Interlaboratory study of the ion source memory effect in 36Cl accelerator mass spectrometry

doi:10.1016/j.nimb.2014.02.130

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms

Volume 329, 15 June 2014, Pages 22-29

https://doi.org/10.1016/j.nimb.2014.02.130 Get rights and content

Highlights

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Long-term memory effect in negative ion sources investigated for chlorine isotopes.
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Interlaboratory comparison of four up-to date negative ion sources.
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Ion source improvement at DREAMS for minimization of long-term memory effect.
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Long-term memory effect is the limitation for precise AMS data of volatile elements.
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Findings to be considered for samples with highly variable ratios of ³⁶Cl/Cl & ¹²⁹I/I.

Abstract

Understanding and minimization of contaminations in the ion source due to cross-contamination and long-term memory effect is one of the key issues for accurate accelerator mass spectrometry (AMS) measurements of volatile elements. The focus of this work is on the investigation of the long-term memory effect for the volatile element chlorine, and the minimization of this effect in the ion source of the Dresden accelerator mass spectrometry facility (DREAMS). For this purpose, one of the two original HVE ion sources at the DREAMS facility was modified, allowing the use of larger sample holders having individual target apertures. Additionally, a more open geometry was used to improve the vacuum level. To evaluate this improvement in comparison to other up-to-date ion sources, an interlaboratory comparison had been initiated. The long-term memory effect of the four Cs sputter ion sources at DREAMS (two sources: original and modified), ASTER (Accélérateur pour les Sciences de la Terre, Environnement, Risques) and VERA (Vienna Environmental Research Accelerator) had been investigated by measuring samples of natural ³⁵Cl/³⁷Cl-ratio and samples highly-enriched in ³⁵Cl (³⁵Cl/³⁷Cl ∼ 999). Besides investigating and comparing the individual levels of long-term memory, recovery time constants could be calculated. The tests show that all four sources suffer from long-term memory, but the modified DREAMS ion source showed the lowest level of contamination. The recovery times of the four ion sources were widely spread between 61 and 1390 s, where the modified DREAMS ion source with values between 156 and 262 s showed the fastest recovery in 80% of the measurements.

Introduction

Accelerator mass spectrometry (AMS) is an ultrasensitive method for the measurement of isotopic ratios. Usually, ratios of radioactive nuclei to their stable isotopes are measured [1], [2], [3]. Due to the direct detection and counting of the radioisotopes, AMS does not depend primarily on radioactive decay parameters such as branching ratios and half-lives. The suppression of interfering molecular isobars enables, depending on the measured isotope, the measurement of isotopic ratios down to 10⁻¹⁶–10⁻¹² which cannot be achieved by any other analytical technique. Therefore, AMS covers a wide spectrum of applications which range from dating organic material to geomorphology and astrophysical studies [1], [2], [3].

Chlorine-36 (half-life = (3.013 ± 0.015) × 10⁵ a [4]) is a cosmogenically and anthropogenically produced radionuclide. It is commonly used for groundwater dating (e.g. [5], [6]), surface rock exposure dating (e.g. [7]), bomb peak studies (e.g. [8]) and extra-terrestrial material and cosmic ray variations studies (e.g. [9]). Due to the necessity of reaching the charge state 5+ to insure an unambiguous characterization and detection of ³⁶Cl, AMS measurements of ³⁶Cl were usually performed with large accelerators capable of producing terminal voltages of at least 5 MV that in addition provide the necessary energy for separating ³⁶Cl from the naturally superabundant ³⁶S. Recently, Martschini et al. [10], [11] showed that measurements of ³⁶Cl with sufficient sulfur suppression are also possible at lower energies.

Besides the suppression of sulfur, a well working ion source is crucial to gain high-quality data in ³⁶Cl-AMS. AMS ion sources are usually based on the Middleton concept of sputtering negative ions with Cs [12], [13]. Negative ions are emitted from the sample material by sputtering with Cs⁺ -ions and using neutral Cs deposited on the cooled target as electron donator. According to Middleton [13] and Kilius et al. [14] the requirements for a good working AMS ion source are:

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High and stable current.
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Low emittance.
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High ionization efficiency.
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Low memory effect.
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Fast sample changing.

Chlorine, as a halogen, easily forms negative ions resulting in a very high sputtering efficiency, which is, according to Middleton [15], in the range of 16%. This means that the high-efficiency and high-current requirements are easily satisfied for Cl. With sample changing times of a few seconds, the criteria of fast sample changing is also met in up-to date ion sources. Despite the high electron affinity, most of the sputtered Cl, however, remains neutral in the ion source [15]. This gaseous neutral Cl either stagnates in the ion source, is pumped or condensates on cold surfaces in the ion source. This implies the build-up of an ion source Cl reservoir, which isotopic ratio is a mixture of the ratios of the previously sputtered targets and may affect all following measurements. This so-called long-term memory effect has to be considered in addition to the short-term memory effect that concerns the contamination of one sample by the immediately preceding one. These ion source memory effects (short- and long-term) were recently discussed in Wilcken et al. [16], [17], Arnold et al. [18] and Finkel et al. [19]. Based on these works, we focused on optimizing the High Voltage Engineering (HVE) SO110 ion source at the Dresden accelerator mass spectrometry (DREAMS) facility for the measurement of volatile elements with the aim of minimizing the long-term memory effect. The performance of the modified ion source were then compared to other up-to-date ion sources.

Minimizing the long-term memory effect is also crucial for AMS measurements of other volatile elements such as iodine, i.e. ¹²⁹I/¹²⁷I ratios, and therefore the performed study also has important implications for smaller machines, which do not measure ³⁶Cl. However, chlorine unlike iodine has two stable isotopes, i.e. ³⁵Cl and ³⁷Cl, which makes Cl more pertinent considering the measurement procedures developed for this work. The focus of this work is therefore the measurement of the ion source long-term memory effect using the two stable isotopes of chlorine.

Section snippets

Experimental

In order to determine the long-term memory effect, a protocol using AgCl-samples of different ³⁵Cl/³⁷Cl-ratios was developed. Measuring the ^35,37Cl currents avoids the limitations linked to low counting statistics of the rare radionuclide, and thus allows to perform precise measurement of the change of the ³⁵Cl/³⁷Cl-isotopic ratios over time for the determination of recovery times. This protocol was followed to compare the long-term memory effect of the ion sources of the following AMS

Data evaluation

Due to the different time structure of the raw data, the data evaluation at VERA was slightly different from the evaluations at ASTER and DREAMS.

Conclusion

The interlaboratory measurements have shown that all participating ion sources suffer from the long-term memory effect. Whereas the initial contamination, the difference between r_enr (³⁵Cl-enriched sample ratio) and A (saturation value) measured in the first run of each ion source is partially caused by different tuning periods with natural Cl samples (see Table 1) different target preparation and also different target holder material, the further decrease of the saturation value can be

Outlook

³⁵Cl/³⁷Cl measurements at different AMS-facilities were performed under standard conditions for the particular facility and ion source. Hence, ion source parameters like Cs-temperature, ionizer power and applied voltages differ for all four ion sources, resulting in different currents. For further studies of the integrative effect of the long-term memory, parameters like the amount of vaporized sample material and the efficiency of production of negative Cl, which are hardly accessible with the

Acknowledgements

We would like to thank the operator crew of the ion beam center (HZDR) and R. Ziegenrücker (HZDR) for the good cooperation and their helping hand during beam times at the DREAMS-facility. The help of R. Aniol from the HZDR mechanical workshop during the planning and construction phase of the modified ion source is much appreciated. This work was partially funded by German – French exchange programmes from DAAD/ÊGIDE (Project Nos. 500 888 61 and 22077QC) and DFG (Project No. RU 1832/1-1). The

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Natural ¹⁰Be (t_1/2 = 1.387 ± 0.012 Ma) is produced by cosmic rays and is present on Earth's surface only at ultratrace concentrations (typically 10⁴ to 10¹⁰ atoms/g). Its cosmogenic origin makes it an important tracer for many applications in Earth and environmental sciences. An improved accelerator mass spectrometry (AMS) method has been developed at the Vienna Environmental Research Accelerator (VERA) at the University of Vienna to detect the long-lived radionuclide ¹⁰Be and separate it from its isobar ¹⁰B. Recently installed and projected AMS facilities mainly apply a degrader foil followed by an electrostatic or magnetic separator to remove ¹⁰B from the ion beam. This provides the highest suppression of ¹⁰B, but suffers from significant transmission losses of ¹⁰Be ions. The new technique described here achieves comparable ¹⁰B suppression with a passive absorber, consisting of a stack of silicon nitride foils. Compared to a gas absorber, the smaller energy straggling in foils allows separation at lower energies. For a tandem accelerator operated at 3 MV, the charge state 2 + instead of 3 + can be used, with a stripping yield as high as 55%. This way, a high overall efficiency is gained. The setup is simple to operate, and provides good precision and accuracy. We compare this new approach with other methods used at VERA and at other AMS facilities. The foil stack setup was fully characterized with artificial samples from chemically and isotopically well-defined reagents, and is now routinely applied to real samples in various research projects at VERA. The new method is straightforward to be implemented, and was already adopted at another AMS facility with higher terminal voltage, the potential use at tandem accelerators with lower terminal voltage is under exploration.
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Cliff erosion may be a major problem in settled areas affecting populations and producing economic and ecological losses. In this paper we present a procedure to calculate the long-term retreat rate of a cliff affected by rockfalls in the Montsec Range, Eastern Pyrenees (Spain). It is composed of low, densely fractured limestones; and the rockwall is affected by rockfalls of different sizes. The rockfall scars are clearly distinguishable by their regular boundaries and by their orange colour, which contrast with the greyish old reference surface (S₀) of the cliff face. We have dated different stepped surfaces of the rockwall, including S₀, using cosmogenic ³⁶Cl. The total amount of material released by rockfall activity was calculated using a high definition point cloud of the slope face obtained with a terrestrial laser scanner (TLS). The present rockwall surface has been subtracted from the reconstructed old cliff surface. This has allowed the calculation of the total volume released by rockfalls and of the retreat rate. The latter ranges from 0.31 to 0.37 mm·a^− 1. This value is of the same order of magnitude as that obtained by other researchers in neighbouring regions in Spain, having similar geology and affected by rockfalls.

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Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms

Highlights

Abstract

Introduction

Section snippets

Experimental

Data evaluation

Conclusion

Outlook

Acknowledgements

References (29)

Nucl. Instr. Meth. B

Nucl. Instr. Meth. B

Nucl. Instr. Meth. B

Nucl. Data Sheets

Appl. Geochem.

Quat. Geochronol.

Nucl. Instr. Meth.

Earth Planet. Sci. Lett.

Nucl. Instr. Meth. B

Nucl. Instr. Meth. B

Nucl. Instr. Meth.

Nucl. Instr. Meth. B

Nucl. Instr. Meth. B

Nucl. Instr. Meth. B

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Carbonate and silicate intercomparison materials for cosmogenic <sup>36</sup>Cl measurements

Chlorine measurements at the 5MV French AMS national facility ASTER: Associated external uncertainties and comparability with the 6MV DREAMS facility

Calculation of the rockwall recession rate of a limestone cliff, affected by rockfalls, using cosmogenic chlorine-36. Case study of the Montsec Range (Eastern Pyrenees, Spain)