Synopsis: Clocking the last century

Atom trap analysis has reached new sensitivity limits in measuring the abundance of argon-39, a desirable isotope for dating environmental samples on the time scale of a few hundred years.

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 10500 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


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