Atom Trap Trace Analysis of 39Ar

Atom Trap Trace Analysis (ATTA) is an ultra-sensitive counting method for rare and long-lived isotopes. It is based on the high selectivity of resonant photon scattering during laser cooling and trapping in order to distinguish the rare isotope from the abundant ones. It has been developed during the past two decades for the rare krypton isotopes and is now routinely available for the earth science community. We have focused on the rare argon isotope ³⁹Ar. As an inert noble gas and with a half-life of 269 years it is the perfect tracer to fill the dating gap for ice and water samples between 50 and 1,000 years before present, for which time period no other tracers exist. ³⁹Ar data can therefore strongly improve the information about the age of a sample in groundwater, ice and ocean studies. The experimental challenge lies in the low atmospheric abundance of ³⁹Ar (³⁹Ar/Ar = 8.23·10⁻¹⁶), which is more than 600 times lower compared to ⁸¹Kr, and in the absence of an abundant reference isotope with hyperfine structure. Using atmospheric ³⁹Ar as a reference requires a stable and reproducible performance of the apparatus leading to a robust ³⁹Ar detection efficiency. An atmospheric count rate of 3.6 atoms/h allowed the first demonstration of its applicability for groundwater dating. In order to avoid any cross-sample contamination, several tons of water (corresponding to 500 – 800 ml STP of argon) were sampled and degassed for this first demonstration. Current developments reduced the needed sample size down to 4 - 10 ml STP of argon, which corresponds to 10 - 25 L of water or 4 - 10 kg of ice. Such sample sizes are a prerequisite for a convenient application of ³⁹Ar dating in most environmental systems. I will introduce the technique of ATTA and report on the first demonstration using groundwater samples and the status of dating with small samples. Finally, I will show current developments such as bichromatic cooling and optical pumping with the aim of enhancing the efficiency of the metastable noble gas source and the beam manipulation.