Abstract
Atomic mass measurements are essential for obtaining several of the fundamental constants. The most precise atomic mass measurements, at the 10−10 level of precision or better, employ measurements of cyclotron frequencies of single ions in Penning traps. We discuss the relation of atomic masses to fundamental constants in the context of the revised SI. We then review experimental methods, and the current status of measurements of the masses of the electron, proton, neutron, deuteron, tritium, helium-3, helium-4, oxygen-16, silicon-28, rubidium-87, and cesium-133. We conclude with directions for future work.
Highlights
The fundamental constants enable the quantitative unification of physical science
While the atomic masses of the electron, the isotopes of hydrogen and helium, and the neutron are themselves regarded as fundamental constants, others are necessary for various routes to obtaining other constants
Acoustic gas thermometry (AGT) [2], which formerly measured the Boltzmann constant kB and which realizes the kelvin, generally uses either 4 He or 40 Ar, which may have fractions of 36,38 Ar, requiring atomic masses for these too. Since all these masses have uncertainties in the Atomic MassEvaluations (AME) 2016 less than 10−9, except 38 Ar which has 5.5 × 10−9, and the precision of the XRCD technique is at the 10−8 level, and of AGT is at the 10−6 level, the uncertainties in these atomic masses are not an issue
Summary
The fundamental constants enable the quantitative unification of physical science. At the highest level of precision, values for dimensionless constants such as the fine-structure constant, obtained from different types of measurement, can be used to test the validity of fundamental theory and search for physics beyond the so-called Standard Model [1]. We discuss precision atomic mass measurements that are relevant to the determination of fundamental constants as in the Committee on Data for Science and Technology (CODATA) evaluations [2,3]. Kg, we review current techniques for precision atomic mass measurements These generally involve measuring cyclotron frequency ratios of ions in Penning ion traps. We discuss mass measurements of the “light ions”, i.e., the isotopes of hydrogen and helium; and of the alkali metal atoms 87 Rb and 133 Cs, which are important for the atom-recoil method for the fine-structure constant. The most precise measurements of the masses of atoms are obtained using Penning ion traps, which yield mass ratios of ions from (inverse) cyclotron frequency ratios (CFRs). As a point of notation, in this article we use upper case, e.g., M[e] or M[3 He] to denote (relative) masses expressed in u, and lower case, e.g., me , when this distinction is not needed
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