Physics Division Research Highlights
Ar-39 detection at the part-per-quadrillion level with atom trap trace analysis: a powerful new tool for radioisotope dating
Atom Trap Trace Analysis (ATTA), a laser-based atom counting method, has been applied to analyze atmospheric 39Ar (half-life = 269 yr), a cosmogenic isotope with an isotopic abundance of only 0.8 parts-per-quadrillion (8x10-16) in the atmosphere. The ATTA apparatus consists of lasers and vacuum systems of table-top size. At its center is a magneto-optical trap (MOT) 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 as 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 caused by changes in nuclear mass and moments – are sufficient to perfectly distinguish between the isotopes. ATTA is unique among trace analysis techniques as it is virtually free of interferences from other isotopes, isobars, or molecular species.
Recent work within the Physics Division  has now demonstrated that the possibility of contamination counts due to other atomic or molecular species above the 1x10-16 level is excluded. In addition to the superior selectivity demonstrated in this work, both the counting rate and counting efficiency of the ATTA apparatus have been improved by two orders of magnitude over prior results. Along with the previously demonstrated detection of 81Kr (229,000 yr) and 85Kr (10.8 yr) at the 10-12 level, ATTA can now be used to analyze all three long-lived noble gas radioisotopes covering a wide range of ages and applications.
Figure 1: Geological age ranges covered by the tracer isotopes 85Kr, 39Ar, 81Kr and other established radioisotope tracers. ATTA can now be used to analyze all three long-lived noble gas radioisotopes covering a wide range of ages of importance for applications in the earth sciences and in dark matter searches, for example.
The three long-lived noble-gas isotopes, 85Kr, 39Ar and 81Kr, possess ideal geophysical and geochemical properties for radioisotope dating and are particularly significant for applications in the earth sciences. The age range covered by a radioisotope tracer follows closely its radioactive half-life. The half-lives of the three noble gas isotopes have different orders of magnitude, allowing them to cover a wide range of ages (see Fig. 1). In particular, 39Ar conveniently fills an apparent gap between 85Kr, 3H/3He on the shorter side and 14C on the longer side. This makes 39Ar a much desired isotope for dating environmental samples on the time scale of a few hundred years. For example, 39Ar dating in combination with 14C dating can be used to study mixing processes in ocean water or groundwater, with implications for modeling global and regional climate changes. Another application is in the dating of ice for climatology. On a different front, ultrasensitive detection of 39Ar is critical for developing the next-generation dark matter detectors based on liquid argon, where 39Ar is a major source of unwanted radioactive background. A paper reporting these results has been published in Physical Review Letters .
- W. Jiang, W. D. Williams, K. Bailey, A.M. Davis, S.-M. Hu, Z.-T. Lu, T.P. O’Connor, R. Purtschert, N.C. Sturchio, Y.R. Sun, and P. Mueller, Phys. Rev. Lett. 106, 103001 (2011)