Laboratory for Radio-Krypton Dating

We have developed the Atom Trap Trace Analysis (ATTA) method to analyze ⁸¹Kr and ⁸⁵Kr at and below the part-per-trillion (PPT) level [Chen et al., 1999; Du et al., 2003], and ³⁹Ar at and below the part-per-quadrillion (PPQ) level [Jiang et al., 2011].

Sample size required for ⁸¹Kr dating: 10 micro-liter STP of krypton gas
Contact: Zheng-Tian Lu (lu@anl.gov, 630-252-0583)

⁸⁵Kr, ³⁹Ar, ⁸¹Kr – Long-lived noble-gas isotopes in the environment

Ultrasensitive trace analysis of radioactive isotopes has enabled a wide range of applications in both fundamental and applied sciences [Collon et al., 2004]. The three long-lived noble-gas isotopes, ⁸⁵Kr, ³⁹Ar and ⁸¹Kr, are particularly significant for applications in the earth sciences. Being immune to chemical reactions, these three isotopes are predominantly stored in the atmosphere, they follow relatively simple mixing and transport processes in the environment, and they can be easily extracted from a large quantity (tons) of water or ice samples. Indeed they possess ideal geophysical and geochemical properties for radioisotope dating. Dating ranges of radioisotope tracers follow closely their radioactive half-lives. The half-lives of the three noble gas isotopes have different orders of magnitude, allowing them to cover a wide range of ages.

Fig. 1: Dating ranges of ⁸⁵Kr, ³⁹Ar, ⁸¹Kr and other established radioisotope tracers.

Atom Trap Trace Analysis (ATTA)

ATTA is a laser-based atom counting method [Chen et al., 1999]. Its apparatus consists of lasers and vacuum systems of table-top size. At its center is a magneto-optical trap to capture atoms of the desired isotope using laser beams. A sensitive CCD camera detects the laser induced fluorescence emitted by the atoms held in vacuum. Trapping force and fluorescence detection require the atom to repeatedly scatter photons at a high rate (~10⁷ s^-1). This is the key to the superior selectivity of ATTA because it only occurs when the laser frequency precisely matches the resonance frequency of a particular atomic transition. Even the small changes in the atomic transition frequency between isotopes of the same element – the so called isotope shifts – are sufficient to perfectly distinguish between the isotopes. ATTA is unique among trace analysis techniques as it is free of interferences from other isotopes, isobars, or molecular species.

Fig. 2: Schematic layout of the ATTA-3 apparatus.

ATTA-3 Instrument

ATTA-3 is the most recently developed instrument. Compared to the previously reported ATTA-2 system, the counting rates of ATTA-3 are two orders of magnitude higher, and the sample size one order of magnitude lower. The required sample size for ⁸¹Kr dating, depending on its age and the desired precision (Fig. 3), is approximately 5 – 10 micro-liter STP of krypton gas, which can be extracted from approximately 100 – 200 kg of water or 40 – 80 kg of ice. For ⁸⁵Kr dating, the required sample size is smaller due to the isotope’s higher abundance. While a proof-of-principle measurement of ³⁹Ar/Ar has been demonstrated, routine ³⁹Ar dating is not yet available.

Fig. 3: Sample size requirement vs. sample age precision for ATTA-3

Applications

Nubian Aquifer by Sturchio et al. (2004) – For the first real-world application of ATTA, measurements of ⁸¹Kr/Kr in deep groundwater from the Nubian Aquifer in the Western Desert of Egypt were performed (Fig. 4). 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.

Fig. 4: Sampling locations and ⁸¹Kr ages of groundwater extracted from the Nubian Aquifer.

 

Wanted: VUV Light Source

Laser trapping of krypton atoms can only be realized with atoms in the metastable 5s[3/2]⁰₂ state. In the existing ATTA instrument, the excitation of krypton atoms into the metastable level is done in a gas discharge, a process that both limits counting efficiency and induces memory effect. Alternatively, atoms can be transferred into the metastable state via the following scheme involving three E1 transitions (Fig. 4): excitation with a photon at 124 nm and a photon at 819 nm, followed by a spontaneous decay at 760 nm. For this reason, we are seeking a 124 nm light source, either a lamp or a laser.

Fig. 4: Possible pathways to optically excite the krypton metastable state.

Acknowledgement – This work is supported by the Department of Energy, Office of Nuclear Physics, and by the National Science Foundation, Division of Earth Sciences.

References

• Chen, C. Y. et al. (1999),Ultrasensitive isotope trace analyses with a magneto-optical trap, Science 286, 1139-1141.
• Collon, P., W. Kutschera, and Z.-T. Lu (2004), Tracing noble gas radionuclides in the environment, Ann. Rev. Nucl. Part. Sci. 54, 39-67.
• Du, X. et al. (2003), A new method of measuring Kr-81 and Kr-85 abundances in environmental samples, Geophys. Res. Lett. 30, 2068.
• Sturchio, N. C. et al. (2004), One million year old groundwater in the Sahara revealed by krypton-81 and chlorine-36, Geophys. Res. Lett. 31, L05503.
• Jiang, W. et al. (2011), Ar-39 detection at the 10^-16 isotopic abundance level with Atom Trap Trace Analysis, PRL 106, 103001 (2011).
• W. Jiang et al. (2012), An Atom Counter for Measuring ⁸¹Kr and ⁸⁵Kr in Environmental Samples, Geochimica et Cosmochimica Acta, in Press