Radioargon dating with atom trap trace analysis

The noble gas radioisotope 39Ar (half-life = 269 yrs.) is the only reliable dating isotope for water and ice in the range of 50-1,400 years, a time range particularly important for tracing ocean circulation, groundwater and alpine glacier ice [3]. However, its use especially in oceanography has been hampered by the large sample size required by Low-Level Counting, so far the only available detection technique for 39Ar. Atom Trap Trace Analysis (ATTA) is a laser-based atom counting method with ultra-high selectivity and sensitivity that has been developed for analyzing the environmental isotope tracers 85Kr and 81Kr [1]. Its realization for 39Ar presents a major challenge due to its extremely low abundance of only 8×10^(-16) [2], which is more than 800 times lower than for 81Kr. The applicability of ATTA to 39Ar has been demonstrated in a proof-of-principle experiment [4]. Practical dating of groundwater and ocean samples could later be demonstrated with 39Ar count rates reaching up to 7 atoms/h [5, 6]. Here, we present the latest status of the ATTA system for 39Ar at the USTC. We currently achieve an atmospheric 39Ar count rate of 10 atoms/h. In order to obtain a higher 39Ar count rate, together with the Institute of modern physics in Lanzhou we are working on 39Ar pre-enrichment. In preliminary tests, we could demonstrate an enrichment by a factor 10. We plan to test the water samples in a few months