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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.

A small, low-energy cyclotron (“cyclotrino”) has been designed and built at Berkeley for direct detection dating of 14 C. The system combines the use of a negative ion source to reject 14 N with the high resolution of a cyclotron to reject other background ions. In order to allow the dating of old and small samples, the present system incorporates a high-current external ion source and injection beamline. The system is expected to be operational by mid-1988.

We have built and begun testing a small low energy negative ion cyclotron for direct detection of 14C. At present, the cyclotron is operated in a high resolution mode at the 31st harmonic, with 1–2 kV on the dees. The high harmonic and a minimum number of turns of approximately 100, should give a fwhm mass resolution of about 130000 — sufficient to suppress the background from molecular ions such as 13CH−. Background such as scattered ions of 12C− and 13C− should be totally suppressed by the cyclotron acceleration process. (At the 88″ cyclotron at LBL we found that ions only 1% off-resonance are suppressed by more than a factor of 1017.) A miniature Cs sputter source located at the center of the cyclotron is expected to provide more than 1 μA of negative carbon ions. Negative ions are used in order to eliminate the interference from 14N. Unlike high energy cyclotrons, focussing is obtained solely from the axial components of the accelerating electric field. The magnetic field is kept flat to within 1 part in 104 in order to maintain exact isochronism throughout the several thousand accelerating rf cycles. The low final energy of 40 keV eliminates any danger from radiation or need for shielding, and the final orbit radius of only 10.5 cm, reduce the size and cost of the machine to that of conventional mass spectrometers.