A two-ion balance for high precision mass spectrometry

Abstract

This thesis describes the demonstration of a new technique that allows masses to be compared with fractional uncertainty at or below 1 x 10-11, an order of magnitude improvement over our previous results. By confining two different ions in a Penning trap we can now simultaneously measure the ratio of their two cyclotron frequencies, making our mass comparisons insensitive to many sources of fluctuations (e.g. of the magnetic field). To minimize the systematic error associated with the Coulomb interaction between the two ions, we keep them about 1 mm apart from each other, on a common magnetron orbit. We have developed novel techniques to measure and control all three normal modes of motion of each ion, including the two strongly coupled magnetron modes. With the help of a new computer control system we have characterized the electric field anharmonicities and magnetic field inhomogeneities to an unprecedented level of precision. This allows us to optimize the trap so that our measurement of the cyclotron frequency ratio is to first order insensitive to the field imperfections. Using the ions 13C2H2+ and 14N2+, we performed many tests of our understanding of the ions dynamics and of the various sources of errors in this technique. From these we conclude that there should be no systematic error in our measurements at the level of 5 x 10-12. Thus we feel confident reporting a value for the mass ratio of these ions with an uncertainty of 10-11.

(cont.) In this thesis, we also report measurements of the two mass ratios m[33S+]/m[32SH+] and m[29Si+]/m[28SiH+] with a relative uncertainty of less than 10-1l, which makes them the best known mass ratios to date. These can be combined with precise measurements of high-energy gamma-rays to provide a direct test of the relation E = mc2. This is a test of special relativity which does not rely on the assumption of a preferred reference frame. The uncertainty on the atomic mass of 29Si is also reduced by about an order of magnitude.