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 1.2σ 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.