Atom Trap Trace Analysis LogogLaboratory 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; Jiang et al., 2012], and 39Ar at and below the part-per-quadrillion (PPQ) level [Jiang et al., 2011]. An overview of our effort is provided below; a list of earth science projects applying radiokrypton dating is at Radiokrypton Dating for Earth Sciences.

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 [Lu et al., 2013]. 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 (10-100 kg) 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.

Dating Ranges of Radioactive Isotopes

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.

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

ATTA-3 Instrument

ATTA-3 is the most recently developed instrument [Jiang et al., 2012]. 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 [Jiang et al., 2011], routine 39Ar dating is not yet available.

ATTA 3 sample size requirement vs sample age

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

Applications

With the advent of ATTA as a new tool, earth scientists gathered at Argonne in June, 2012, for the first International Workshop on Tracer Applications of Noble-Gas Radionuclides in the Geosciences (TANGR2012). The workshop whitepaper, published in the journal of Earth-Science Reviews, outlined applications of ATTA to compelling scientific problems in the earth and environmental sciences [Lu et al., 2013]. In collaboration with earth scientists worldwide, we have so far analyzed ~ 80 samples extracted from 6 different continents. The sampling sites are marked with red/yellow dots on the map shown in Fig. 4.  A list of projects is available at Radiokrypton Dating for Earth Sciences.

map of ATTA sample locations

Fig. 4: Sampling sites covered by ATTA Kr-81 analysis as of 2013.
(Map source: BGR & UNESCO)

 

Acknowledgement

Laboratory for Radiokrypton Dating is supported by the Department of Energy, Office of Nuclear Physics, and Argonne LDRD grants. Development of the ATTA-3 instrument was supported in part by the National Science Foundation, Division of Earth Sciences. Sampling and sample processing effort are supported by the National Science Foundation, International Atomic Energy Agency, and other agencies worldwide.

 

References