Precision Mass Spectrometry Group

We can now compare the masses of single ions with relative accuracy below 1 part in 1011. This is an order of magnitude more precise than our previous comparisons, and an advance in the state-of-the-art by nearly 4 orders of magnitude since we started this experiment in 1983. It firmly establishes our atomic mass measurements as the most accurate in the world. This dramatic improvement was achieved by implementing simultaneous cyclotron frequency comparisons of two ions in a Penning trap in order to eliminate the effect of magnetic field noise on the measured ratio of frequencies. This elegant and novel technique has allowed us to:

(*) demonstrate an unprecedented level of control of the motion of two different ions in a Penning trap;

(*) measure four mass ratios to about 1 part in 1011 with diverse applications

(*) discover a novel cyclotron frequency shift arising from polarization forces that allows us to monitor the quantum state of a single molecular ion nondestructively

(*) perform a test of E=mc2 at 3 parts in 107, an improvement by a factor of 40 over previous tests

(*) determine the dipole moment of a molecular ion more accurately than in previous work. In previous measurements made with single ions, we obtained a total of 14 neutral atomic masses with typical accuracies of 1 part in 1010, ranging from the masses of the proton and neutron to the mass of 133Cs, all with accuracies one to three orders of magnitude higher than the previously accepted values. The atomic mass measurements made important contributions in both fundamental physics and metrology, including:

(*) an 80-fold improvement of the gamma-ray wavelength standard by using E = DELTA-mc2 to determine the energies of (14)N neutron capture gamma-rays (widely used as £^-ray calibration lines);

(*) opened the way for an atomic standard of mass by replacing the “artifact” kilogram mass standard with a crystal of pure silicon and our accurate determination of the atomic weight of 28Si.

We achieved the previous accuracy of roughly 10(-10) by measuring the cyclotron frequency of a single molecular or atomic ion in a Penning trap. A Penning trap consists of a highly uniform magnetic field, which provides radial confinement, combined with a much weaker electric field that confines ions harmonically along the magnetic field lines. We measured a mass ratio by comparing the cyclotron frequencies of two ions alternately confined in the trap, with preferably one of them rich in the atomic mass standard, (12)C. We monitor an ion's axial oscillation by detecting the tiny currents (~10(-14) A)) induced in the trap electrodes. Measuring such a small current requires an extremely sensitive detector, and we are fortunate to have improved the ultrasensitive superconducting electronics we developed for this application by switching to an order of magnitude quieter DC SQUID. We have also developed techniques for quickly isolating single ions in the trap by selectively driving the axial motion of the unwanted ions so that they neutralize on the trap electrodes. The entire ion-making process is fully computer controlled, and we can cycle from an empty trap to having a cooled single ion in about 3 minutes under optimal conditions.

We have developed a π-pulse method to coherently swap the phase and action of the cyclotron and axial modes. Therefore, although we detect only the axial motion directly, we can determine the cyclotron frequency by measuring the phase accumulated in the cyclotron motion in a known time interval. We can measure the phase of the cyclotron motion to about 10 degrees, yielding a precision of 10-10 in the cyclotron frequency for a one minute measurement. By measuring the frequencies of the other two normal modes of ion motion in a Penning trap, we can correct for electrostatic shifts in the cyclotron frequency ratio to better than 10(-12).

The method for making simultaneous cyclotron frequency comparisons has eliminated the effects of magnetic field fluctuations, bringing us within reach of our long-term goal of attaining an accuracy of a few parts in 10(-12). Further studies of systematic errors might reveal that this technique already allows this level of precision. Improving accuracy to this level could also be achieved in the future by making a double trap wherein the two ions are contained in neighboring wells, measured simultaneously, and swapped between them to eliminate systematic errors. In either case, the current and improved levels of accuracy will allow further contributions to fundamental physics including:

(*) Measurement of the 3H - 3He mass difference, which is important in ongoing experiments to determine the electron neutrino rest mass.

(*) Determination of excitation and binding energies of atomic and molecular ions by weighing the associated small decrease in mass, £Gm = Ebind / c2 (we must reach our ultimate goal of a few parts °— 10-12 to make this a generally useful technique).

(*) Improvement of traditional applications of mass spectrometry resulting from our orders of magnitude improvement in both accuracy and sensitivity.