A small low energy cyclotron for radioisotope measurements

Direct detection of 14C by accelerator mass spectrometry has proved to be a much more sensitive method for radiocarbon dating than the decay counting method invented earlier by Libby. A small cyclotron (the ''cyclotrino'') was proposed for direct detection of radiocarbon in 1980. This combined the suppression of background through the use of negative ions, which had been used effectively in tandem accelerators, with the high intrinsic mass resolution of a cyclotron. Development of a small electrostatically-focused cyclotron for use as a mass spectrometer was previously reported but the sensitivity needed for detection of 14C at natural abundance was not achieved. The major contributions of this work are the integration of a high current external ion source with a small flat-field, electrostatically-focused cyclotron to comprise a system capable of measuring 14C at natural levels, and the analysis of ion motion in such a cyclotron, including a detailed analysis of phase bunching and its effect on mass resolution. A high current cesium sputter negative ion source generates a beam of carbon ions which is pre-separated with a Wien filter and is transported to the cyclotron via a series of electrostatic lenses. Beam is injected radially into the cyclotron using electrostatic deflectors and an electrostatic mirror. Axial focusing is entirely electrostatic. A microchannel plate detector is used with a phase-grated output. In its present form the system is capable of improving the sensitivity of detecting 14C in some biomedical experiments by a factor of 104. Modifications are discussed which could bring about an additional factor of 100 in sensitivity, which is important for archaeological and geological applications. Possibilities for measurements of other isotopes, such as 3H, and 10Be, and 26Al, are discussed. 70 refs