# Synopsis: Clocking the last century

#### 39Ar Detection at the 10-16 Isotopic Abundance Level with Atom Trap Trace Analysis

W. Jiang, W. 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

Published March 9, 2011

With a half-life of $5730$ years, carbon-$14$ is well suited for radioisotope dating of fossils and other archeological finds. On the other end of the time spectrum, tritium (half-life of $12.3$ years) and krypton-$85$ (half-life of $10.7$ years) are useful for dating ice and water samples over the course of several decades. There are, however, many geological changes that occur on a timescale of $10$$500$ years. Mixing processes in the ocean and groundwater, for example, have implications for modeling global and regional climate, but a good radioactive “clock” for monitoring these changes is not available.

Argon-$39$, which is produced in the atmosphere by cosmic rays and has a half-life of $269$ years, would seem an ideal isotope to fill this niche. Unfortunately, the equilibrium isotopic abundance of argon-$39$ is only $8×{10}^{-16}$, making it difficult to detect without expensive or time-consuming techniques. Writing in Physical Review Letters, a team of scientists working at Argonne National Laboratory, US, reports they have reached an isotopic sensitivity of ${10}^{-16}$ for argon-$39$ using a specialized magneto-optical atom trap that allows them to detect single atoms. In their setup, the team laser-cools and traps argon atoms with a laser tuned to the vicinity of an argon-$39$ atomic resonance. Since it takes many cycles of absorption to trap the atoms, there is a nearly complete rejection of the other isotopes from the trap and only the remaining argon-$39$ atoms are detected.

Alternative methods to analyze the abundance of argon-$39$ exist, such as accelerator mass spectrometry. But with further development, the Argonne group’s technique offers a promising way to perform trace analysis of this important isotope with a table-top apparatus. – Gene Sprouse