Setting-up chemistry labs for accelerator mass spectrometry


Setting-up chemistry labs for accelerator mass spectrometry

Merchel, S.; Andermann, C.; Arnold, M.; Aumaître, G.; Bourlès, D.; Braucher, R.; Fuchs, M.; Gloaguen, R.; Klemm, K.; Martschini, M.; Schildgen, T. F.; Steier, P.; Wallner, A.; Yildirim, C.

Introduction: The AMS business is booming: Many low-energy (< 1 MV) facilities, which are fully dedicated for 14C-analysis, are under construction or in funding status. Additionally, medium-energy accelerators such as the British 5 MV-NEC machine at “SUERC” Glas-gow, the French 5 MV-HVEE-machine “ASTER” at Aix-en-Provence [1] and the two German 6 MV-HVEE-machines “DREAMS” at Dresden [2] and “Cologne AMS” have been recently installed or are still in testing mode in Central Europe. Of course, these bigger machines need not only experienced physicists and technicians to get them running. It also seems to be advisable to have some experienced scientists around, who knows how to prepare AMS targets for 10Be, 26Al, 36Cl, 41Ca, and 129I measurements.
Quality assurance: In contrast to the 14C-community, where e.g. round-robin exercises are routine business, the idea of quality assurance and traceable standards has only been brought up lately for the other cosmogenic radionuclides. Thus, world-wide accepted standards issued by metrology institutes are rare: NIST is selling two kinds of 129I/127I-standards, and the Institute for Reference Materials and Measurements (IRMM) provides one set of 41Ca/40Ca solutions having eight different ratios [3]. Unfortunately, the most commonly used 10Be/9Be standard provided by NIST has been recently sold-out and will not be reissued. Other primary standard-type materials (26Al,36Cl), which are not commercially available, have been prepared by diluting certified activities and subsequent analysis within round-robin exercises [4,5]. After production of big quantities of in-house secondary standards for all nuclides, cross-calibration vs. primary standard-type materials has to be performed in-house and elsewhere [1].
Finally, as commercial 9Be contains intrinsic 10Be up to a level of 4x10-14 [6] sophisticated production of in-house carriers, used as machine blanks and for processing samples, such as Be2SiO4 originating from a deep mine in Brazil (Fig. 1), is needed.

Fig. 1: Origins of terrestrial samples yet processed at FZD chemistry labs: Antarctica, Brazil (carrier), Italy, Macedonia, Nepal, Slovenia, Tajikistan and Turkey.

Only after production and measurement of all these materials, the AMS facility is ready for real sample measurements.
Real samples: As it is always not advisable to change two important “things” at the same time, here machine and chemistry, first and foremost, a close cooperation with the AMS teams of “ASTER” and “VERA” (Vienna Environmental Research Accelerator) needed to check the quality of the new chemical sample preparation at FZD.
A “good” AMS sample is defined by two main features: high stable isotope current and low isobar concentration. A high chemical yield and low concentrations of other elements – originating either from the matrix or chemical products used – are less important. A corresponding low processing blank, i.e. with a very low radionuclide/stable nuclide ratio, is, however, essential for projects working near the detection limit. For high sample throughput and reasonable costs a fast, easy and cheap chemical separation is also favorable. Though, as it is the case for most destructive analytical methods, AMS sample preparation takes much longer, i.e. 24 h (ice, water) to 2-4 weeks (sediments, rocks), and is more expensive than the actual AMS measurement [7], which takes about 10-60 min.
For 10Be-AMS-targets isolated from quartz-rich river sediments (Himalaya, erosion rate study) and a calcite-rich boulder from a Slovenian rockfall area, 9Be-currents had been in the range of “ASTER” standards and machine blanks. The processing blanks produced at FZD were in the same order as the machine blank (1x10-15), thus, more than one order of magnitude lower than the lowest sample ratio.
Ten 36Cl-AMS-targets prepared from river terraces (Anatolian Plateau, Turkey, uplift rate study) will be investigated in June at “VERA”, hopefully validating the high quality of 36Cl-AMS targets.
Conclusion: We successfully started AMS chemistry in 2009 at FZD and are now open to more collaborations with research institutes and universities.
Acknowledgments: Thanks to E. Strub (26Al-activity), M. Bichler (neutron-irradiation of 9Be), C. Varajão (Be2SiO4 crystals), AWI & U Heidelberg (Antarctic & Italian ice/snow samples), DAAD & DFG (cash) & L. Benedetti, R.C. Finkel, I. Mrak, W. Möller, HVEE, FZD-AMS-team (great cooperation).
References: [1] Arnold M. et al. NIMB 268 (2010) 1954. [2] Akhmadaliev Sh. et al. this meeting. [3] Hennessy C. et al. NIMB 229 (2005) 281. [4] Merchel S., Bremser W. NIMB 223–224 (2004) 393. [5] Merchel S. et al. GCA 73 (2009) A871. [6] Merchel S. et al. NIMB 266 (2008) 4921. [7] Merchel S., Herpers U. RCA 84 (1999) 215.

Keywords: AMS; cosmogenic radionuclides; TCN

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