The first four years of the AMS-facility DREAMS: Status and developments for more accurate radionuclide data

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Highlights

  • DREAMS is performing routine AMS of 10Be, 26Al, 36Cl, 41Ca, and 129I.

  • Data is normalised directly to primary standards or traceable to those.

  • Improved accuracy due to low-memory ion source and new tuning strategy for 129I.

  • Smaller statistical uncertainty due to optimised pressure, metals and mixing ratios.

  • Cross-contamination and long-term memory generally underestimated problem.

Abstract

DREAMS, the DREsden AMS-facility, is performing routine accelerator mass spectrometry of 10Be, 26Al, 36Cl, 41Ca, and 129I for a wide range of applications. All DREAMS-data is normalised directly to primary standards or traceable to those via cross-calibration of secondary standards. Recent technical developments such as a low-memory ion source for 36Cl and 129I and sophisticated tuning strategies for 129I led to improved-accuracy data. Tests of ion source output have been performed with different metal binders, sample-to-binder mixing ratios, and compaction pressures in order to find optimal parameters. The highest and most stable outputs have been obtained for 10Be, 26Al, and 41Ca for the following binders and mixing ratios (by weight): BeO:Nb, 1:4; Al2O3:Ag, 1:1; CaF2:Ag, 1:4. Higher beam currents generally result in reduced statistical uncertainty. Cross-contamination and long-term memory seem to be underestimated problems asking for further tests and improvements such as the development of low-level in-house-standards.

Introduction

The DREsden Accelerator Mass Spectrometry facility (DREAMS) is in routine operation since autumn 2011 [1]. Here, long-lived so-called cosmogenic radionuclides such as 10Be, 26Al, 36Cl, 41Ca, and 129I can be quantified at the 10−14 (radionuclide/stable nuclide) level. Applications are performed within interdisciplinary cooperations with users from universities and research centres as DREAMS is part of a Helmholtz large-scale facility providing access to external users on the basis of positively-evaluated scientific proposals. Hence, research is focussed on diverse topics such as astrophysics [2], [3], climate [4], cosmochemistry [5], [6], geomorphology [7], [8], [9], hydrogeology [10] and nuclear decommissioning [11]. The benefits from using AMS for research in other fields of applications like radiation protection, nuclear safety/waste/forensics, radioecology, phytology, nutrition, toxicology, and pharmacology are the same: Smaller sample sizes, easier and faster sample preparation, higher sample throughput and the redundancy for cost-intensive radiochemistry laboratories are reducing expenses for the AMS-facility and users.

Two dedicated AMS chemistry labs, one for volatile 36Cl and 129I, the other for all other nuclides, are accessible to users. Together with excellent quality assurance based on the use of primary or traceable standards, these make DREAMS especially attractive for experienced cosmogenic nuclide researchers but even more for newcomers. Thus, training on the job e.g. in the chemistry labs is an essential tool of our “mission” to widen the fields of AMS applications and user communities.

To keep DREAMS a state-of-the-art facility, in-house research is an important component of our operation. This has included the development of an ion source with low cross-contamination and memory-effect [12]. The capability to measure rare stable nuclides with ultra-sensitivity via spatially-resolved Super-Secondary Ion Mass Spectrometry (Super-SIMS) or, if spatial resolution is not required, via Trace Element AMS (TEAMS) is currently under development. This technology development is of course partially based on our own research interests, but also driven by the steadily increasing demands of our users.

Section snippets

System set-up

The original DREAMS-system with the main focus on 10Be and 26Al has been described earlier [1], thus, mainly changes are described in the following. One of the two Cs-sputter ion sources (Fig. 1) has been further developed to reduce sample-to-sample cross-contamination and long-term memory for volatiles like 36Cl and 129I, which has been introduced in greatest detail in [12]. For further improvement the shape of the cathodes used in this ion source has been modified from the original design [13]

AMS-measurements

Chemical separation schemes for production of AMS-targets from samples of different origin for all nuclides in use at DREAMS, with the exception of 129I, have been published earlier [e.g. [2], [14], [15], and references therein]. Resulting materials are mixed with metal binders (see below). They are pressed in Cu-cathodes from the back against disposable steel balls giving the surfaces always the same concave shape. Finally, a stainless steel pin fixes the material from the back. This way,

Summary

DREAMS is performing routine AMS-measurements of 10Be, 26Al, 36Cl, 41Ca and 129I to produce highly-accuracy data for a diverse range of applications. Recent developments and tests led to generally improved performance such as lower background, allowing quantification of lower-ratio samples. All DREAMS-data is normalised directly to primary standards or traceable via secondary standards to those. Further work for low-level standards and repetition of cross-calibration of 129I-standards at DREAMS

Acknowledgements

The authors are grateful to the DREAMS-operators and external users, in particular Jenny Feige (VERA, now TU Berlin), Peter Ludwig (TU Munich), Samuel Niedermann (GFZ Potsdam), Thomas Smith (U Bern), and Cornelia Wilske (UFZ Halle). We would like to thank Christoph Vockenhuber (ETH Zurich) for helpful discussions and suggestions while an earlier 129I beam time and Peter Steier (VERA) for providing us with low-129I-NaI for producing machine blank material. Thanks to Erik Strub (University of

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Present address: Australian National University, Canberra, ACT 0200, Australia.

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