Exposure Experiment, South Pole
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.