Atom Trap, Krypton-81, and Saharan Water

X. Du,¹ K. Bailey,¹ Z. El Alfy,² B. El-Kaliouby,³ B. E. Lehmann,⁴ R. Lorenzo,⁴ Z.-T. Lu,¹ P. Mueller,¹ T. P. O'Connor,¹ R. Purtschert,⁴ N. C. Sturchio,⁵ M. Sultan,⁶ L. Young^7
1Physics Division, Argonne National Laboratory; ²Egyptian Geological Survey and Mining Authority, Cairo, Egypt; ³Geology Department, Ain Shams University, Cairo, Egypt; ⁴Physics Institute, University of Bern, Switzerland; ⁵Department of Earth and Environmental Sciences, University of Illinois at Chicago; ⁶Geology Department, State University of New York, Buffalo; ⁷Chemistry Division, Argonne National Laboratory

Motivation

We are developing the Atom Trap Trace Analysis (ATTA) method for the trace analysis of two long-lived krypton isotopes, ⁸¹Kr (t_1/2 = 2.3x10⁵ years, isotopic abundance ~ 10^-13) and ⁸⁵Kr (t_1/2 = 10.8 years, isotopic abundance ~ 10^-12).

81Kr is a cosmogenic isotope produced in the upper atmosphere. Its analysis allows age determination for dating polar ice to study the climate history of the Earth, and dating old groundwater to study the source, sink, and flow pattern of aquifers. The applicable age range of 100 kyr � 1 Myr  is beyond the reach of 14C-dating.

85Kr is an anthropogenic isotope produced  through fission of uranium and plutonium. Its  analysis in the atmosphere can help in verifying compliance with the Nuclear Non-Proliferation Treaty by monitoring nuclear fuel re-processing activities as well as detecting leaks from fuel containers for nuclear safety issues.

Method

In ATTA, atoms of the desired isotope are selectively captured into a magneto-optical trap (MOT) and detected by observing the fluorescence of the trapped atoms. This detection scheme is so sensitive, that a single atom can be easily discerned when captured in the MOT. The required isotopic selectivity comes from the isotope shift of the trapping transition at 811 nm. Each isotope is trapped only when the laser is precisely tuned to its respective transition frequency. This allows for the background free detection of the extremely rare isotopes 81Kr and 85Kr in a sample of natural krypton gas. The overall counting efficiency determines the required sample size and is a key system parameter. At present, we have completed the development of a second-generation ATTA machine with a counting efficiency of 1x10-4.

Results

We have determined the ages of groundwater at six sites, ranging from 200,000 to 1,000,000 years, in the Nubian Aquifer underneath the Eastern Sahara Desert. These results revealed hydrologic behavior of this huge aquifer, with important implications for climate history and water resource management in the region. With this demonstration, widespread application of 81Kr in Earth sciences now appears feasible.

This work is supported by NSF, Division of Earth Sciences, under Award No. EAR-0651161, and by DOE, Office of Nuclear Physics, under Contract No. DE-AC02-06CH11357.

Selected publications

One million year old groundwater in the Sahara revealed by krypton-81 and chlorine-36 GRL Abstract / Preprint (physics/0402092)

A new method of measuring ⁸¹Kr and ⁸⁵Kr abundances in environmental samples GRL Abstract / Preprint (physics/0311118)

Ultrasensitive isotope trace analyses with a magneto-optical trap