Radioactive Noble Gas Analysis: the ATTA facility

The radioactive noble gas isotopes of argon (³⁹Ar) and krypton (⁸⁵Kr  and ⁸¹Kr) are amongst the most difficult to measure tracers because of their very low abundance: the atmospheric isotopic abundance is 2×10^-11 for ⁸⁵Kr, 6×10^-13 for ⁸¹Kr and 8×10^-16 for ³⁹Ar. ³⁹Ar and ⁸¹Kr are produced cosmogenically while ⁸⁵Kr is produced during nuclear reactions and released into the atmosphere from fuel reprocessing. They are used to determine the origin of recharge water tracing back decades (⁸⁵Kr), centuries (³⁹Ar) or up to one million years (⁸¹Kr). The challenge with traditional measurement technology for ⁸⁵Kr and ³⁹Ar is that large amounts of water (several tons) is needed to be sampled to achieve sufficient accuracy making this technique inefficient and time consuming. The traditional counting technologies are also unable to measure ⁸¹Kr. The Atom Trap Trace Analysis (ATTA) technology combined with an efficient automated sample preparation system at the Noble Gas Isotope Hydrogeology Facility, means we now only need to collect approximately 20 L of groundwater to be able to extract sufficient gas for subsequent purification into argon and krypton gas samples.

The ATTA facility at The University of Adelaide’s Institute for Photonics and Advanced Sensing uses the latest laser technologies to drive the successful application of radioactive noble gas tracers for natural groundwater systems. The ATTA technology combines laser-physics based atom excitation, trapping and detection to selectively separate and individually count the targeted atoms of ⁸⁵Kr, ³⁹Ar and ⁸¹Kr. These technologies, derived from recent years of high profile applied physics, quantum information and computing, can take direct measurements of the Ar and Kr fractions of environmental samples purified at CSIRO. Since the technique captures single atoms in a magnetic field and crossed laser beams, it is labelled Atom Trap Trace Analysis (ATTA). This unique facility – only the second of its kind to measure both radioargon and radiokrypton – is a leap forward in groundwater research.

To derive the input function of atmospheric ⁸⁵Kr on the southern hemisphere for dating modern water, ⁸⁵Kr in air samples is measured on a weekly basis using air samples collected at the Waite Campus, Adelaide.

To improve access to the ATTA facility for analysis of radioactive noble gas isotopes ⁸⁵Kr, ⁸¹Kr and ³⁹Ar, a new groundwater sampling methodology has been developed. This involves sampling of standard 20 litre propane gas bottles followed by dedicated sample preparation at the Noble Gas Isotope Hydrogeology Facility by automated extraction of the Kr and Ar fractions. Samples can then be measured at an ATTA facility.