High precision Penning trap mass spectroscopy and a new measurement of the proton’s “atomic mass”

Abstract

The Penning trap mass spectrometer (PTMS) at the University of Washington has been rebuilt into a new state-of-the-art magnet/cryostat system with external Helmholtz compensation coils (controlled by a nearby flux-gate sensor). This system gives a total magnetic shielding factor of ${\text{∼10}}^4$ (which includes the effects of a passive internal flux-stabilizing coil supplied by the manufacturer). When the new magnet/cryostat is fitted with a system to control its boil-off pressure, the typical temporal field stability is ∼−0.017(2) ppb/h. The ultimate resolution of this improved spectrometer is expected to exceed 0.020 ppb with 100 hours of data using a single ${\mathrm{C}}^{\mathrm{4+}}$ ion. The comparison of a ${\mathrm{C}}^{\mathrm{4+}}$ ion with a ${\mathrm{C}}^{\mathrm{5+}}$ ion suggests that the spectrometer’s accuracy may indeed match its resolution. To demonstrate the spectrometer’s improved performance over its previous version, the cyclotron frequency of a single proton is compared to the corresponding frequency of a single ${\mathrm{C}}^{\mathrm{4+}}$ ion, yielding a determination of the proton’s atomic mass given by $M_{\mathrm{P}}\text{=1,007,276,466.89(14)\hspace{0.05em}}\mathrm{nu}.$ The primary systematic error (overcome in this case) is due to a position sensitivity within the non-uniform magnetic field, enhanced by the electrostatic trapping potentials of these ions which differ by a factor of three. A residual limitation to the overall field stability appears to be due to a long-term temperature-dependent temporal wander in the magnetic field.