Precision Penning Trap

Penning trap mass spectrometer

A Penning ion trap mass spectrometer measures atomic masses by measuring ratios of cyclotron frequencies of ions trapped in a strong magnetic field. The Florida State Precision Penning Trap, which uses single ions and cryogenic electronics, has produced many of the world's most precise atomic masses. Brought into operation at FSU by Edmund Myers in 2003, it was originally developed by the ICR group of David Pritchard at MIT. At FSU it has been further developed and used to measure the atomic masses of over 40 isotopes with fractional uncertainties near of below 10^-10.

Precise atomic masses have applications to several areas of physical science. These applications include the quest to determine the absolute mass of neutrinos, the determination of more precise values of fundamental constants - such as the fine structure constant and the proton/electron mass ratio, the testing of QED atomic theory, and the testing of molecular structure theory. More generally, precise masses of stable isotopes provide the "backbone" of the global evaluation of all atomic mass data. They are often used to calibrate less precise mass spectrometers such as those used to measure short-lived isotopes produced at accelerators.

Measurement of g-factors and hyperfine structure

With the addition of a microwave input and using the “continuous Stern-Gerlach effect” to detect an electron spin-flip, a Penning trap can also precisely measure g-factors (i.e., magnetic moments) in atomic and molecular ions. Using the fact that the frequency of electron spin-flips can depend on the nuclear spin sub-states (atomic ions), and on the nuclear spin and rotational sub-states (molecular ions), direct transitions between different hyperfine-Zeeman and rotational sates can also be detected. When applied to one-electron atomic ions, i.e. hydrogen-like ions such as ⁴He⁺, ¹²C⁵⁺, where the g-factor can be accurately calculated, such measurements can be used to determine the atomic mass of the electron. When applied to diatomic hydrogen ions, e.g. H₂⁺, HD⁺, they can be used to test modern, high-precision ab initio molecular theory. A comparison between experiment and theory for diatomic hydrogen ions can also be used to set limits on a hypothetical Angstrom-range interaction between nucleons, that is postulated by some extensions of the Standard Model. Further, the same techniques could be applied to the prototype antimatter molecule, anti-H₂⁺ (consisting of two antiprotons bound by a positron), enabling a test of matter-antimatter symmetry with atomic clock precision.

Acknowledgements: This work was supported in part by the National Science Foundation and by the the National Institute of Standards and Technology (Precision Measurement Grants Program).