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26Al interferences in accelerator mass spectrometry measurements
Nuclear Instruments and Methods in Physics Research B 333 (2014) 42–45
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
26
Al interferences in accelerator mass spectrometry measurements
Sheng Xu ⇑, Stewart P.H.T. Freeman, Dylan H. Rood, Richard P. Shanks Scottish Universities Environmental Research Centre (SUERC), East Kilbride, G75 0QF, UK
a r t i c l e
i n f o
Article history: Received 27 October 2013 Received in revised form 31 March 2014 Accepted 18 April 2014 Available online 20 May 2014 Keywords: 26 Al Cl interference Accelerator mass spectrometer
a b s t r a c t The identification of interferences to 26Al was conducted with a 5 MV tandem accelerator mass spectrometer. In addition to 9Be1+, 17O2+ and 35Cl4+ ions observed previously, this study confirmed existence of the most significant interference 37Cl4+ continuum ion to 16 MeV 26Al3+ by measuring primary standard mixed with Cl with various 37Cl/35Cl ratios. The 37Cl ions were formed by 37Cl16O molecular-dissociation before the injection magnet, resulting in 0.7% of 26Al magnetic rigidity. Subsequently, the 37Cl4+ ions have ME/q2 value that differ from 26Al3+ by 0.1%. These allow the 37Cl and 37Cl4+ to simultaneously pass through injection magnet, analytical magnet and high-energy analyser, and finally reach the detector with 26Al3+. Further investigations on high charge states (26Al5+ and 26Al7+) indicate that the problem of interferences is generic. That is, interferences closest to 24 MeV 26Al5+ ions include 10B2+, 16O3+, 35Cl7+ and 37 7+ Cl ions, while 32 MeV 26Al7+ ions may be interfered by 7Li2+, 16O4+, 18O5+, 35Cl9+ and 37Cl9+. However, it remains unclear that 37Cl continuum events observed in 26Al3+-AMS do not exist in 26Al5+ and 26Al7+AMS operations. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction 26 Al (T1/2 = 7.2 105 year) is one of the most important cosmogenic radionuclide generated in the Earth’s atmosphere and surface rocks. The high detection sensitivity provided by accelerator mass spectrometer (AMS) since the 1980’s has dramatically expanded the utility of 26Al. It is now widely applied in geological and environmental science [1–4], but also in biological and biomedical studies [5,6]. The 26Al-AMS measurement is often thought to be straightforward as negative ion source does not produce stable 26Mg ions, the isobar of 26Al. Obviously, any even charge state, 26Al4+ for example, would be interfered by the intense counting rates of 13 2+ C ions which are broken from the injected 13C2 ions. Therefore, odd charge states are always employed in an 26Al-AMS operation. Due to the highest ion transmission efficiency the 26Al3+ is usually selected for routine measurements at the middle size of accelerators (i.e., 3–6 MV [7–11]). There are few laboratories with large size accelerator where high 26Al charge states are alternatively chosen, i.e., 26Al5+ [12], 26Al7+ [13,14], and 26Al13+ [15]. Indeed, our initial operational conditions for 26Al3+ measurement with a conventional thick Mylar entrance detector window suggested that the 26Al/27Al ratios of instrumental and processed
⇑ Corresponding author. Tel.: +44 1355270189. E-mail address: [email protected] (S. Xu). http://dx.doi.org/10.1016/j.nimb.2014.04.009 0168-583X/Ó 2014 Elsevier B.V. All rights reserved.
backgrounds are typically a few times 10 15 [8]. However, later measurement on a chemical process blank sample has once showed an extremely high 26Al/27Al ratio up to 10 11. In addition, it was observed that the measured 26Al/27Al ratio of the reference material Al01-5-3 is occasionally higher than the nominal value by >10%. These observations led us to consider the existence of interference to 26Al3+ [16], and subsequently to conduct the preliminary investigations [17]. Our initial measurements with the Mylar detector window indicated only one species in the 26Al3+ energy loss spectra. However, with the installation of thin SiN membrane windows (Silson Ltd., UK) the detector resolution was significantly increased so that multiple interferences were discerned. The early investigation indicated that 9Be1+ and 17O2+ ions, injected as 9Be17O and/or 9 Be16OH , correspond to the two interferences with lower energy [16]. Further experiments suggested that 35Cl4+, extracted as 35 12 Cl C , might contribute both higher energy peak events and continuum events with variable energy [17]. However, it remains uncertain whether or not 35Cl4+ contributed both peak and continuum events, and how the continuum events were formed. As the continuum events have energy distribution overlapping the 26 3+ Al , they can significantly interfere 26Al3+ measurement. Hence, this study focused on identifying the continuum events observed in our earlier study [16,17]. For alternative candidates, this study also testified whether such interferences exist in other high charge state 26Al-AMS analysis.
43
S. Xu et al. / Nuclear Instruments and Methods in Physics Research B 333 (2014) 42–45
2. Materials and experiments Description of the SUERC AMS instrument can be found in [8]. Detailed sample preparation and AMS conditions have been previously described in [16,17]. In this study, the primary standard Z92-0222 and blank Aristar Al2O3–Ag mixtures were further mixed with AgCl and graphite powder (Al:Cl:C = 4:1:1 by weight). Six AgCl samples have 36Cl/Cl < 5 10 15 and 37Cl/35Cl ratios varying from 0.001 to 54.56. 26Al and other interference ions were counted in 26Al3+-, 26Al5+-, and 26Al7+-AMS operations. The major parameters include 66 keV for the extracted negative ions, 4 MV terminal voltage, Ar gas stripper, and the thinnest 30 nm SiN detector window. 3. Results and discussion The 26Al and other interferences gate is set in the two dimensional Etotal versus dE1 histogram. Fig. 1 shows the detector ion energy loss spectra for Z92-0222 primary standard mixed with AgCl and BeO for 26Al3+, 26Al5+ and 26Al7+. A distinct feature is that the continuum events were observed in 26Al3+-AMS (Fig. 1a), neither 26Al5+- nor 26Al7+-AMS operations (Fig. 1b and c). 3.1.
37
Cl4+ interferences to
26
Al3+
In 26Al3+ spectra (Fig. 1a), three discrete peaks and one continuum were close to 26Al3+ ions. Two discrete peaks with lowerenergy were previously identified as 9Be+ and 17O2+ [16], while the discrete peak with higher-energy was attributed as 35Cl4+ [17]. The continuum events have wide energy distribution overlapping between 9Be+ and 35Cl4+. The measured ratios of the continuum events over 35Cl4+ discrete peak events are listed in Table 1 and plotted against the nominal 37Cl/35Cl ratios in Fig. 2. It is clear that there is an excellent positive correlation between the ratio of continuum over 35Cl4+ events and the nominal 37Cl/35Cl ratio varying from 0.001 to 54.56. The high correlation coefficient (R2 = 0.999) strongly indicates that the 37Cl mainly contributes the continuum events. This conclusion is supported by the Etotal vs dE1 spectra obtained by directly injecting mass 35 and 37 from a mixture of background Al2O3 sample and AgCl. It was found that mass 35 produced a peak with few continuum events while mass 37 dominated the continuum. Then question arises as how the 37Cl ions were injected into the accelerator; how the Cl continuum ions were generated and reached the detector; and why the 35Cl4+ and 37Cl ions have different behaviour. 4
4
a 35Cl4+ (37Cl4+)
3
dE1 (V)
37Cl4+
It is known that 37Cl cannot be directly injected into the accelerator due to the large difference of magnetic rigidity from 26 Al . For instance, the magnetic rigidity of 37Cl is 42% higher than that of 26Al if they have the same injection energy. Therefore, the only way for 37Cl to be injected is that 37Cl has less energy than 26 Al . That is, like 35Cl from the dissociation of 35Cl12C [16], 37Cl must be dissociated from a molecular ion before the injection magnet. Although 37Cl12C and/or 37Cl13C should be firstly considered as a candidate because 35Cl12C existence was confirmed in the sample [16], the magnetic rigidity of 37Cl from both 37Cl12C and 37Cl13C dissociation is 7.5% and 5.3% high than that of 26Al , respectively. Such large differences in magnetic rigidity exclude the possibility of 37Cl dissociated from 37Cl12C or 37Cl13C to simultaneously pass through injection magnet with 26Al . Thus, taking into account of abundant oxygen in the sample, we considered 37Cl16O as a potential compound. Indeed, theoretical calculation indicates that any 37Cl16O with energy of 66 keV from the ion source that dissociates between the low-energy ESA and the injection magnet would produce 37Cl with energy of 46 keV, resulting in magnetic rigidity of 1.705. This is common to 0.7% with that of 26 Al , which is close enough to allow the 37Cl to simultaneously pass through the injection magnet with 26Al . This consideration was further supported by mass scanning injection magnet from which ion current intensity of mass 53 (most likely 37Cl16O ) was seen in range of nA. Subsequently, among the multiple change states after stripper, the 37Cl4+ ions have ME/q2 of 46.36 and E/q of 5.01 that differ from 26Al3+ by 0.1% and 6.4%, respectively. These allow the 37Cl4+ to simultaneously pass through analytical magnet and high-energy analyser, and finally reach the detector with 26 3+ Al . However, the 37Cl4+ ions show continuum events instead of normal single peak like 35Cl4+. In theory, the 37Cl4+ ions have almost the same energy as 35Cl4+, and would be expected a discrete peak overlapping the 35Cl4+. Indeed, when the high-energy electrostatic cylindrical analyser (ECA) was further scanned to 40.1 kV, it was found that the 37Cl4+ continuum ions are ultimately changed into a discrete peak where 35Cl4+ ions would be expected. Identification of the 37Cl4+ discrete peak was further supported by a good agreement between the observed and theoretical ECA values. The ECA tune for the 16.066 MeV 26Al3+ ions is at 42.8 kV and the other signals are centred at 40.1 kV, 41.2 kV, 42.4 kV and 43.7 kV (Fig. 3a). They match the expected values for 9 Be1+, 17O2+, 35Cl4+ and 37Cl4+ ions of 41.2 kV, 43.6 kV, 42.4 kV and 40.1 kV presupposing that they share the 26Al3+ magnetic rigidity and therefore also pass the post-accelerator analysis magnet. It is not clear so far how the 37Cl4+ continuum events formed. Molecular fragmentation, charge changing and gas scattering can 4
b
3
c
3 35Cl7+&37Cl7+
35Cl9+&37Cl9+
continuum 26Al3+
2
2
2
26Al5+
26Al7+
18O5+ 17O2+
1
16O3+
1
16O4+
1
10B2+
7Li2+
9Be1+
0
0
1
2
3
Etotal (V)
4
5
0
0
1
2
Etotal (V)
3
0
0
1
2
3
Etotal (V)
Fig. 1. 26Al3+, 26Al5+ and 26Al7+ spectra of Z92-0222 primary standard (4.11 10 11 26Al/27Al) mixed with AgCl and BeO (with Al:Cl:Be = 20:5:1). Note that all interferences were individually optimized by adjusting high-energy ECA voltages from 26Al tuning value.
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S. Xu et al. / Nuclear Instruments and Methods in Physics Research B 333 (2014) 42–45
Table 1 Measured ratios of Cl continuum events over peak events.a
a
37
Target
Lab code
Material
Nominal
1 2 3 4 5 6
SUERC-a1575 SUERC-a1579 SUERC-a1584 SUERC-a1586 SUERC-a1588 SUERC-a1590
Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3
0.0892 0.05099 0.001 0.297 2.83 54.56
blank mixed with AgCl and graphite (Al:Cl:C = 4:1:1) blank mixed with AgCl and graphite (Al:Cl:C = 4:1:1) primary standard mixed with AgCl and graphite (Al:Cl:C = 4:1:1) primary standard mixed with AgCl and graphite (Al:Cl:C = 4:1:1) primary standard mixed with AgCl and graphite (Al:Cl:C = 4:1:1) primary standard mixed with AgCl and graphite (Al:Cl:C = 4:1:1)
Cl/35Cl
Ratio of Cl continuum over peak 0.275 ± 0.021 0.152 ± 0.01 0.0163 ± 0.0011 0.785 ± 0.034 6.85 ± 0.12 86 ± 4
Operated at 4 MV with the 30 nm SiN membrane window.
this is the case, different behaviour between 35Cl4+ and 37Cl4+ ions may be caused by the large difference in M/q ratios. For instance, 35 4+ Cl and 37Cl4+ have M/q ratios of 8.75 and 9.25, respectively. The 37Cl4+ ions were expected more offset from the central trajectory than 35Cl4+. Subsequently, the 37Cl4+ ions might hit the ECA plates resulting in ion scattering in a condition of critical angle. It should be pointed out that the 37Cl4+ continuum events were visible not only in 4 MV terminal voltages operation, but also observed in conditions of other energies (i.e., 3 MV and 5 MV). Moreover, we also testified if vacuum affects the breakdown of 37 16 Cl O (and 35Cl12C ) in low energy side by fully closing the stripper gas and instead using carbon foil stripping. As a result, the vacuum in low energy side improved from 1 10 7 mbar to 3 10 8 mbar. However, the continuum events remain in 26 3+ Al -AMS spectra.
100 2
80
60
37
4+ 35
4+
( Cl / Cl )
Cl continuum over peak events
y = 0.57258 + 1.5665x R = 0.99929
40
20
0
0
10
20
30 37
40
50
60
35
Nominal Cl/ Cl Fig. 2. Correlation between the measured ratio of Cl continuum over events and the nominal 37Cl/35Cl ratios.
35
Cl
4+
3.2. Identification of interferences to high charge states (26Al5+ and Al7+)
26
peak
Identification of interferences to 26Al5+ and 26Al7+ was achieved by comparing the optimised ECA setting (Fig. 3b and c) with the theoretical ECA values taking into account of the same ME/q2 ratio as 26Al5+ and 26Al7+ for all ions. For example, the ECA tune for the 24 MeV 26Al5+ ions is at 38.6 kV and the other signals are centred at 37.7 kV, 38.0 kV, 40.0 kV and 40.1 kV (Fig. 3b). These match the expected values for 10B2+, 16O3+, 35Cl7+ and 37Cl7+ ions of 40.2 kV, 37.7 kV, 40.2 kV and 38.0 kV presupposing that they share the 26 5+ Al magnetic rigidity. Such a consistence allows us to conclude that 10B2+ ions were injected as 10B16O , and 16O3+ as 10B16O and/or 9Be16OH . Like 26Al3+-AMS operation, the 35Cl7+ and 37Cl7+ ions were originated from 35Cl12C and 37Cl16O ions, respectively, which were dissociated between the low energy analyser and the injection magnet.
Normalized deadtime corrected count rate
combine to complicate the post acceleration spectrum. A continuum of ions can be generated within the accelerator, some of which will have the required ME/q2 to pass through the 90° analytical magnet and the lower resolution electrostatic analysers. Several types of such interferences were noted by [18]. Vockenhuber et al. [19] pointed out the high-energy charge changes occurred at the accelerator tube and the high energy electrostatic analyser plates in a condition of critical vacuum. In Fig. 3a, the ECA setting for the 37Cl4+ continuum events were wide from 41.5 kV to 44 kV. Although we do not have convincing explanations at present, this fact suggests that the 37Cl4+ continuum events might have been caused by angle scattering that happened in the ECA chamber. If
a
1
37Cl4+
9Be1+
35Cl4+
26Al3+
17O2+
b
1
0.8
16O3+
37Cl7+
26Al5+
10B2+
35Cl7+
c
1
0.8
0.8
0.6
0.6
0.4
0.4
0.4
0.2
0.2
0.2
37Cl4+ continuum
0.6
0
39
40
41
42
ECA (KV)
43
44
45
0
37Cl9+ 16O4+
35Cl9+
26Al7+
18O5+
7Li2+
0 37
38
39
ECA (KV)
40
41
31
32
33
34
35
36
37
38
39
40
41
ECA (KV)
Fig. 3. Summary of interferences closest to 26Al at different charge states (a) 26Al3+, (b) 26Al5+, (c) 26Al7+ by measuring Z92-0222 primary standard mixed with AgCl and BeO (with Al:Cl:Be = 20:5:1).
S. Xu et al. / Nuclear Instruments and Methods in Physics Research B 333 (2014) 42–45
On the other hand, the ECA tune for the 32 MeV 26Al7+ ions is at 36.7 kV and the other signals are centred at 33.1 kV, 34.0 kV, 34.9 kV, 37.8 kV and 38.9 kV (Fig. 3c). Like 26Al3+ and 26Al5+, these match the expected values for 7Li2+, 16O4+, 18O5+, 35Cl9+ and 37Cl9+ ions of 38.9 kV, 34.0 kV, 37.8 kV, 35.0 kV and 33.1 kV presupposing that they share the 26Al7+ magnetic rigidity. The 7Li2+ and 18O5+ ions were most likely injected as 7Li18OH , and 16O4+ as 10B16O and/or 9 Be16OH . The 35Cl9+ and 37Cl9+ ions were originated from molecular-dissociation of 35Cl12C and 37Cl16O as discussed above. Thus, it can be concluded that the issues of the lighter and heavier-than-26Al interferences are generic in all charge states. However, unlike 26Al3+ spectra, no 37Cl7+ and 37Cl9+ continuum events were observed in 26Al5+ and 26Al7+ spectra, respectively. This distinct difference remains unclear. 4. Conclusions The interference of the continuum events observed in the 26Al3+ spectra was identified as 37Cl4+ which was injected as 37Cl by 37 16 Cl O molecular-dissociation between the low energy electrostatic analyser and the injection magnet. The 37Cl4+ continuum events might have been caused by a critical angle scattering that happened in the high-energy electrostatic analyser chamber. Investigations of high charge state 26Al5+- and 26Al7+-AMS suggested that the issues of the lighter and heavier-than-26Al interferences are generic. However, no 37Cl continuum events were found in 26Al5+- and 26Al7+ operations. Acknowledgements We are grateful to A. Dougans and M. Miguéns-Rodríguez of SUERC for laboratory assistance. L. Benedetti and V. Guillou of CEREGE are greatly appreciated for kindly providing some AgCl materials with various 37Cl/35Cl ratios. References [1] G.M. Raisbeck, F. Yiou, J. Klein, R. Middleton, Accelerator mass spectrometry measurement of cosmogenic 26Al in terrestrial and extraterrestrial matter, Nature 301 (1983) 690–692. [2] K. Nishiizumi, J. Klein, R. Middleton, D. Elmore, P.W. Kubik, J.R. Arnold, Exposure history of Shergottites, Geochim. Cosmochim. Acta 50 (1986) 1017– 1021.
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[3] J.C. Gosse, F.M. Phillips, Terrestrial in situ cosmogenic nuclides: theory and application, Quat. Sci. Rev. 20 (2001) 1475–1560. [4] M. Auer, D. Wagenbach, E.M. Wild, A. Wallner, A. Priller, H. Miller, C. Schlosser, W. Kutschera, Cosmogenic 26Al in the atmosphere and the prospect of a 26 Al/10Be chronometer to date old ice, Earth Planet. Sci. Lett. 287 (2009) 453– 462. [5] G. Kislinger, C. Steinhausen, M. Alvarez-Brückmann, C. Winklhofer, T.H. Ittel, E. Nolte, Investigations of the human aluminium biokinetics with 26Al and AMS, Nucl. Instrum. Meth. B 123 (1997) 259–265. [6] S. Yumoto, H. Nagai, K. Kobayashi, W. Tada, T. Horikawa, H. Matsuzaki, 26Al incorporation into the tissues of suckling rats through maternal milk, Nucl. Instrum. Meth. B 223–224 (2004) 754–758. [7] P. Sharma, M. Bourgeois, D. Elmore, D. Granger, M.E. Lipschutz, X. Ma, T. Miller, K. Mueller, F. Rickey, P. Simms, S. Vogt, PRIME lab AMS performance, upgrades and research applications, Nucl. Instrum. Meth. B 172 (2000) 112–123. [8] C. Maden, P. Anastas, A. Dougans, S. Freeman, R. Kitchen, G. Klody, C. Schnabel, M. Sundquist, K. Vanner, S. Xu, SUERC AMS ion detection, Nucl. Instrum. Meth. B 259 (2007) 131–139. [9] P. Steier, R. Golser, W. Kutschera, A. Priller, C. Vockenhuber, S. Winkler, VERA, an AMS facility for ‘‘all’’ isotopes, Nucl. Instrum. Meth. B 223–224 (2004) 67– 71. [10] H. Matsuzaki, C. Nakano, Y. Tsuchiya, K. Kato, Y. Maejima, Y. Miyairi, S. Wakasa, T. Aze, Multi-nuclide AMS performances at MALT, Nucl. Instrum. Meth. B 259 (2007) 36–40. [11] P.W. Kubik, M. Christl, 10Be and 26Al measurements at the Zurich 6 MV Tandem AMS facility, Nucl. Instrum. Meth. B 268 (2010) 880–883. [12] D. Fink, M. Hotchkis, Q. Hua, G. Jacobsen, A.M. Smith, U. Zoppi, D. Child, C. Mifsud, H. van der Gaast, A. Williams, M. Williams, The ANTARES AMS facility at ANSTO, Nucl. Instrum. Meth. B 223–224 (2004) 109–115. [13] A.L. Hunt, G.A. Petrucci, P.R. Bierman, R.C. Finkel, Investigation of metal matrix systems for cosmogenic 26Al analysis by accelerator mass spectrometry, Nucl. Instrum. Meth. B 260 (2007) 633–636. [14] L.K. Fifield, S.G. Tims, T. Fujioka, W.T. Hoo, S.E. Everett, Accelerator mass spectrometry with the 14UD accelerator at the Australian National University, Nucl. Instrum. Meth. B 268 (2010) 858–862. [15] K. Sasa, T. Takahashi, Y. Tosaki, Y. Matsushi, K. Sueki, M. Tamari, T. Amano, T. Oki, S. Mihara, Y. Yamato, Y. Nagashima, K. Bessho, N. Kinoshita, H. Matsumura, Status and research programs of the multinuclide accelerator mass spectrometry system at the University of Tsukuba, Nucl. Instrum. Meth. B 268 (2010) 871–875. [16] S. Xu, A. Dougans, S. Freeman, C. Schnabel, K. Wilcken, Improved 10Be and 26AlAMS with a 5 MV spectrometer, Nucl. Instrum. Meth. B 268 (2010) 736–738. [17] S. Xu, S. Freeman, D. Sanderson, R. Shanks, K. Wilcken, Cl can interfere with Al3+ AMS but B need not matter to Be measurement, Nucl. Instrum. Meth. B 294 (2013) 403–405. [18] M.J. Nadeau, H.W. Lee, A.E. Litherland, K.H. Purser, X.L. Zhao, E/Q and ME/Q2 interference in the two models of 14C Tandetron systems: towards the 21st century, Nucl. Instrum. Meth. B 223–224 (2004) 328–332. [19] C. Vockenhuber, I. Ahmad, R. Golser, W. Kutschera, V. Liechtenstein, A. Priller, P. Steier, S. Winkler, Accelerator mass spectrometry of heavy long-lived radionuclides, Int. J. Mass Spectrom. 223–224 (2003) 713–732.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
26
Al interferences in accelerator mass spectrometry measurements
Sheng Xu ⇑, Stewart P.H.T. Freeman, Dylan H. Rood, Richard P. Shanks Scottish Universities Environmental Research Centre (SUERC), East Kilbride, G75 0QF, UK
a r t i c l e
i n f o
Article history: Received 27 October 2013 Received in revised form 31 March 2014 Accepted 18 April 2014 Available online 20 May 2014 Keywords: 26 Al Cl interference Accelerator mass spectrometer
a b s t r a c t The identification of interferences to 26Al was conducted with a 5 MV tandem accelerator mass spectrometer. In addition to 9Be1+, 17O2+ and 35Cl4+ ions observed previously, this study confirmed existence of the most significant interference 37Cl4+ continuum ion to 16 MeV 26Al3+ by measuring primary standard mixed with Cl with various 37Cl/35Cl ratios. The 37Cl ions were formed by 37Cl16O molecular-dissociation before the injection magnet, resulting in 0.7% of 26Al magnetic rigidity. Subsequently, the 37Cl4+ ions have ME/q2 value that differ from 26Al3+ by 0.1%. These allow the 37Cl and 37Cl4+ to simultaneously pass through injection magnet, analytical magnet and high-energy analyser, and finally reach the detector with 26Al3+. Further investigations on high charge states (26Al5+ and 26Al7+) indicate that the problem of interferences is generic. That is, interferences closest to 24 MeV 26Al5+ ions include 10B2+, 16O3+, 35Cl7+ and 37 7+ Cl ions, while 32 MeV 26Al7+ ions may be interfered by 7Li2+, 16O4+, 18O5+, 35Cl9+ and 37Cl9+. However, it remains unclear that 37Cl continuum events observed in 26Al3+-AMS do not exist in 26Al5+ and 26Al7+AMS operations. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction 26 Al (T1/2 = 7.2 105 year) is one of the most important cosmogenic radionuclide generated in the Earth’s atmosphere and surface rocks. The high detection sensitivity provided by accelerator mass spectrometer (AMS) since the 1980’s has dramatically expanded the utility of 26Al. It is now widely applied in geological and environmental science [1–4], but also in biological and biomedical studies [5,6]. The 26Al-AMS measurement is often thought to be straightforward as negative ion source does not produce stable 26Mg ions, the isobar of 26Al. Obviously, any even charge state, 26Al4+ for example, would be interfered by the intense counting rates of 13 2+ C ions which are broken from the injected 13C2 ions. Therefore, odd charge states are always employed in an 26Al-AMS operation. Due to the highest ion transmission efficiency the 26Al3+ is usually selected for routine measurements at the middle size of accelerators (i.e., 3–6 MV [7–11]). There are few laboratories with large size accelerator where high 26Al charge states are alternatively chosen, i.e., 26Al5+ [12], 26Al7+ [13,14], and 26Al13+ [15]. Indeed, our initial operational conditions for 26Al3+ measurement with a conventional thick Mylar entrance detector window suggested that the 26Al/27Al ratios of instrumental and processed
⇑ Corresponding author. Tel.: +44 1355270189. E-mail address: [email protected] (S. Xu). http://dx.doi.org/10.1016/j.nimb.2014.04.009 0168-583X/Ó 2014 Elsevier B.V. All rights reserved.
backgrounds are typically a few times 10 15 [8]. However, later measurement on a chemical process blank sample has once showed an extremely high 26Al/27Al ratio up to 10 11. In addition, it was observed that the measured 26Al/27Al ratio of the reference material Al01-5-3 is occasionally higher than the nominal value by >10%. These observations led us to consider the existence of interference to 26Al3+ [16], and subsequently to conduct the preliminary investigations [17]. Our initial measurements with the Mylar detector window indicated only one species in the 26Al3+ energy loss spectra. However, with the installation of thin SiN membrane windows (Silson Ltd., UK) the detector resolution was significantly increased so that multiple interferences were discerned. The early investigation indicated that 9Be1+ and 17O2+ ions, injected as 9Be17O and/or 9 Be16OH , correspond to the two interferences with lower energy [16]. Further experiments suggested that 35Cl4+, extracted as 35 12 Cl C , might contribute both higher energy peak events and continuum events with variable energy [17]. However, it remains uncertain whether or not 35Cl4+ contributed both peak and continuum events, and how the continuum events were formed. As the continuum events have energy distribution overlapping the 26 3+ Al , they can significantly interfere 26Al3+ measurement. Hence, this study focused on identifying the continuum events observed in our earlier study [16,17]. For alternative candidates, this study also testified whether such interferences exist in other high charge state 26Al-AMS analysis.
43
S. Xu et al. / Nuclear Instruments and Methods in Physics Research B 333 (2014) 42–45
2. Materials and experiments Description of the SUERC AMS instrument can be found in [8]. Detailed sample preparation and AMS conditions have been previously described in [16,17]. In this study, the primary standard Z92-0222 and blank Aristar Al2O3–Ag mixtures were further mixed with AgCl and graphite powder (Al:Cl:C = 4:1:1 by weight). Six AgCl samples have 36Cl/Cl < 5 10 15 and 37Cl/35Cl ratios varying from 0.001 to 54.56. 26Al and other interference ions were counted in 26Al3+-, 26Al5+-, and 26Al7+-AMS operations. The major parameters include 66 keV for the extracted negative ions, 4 MV terminal voltage, Ar gas stripper, and the thinnest 30 nm SiN detector window. 3. Results and discussion The 26Al and other interferences gate is set in the two dimensional Etotal versus dE1 histogram. Fig. 1 shows the detector ion energy loss spectra for Z92-0222 primary standard mixed with AgCl and BeO for 26Al3+, 26Al5+ and 26Al7+. A distinct feature is that the continuum events were observed in 26Al3+-AMS (Fig. 1a), neither 26Al5+- nor 26Al7+-AMS operations (Fig. 1b and c). 3.1.
37
Cl4+ interferences to
26
Al3+
In 26Al3+ spectra (Fig. 1a), three discrete peaks and one continuum were close to 26Al3+ ions. Two discrete peaks with lowerenergy were previously identified as 9Be+ and 17O2+ [16], while the discrete peak with higher-energy was attributed as 35Cl4+ [17]. The continuum events have wide energy distribution overlapping between 9Be+ and 35Cl4+. The measured ratios of the continuum events over 35Cl4+ discrete peak events are listed in Table 1 and plotted against the nominal 37Cl/35Cl ratios in Fig. 2. It is clear that there is an excellent positive correlation between the ratio of continuum over 35Cl4+ events and the nominal 37Cl/35Cl ratio varying from 0.001 to 54.56. The high correlation coefficient (R2 = 0.999) strongly indicates that the 37Cl mainly contributes the continuum events. This conclusion is supported by the Etotal vs dE1 spectra obtained by directly injecting mass 35 and 37 from a mixture of background Al2O3 sample and AgCl. It was found that mass 35 produced a peak with few continuum events while mass 37 dominated the continuum. Then question arises as how the 37Cl ions were injected into the accelerator; how the Cl continuum ions were generated and reached the detector; and why the 35Cl4+ and 37Cl ions have different behaviour. 4
4
a 35Cl4+ (37Cl4+)
3
dE1 (V)
37Cl4+
It is known that 37Cl cannot be directly injected into the accelerator due to the large difference of magnetic rigidity from 26 Al . For instance, the magnetic rigidity of 37Cl is 42% higher than that of 26Al if they have the same injection energy. Therefore, the only way for 37Cl to be injected is that 37Cl has less energy than 26 Al . That is, like 35Cl from the dissociation of 35Cl12C [16], 37Cl must be dissociated from a molecular ion before the injection magnet. Although 37Cl12C and/or 37Cl13C should be firstly considered as a candidate because 35Cl12C existence was confirmed in the sample [16], the magnetic rigidity of 37Cl from both 37Cl12C and 37Cl13C dissociation is 7.5% and 5.3% high than that of 26Al , respectively. Such large differences in magnetic rigidity exclude the possibility of 37Cl dissociated from 37Cl12C or 37Cl13C to simultaneously pass through injection magnet with 26Al . Thus, taking into account of abundant oxygen in the sample, we considered 37Cl16O as a potential compound. Indeed, theoretical calculation indicates that any 37Cl16O with energy of 66 keV from the ion source that dissociates between the low-energy ESA and the injection magnet would produce 37Cl with energy of 46 keV, resulting in magnetic rigidity of 1.705. This is common to 0.7% with that of 26 Al , which is close enough to allow the 37Cl to simultaneously pass through the injection magnet with 26Al . This consideration was further supported by mass scanning injection magnet from which ion current intensity of mass 53 (most likely 37Cl16O ) was seen in range of nA. Subsequently, among the multiple change states after stripper, the 37Cl4+ ions have ME/q2 of 46.36 and E/q of 5.01 that differ from 26Al3+ by 0.1% and 6.4%, respectively. These allow the 37Cl4+ to simultaneously pass through analytical magnet and high-energy analyser, and finally reach the detector with 26 3+ Al . However, the 37Cl4+ ions show continuum events instead of normal single peak like 35Cl4+. In theory, the 37Cl4+ ions have almost the same energy as 35Cl4+, and would be expected a discrete peak overlapping the 35Cl4+. Indeed, when the high-energy electrostatic cylindrical analyser (ECA) was further scanned to 40.1 kV, it was found that the 37Cl4+ continuum ions are ultimately changed into a discrete peak where 35Cl4+ ions would be expected. Identification of the 37Cl4+ discrete peak was further supported by a good agreement between the observed and theoretical ECA values. The ECA tune for the 16.066 MeV 26Al3+ ions is at 42.8 kV and the other signals are centred at 40.1 kV, 41.2 kV, 42.4 kV and 43.7 kV (Fig. 3a). They match the expected values for 9 Be1+, 17O2+, 35Cl4+ and 37Cl4+ ions of 41.2 kV, 43.6 kV, 42.4 kV and 40.1 kV presupposing that they share the 26Al3+ magnetic rigidity and therefore also pass the post-accelerator analysis magnet. It is not clear so far how the 37Cl4+ continuum events formed. Molecular fragmentation, charge changing and gas scattering can 4
b
3
c
3 35Cl7+&37Cl7+
35Cl9+&37Cl9+
continuum 26Al3+
2
2
2
26Al5+
26Al7+
18O5+ 17O2+
1
16O3+
1
16O4+
1
10B2+
7Li2+
9Be1+
0
0
1
2
3
Etotal (V)
4
5
0
0
1
2
Etotal (V)
3
0
0
1
2
3
Etotal (V)
Fig. 1. 26Al3+, 26Al5+ and 26Al7+ spectra of Z92-0222 primary standard (4.11 10 11 26Al/27Al) mixed with AgCl and BeO (with Al:Cl:Be = 20:5:1). Note that all interferences were individually optimized by adjusting high-energy ECA voltages from 26Al tuning value.
44
S. Xu et al. / Nuclear Instruments and Methods in Physics Research B 333 (2014) 42–45
Table 1 Measured ratios of Cl continuum events over peak events.a
a
37
Target
Lab code
Material
Nominal
1 2 3 4 5 6
SUERC-a1575 SUERC-a1579 SUERC-a1584 SUERC-a1586 SUERC-a1588 SUERC-a1590
Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3
0.0892 0.05099 0.001 0.297 2.83 54.56
blank mixed with AgCl and graphite (Al:Cl:C = 4:1:1) blank mixed with AgCl and graphite (Al:Cl:C = 4:1:1) primary standard mixed with AgCl and graphite (Al:Cl:C = 4:1:1) primary standard mixed with AgCl and graphite (Al:Cl:C = 4:1:1) primary standard mixed with AgCl and graphite (Al:Cl:C = 4:1:1) primary standard mixed with AgCl and graphite (Al:Cl:C = 4:1:1)
Cl/35Cl
Ratio of Cl continuum over peak 0.275 ± 0.021 0.152 ± 0.01 0.0163 ± 0.0011 0.785 ± 0.034 6.85 ± 0.12 86 ± 4
Operated at 4 MV with the 30 nm SiN membrane window.
this is the case, different behaviour between 35Cl4+ and 37Cl4+ ions may be caused by the large difference in M/q ratios. For instance, 35 4+ Cl and 37Cl4+ have M/q ratios of 8.75 and 9.25, respectively. The 37Cl4+ ions were expected more offset from the central trajectory than 35Cl4+. Subsequently, the 37Cl4+ ions might hit the ECA plates resulting in ion scattering in a condition of critical angle. It should be pointed out that the 37Cl4+ continuum events were visible not only in 4 MV terminal voltages operation, but also observed in conditions of other energies (i.e., 3 MV and 5 MV). Moreover, we also testified if vacuum affects the breakdown of 37 16 Cl O (and 35Cl12C ) in low energy side by fully closing the stripper gas and instead using carbon foil stripping. As a result, the vacuum in low energy side improved from 1 10 7 mbar to 3 10 8 mbar. However, the continuum events remain in 26 3+ Al -AMS spectra.
100 2
80
60
37
4+ 35
4+
( Cl / Cl )
Cl continuum over peak events
y = 0.57258 + 1.5665x R = 0.99929
40
20
0
0
10
20
30 37
40
50
60
35
Nominal Cl/ Cl Fig. 2. Correlation between the measured ratio of Cl continuum over events and the nominal 37Cl/35Cl ratios.
35
Cl
4+
3.2. Identification of interferences to high charge states (26Al5+ and Al7+)
26
peak
Identification of interferences to 26Al5+ and 26Al7+ was achieved by comparing the optimised ECA setting (Fig. 3b and c) with the theoretical ECA values taking into account of the same ME/q2 ratio as 26Al5+ and 26Al7+ for all ions. For example, the ECA tune for the 24 MeV 26Al5+ ions is at 38.6 kV and the other signals are centred at 37.7 kV, 38.0 kV, 40.0 kV and 40.1 kV (Fig. 3b). These match the expected values for 10B2+, 16O3+, 35Cl7+ and 37Cl7+ ions of 40.2 kV, 37.7 kV, 40.2 kV and 38.0 kV presupposing that they share the 26 5+ Al magnetic rigidity. Such a consistence allows us to conclude that 10B2+ ions were injected as 10B16O , and 16O3+ as 10B16O and/or 9Be16OH . Like 26Al3+-AMS operation, the 35Cl7+ and 37Cl7+ ions were originated from 35Cl12C and 37Cl16O ions, respectively, which were dissociated between the low energy analyser and the injection magnet.
Normalized deadtime corrected count rate
combine to complicate the post acceleration spectrum. A continuum of ions can be generated within the accelerator, some of which will have the required ME/q2 to pass through the 90° analytical magnet and the lower resolution electrostatic analysers. Several types of such interferences were noted by [18]. Vockenhuber et al. [19] pointed out the high-energy charge changes occurred at the accelerator tube and the high energy electrostatic analyser plates in a condition of critical vacuum. In Fig. 3a, the ECA setting for the 37Cl4+ continuum events were wide from 41.5 kV to 44 kV. Although we do not have convincing explanations at present, this fact suggests that the 37Cl4+ continuum events might have been caused by angle scattering that happened in the ECA chamber. If
a
1
37Cl4+
9Be1+
35Cl4+
26Al3+
17O2+
b
1
0.8
16O3+
37Cl7+
26Al5+
10B2+
35Cl7+
c
1
0.8
0.8
0.6
0.6
0.4
0.4
0.4
0.2
0.2
0.2
37Cl4+ continuum
0.6
0
39
40
41
42
ECA (KV)
43
44
45
0
37Cl9+ 16O4+
35Cl9+
26Al7+
18O5+
7Li2+
0 37
38
39
ECA (KV)
40
41
31
32
33
34
35
36
37
38
39
40
41
ECA (KV)
Fig. 3. Summary of interferences closest to 26Al at different charge states (a) 26Al3+, (b) 26Al5+, (c) 26Al7+ by measuring Z92-0222 primary standard mixed with AgCl and BeO (with Al:Cl:Be = 20:5:1).
S. Xu et al. / Nuclear Instruments and Methods in Physics Research B 333 (2014) 42–45
On the other hand, the ECA tune for the 32 MeV 26Al7+ ions is at 36.7 kV and the other signals are centred at 33.1 kV, 34.0 kV, 34.9 kV, 37.8 kV and 38.9 kV (Fig. 3c). Like 26Al3+ and 26Al5+, these match the expected values for 7Li2+, 16O4+, 18O5+, 35Cl9+ and 37Cl9+ ions of 38.9 kV, 34.0 kV, 37.8 kV, 35.0 kV and 33.1 kV presupposing that they share the 26Al7+ magnetic rigidity. The 7Li2+ and 18O5+ ions were most likely injected as 7Li18OH , and 16O4+ as 10B16O and/or 9 Be16OH . The 35Cl9+ and 37Cl9+ ions were originated from molecular-dissociation of 35Cl12C and 37Cl16O as discussed above. Thus, it can be concluded that the issues of the lighter and heavier-than-26Al interferences are generic in all charge states. However, unlike 26Al3+ spectra, no 37Cl7+ and 37Cl9+ continuum events were observed in 26Al5+ and 26Al7+ spectra, respectively. This distinct difference remains unclear. 4. Conclusions The interference of the continuum events observed in the 26Al3+ spectra was identified as 37Cl4+ which was injected as 37Cl by 37 16 Cl O molecular-dissociation between the low energy electrostatic analyser and the injection magnet. The 37Cl4+ continuum events might have been caused by a critical angle scattering that happened in the high-energy electrostatic analyser chamber. Investigations of high charge state 26Al5+- and 26Al7+-AMS suggested that the issues of the lighter and heavier-than-26Al interferences are generic. However, no 37Cl continuum events were found in 26Al5+- and 26Al7+ operations. Acknowledgements We are grateful to A. Dougans and M. Miguéns-Rodríguez of SUERC for laboratory assistance. L. Benedetti and V. Guillou of CEREGE are greatly appreciated for kindly providing some AgCl materials with various 37Cl/35Cl ratios. References [1] G.M. Raisbeck, F. Yiou, J. Klein, R. Middleton, Accelerator mass spectrometry measurement of cosmogenic 26Al in terrestrial and extraterrestrial matter, Nature 301 (1983) 690–692. [2] K. Nishiizumi, J. Klein, R. Middleton, D. Elmore, P.W. Kubik, J.R. Arnold, Exposure history of Shergottites, Geochim. Cosmochim. Acta 50 (1986) 1017– 1021.
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