We are continuing with the exposure experiment for ¹⁴C production rates at the South Pole.

The scope of this project is to directly detect and quantify the production rate of atmospheric ¹⁴C. Carbon-14 is not only used for dating organic materials, it can also be used as a tracer of OH chemistry (in the form of ¹⁴CO). In the latter case, if the inventory of ¹⁴CO is measured and the production rate known, then the sink rate, which is oxidation by OH, can be calculated. Although cascade calculations have constrained the ¹⁴CO production rate to 120%, it is of interest to measure this rate directly.

Carbon-14 is produced from the reaction:

which is immediately followed by:

The methodology takes advantage of gas-handling techniques previously developed by these authors. A known amount of CO carrier gas was mixed in with zero air and compressed into a suite of cylinders, some of which were placed 1 m above the surface at SPO. There they sit, exposed to incoming cosmic rays. After a certain length of time the cylinders are brought back to the isotope laboratory where the CO is extracted and measured for ¹⁴C content at the Lawrence Livermore Center for Accelerator Mass Spectrometry.

Monte Carlo simulations are currently being performed at LLNL to estimate any effects from the mass of the cylinders used as well as the ground effect. These simulations indicate that the effect from the cylinders is small. The experimental results are still being analyzed, and preliminary analyses show that the amounts of ¹⁴C produced at the South Pole would be easily detectable for an exposure time of about 6 months. To our knowledge, this is the second time direct ¹⁴C detection has been achieved and the first time a ground effect has been accounted for by direct measurement. This is an ongoing cooperative project.

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