Steven L. Zafonte

Ultrastable superconducting magnet system for a penning trap mass spectrometer

A custom-designed magnet/cryostat system is described which has demonstrated remarkably improved field stability over previous designs. To shield from external magnetic noise, a custom-fabricated flux-gate device remotely senses the changes in magnetic field and cancels them out at the site of the magnet/cryostat via a 1.7-m-diam Helmholtz coil. To provide further shielding, the basic superconducting solenoid includes a passive flux-stabilizing coil. To stabilize internal field shifts, the temperature of the materials in the immediate vicinity of the solenoid (which have a temperature-dependent susceptibility) is stabilized via the new cryostat geometry and by controlling the pressure of the evaporating liquid helium to a few parts per million. As a result, the total system now has a composite shielding factor of approximately 104 and an overall temporal stability on the order of 17(2) parts in 1012 per hour. This instrument, the heart of our new Penning trap mass spectrometer, has recently been used to d...

Ultra-Precise Mass Measurements Using the UW-PTMS

Based on the use of a single ion, isolated at the center of a cryogenically cooled Penning trap, an environment is produced which makes this mass spectrometer remarkably free of systematic errors. The most notable developments in our quest for an ultra-high accuracy instrument were (a) the compensation of the trapping potential, (b) the discovery that motional sidebands could manipulate radial energies, (c) the use of multiply-charged ions that could improve signal-to-noise, and (d) the use of an ultra-stable superconducting magnet/cryostat system with drift <0.010 ppb/h. The dominant systematic errors are associated with radial electric fields caused by image charges in the trap electrodes and with the rf-electrical drive field used to determine the harmonic axial resonance. To illustrate the potential of this improved spectrometer, the four-fold improved measurement of the protons mass and the eight-fold improved measurement of oxygens atomic mass will be described.

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

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 ∼104 (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 C4+ ion. The comparison of a C4+ ion with a C5+ 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 C4+ ion, yielding a determination of the proton’s ato...

Stable voltage source for Penning trap experiments

A voltage reference has been developed to bias ring electrodes of two Penning traps between -90 and 0 V. For output voltages near -90 V, the Allan deviation of the systems voltage instability is less than 1 part in 10(8) over all time scales shorter than 10(4) s. For averaging times longer than several seconds, the systems stability is determined almost completely by the noise, drift, and aging of the zener diodes in the array of voltage reference integrated circuits. For shorter averaging times, active filters built into the new system significantly reduce the intrinsic noise of the zener diodes. The system makes it possible to continuously adjust the ring voltages for frequency locking the axial motion in the two Penning traps. By keeping electrical noise highly correlated between the two traps, measurement uncertainty should be reduced for precision experiments such as Penning trap mass spectrometry.

Ultra-precise single-ion atomic mass measurements on deuterium and helium-3

The former University of Washington Penning Trap Mass Spectrometer (UW-PTMS), now located at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, was used in the decade before the move to determine new values for the deuteron atomic mass, M (2H+) = 2.013 553 212 745(40) u, and the deuterium atomic mass, M (2H) = 2.014 101 778 052(40) u, both of which are now more than an order-of-magnitude more accurate than the previous best 1994-MIT measurements of these quantities. The new value for the deuterons mass can then be used with the accepted 2010-CODATA proton mass and the most recent 1999-measurement of the 2.2 MeV gamma-ray binding energy of the deuteron to refine the neutrons mass to mn = 1.008 664 916 018(435) u which has about half the uncertainty relative to the value computed using that previous 1994-MIT deuterium measurement. As a result, further improvements of mn must now come from a more accurate determination of the wavelength of this gamma ray.In this same period of time, this spectrometer has also been used to determine new values for the helion atomic mass, M (3He2+) = 3.014 932 246 668(43) u, and the neutral helium-3 atomic mass, M (3He) = 3.016 029 321 675(43) u, which are both about 60 times more accurate than the 2006-SMILETRAP measurements, but disagree with the 4.4-times less-accurate 2015-Florida-State measurements by 0.76 nu. It is expected that these helium-3 results will be used in the future 3H/3He mass ratio (to be determined by the Heidelberg, Germany version of the old UW-PTMS) in order to generate a more accurate value for the tritium atomic mass.