Atomic mass determination of uranium-238

The atomic mass of uranium-238 has been determined to be $238.050787618\left(15\right)$ u, improving the literature uncertainty by two orders of magnitude. It is obtained from a measurement of the mass ratio of ${}^{238}\mathrm{U}^{47+}$ and ${}^{132}\mathrm{Xe}^{26+}$ ions with an uncertainty of $3.5\times {10}^{-12}$. The measurement was carried out with the Penning-trap mass spectrometer Pentatrap and was accompanied by a calculation of the binding energies $E_{\text{U}}$ and $E_{\text{Xe}}$ of the 47 and 26 missing electrons of the two highly charged ions, respectively. These binding energies were determined using an ab initio multiconfiguration Dirac-Hartree-Fock method to be $E_{\text{U}}=39927\left(10\right)$ eV and $E_{\text{Xe}}=8971.2\left(21\right)$ eV. The new mass value will serve as a reference for high-precision mass measurements in the heavy mass region of the nuclear chart up to transuranium nuclides.

Figure 1

(a) Sectional view of the Penning-trap tower with three ions in configuration 1. Configuration 2 is indicated by arrows. (b) Axial spectrum of trap 2 showing an overlay of a resonator with dip signal of the Xe ion and of the U ion at their respectively different resonator frequencies varied by the varactor. (c) Exemplary measurement run showing the determined cyclotron frequencies of both ions ($y$ axis of the xenon ion is on the right in blue) and the ratios that can be formed by interpolation in time of the cyclotron frequencies (lower panel).