High-precision mass measurement of doubly magic 208 Pb.

The absolute atomic mass of 208 Pb has been determined with a fractional uncertainty of 7 × 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 Pb and E 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 × 10 - 12 . E Pb and E Xe have been computed with an uncertainty of 9.1 eV and 2.1 eV, respectively, yielding 207.976 650 571 ( 14 )  u ( u = 9.314 941 024 2 ( 28 ) × 10 8  eV/c 2 ) for the 208 Pb neutral atomic mass. This result agrees within 1.2 σ 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.