Laboratory for Radiokrypton Dating

Abstract:

Due to its simple production and transport in the terrestrial environment, ⁸¹Kr (half-life = 230,000 yr) is the ideal tracer for old water and ice with mean residence times in the range of 10⁵-10⁶ years, a range beyond the reach of ¹⁴C. ⁸¹Kr-dating is now available to the earth science community at large thanks to the development of an efficient and selective atom counter based on the Atom Trap Trace Analysis (ATTA) method. ATTA is a laser-based atom counting method where individual neutral atoms of the desired isotope are selectively captured by laser beams, and their fluorescence detected via a CCD camera. ATTA is unique among trace analysis techniques in that it is free of interferences from any other isotopes, isobars, atomic or molecular species.

The ATTA instrument at Argonne’s Laboratory for Radiokrypton Dating is capable of measuring both ⁸¹Kr/Kr and ⁸⁵Kr/Kr ratios of environmental samples in the range of 10^-14-10^-10. For ⁸¹Kr-dating in the age range of 150 kyr – 1500 kyr, the required sample size is 5 micro-L STP of krypton gas, which can be extracted from approximately 100 kg of water or 40 kg of ice. For ⁸⁵Kr/Kr analysis, the sample size can be smaller by an order of magnitude. We are continually developing the method towards higher counting efficiency, smaller sample sizes requirements, and higher sample throughput rates. In the past four years, we have performed radiokrypton analysis of over 150 groundwater and ice samples extracted by collaborators from all seven continents. Sample collection and purification was performed by groups including the University of Chicago, the University of Illinois at Chicago, the University of Bern, and the International Atomic Energy Agency.

This work is supported by the U.S. DOE, Office of Nuclear Physics, under contract DE-AC02-06CH11357.