Proton/electron mass ratio and the electron's "atomic mass"

Summary form only given. The heart of the Penning trap mass spectrometer (PTMS) is a well-machined, quadrupole trap with hyperbolic surfaces of revolution formed about a common axis of symmetry. Equally as important from the point of view of precision, a strong magnetic field, aligned along this trap axis, must be highly stable and uniform. For our present solenoid, the long term drift is /spl sim/0.1-0.3 ppb/h and short-term fluctuations are <1 ppb. We are now using a quite uniform magnetic field (with B/sub 2/<2 G/cm/sup 2/). Also, a significant improvement in the X-band microwave system (multiplied up to 160 GHz) now allows us to use the relativistic mass shift (equivalent to /spl sim/-10 G/cm/sup 2/) in order to observe the electron's cyclotron frequency with a precision of /spl plusmn/5 ppb per measurement and with systematics that are expected to be at least an order of magnitude less than that obtained in our previous work. The comparison ion in this case is a fully stripped carbon atom such that the electron's atomic mass can be directly determined upon correcting for lost electrons and their binding energies. At the present time, our typical single measurement precision for the C/sup 6+/ cyclotron frequency is about /spl plusmn/1 ppb which yields a relative precision on the order of 0.2 ppb after a 24-h data-period. Altogether, the typical measurement precision of the electron/carbon comparison now exceeds 1 ppb (using about 100 h of data). To obtain m/sub p//m/sub e/, a comparison is made with the proton's atomic mass which has an accepted value with an accuracy of 0.5 ppb. Thus, an improvement in this important fundamental constant is expected to be an order of magnitude, limited only by our estimate of residual systematics.<>