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Improved 36Cl performance at the ASTER HVE 5 MV accelerator mass spectrometer national facility
Nuclear Instruments and Methods in Physics Research B 294 (2013) 121–125
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Improved 36Cl performance at the ASTER HVE 5 MV accelerator mass spectrometer national facility R. Finkel a,⇑, M. Arnold a,b, G. Aumaître a,b, L. Benedetti a, D. Bourlès a,b, K. Keddadouche a,b, S. Merchel a,c a
CEREGE, CNRS-IRD- Aix-Marseille Université, F-13545 Aix-en-Provence, France ASTER-Team, CNRS-IRD- Aix-Marseille Université, F-13545 Aix-en-Provence, France c Helmholtz-Zentrum Dresden-Rossendorf, D-01314 Dresden, Germany b
a r t i c l e
i n f o
Article history: Received 1 June 2011 Received in revised form 18 April 2012 Available online 16 May 2012 Keywords: Accelerator mass spectrometry 36 Cl capability ASTER Ion source memory
a b s t r a c t The HVE 5 MV ASTER AMS national facility at CEREGE was accepted in 2007. Since then we have continued to optimize performance for 36Cl. Cl-36 analyses use AgCl, a Cs negative ion sputter source, Ar stripping to +5 in the terminal of the Tandetron™ accelerator at 5 MV and a silicon nitride post-acceleration stripper foil to enhance suppression of 36S relative to 36Cl. The major challenges to obtaining the desired performance for Earth science applications are ion source memory, mass fractionation effects, 36S isobar suppression and sensitivity. Redesign of the SO110 ion source by HVE to change the size of the aperture and the shape of cathode significantly reduced ion source memory to less than 0.1%, a level that can be compensated for by matching standards to samples. We observe small systematic drifts in 35Cl/37Cl ratios over time, the source of which is not yet determined. Measurement of standards indicates that this effect limits the precision of 35Cl/37Cl ratio determination to about 2%. 36S is suppressed in several ways. First, the sample chemistry has been designed to reduce S to low levels. Second, cathodes are constructed of low-S nickel, enabling direct target loading without the use of AgBr pre-packing. Third, a post-acceleration Si3N4 stripper foil differentially absorbs energy from 36Cl and 36S. A subsequent electrostatic deflector is then able to suppress 36S by a factor of 240 relative to 36Cl. Differential energy loss in the detector further suppresses 36S by about 10 4, for an overall suppression factor of 4 10 7. 36S count rates are typically equivalent to a background 36Cl/Cl of 10 15. At typical 35Cl currents of 20 lA Cl5+ samples with 36 Cl/35Cl of 6 10 14 can be measured to ±5% statistical uncertainty with 1 h of analysis time. Typical machine blanks have 36Cl/Cl 2 10 15. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction The ASTER AMS (accelerator mass spectrometer) national facility at CEREGE (Centre Européen de Recherche et d’Enseignement des Géosciences de l’Environnement) was inaugurated in 2007 as a multi-nuclide accelerator mass spectrometer emphasizing environmental and Earth science applications. The heart of the ASTER facility is an HVE (High Voltage Engineering Europa) 5 MV Tandetron™ accelerator mass spectrometer. Nuclides determined include 10Be, 26Al, 36Cl, 41Ca and 129I. We report here performance parameters achieved in utilizing the ASTER AMS for 36Cl measurements. 36Cl is determined utilizing AgCl in the cathode, a Cs negative ion sputter source, Ar stripping to +5 in the terminal of the Tandetron™ accelerator at 5 MV and a silicon nitride post-acceleration stripper foil to enhance suppression of 36S relative to 36Cl. The major challenges to obtaining the desired performance for Earth science applications are ion source memory, mass fractionation ⇑ Corresponding author. E-mail address: fi[email protected] (R. Finkel). 0168-583X/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2012.05.019
effects, 36S isobar suppression and sensitivity. The results given here show that these challenges have been met and demonstrate that ASTER is capable of producing 36Cl analyses with precision of a few percent and 35Cl/37Cl with 2% precision. 2. Operational parameters 36
Cl is measured at the ASTER HVE 5 MV Tandetron™ facility using AgCl cathode material [1]. Negative Cl ions are produced using an SO110B Cs-sputter ion source. The 36Cl produced is injected into the accelerator. Charge state 5+ 36Cl at 30 MeV is selected after argon gas stripping at 5 MV in the terminal of the accelerator. Discrimination of 36Cl and 36S is enhanced by taking advantage of nuclear charge dependent energy loss in a 1 lm Si3N4 foil following the 90° high-energy magnet. Cl10+ output from this foil is subjected to subsequent energy selection in a 35° electrostatic deflector followed by a 30° vertical magnet. The Cl10+ then passes through a 50 nm Si3N4 window to a 4-anode detector where 36 Cl and 36S are further separated by differential energy loss. Fast bouncing of the electric field before the accelerator is used to
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briefly change the energy of the beam to inject 35Cl and 37Cl into the accelerator. Currents of both these stable chlorine isotopes can then be measured in off axis Faraday cups following the high energy 90° analyzing magnet. Particle transmission to 35Cl5+ is 17%. 3. Performance 3.1. Ion source memory Silver chloride is, in many ways, an ideal compound for 36Cl AMS. Its chemical separation and purification are relatively straightforward [e.g. 2] and it produces large and stable negative ion currents in the Cs-sputter ion sources commonly used in AMS [3]. AgCl does, however, have the disadvantage of being a relatively volatile compound compared to other cathode materials commonly used in AMS such at BeO or Al2O3. Because of this volatility, AgCl can migrate from the sample cathode, which becomes very warm during sputtering, to one of the many cooler surfaces found in the Cs-sputter source. This condensed material has a finite probability of being re-introduced to the ion source during subsequent analyses, thereby appearing as a source of contamination for subsequent measurements. With the version of the SO110 ion source originally used at ASTER, this memory effect was found to be a serious problem for 36Cl measurements, causing elevated and variable 36Cl backgrounds and leading to perturbations of the stable 35Cl/37Cl measurements required for isotope dilution determination of the stable chloride concentrations [1]. Collaboration between HVE and CEREGE led to several improvements in the SO110 design to reduce these effects. The new SO110-
B source geometry was altered to produce better focusing of the Cs ions on the sample material in the cathode, thus avoiding sputtering of surrounding material in the source that could lead to increased measurement background and memory [1]. The source was redesigned for a more open configuration. This change improved pumping efficiency at the target cathode, which reduced scattering and memory. Cooling was improved by enhancing liquid flow at the base of the source head, thus reducing Cs ionization at locations other than the ionizer. While these changes all led to a reduction in memory, a notable effect could still be observed. Memory is most easily studied by following 35Cl/37Cl ratio measurements, which, being determined by current measurements in Faraday cups, are more precisely monitored than are 36Cl/35Cl ratios, the precision of which are limited by counting statistics. Using 35 Cl/37Cl, the memory effect is illustrated by Fig. 1a. This plot shows a time series of 35Cl/37Cl measured in Faraday cups after the accelerator using the standard, unshielded cathode configuration, a cylindrical cathode with a 1.3 mm hole containing the AgCl sample material. In this configuration when successive cathodes had isotope ratios that differed by more than an order of magnitude, it took several minutes for the measured ratio to reach a plateau indicative of its true value. This occurred both when a high 35 Cl/37Cl ratio followed a low one, and vice versa. We hypothesized that this residual memory might be at least partially the result of sputtered material adhering to the immersion lens, which surrounded the cathode and served as an aperture to focus the negative ion beam to pass through the hole in the center of the spherical ionizer. In an attempt to reduce this effect the ion source was redesigned so that the immersion lens was incorporated into
Fig. 1. Effect of immersion lens geometry on ion source memory in 35Cl/37Cl measurements. The graphs show 35Cl/37Cl ratios vs time as measured in Faraday cups after the accelerator for cathode material of very different ratio. The slope of the peaks is indicative of memory. (a) Memory effect in passing from very different 35Cl/37Cl ratios with the cathode geometry shown schematically in the sketch. In this case each cathode sees the same immersion lens. (b) Reduced memory is seen when utilizing the cathode geometry illustrated in the sketch. In this case the immersion lens is shielded from the cathode by a lip intrinsic to each cathode.
R. Finkel et al. / Nuclear Instruments and Methods in Physics Research B 294 (2013) 121–125
each cathode. The effect of this change is seen in Fig. 1b, which shows that a plateau ratio is reached much more quickly when changing between cathodes with widely different ratios. The influence of these design changes on source memory was further investigated by measuring a sequence of 35Cl/37Cl samples prepared with 35Cl and 37Cl enriched material to give widely different ratios [4]. Three samples were used with 35Cl/37Cl of 0.019, 3.127 (=natural), and 370. During these tests, samples were measured in the sequence shown in Fig. 2 where the measured ratios are plotted against time. The measurements of Fig. 2 provide quantitative data on the sample-to-sample memory of the ion source
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when AgCl is being sputtered. When measuring a 35Cl/37Cl ratio of 370 just after a ratio of 0.019 the decay constant determined from an exponential fit was 35 s. When moving from 0.019 to 370, it was 45 s, a marked improvement over the data plotted in Fig. 1a, which have a decay constant of 160 s. These results indicate that the analysis of samples with isotopic ratios that differ by a factor of 2 104 can be measured consecutively with minimal memory effect. Memory was likewise examined during several measurement sequences in which a KNSTD5000 standard (36Cl/Cl = 5 10 12) [5] was measured followed by three AgCl blanks in succession.
Fig. 2. 35Cl/37Cl memory in the HVE SO110B ion source with cathode geometry illustrated in Fig. 1b. (a) 35Cl/37Cl measured in Faraday cups after the accelerator. (b) Memory on passing from a low to a high 35Cl/37Cl ratio as indicated on the figure. The time constant is derived from an exponential fit. (c) Memory on passing from a high to a low 35 Cl/37Cl ratio as indicated on the figure. The decay time constant t0 is derived from an exponential fit.
Fig. 3. 36Cl/35Cl memory in the SO110B ion source. The cathode geometry is the same as in Fig. 1b. After running a standard with a 36Cl/Cl ratio of 5000 10 15, three blank AgCl cathodes were run for 20 min each. The measured ratio and its uncertainty are plotted. Six sequences are illustrated. A memory of 2–3 10 15 is observed as explained in the text.
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The results are shown in the Fig. 3, which shows the blank values measured in three 20 min runs for six sets of three AgCl blanks run in succession after a KNSTD5000 standard. It appears that the first run of each blank is elevated above the second and third runs because of memory from the standard with its high 36Cl/Cl. The magnitude of the memory following the standard run can be estimated by subtracting the last run of each group of three from the first run. This test shows that the memory is about 2–3 10 15 or about 5 10 4 of the KNSTD5000.
3.2. Mass fractionation Variable mass fractionation in the spectrometer is a potential source of non-statistical uncertainty in AMS measurements in addition to memory. We investigated possible current-dependent mass fractionation in 35Cl/37Cl isotope ratios measured at ASTER by examining the measured 35Cl/37Cl isotopic ratios as a function of time for unspiked samples having the natural 35Cl/37Cl isotopic ratio of 3.127. Fig. 4 shows individual 35Cl/37Cl ratios measured in the off-axis Faraday cup after the accelerator plotted versus 35 Cl current. The ratios have been normalized using a factor of 3.127/3.57, which reflects the ratio between 3.127, the natural 35 Cl/37Cl ratio, and 3.57, the mean of the measured 35Cl/37Cl for the KNSTD1600 standard. Each point represents one block of data accumulated during a 26 s measurement interval. As a result of employing fast bouncing, during 97% of this interval 36Cl was being injected into the accelerator (9.75 ms per bounce cycle), while 35Cl and 37Cl were injected for the remaining period. (each 0.1 ms per bounce cycle). All the cathodes contained either standard or blank AgCl with natural 35Cl/37Cl ratios. Each point represents one block, about 30 s, of data. The lines connect data points in the sequence that they were collected. For the cathodes represented in black, the average ratio is 3.101 with a standard deviation of 1.3%. This range is indicated by the vertical bar. A few of the cathodes, represented as dotted, behaved anomalously, exhibiting an overall lower ratio and somewhat higher standard deviation. These data appear to show a current-dependent mass fractionation for which we have no good explanation at the moment. Nevertheless these data suggest that 35Cl/37Cl can be determined with a precision of about 2%. Further work is underway to characterize this current dependent fractionation and to understand the source of the anomalous behavior.
3.3.
36
S isobar suppression
With an isotopic abundance of 0.02%, interference from 36S has been the most serious impediment to routine 36Cl measurements. The ASTER spectrometer takes a multi-pronged approach to managing this interference. First, the separative chemistry has been designed to reduce sulfur to low levels both by strictly controlling possible sulfur contamination during sample processing, by using multiple AgCl dissolutions and precipitations and by utilizing a specific Ba(NO3)2–Ba(CO3)2 scavenging precipitation to remove sulfur [2]. Second, a materials test was undertaken to identify cathode material that might be low in sulfur and machinable. Nickel proved to be the best material and the ASTER cathodes for 36Cl are constructed from Ni 201 alloy supplied by CLAL-MSX, with a sulfur content of <20 ppm from manufacturer’s specifications. The use of this material lowered the sulfur rate sufficiently that it was possible to dispense with pre-packing cathodes with AgBr as is done in most other laboratories. The AgCl sample material is packed directly in a 1.3 mm diameter hole drilled in the Ni cathode and backed with a nickel pin of the same alloy. This is accomplished by compressing the AgCl sample material between a spherical stainless steel ball, whose shape defines the surface of the sample and a nickel pin that serves as a plug to keep the sample from dropping out of the back of the cathode. A metered hydraulic press is used to ensure uniform packing. Third, nuclear-charge-dependent energy loss and charge state stripping using a 1 lm silicon nitride degrader foil are employed to further separate the 36Cl and 36S isobars. For 36Cl+5 and 36S+5 incident at 30 MeV, the charge state yield of 36Cl+10 and 36S+10 exiting the degrader foil is somewhat more than 30% [7]. Because of its smaller Z, 36S leaves the foil with about 1.2% more average energy than does 36Cl [7]. The difference in charge state yield of 36Cl+10 and 36 +10 S is small and does not significantly influence the suppression of S [6]. The 35° ESA following the high-energy foil is tuned to the energy of 36Cl+10 and thus discriminates against 36S+10 with its higher energy. The slits before the detector are set asymmetrically tight on the high-energy side to take advantage of this energy difference. A fraction of 7 10 4 of 36S incident on the foil reaches the detector. 36Cl and 36S are further discriminated using a 4 anode (DE1, DE2, DE3, Efinal) gas ionization detector. Coincidence spectra from each combination of two anodes are examined to define regions of interest that filter 36S from 36Cl. In the analysis of the
Fig. 4. 35Cl/37Cl is plotted vs 35Cl current measured in post accelerator Faraday cups for several cathodes. All cathode material had the natural 35Cl/37Cl ratio of 3.127. See text for explanation. Dotted curves show anomalous behavior.
R. Finkel et al. / Nuclear Instruments and Methods in Physics Research B 294 (2013) 121–125 Table 1 Transmission of 36S vs 36Cl for the energy degrader technique and for multiparameter gating with the segmented particle detector. 36
S Transmission Absorber foil, ESA & magnet Detector Total: high-energy spectrometer
7 10 5 10 3.5 10
4 5 8
36
Cl Transmission
36
S/36Cl
17% 54% 9%
1/240 10 4 4 10
7
multiparameter data [7], the 36Cl windows are set to accept 54% of the 36Cl ions entering the detector, but only 5 10 5 of the 36S, leading to an overall suppression of 36S relative to 36Cl on the high-energy side of the accelerator of about 4 10 7 (Table 1). By analyzing an empty cathode with a stainless steel pin that contains rather high sulfur, we measured the contribution of the 36S tail to the 36Cl count rate. About 5 10 5 of the total 36S counts passed through the multiple region of interest filter that defines the 36Cl gate. As an example, in an analysis of a KNSTD5000 standard there were about 30,000 total counts in the 36S region, which, based on the above measurement, contributed about 1–2 counts to the 36Cl region. Since there were 15,000 total counts in the 36Cl region for this standard, which had a 36Cl/Cl ratio of 5 10 12, the background from sulfur was equivalent to a 36Cl/Cl ratio of about 10 15. The relative importance of the sulfur correction depends on the 36Cl content of the sample. For blanks, the sulfur correction could be up to 50%. For samples in the 10 13 range it was generally a few percent. 36S count rates are typically equivalent to a background 36Cl/Cl of 10 15. 3.4. Sensitivity A sample consisting of 6–8 mg of AgCl (equivalent to 1.5– 2.0 mg Cl) gave a 35Cl5+ current of about 23 lA that was sustained for an hour or more. This sample with a 36Cl/35Cl ratio of 6 10 14 gave about 460 counts of 36Cl in 60 min, yielding a 36 Cl content with a 5% uncertainty. This result implies that samples containing 1 106 atoms of 36Cl can be measured with better than 10% uncertainty.
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3.5. Throughput Following an extended period in which analytical techniques and protocols were developed and instituted, ASTER has now entered a stage in which routine sample analyses are combined with developmental runs designed to better characterize and improve the reliability of the spectrometer. During 2010, 758 samples 36Cl including 483 environmental and geoscience unknowns were analyzed. Acknowledgments We thank Bruno Hamelin and Pierre Deschamps for fruitful discussions during the course of this work. Khemrak Pou provided invaluable assistance in preparing cathodes. The careful comments of three anonymous reviewers greatly improved the quality of the manuscript. The ASTER AMS national facility (CEREGE, Aix en Provence) is supported by the INSU/CNRS, the French Ministry of Research and Higher Education, IRD and CEA. References [1] M. Arnold, S. Merchel, D.L. Bourlès, R. Braucher, L. Benedetti, R. Finkel, G. Aumaître, A. Gottdang, M. Klein, The French accelerator mass spectrometry facility ASTER: improved performance and developments, Nucl. Instr. Meth. B 268 (11–12) (2010) 1954–1959. [2] I. Schimmelpfennig, L. Benedetti, R. Finkel, R. Pik, P.H. Blard, D. Bourlès, P. Burnard, A. Williams, Sources of in-situ 36Cl in basaltic rocks: implications for calibration of production rates, Quat. Geochronol. 4 (6) (2009) 441–461. [3] R. Middleton, A Negative Ion Cookbook, Brookhaven National Laboratory, 1989. [4] M.G. Klein, W.E. Kieser, X.-L. Zhao, D.J.W. Mous, A. Gottdang, I.D. Clark, A.E. Litherland, A novel 3 MV multi-element AMS system, these proceedings. [5] P. Sharma, P.W. Kubik, U. Fehn, H.E. Gove, K. Nishiizumi, D. Elmore, Development of Cl-36 standards for AMS, Nucl. Instr. Meth. B 52 (3–4) (1990) 410–415. [6] E. Nottoli, M. Arnold, G. Aumaître, K. Keddadouche, D.L. Bourlès, M. Suter, The physics behind the isobar separation of 36Cl and 10Be at the AMS facility ASTER, These proceedings. [7] M. Klein, A. Gottdang, D. Mous, D. Bourlès, M. Arnold, B. Hamelin, G. Aumaître, R. Braucher, S. Merchel, F. Chauvet, Performance of the HVE 5 MV AMS system at CEREGE using an absorber foil for isobar suppression, Nucl. Instr. Meth. B 266 (8) (2008) 1828–1832.
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Improved 36Cl performance at the ASTER HVE 5 MV accelerator mass spectrometer national facility R. Finkel a,⇑, M. Arnold a,b, G. Aumaître a,b, L. Benedetti a, D. Bourlès a,b, K. Keddadouche a,b, S. Merchel a,c a
CEREGE, CNRS-IRD- Aix-Marseille Université, F-13545 Aix-en-Provence, France ASTER-Team, CNRS-IRD- Aix-Marseille Université, F-13545 Aix-en-Provence, France c Helmholtz-Zentrum Dresden-Rossendorf, D-01314 Dresden, Germany b
a r t i c l e
i n f o
Article history: Received 1 June 2011 Received in revised form 18 April 2012 Available online 16 May 2012 Keywords: Accelerator mass spectrometry 36 Cl capability ASTER Ion source memory
a b s t r a c t The HVE 5 MV ASTER AMS national facility at CEREGE was accepted in 2007. Since then we have continued to optimize performance for 36Cl. Cl-36 analyses use AgCl, a Cs negative ion sputter source, Ar stripping to +5 in the terminal of the Tandetron™ accelerator at 5 MV and a silicon nitride post-acceleration stripper foil to enhance suppression of 36S relative to 36Cl. The major challenges to obtaining the desired performance for Earth science applications are ion source memory, mass fractionation effects, 36S isobar suppression and sensitivity. Redesign of the SO110 ion source by HVE to change the size of the aperture and the shape of cathode significantly reduced ion source memory to less than 0.1%, a level that can be compensated for by matching standards to samples. We observe small systematic drifts in 35Cl/37Cl ratios over time, the source of which is not yet determined. Measurement of standards indicates that this effect limits the precision of 35Cl/37Cl ratio determination to about 2%. 36S is suppressed in several ways. First, the sample chemistry has been designed to reduce S to low levels. Second, cathodes are constructed of low-S nickel, enabling direct target loading without the use of AgBr pre-packing. Third, a post-acceleration Si3N4 stripper foil differentially absorbs energy from 36Cl and 36S. A subsequent electrostatic deflector is then able to suppress 36S by a factor of 240 relative to 36Cl. Differential energy loss in the detector further suppresses 36S by about 10 4, for an overall suppression factor of 4 10 7. 36S count rates are typically equivalent to a background 36Cl/Cl of 10 15. At typical 35Cl currents of 20 lA Cl5+ samples with 36 Cl/35Cl of 6 10 14 can be measured to ±5% statistical uncertainty with 1 h of analysis time. Typical machine blanks have 36Cl/Cl 2 10 15. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction The ASTER AMS (accelerator mass spectrometer) national facility at CEREGE (Centre Européen de Recherche et d’Enseignement des Géosciences de l’Environnement) was inaugurated in 2007 as a multi-nuclide accelerator mass spectrometer emphasizing environmental and Earth science applications. The heart of the ASTER facility is an HVE (High Voltage Engineering Europa) 5 MV Tandetron™ accelerator mass spectrometer. Nuclides determined include 10Be, 26Al, 36Cl, 41Ca and 129I. We report here performance parameters achieved in utilizing the ASTER AMS for 36Cl measurements. 36Cl is determined utilizing AgCl in the cathode, a Cs negative ion sputter source, Ar stripping to +5 in the terminal of the Tandetron™ accelerator at 5 MV and a silicon nitride post-acceleration stripper foil to enhance suppression of 36S relative to 36Cl. The major challenges to obtaining the desired performance for Earth science applications are ion source memory, mass fractionation ⇑ Corresponding author. E-mail address: fi[email protected] (R. Finkel). 0168-583X/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2012.05.019
effects, 36S isobar suppression and sensitivity. The results given here show that these challenges have been met and demonstrate that ASTER is capable of producing 36Cl analyses with precision of a few percent and 35Cl/37Cl with 2% precision. 2. Operational parameters 36
Cl is measured at the ASTER HVE 5 MV Tandetron™ facility using AgCl cathode material [1]. Negative Cl ions are produced using an SO110B Cs-sputter ion source. The 36Cl produced is injected into the accelerator. Charge state 5+ 36Cl at 30 MeV is selected after argon gas stripping at 5 MV in the terminal of the accelerator. Discrimination of 36Cl and 36S is enhanced by taking advantage of nuclear charge dependent energy loss in a 1 lm Si3N4 foil following the 90° high-energy magnet. Cl10+ output from this foil is subjected to subsequent energy selection in a 35° electrostatic deflector followed by a 30° vertical magnet. The Cl10+ then passes through a 50 nm Si3N4 window to a 4-anode detector where 36 Cl and 36S are further separated by differential energy loss. Fast bouncing of the electric field before the accelerator is used to
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briefly change the energy of the beam to inject 35Cl and 37Cl into the accelerator. Currents of both these stable chlorine isotopes can then be measured in off axis Faraday cups following the high energy 90° analyzing magnet. Particle transmission to 35Cl5+ is 17%. 3. Performance 3.1. Ion source memory Silver chloride is, in many ways, an ideal compound for 36Cl AMS. Its chemical separation and purification are relatively straightforward [e.g. 2] and it produces large and stable negative ion currents in the Cs-sputter ion sources commonly used in AMS [3]. AgCl does, however, have the disadvantage of being a relatively volatile compound compared to other cathode materials commonly used in AMS such at BeO or Al2O3. Because of this volatility, AgCl can migrate from the sample cathode, which becomes very warm during sputtering, to one of the many cooler surfaces found in the Cs-sputter source. This condensed material has a finite probability of being re-introduced to the ion source during subsequent analyses, thereby appearing as a source of contamination for subsequent measurements. With the version of the SO110 ion source originally used at ASTER, this memory effect was found to be a serious problem for 36Cl measurements, causing elevated and variable 36Cl backgrounds and leading to perturbations of the stable 35Cl/37Cl measurements required for isotope dilution determination of the stable chloride concentrations [1]. Collaboration between HVE and CEREGE led to several improvements in the SO110 design to reduce these effects. The new SO110-
B source geometry was altered to produce better focusing of the Cs ions on the sample material in the cathode, thus avoiding sputtering of surrounding material in the source that could lead to increased measurement background and memory [1]. The source was redesigned for a more open configuration. This change improved pumping efficiency at the target cathode, which reduced scattering and memory. Cooling was improved by enhancing liquid flow at the base of the source head, thus reducing Cs ionization at locations other than the ionizer. While these changes all led to a reduction in memory, a notable effect could still be observed. Memory is most easily studied by following 35Cl/37Cl ratio measurements, which, being determined by current measurements in Faraday cups, are more precisely monitored than are 36Cl/35Cl ratios, the precision of which are limited by counting statistics. Using 35 Cl/37Cl, the memory effect is illustrated by Fig. 1a. This plot shows a time series of 35Cl/37Cl measured in Faraday cups after the accelerator using the standard, unshielded cathode configuration, a cylindrical cathode with a 1.3 mm hole containing the AgCl sample material. In this configuration when successive cathodes had isotope ratios that differed by more than an order of magnitude, it took several minutes for the measured ratio to reach a plateau indicative of its true value. This occurred both when a high 35 Cl/37Cl ratio followed a low one, and vice versa. We hypothesized that this residual memory might be at least partially the result of sputtered material adhering to the immersion lens, which surrounded the cathode and served as an aperture to focus the negative ion beam to pass through the hole in the center of the spherical ionizer. In an attempt to reduce this effect the ion source was redesigned so that the immersion lens was incorporated into
Fig. 1. Effect of immersion lens geometry on ion source memory in 35Cl/37Cl measurements. The graphs show 35Cl/37Cl ratios vs time as measured in Faraday cups after the accelerator for cathode material of very different ratio. The slope of the peaks is indicative of memory. (a) Memory effect in passing from very different 35Cl/37Cl ratios with the cathode geometry shown schematically in the sketch. In this case each cathode sees the same immersion lens. (b) Reduced memory is seen when utilizing the cathode geometry illustrated in the sketch. In this case the immersion lens is shielded from the cathode by a lip intrinsic to each cathode.
R. Finkel et al. / Nuclear Instruments and Methods in Physics Research B 294 (2013) 121–125
each cathode. The effect of this change is seen in Fig. 1b, which shows that a plateau ratio is reached much more quickly when changing between cathodes with widely different ratios. The influence of these design changes on source memory was further investigated by measuring a sequence of 35Cl/37Cl samples prepared with 35Cl and 37Cl enriched material to give widely different ratios [4]. Three samples were used with 35Cl/37Cl of 0.019, 3.127 (=natural), and 370. During these tests, samples were measured in the sequence shown in Fig. 2 where the measured ratios are plotted against time. The measurements of Fig. 2 provide quantitative data on the sample-to-sample memory of the ion source
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when AgCl is being sputtered. When measuring a 35Cl/37Cl ratio of 370 just after a ratio of 0.019 the decay constant determined from an exponential fit was 35 s. When moving from 0.019 to 370, it was 45 s, a marked improvement over the data plotted in Fig. 1a, which have a decay constant of 160 s. These results indicate that the analysis of samples with isotopic ratios that differ by a factor of 2 104 can be measured consecutively with minimal memory effect. Memory was likewise examined during several measurement sequences in which a KNSTD5000 standard (36Cl/Cl = 5 10 12) [5] was measured followed by three AgCl blanks in succession.
Fig. 2. 35Cl/37Cl memory in the HVE SO110B ion source with cathode geometry illustrated in Fig. 1b. (a) 35Cl/37Cl measured in Faraday cups after the accelerator. (b) Memory on passing from a low to a high 35Cl/37Cl ratio as indicated on the figure. The time constant is derived from an exponential fit. (c) Memory on passing from a high to a low 35 Cl/37Cl ratio as indicated on the figure. The decay time constant t0 is derived from an exponential fit.
Fig. 3. 36Cl/35Cl memory in the SO110B ion source. The cathode geometry is the same as in Fig. 1b. After running a standard with a 36Cl/Cl ratio of 5000 10 15, three blank AgCl cathodes were run for 20 min each. The measured ratio and its uncertainty are plotted. Six sequences are illustrated. A memory of 2–3 10 15 is observed as explained in the text.
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The results are shown in the Fig. 3, which shows the blank values measured in three 20 min runs for six sets of three AgCl blanks run in succession after a KNSTD5000 standard. It appears that the first run of each blank is elevated above the second and third runs because of memory from the standard with its high 36Cl/Cl. The magnitude of the memory following the standard run can be estimated by subtracting the last run of each group of three from the first run. This test shows that the memory is about 2–3 10 15 or about 5 10 4 of the KNSTD5000.
3.2. Mass fractionation Variable mass fractionation in the spectrometer is a potential source of non-statistical uncertainty in AMS measurements in addition to memory. We investigated possible current-dependent mass fractionation in 35Cl/37Cl isotope ratios measured at ASTER by examining the measured 35Cl/37Cl isotopic ratios as a function of time for unspiked samples having the natural 35Cl/37Cl isotopic ratio of 3.127. Fig. 4 shows individual 35Cl/37Cl ratios measured in the off-axis Faraday cup after the accelerator plotted versus 35 Cl current. The ratios have been normalized using a factor of 3.127/3.57, which reflects the ratio between 3.127, the natural 35 Cl/37Cl ratio, and 3.57, the mean of the measured 35Cl/37Cl for the KNSTD1600 standard. Each point represents one block of data accumulated during a 26 s measurement interval. As a result of employing fast bouncing, during 97% of this interval 36Cl was being injected into the accelerator (9.75 ms per bounce cycle), while 35Cl and 37Cl were injected for the remaining period. (each 0.1 ms per bounce cycle). All the cathodes contained either standard or blank AgCl with natural 35Cl/37Cl ratios. Each point represents one block, about 30 s, of data. The lines connect data points in the sequence that they were collected. For the cathodes represented in black, the average ratio is 3.101 with a standard deviation of 1.3%. This range is indicated by the vertical bar. A few of the cathodes, represented as dotted, behaved anomalously, exhibiting an overall lower ratio and somewhat higher standard deviation. These data appear to show a current-dependent mass fractionation for which we have no good explanation at the moment. Nevertheless these data suggest that 35Cl/37Cl can be determined with a precision of about 2%. Further work is underway to characterize this current dependent fractionation and to understand the source of the anomalous behavior.
3.3.
36
S isobar suppression
With an isotopic abundance of 0.02%, interference from 36S has been the most serious impediment to routine 36Cl measurements. The ASTER spectrometer takes a multi-pronged approach to managing this interference. First, the separative chemistry has been designed to reduce sulfur to low levels both by strictly controlling possible sulfur contamination during sample processing, by using multiple AgCl dissolutions and precipitations and by utilizing a specific Ba(NO3)2–Ba(CO3)2 scavenging precipitation to remove sulfur [2]. Second, a materials test was undertaken to identify cathode material that might be low in sulfur and machinable. Nickel proved to be the best material and the ASTER cathodes for 36Cl are constructed from Ni 201 alloy supplied by CLAL-MSX, with a sulfur content of <20 ppm from manufacturer’s specifications. The use of this material lowered the sulfur rate sufficiently that it was possible to dispense with pre-packing cathodes with AgBr as is done in most other laboratories. The AgCl sample material is packed directly in a 1.3 mm diameter hole drilled in the Ni cathode and backed with a nickel pin of the same alloy. This is accomplished by compressing the AgCl sample material between a spherical stainless steel ball, whose shape defines the surface of the sample and a nickel pin that serves as a plug to keep the sample from dropping out of the back of the cathode. A metered hydraulic press is used to ensure uniform packing. Third, nuclear-charge-dependent energy loss and charge state stripping using a 1 lm silicon nitride degrader foil are employed to further separate the 36Cl and 36S isobars. For 36Cl+5 and 36S+5 incident at 30 MeV, the charge state yield of 36Cl+10 and 36S+10 exiting the degrader foil is somewhat more than 30% [7]. Because of its smaller Z, 36S leaves the foil with about 1.2% more average energy than does 36Cl [7]. The difference in charge state yield of 36Cl+10 and 36 +10 S is small and does not significantly influence the suppression of S [6]. The 35° ESA following the high-energy foil is tuned to the energy of 36Cl+10 and thus discriminates against 36S+10 with its higher energy. The slits before the detector are set asymmetrically tight on the high-energy side to take advantage of this energy difference. A fraction of 7 10 4 of 36S incident on the foil reaches the detector. 36Cl and 36S are further discriminated using a 4 anode (DE1, DE2, DE3, Efinal) gas ionization detector. Coincidence spectra from each combination of two anodes are examined to define regions of interest that filter 36S from 36Cl. In the analysis of the
Fig. 4. 35Cl/37Cl is plotted vs 35Cl current measured in post accelerator Faraday cups for several cathodes. All cathode material had the natural 35Cl/37Cl ratio of 3.127. See text for explanation. Dotted curves show anomalous behavior.
R. Finkel et al. / Nuclear Instruments and Methods in Physics Research B 294 (2013) 121–125 Table 1 Transmission of 36S vs 36Cl for the energy degrader technique and for multiparameter gating with the segmented particle detector. 36
S Transmission Absorber foil, ESA & magnet Detector Total: high-energy spectrometer
7 10 5 10 3.5 10
4 5 8
36
Cl Transmission
36
S/36Cl
17% 54% 9%
1/240 10 4 4 10
7
multiparameter data [7], the 36Cl windows are set to accept 54% of the 36Cl ions entering the detector, but only 5 10 5 of the 36S, leading to an overall suppression of 36S relative to 36Cl on the high-energy side of the accelerator of about 4 10 7 (Table 1). By analyzing an empty cathode with a stainless steel pin that contains rather high sulfur, we measured the contribution of the 36S tail to the 36Cl count rate. About 5 10 5 of the total 36S counts passed through the multiple region of interest filter that defines the 36Cl gate. As an example, in an analysis of a KNSTD5000 standard there were about 30,000 total counts in the 36S region, which, based on the above measurement, contributed about 1–2 counts to the 36Cl region. Since there were 15,000 total counts in the 36Cl region for this standard, which had a 36Cl/Cl ratio of 5 10 12, the background from sulfur was equivalent to a 36Cl/Cl ratio of about 10 15. The relative importance of the sulfur correction depends on the 36Cl content of the sample. For blanks, the sulfur correction could be up to 50%. For samples in the 10 13 range it was generally a few percent. 36S count rates are typically equivalent to a background 36Cl/Cl of 10 15. 3.4. Sensitivity A sample consisting of 6–8 mg of AgCl (equivalent to 1.5– 2.0 mg Cl) gave a 35Cl5+ current of about 23 lA that was sustained for an hour or more. This sample with a 36Cl/35Cl ratio of 6 10 14 gave about 460 counts of 36Cl in 60 min, yielding a 36 Cl content with a 5% uncertainty. This result implies that samples containing 1 106 atoms of 36Cl can be measured with better than 10% uncertainty.
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3.5. Throughput Following an extended period in which analytical techniques and protocols were developed and instituted, ASTER has now entered a stage in which routine sample analyses are combined with developmental runs designed to better characterize and improve the reliability of the spectrometer. During 2010, 758 samples 36Cl including 483 environmental and geoscience unknowns were analyzed. Acknowledgments We thank Bruno Hamelin and Pierre Deschamps for fruitful discussions during the course of this work. Khemrak Pou provided invaluable assistance in preparing cathodes. The careful comments of three anonymous reviewers greatly improved the quality of the manuscript. The ASTER AMS national facility (CEREGE, Aix en Provence) is supported by the INSU/CNRS, the French Ministry of Research and Higher Education, IRD and CEA. References [1] M. Arnold, S. Merchel, D.L. Bourlès, R. Braucher, L. Benedetti, R. Finkel, G. Aumaître, A. Gottdang, M. Klein, The French accelerator mass spectrometry facility ASTER: improved performance and developments, Nucl. Instr. Meth. B 268 (11–12) (2010) 1954–1959. [2] I. Schimmelpfennig, L. Benedetti, R. Finkel, R. Pik, P.H. Blard, D. Bourlès, P. Burnard, A. Williams, Sources of in-situ 36Cl in basaltic rocks: implications for calibration of production rates, Quat. Geochronol. 4 (6) (2009) 441–461. [3] R. Middleton, A Negative Ion Cookbook, Brookhaven National Laboratory, 1989