Atom trap, krypton-81, and the groundwater underneath the Sahara Desert

Since radiocarbon dating was first demonstrated in 1949, the field of trace analyses of long-lived cosmogenic isotopes has seen steady growth in both analytical methods and applicable isotopes. The impact of such analyses has reached a wide range of scientific and technological areas. In this review article, we discuss a new method, named atom trap trace analysis (ATTA), which was used to analyze 81Kr (t1/2=2.3×105 years, isotopic abundance～10-12) and 85Kr (t1/2=10.8 years, isotopic abundance 2×10-11) in environmental samples. 81Kr is produced by cosmic rays in the upper atmosphere. It is the ideal tracer for dating ice and groundwater in the age range from 50,000 years to 1 million years. On the other hand, analyses of 85Kr, a fission product of uranium-235 and plutonium-239, can serve as a means to help verify compliance with the Nuclear Non-Proliferation Treaty. In ATTA, individual atoms of the desired isotope are selectively captured into an atom trap that consists of laser beams and magnetic fields and detected by observing the fluorescence of trapped atoms. As the first real-world application of ATTA, the age of the groundwater (200,000 years to 1 million years) in the Nubian Aquifer located underneath the Sahara Desert was determined. With the success of this demonstration, wide spread use of 81Kr-dating in Earth sciences seems feasible.