One of the major goals of this research program is the long- term isolation of a single ionized atom nearly at rest in free space. Using only rf techniques for observing the normal mode frequencies of the ion, bound to a small electromagnetic cage called a Penning trap, it will become possible to measure light- ion masses relative to an appropriate calibration mass with a precision approaching one part in ten billion. An applied magnetic field forces the ion to revolve in circular orbits about the field direction at a rate which can be measured and compared with the corresponding calibration frequency (which may be obtained from either a single electron, proton, or carbon 12 ion depending on the particular experiment). The ion of interest and the calibration-ion are alternately stored in the same apparatus; the ratio of their respective cyclotron frequencies, corrected for the trapping fields, yields the inverse mass ratio. A variable magnetic bottle has been incorporated into this trap which has the effect of externally adjusting the quadratic magnetic field dependence at trap center. Using this new variable bottle, the proton to electron mass ratio has been measured as a function of the magnetic field dependence to yield a uniform-field value accurate to 20 ppb. This ratio will be used primarily in a consistency check for the next least squares adjustment of the fundamental constants. Present efforts are concentrating on measuring the tritium helium-three mass difference which will have a direct bearing on the measurement of the antineutrino rest mass. On one occasion, one helium-three and two tritium ions were isolated at the same time yielding the expected result that the effective cyclotron resonance was shifted according to the ions' resultant center of mass. In yet another experiment, the deuteron-proton mass ratio will be measured in order to indirectly yield an independent determination of the neutron-proton mass ratio.
