Zheng-tian Lu
Argonne National Laboratory
Atom Trap, Krypton-81, and Egypt

Since radio-carbon dating was first demonstrated in 1949, the field of trace analysis of long-lived radioactive isotopes has seen steady growth on both analytical methods and applicable isotopes. The impact of such analyses has reached a wide range of scientific and technological areas. A new method, named Atom Trap Trace Analysis (ATTA), was developed by our group and used to analyze ⁸¹Kr (t_1/2 = 2.3 10⁵ years, isotopic abundance ~ 1 10^-12) in environmental samples. In this method, individual ⁸¹Kr atoms are selectively captured and detected with a laser-based atom trap. ⁸¹Kr is produced in the upper atmosphere by cosmic-ray induced spallation and neutron activation of stable krypton isotopes. It is the ideal tracer for dating ice and groundwater in the age range of 10⁴�10⁶ years. As the first real-world application of ATTA, we have determined the mean residence time of the old groundwater in the Nubian Aquifer located underneath the Western Desert of Egypt.