Cs/137Cs as a forensic tool for identifying source of radioactive contamination, J. Environ. Radioact. 99 (1) (2008) 109–118. [3] C.M. MacDonald, C.R.J. Charles, R.J. Cornett, X.-L. Zhao, W.E. Kieser, A.E. Litherland, Detection of 135Cs by accelerator mass spectrometry, Rapid Commun. Mass Spectrom. 29 (1) (2015) 115–118. [4] J. Eliades, X.-L. Zhao, A.E. Litherland, W.E. Kieser, On-line ion chemistry for the ams analysis of 90Sr and 135,137Cs, Nucl. Instr. Meth. B 294 (2013) 361–363. [5] A.E. Litherland, J. Doupé, W.E. Kieser, X.L. Zhao, G. Javaheri, L. Cousins, I. Tomski, A negative ion source with isobar selection by chemical reaction cell, Patent US 7 439 498, 10 21, 2008. [6] J. Zheng, W. Bu, K. Tagami, Y. Shikamori, K. Nakano, S. Uchida, N. Ishii, Determination of 135Cs and 135Cs/137Cs atomic ratio in environmental samples by combining ammonium molybdophosphate (amp)-selective Cs adsorption and ion-exchange chromatographic separation to triple-quadrupole inductively coupled plasma–mass spectrometry, Anal. Chem. 14 (86) (2014) 7103–7110. [7] T. Lee, K. Teh-Lung, L. Hsiao-Ling, C. Ju-Chin, First detection of fallout Cs-135 and potential applications of 137Cs/135Cs ratios, Geochim. Cosmochim. Acta 57 (14) (1993) 3493–3497. [8] X.-L. Zhao, A.E. Litherland, J. Eliades, W.E. Kieser, Q. Liu, Studies of anions from sputtering I: survey of MFn, Nucl. Instr. Meth. B 268 (7) (2010) 807–811. [9] R. Kresse, U. Baudis, P. Jäger, H.H. Riechers, H. Wagner, J. Winkler, H.U. Wolf, Barium and barium compounds, in: Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA, 2007.
135
Cs by accelerator mass spectrometry, Nucl. Instr. Meth. B (2015), http://
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Determination of
135
Cs by accelerator mass spectrometry
C.M. MacDonald a,b,⇑, C.R.J. Charles a,b, X.-L. Zhao a,c, W.E. Kieser a,c, R.J. Cornett a,b, A.E. Litherland d a
Andre. 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 d IsoTrace Laboratory, University of Toronto, 60 St. George St., Toronto, ON M5S 1A7, Canada b
a r t i c l e
i n f o
Article history: Received 11 December 2014 Received in revised form 2 March 2015 Accepted 5 March 2015 Available online xxxx Keywords: AMS Caesium Isobar
a b s t r a c t The ratio of anthropogenic 135Cs and 137Cs isotopes is characteristic of a uranium fission source. This research evaluates the technique of isotope dilution (yield tracing) for the purpose of quantifying 135Cs by accelerator mass spectrometry with on-line isobar separation. Interferences from Ba, Zn2, and isotopes of equal mass to charge ratios were successfully suppressed. However, some sample crosstalk from source contamination remains. The transmission and di-fluoride ionization efficiencies of Cs isotopes were found to be 8 10 3 and 1.7 10 7 respectively. This quantification of 135Cs using yield tracing by accelerator mass spectrometry shows promise for future environmental sample analysis once the issues of sample crosstalk and low efficiency can be resolved. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Anthropogenic Cs isotopes have been released into the environment from nuclear weapons tests and reactor emissions. As a result, 137Cs and 134Cs have been used in many applications including sediment dating and nuclear forensics [1,2]. 135Cs, which is also produced, can provide additional information about the source of the Cs isotopes by observing the 135Cs:137Cs ratio. However, since the established techniques rely on radiometric measurement of the Cs isotopes, new techniques for the measurement of 135Cs must be developed because of its long half life (2.3 Ma) and pure beta emission. The first detection of 135Cs by accelerator mass spectrometry (AMS) was accomplished using an Isobar Separator for Anions (ISA) which eliminated the ubiquitous Ba isobar interference in Cs samples [3–5]. ISA-AMS has the potential to enhance nuclear contamination forensics that has been pioneered using inductively coupled mass spectrometry (ICPMS) and chronometric tracing techniques proposed by Lee et al., and oceanic tracing techniques which thus far have relied on the shorter lived 137Cs isotope. [2,6,7] For ISA-AMS to be used as an analytical method the ISA-AMS operating parameters must be optimized, its sample requirements and limitations must be determined, and a method to quantify the amount of 135Cs must be developed. A Cs anion beam must be produced in the ion source with a high efficiency to minimize the use ⇑ Corresponding author at: Andre. E. Lalonde AMS Laboratory, University of Ottawa, 150 Louis Pasteur, Ottawa, ON K1N 6N5, Canada.
of valuable sample material and analysis time. Many factors can affect the efficiency of an isotope measurement by AMS. These factors include: 1. Ionization efficiency: the production of the desired ion by the ion source. 2. Isobar suppressor efficiency: the transmission of analyte through the on-line separation of isobars. 3. System efficiency: the transmission of the ion beam through mass filters, electron stripping canal, and other system components. The objectives of this paper was to examine these factors and to determine the best operating conditions and techniques to measure 135Cs, to outline the limitations of the ISA-AMS system, and to develop a method to make an absolute measurement of 135Cs isotope concentration. 2. Methods These experiments were performed at the IsoTrace Laboratory (Toronto, Canada), the 3 MV AMS facility operated by the University of Ottawa (Ottawa, Canada) (Fig. 1). It was the very last use of this facility before its closure. A common AMS technique for quantifying an isotope is to monitor the rare isotope of interest and the abundant isotope of the same element with as little delay between measurements as possible. This can be done to get a relative value, the ratio of the
http://dx.doi.org/10.1016/j.nimb.2015.03.008 0168-583X/Ó 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: C.M. MacDonald et al., Determination of dx.doi.org/10.1016/j.nimb.2015.03.008
135
Cs by accelerator mass spectrometry, Nucl. Instr. Meth. B (2015), http://
2
C.M. MacDonald et al. / Nuclear Instruments and Methods in Physics Research B xxx (2015) xxx–xxx
Fig. 1. Top-down view of the IsoTrace ISA-AMS system.
isotopes, or an absolute value using isotope dilution. However, this technique could not be used for measuring 135Cs since the samples were sputtered with the abundant isotope, 133Cs. Therefore, to obtain the concentration of 135Cs, a known amount of 134Cs was added to the sample. The total yield of this isotope could be tracked through the sample preparation and measurement procedures and could then be used to calculate the absolute amount of 135Cs in an unknown sample. This process of yield tracing is effective as long as there is no significant isotope fractionation in the procedures. For the purposes of this experiment, the known isotope, analyte, and interferences needed to be monitored throughout the measurement process. Accordingly, masses 134, 135, 136, and 137 were observed throughout each experiment to monitor the radiogenic isotopes of Cs and also Ba interference (mass 136). Each mass was observed for 5 min per target, resulting in a total target analysis time of 20 min. Targets included analyte, spike, analyte–spike mixture, and process blanks so that contamination could be quantified, and corrected for if present. The ISA-AMS operating parameters for the full mass range are listed in Table 1. Samples of 135Cs and 134Cs were required to make this measurement. Furthermore, each sample needed to have low or no contamination of the other isotopes. Since there are no standards of 135Cs, and the amount of 135Cs in the available 134Cs standards was not quantified, the samples for these experiments had to be produced. A sample of 134Cs was produced for these experiments by exposing 80 mg of Caesium Carbonate (Aldrich AB8824CHM), 133Cs2CO3, to the neutron flux of the Slowpoke reactor at the Royal Military College of Canada (Kingston, Canada). By exposing this compound to the neutron flux from the reactor there is a probability,
Table 1 AMS system operating parameters for sequential measurements. The isotopes were analyzed as CsF2 ? Cs+3. B1 and B2 refer to the field generated by the injection and high energy analyzing magnets. Isotope
Mass 1 (amu)
B1 (T)
Terminal (MV)
Mass 2 (amu)
B2 (T)
Cs133 Cs134 Cs135 Ba136 Cs137
170.9022534 171.9035064 172.9026964 173.9013764 174.9038764
0.43390 0.43514 0.43642 0.43770 0.43900
1.3505 1.3500 1.3496 1.3491 1.3487
132.90544 133.90670 134.90589 135.90456 136.90707
0.98656500 0.99026279 0.99397331 0.99762948 1.00132165
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represented by the constituents’ neutron cross section, that other heavier isotopes will be created by neutron capture. It was calculated that, given the reactor parameters, exposing the sample for 210 s would produce 18 kBq of 134Cs without significant creation of 135Cs (<0.01%) from a secondary neutron capture reaction. The sample of 135Cs was obtained by the decay of 135Xe. A section of Xe exhaust pipe was supplied by Chalk River Laboratories (Chalk River, Canada) from their Xe gas collection facility. The Xe gas collected after the dissolution of uranium oxide fuel to capture 99 Mo, the precursor to 99mTc which is used for diagnostic imaging. Due to the relatively short half-lives of 135Xe and 137Xe, and the stability of 134Xe, any radio-Cs that would have collected in this section of pipe by Xe decay would have been either 135Cs or 137 Cs. The pipe was rinsed with 5 ml of distilled water to collect any Cs that had built up in the pipe. This 5 ml rinse was measured radiometrically (beta- and hyper pure germanium gamma-spectroscopy) and the only radioactive Cs contaminant that was observed was 137Cs. Cs atoms do not readily form negative ions. Therefore, samples had to be prepared for measurement by AMS so that the ion source would produce a strong molecular anion of CsF2 [8]. To achieve this, the Cs samples were prepared as follows. PbF2 (Alfa Aesar Puratronic 99.999%) was added into pre-weighed clean 15 ml round-bottom Savillex PFA vials. Solutions containing known amounts of 134Cs, 135Cs and naturally abundant Ba isotopes were drawn into PTFE micro-tubing and weighed on a precision Mettler-Toledo microbalance before being expelled into the Savillex vials. 20 drops of 28.9 M HF were then added from a clean PFA Teflon dropper bottle directly into the Savillex vials containing the PbF2 + Cs or PbF2 + Ba. The mixture was capped tightly and fluxed inside an isolated, vented Plexiglas box at 90 °C on a hotplate in their same Savillex capsules with occasional mixing for at least 2 h. Samples were slowly evaporated to dryness for >5 h at 90 °C by removing the lids of the Savillex capsules; HF fumes were withdrawn under mild suction from inside the Plexiglas box. The final samples of 10 mg of dry material were pressed into 1.3 mm diameter stainless steel targets under 0.6 GPa of pressure. After sample preparation and target pressing, the targets were loaded into an SO-110 sputter source (High Voltage Engineering B.V., the Netherlands) for analysis. The SO-110 operating parameters for these experiments are in Table 2. The beam of ions goes from the source, through a low energy injection magnet, ISA, 135
Cs by accelerator mass spectrometry, Nucl. Instr. Meth. B (2015), http://
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C.M. MacDonald et al. / Nuclear Instruments and Methods in Physics Research B xxx (2015) xxx–xxx Table 2 SO-110 operation parameters for Cs measurements.
Table 3 Samples used in the Cs radioisotope experiments.
Parameter
Value
Sample
PbF2 (mg)
134
Target voltage Extraction voltage Cs boiler Ionizer current
7 kV 10.5 kV 90 °C 17 A
Blank Cs-only Cs-only 134 Cs and 135Cs
46.1 32.3 33.2 33.9
0 1.9 1012 0 1.8 1012
tandem accelerator, high energy bending magnet, electrostatic analyzer (ESA), and finally gas ionization chamber (GIC) detector (Fig. 1). Using these sample preparation and sequential injection procedures, Cs was measured on an ISA-AMS to perform the first assessments of the behavior of radioactive Cs beams and the efficiencies and limitations of different approaches for analysis. 3. Results and discussion 3.1. Caesium beam current and efficiency Cs does not produce a large atomic negative ion current due to its valence electron structure (6s1); this effect immediately diminishes the efficiency of Cs measurement by AMS unless a strong molecular beam can be obtained from the ion source. Initial tests to determine the optimal operating parameters for Cs were done using targets that had high concentrations of CsF in the PbF2 matrix, each target containing approximately (0.5 2) 1019 atoms of 133 Cs. Due to the hygroscopic nature of CsF, targets were only created up to a concentration of 30% CsF to PbF2 by mass. Concentrations higher than this quickly absorbed environmental moisture, compromising the integrity of the sample. Typical CsF2 currents measured at the Faraday cup located just after the injection magnet can be seen in Fig. 2. Concentrations of CsF, 7% by mass, produced around 75 nA. Mixing the samples by boiling in HF, as outlined in the methods, produced more stable currents due to the better homogenization of the target material. During experimentation with Cs radioisotopes, however, samples were produced at much lower Cs concentrations. Targets composed of 134Cs and/or 135Cs contained approximately 2 1012 and 5.5 1013 atoms respectively, a factor of 106 times less than stable Cs samples. Table 3 shows sample compositions for these
134 135
135
Cs atoms
Cs atoms
0 0 5.4 1013 5.6 1013
tests. Mass 134 and 135 measurements were taken at the final GIC detector. Here, count rates of 0.3 ± 0.2 and 4 ± 3 s 1 for 134Cs and 135Cs were recorded. All reported currents and count rates were normalized to the 133Cs current of the first measurement of an experiment to enable a direct comparison between experiments and to account for any variation in machine transmission. Two factors limit the detection of Cs: the transmission efficiency of the ISA-AMS system and the ionization efficiency of Cs. The efficiency of the components of the IsoTrace Laboratory ISAAMS system can be seen in Table 4. The magnet box listed in this table is the section of beam line between the ISA and the tandem that is occupied by the injection magnet of the second injection line. Ionization of Cs and formation of CsF2 has a much larger impact on efficiency. The low concentration radiocaesium targets produced a total measurement efficiency of 1.3 10 9 which, accounting for ISA-AMS transmission efficiencies, results in an ionization efficiency of 1.7 10 7. It should be noted however, that these efficiencies are derived from the sequential measurement method outlined in the methods. This process results in only 25% of the target measurement time being spent on a particular isotope so that the actual efficiency of an isotope is about 4 times higher. 3.2. Interferences and crosstalk All isobaric interferences need to be eliminated in order to measure the concentration of 135Cs. Many factors make Ba a significant interference for Cs analysis. Ba is naturally present at high abundances, the Cs isotopes of interest decay into their Ba isobar, and the efficiency of Ba in AMS is similar to Cs [9]. The ISA was used to suppress the isobaric interferences from 134/135Ba. Using 0.53 Pa of O2 gas in the reaction cell of the ISA was found to be sufficient for removing Ba while maintaining a usable Cs signal [3]. In addition to Ba, there was an unanticipated interference from Zn dimers, which span the mass range of interest (Fig. 3). The mechanism of interference can be seen in Table 5. These molecular interferences were eliminated by increasing the stripper pressure in the tandem accelerator until no counts were seen in mass 136. All dimer interferences were suppressed at a stripper gas pressure that resulted in a pressure of 2 10 5 Pa at the low and high energy ends of the tandem accelerator. This corresponds to a stripper gas thickness of 0.5 lg/cm2. Source memory effects introduce crosstalk between samples inside the source. This occurs when a target ejects some material into the source environment and the material remains in the source contaminating subsequent target measurements. This process was observed in the Cs measurements (Fig. 4).
Table 4 The measured efficiencies of the AMS system components.
Fig. 2. Average 133CsF2 currents over 20 min measured after the injection magnet. Samples marked with (j) were mechanically mixed using a glass stir rod and the samples marked with () were mixed by fluxing with HF. Error bars represent the stability of the beam over the 20 min analysis period.
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135
Component
Efficiency
ISA @ 0.53 Pa of O2 2nd injection magnet box Tandem HE magnet + ESA Total
25% 57% 7% 80% 0.8%
Cs by accelerator mass spectrometry, Nucl. Instr. Meth. B (2015), http://
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C.M. MacDonald et al. / Nuclear Instruments and Methods in Physics Research B xxx (2015) xxx–xxx
Relative Abundance
133
134
135
136
137
138
1.0
1.0
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.0
0.0 133
134
135
136
137
138
Mass (amu)
3.3. Determining the concentration of
Injection magnet Tandem (1.3496 MV) HE magnet ESA
10
1
137 136
as
E/q
Form
mE/q2
E/q
CsF2
3.026
0.01750
Zn2[38]
3.025
0.01750
76.646
1.70278
76.646 76.646
1.70278 1.70278
Cs
76.679
1.70282
Cs+3 Cs+3
76.679 76.679
1.70282 1.70282
Zn+3 2 Zn+3 2
135 15 30 5 4 60 5 7 90 134 5 10 120 35 0 Energy 1 15 Loss (a .u.)
s
0
M
mE/q2
Zn+3 2
Counts
100
Zn dimer
Form
+3
Cs
The sequential analysis of the isotopes of interest, masses 134 through 137, produced an energy spectrum for each mass from a single target. The spectra for 134Cs and 135Cs can be seen in Fig. 5. From these spectra the appropriate region of interest was chosen and the counts in that region were recorded. The amount of 134Cs lost in the preparation and analysis procedures (i.e., atoms put in the sample target less the number of atoms measured in the detector) was used to determine the loss 135Cs. From this loss, the total amount of 135Cs in the target was calculated to be (7 ± 3) 1013 atoms. Currently, the factors limiting the precision of these measurements are the low efficiency of production of the Cs ions, the short life of the targets due to the expulsion of material, and the source memory effect.
Table 5 Comparison of Cs and Zn energy and mass characteristics as they would be analyzed by different AMS mass filters. A source voltage of 17.5 kV and tandem accelerator voltage of 1.3496 MV was used for these calculations. Values expressed in units of elementary charge, MeV, and atomic mass units. The initially accelerated molecule of Zn was not directly measured, therefore the molecule is represented as Zn2[38] . Cs 135
135
m u)
During analysis, a large emission of target material was observed after a few minutes of sputtering. This phenomenon was indicated by a drop in target voltage, due to the emission of ions and electrons, and a spike in target current, caused by the source attempting to maintain voltage. It is hypothesized that this event is due to the inefficient removal of heat from the sputtered surface, causing instability in the target material. Any un-ionized Cs ejected from the target during this emission event creates a partial pressure of sample Cs in the source. This partial pressure remains in the source during the analysis of subsequent targets. Cross contamination occurs when previously ejected Cs from the source environment becomes ionized by the source ionizer. The ion optics of the SO-110 source focuses the ionized Cs to the target surface, allowing the contaminant Cs to be reemitted. The expulsion of large amounts of material at the onset of sputtering is not unique to PbF2 targets; the process also occurs in iodine targets. For iodine, the expulsion is minimized by gradually increasing the sputtering current on the target, a process known as ‘‘conditioning’’. Doing so preserves the structural integrity of the sample material and improves the lifetime of the target. A similar procedure should be developed for the Cs + PbF2 targets. Additionally, copper target pieces, due to their superior thermal conductive properties, in lieu of stainless steel, should be tested on the sputter source since this should also reduce residual contamination.
Fig. 4. 135Cs signal in 3 consecutive samples. The first sample contained 135Cs and the following two contained none so the signals in samples 2 and 3 are from cross contamination. Each measurement was taken approximately 20 min apart.
(a
Fig. 3. Abundance of the interfering Zn dimers that can interfere with Cs isotope measurements. The dimer abundance is calculated by determining the probability that any two Zn isotopes will add to a given mass. These probabilities were then normalized to the most abundant Zn dimer, mass 134. For example, a dimer of mass 136 can be a combination of Zn isotopes of mass 66 (28% abundance) and 70 (6% abundance) or 68 and 68 (19% abundance).
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Fig. 5. Spectrum of the 134 (red bars), 135 (blue bars), 136, and 137 mass spectra. The counts between channels 0 and 40 are 45Sc+1 ions. There is no interference from ions of equal mass to charge ratios in the higher energy channels used to monitor the Cs isotopes. The 136 and 137 spectra have zero counts indicating no Ba or 137Cs interference. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 135
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C.M. MacDonald et al. / Nuclear Instruments and Methods in Physics Research B xxx (2015) xxx–xxx
4. Conclusions These experiments outline a method for measuring 135Cs by AMS. Cs di-fluoride anion formation remains the limiting factor in Cs analysis with a measured efficiency of 10 7. Including the AMS system transmission efficiency of 0.8% the total efficiency of Cs measurement was found to be 1.3 10 9. Using the ISA, all Ba interferences were reduced to 0 counts in 20 min with 0.53 Pa of O2 in the reaction cell. Zn dimer interference was easily removed by the increasing of stripper pressure in the tandem accelerator. The remaining interference stems from sample crosstalk in the source environment. The successful first measurements of 135Cs are an important first step in the advancement of Cs analysis by AMS. References [1] J. Zheng, K. Tagami, W. Bu, S. Uchida, Y. Watanabe, Y. Kubota, S. Fuma, S. Ihara, 135 Cs/137Cs isotopic ratio as a new tracer of radiocesium released from the Fukushima nuclear accident, Environ. Sci. Technol. 10 (48) (2014) 5433–5438.
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[2] V.F. Taylor, R.D. Evans, R.J. Cornett, Preliminary evaluation of 135Cs/137Cs as a forensic tool for identifying source of radioactive contamination, J. Environ. Radioact. 99 (1) (2008) 109–118. [3] C.M. MacDonald, C.R.J. Charles, R.J. Cornett, X.-L. Zhao, W.E. Kieser, A.E. Litherland, Detection of 135Cs by accelerator mass spectrometry, Rapid Commun. Mass Spectrom. 29 (1) (2015) 115–118. [4] J. Eliades, X.-L. Zhao, A.E. Litherland, W.E. Kieser, On-line ion chemistry for the ams analysis of 90Sr and 135,137Cs, Nucl. Instr. Meth. B 294 (2013) 361–363. [5] A.E. Litherland, J. Doupé, W.E. Kieser, X.L. Zhao, G. Javaheri, L. Cousins, I. Tomski, A negative ion source with isobar selection by chemical reaction cell, Patent US 7 439 498, 10 21, 2008. [6] J. Zheng, W. Bu, K. Tagami, Y. Shikamori, K. Nakano, S. Uchida, N. Ishii, Determination of 135Cs and 135Cs/137Cs atomic ratio in environmental samples by combining ammonium molybdophosphate (amp)-selective Cs adsorption and ion-exchange chromatographic separation to triple-quadrupole inductively coupled plasma–mass spectrometry, Anal. Chem. 14 (86) (2014) 7103–7110. [7] T. Lee, K. Teh-Lung, L. Hsiao-Ling, C. Ju-Chin, First detection of fallout Cs-135 and potential applications of 137Cs/135Cs ratios, Geochim. Cosmochim. Acta 57 (14) (1993) 3493–3497. [8] X.-L. Zhao, A.E. Litherland, J. Eliades, W.E. Kieser, Q. Liu, Studies of anions from sputtering I: survey of MFn, Nucl. Instr. Meth. B 268 (7) (2010) 807–811. [9] R. Kresse, U. Baudis, P. Jäger, H.H. Riechers, H. Wagner, J. Winkler, H.U. Wolf, Barium and barium compounds, in: Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA, 2007.
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