With a half-life of years, carbon- is well suited for radioisotope dating of fossils and other archeological finds. On the other end of the time spectrum, tritium (half-life of years) and krypton- (half-life of 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 – 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-, which is produced in the atmosphere by cosmic rays and has a half-life of years, would seem an ideal isotope to fill this niche. Unfortunately, the equilibrium isotopic abundance of argon- is only , 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 for argon- 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- 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- atoms are detected.
Alternative methods to analyze the abundance of argon- 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