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An AMS-world without any chemistry? – Untypical measurements of ²⁶Al, ⁴¹Ca and ⁵⁵Fe

Merchel, S.; Golser, R.; Lachner, J.; Marchhart, O.; Martschini, M.; Rugel, G.; Wallner, A.; Walther, D.; Wieser, A.


AMS measurements typically are lasting minutes to hours, but are usually preceded by time-consuming (typically days to weeks of) chemical preparation. Both, physicists and chemists are dreaming of an AMS-world without any chemistry.

Some of our earlier studies have already proven AMS being reasonable, fast and easily accessible for ⁷Be [1] and ⁴¹Ca [2] analysis if largely reducing radiochemical separation. However, to our knowledge there are only a few cases completely omitting wet chemical separation, e.g., ¹⁰Be/⁹Be in a Be mineral (phenakite) [3] and ¹⁴C/¹²C by laser-ablation AMS of stalagmites and corals [4]. Here, we focus on two new examples for “Instrumental” AMS (IAMS) at the DREsden AMS (DREAMS) facility and the Vienna Environmental Research Accelerator (VERA):

First, a pilot study to quantify ⁵⁵Fe (t₁/₂=2.76 a) in steel from a reactor vessel of a nuclear power plant by IAMS was validated (after radiochemical separation) by liquid scintillation counting (LSC) and AMS [5]. DREAMS reaches an uncertainty <10% at the 1 kBq g(Fe)⁻¹ level within 10 min measuring unprocessed steel chips. The background (<3 Bq g(Fe)⁻¹) is limited by the short measurement time. IAMS for analysing ⁵⁵Fe from neutron-capture production is reasonable and fast compared to other analytical methods.

Secondly, the ILIAMS set-up at VERA (Martschini et al., this meeting) allows to determine ratios of ²⁶Al (t₁/₂=0.7 Ma) to ²⁷Al and ⁴¹Ca (t₁/₂=0.104 Ma) to ⁴⁰Ca in stony meteorites by IAMS. The nearly complete suppression of isobars, i.e., ²⁶MgO⁻, when extracting AlO⁻, and ⁴¹KF₃⁻ when extracting CaF₃⁻, make pressure digestion (HF/HNO₃), ion exchange and precipitations unnecessary. Most stony meteorites contain ~1% Al, mainly in the form of Na-rich-Ca-poor plagioclase ((Na,Ca)(Si,Al)₄O₈)). Additional sources for >1% Ca are pyroxene (CaMgSi₂O₆) and phosphates (mainly apatite: Ca₅(PO₄)₃Cl)) [6]. IAMS has been performed using 1-2 mg representative powder of the previously-investigated chondrite Dhurmsala [7], either pure, mixed with Fe or PbF₂ powder.

For IAMS of ²⁶Al, AlO⁻ currents from Dhurmsala were - independent of mixing with Fe or pure - about 2% of Al₂O₃(Fe) ones. At ²⁶Al/²⁷Al of ~1.3x10⁻¹⁰ statistical uncertainties of 3% are reached within 15 min sputtering while cathodes last several hours. IAMS data at VERA - in the presence of about 15% Mg - are comparable to earlier (chemical processing) AMS results at DREAMS and ETH Zurich.

For IAMS of ⁴¹Ca, very stable CaF₃⁻ currents from Dhurmsala are ~5% of chemically-processed CaF₂ ones (each mixed with PbF₂). At ~1x10⁻¹¹ ⁴¹Ca/Ca, count rates of 1 min⁻¹ sputtering time are reached. IAMS data measured in the presence of about 1‰ K at VERA are comparable to earlier (chemical processing) AMS results at ANU, DREAMS and ETH Zurich.

The major uncertainty for both nuclides, originating from the current differences of standards and samples, will be addressed soon.

References: [1] Tiessen et al., JRNCh 319 (2017) 965. [2] Hampe et al., JRNCh 296 (2013) 617. [3] Merchel et al., JRNCh 298 (2013) 1871. [4] Welte et al., Anal.Chem. 88 (2016) 8570. [5] Merchel et al., JRNCh, submitted. [6] A. Bischoff, pers.comm. (2021.) [7] Merchel, PhD thesis, (1998).

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