High-precision mass measurement of doubly magic 208 Pb

Abstract

The absolute atomic mass of ${}^{208}$ Pb has been determined with a fractional uncertainty of $7\times {10}^{-11}$ by measuring the cyclotron-frequency ratio R of ${}^{208}$ Pb ${}^{41+}$ to ${}^{132}$ Xe ${}^{26+}$ with the high-precision Penning-trap mass spectrometer Pentatrap and computing the binding energies $E_{\text{Pb}}$ and $E_{\text{Xe}}$ of the missing 41 and 26 atomic electrons, respectively, with the ab initio fully relativistic multi-configuration Dirac-Hartree-Fock (MCDHF) method. R has been measured with a relative precision of $9\times {10}^{-12}$ . $E_{\text{Pb}}$ and $E_{\text{Xe}}$ have been computed with an uncertainty of 9.1 eV and 2.1 eV, respectively, yielding $207.976650571\left(14\right)$ u ( $\text{u}=9.3149410242\left(28\right)\times {10}^8$ eV/c ${}^2$ ) for the ${}^{208}$ Pb neutral atomic mass. This result agrees within $1.2\sigma$ with that from the Atomic-Mass Evaluation (AME) 2020, while improving the precision by almost two orders of magnitude. The new mass value directly improves the mass precision of 14 nuclides in the region of Z = 81-84 and is the most precise mass value with $A>200$ . Thus, the measurement establishes a new region of reference mass values which can be used e.g. for precision mass determination of transuranium nuclides, including the superheavies.