Detection of Argon 39 using positive ion AMS - an Overview

The first application of ³⁹Ar Accelerator Mass Spectrometry (AMS) at Argonne National Laboratory was to date ocean water samples relevant to oceanographic studies using the gas-filled magnet technique to separate the ³⁹K-³⁹Ar isobars. In particular the use of a quartz liner in the plasma chamber of the electron cyclotron resonance ion source enabled a ³⁹K reduction of a factor ∼130 compared to previous runs without liners, and allowed us to reach a detection sensitivity of ³⁹Ar/Ar = 4.2×10⁻¹⁷. In order to improve this sensitivity and allow the measurement of lower ratios, higher ion source currents and a lower overall ³⁹K background environments needed to be developed.

The talk will give an overview of our efforts to detect low-concentrations of ³⁹Ar as well as efforts to investigate new methods combining low level potassium cleaning techniques with the use of ultra-pure aluminum liners in the plasma chamber of the ion source, with the aim of gaining the 1 to 2 orders of magnitude in ³⁹Ar detection sensitivity required in the selection of ultra-pure materials for detectors used in weakly interacting massive particle dark matter searches.

This presentation was used for the Low-Radioactivity Underground Argon Workshop held at Pacific Northwest National Laboratory in Richland, Washington on March 19 - 20, 2018.