First High-Precision Direct Determination Of The Atomic Mass Of A Superheavy Nuclide

We present the first direct measurement of the atomic mass of a superheavy nuclide. Atoms of (257)Db (Z = 105) were produced online at the RIKEN Nishina Center for Accelerator-Based Science using the fusion-evaporation reaction Pb-208(V-51, 2n)(257)Db. The gas-filled recoil ion separator GARIS-II was used to suppress both the unreacted primary beam and some transfer products, prior to delivering the energetic beam of (257)Db ions to a helium gas-filled ion stopping cell wherein they were thermalized. Thermalized (257)Db(3+) ions were then transferred to a multireflection time-of-flight mass spectrograph for mass analysis. An alpha particle detector embedded in the ion time-of-flight detector allowed disambiguation of the rare (257)Db(3+) time-of-flight detection events from background by means of correlation with characteristic alpha decays. The extreme sensitivity of this technique allowed a precision atomic mass determination from 11 events. The mass excess was determined to be 100 063(231)(stat)(132)(sys) keV/c(2). Comparing to several mass models, we show the technique can be used to unambiguously determine the atomic number as Z = 105 and should allow similar evaluations for heavier species in future work.