A low energy cyclotron for radiocarbon dating

The measurement of abundances of naturally occurring radioisotopes whose half lives are between a few years and a few hundred million years, provides information about the temporal behavior of human activity and geologic and climatic processes, the history of meteoritic bodies, and the production mechanisms of such radioisotopes. An extremely sensitive technique for measuring these radioisotopes at tandem Van de Graaff and cyclotron facilities has been very successful, though not without high cost and limited availability. We have built and tested a 35 keV cyclotron for radiocarbon dating similar in size to a conventional mass spectrometer. We found no significant background present when the cyclotron was tuned to accelerate 14C negative ions, and adequate transmission efficiency to perform radiocarbon dating on milligram samples of carbon. These tests clearly show that a low energy cyclotron can perform the extremely high sensitivity 14C measurements that are now done at accelerator facilities. The internal ion source used did not produre sufficient current to detect 14C directly at the present atmospheric 14C/12C concentration of 1.3×10−12. A conventional carbon negative ion source located outside the cyclotron magnet could produce sufficient beam and provide for quick sample changing to make feasible radiocarbon dating milligram samples with a modest laboratory instrument.