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 ²⁵⁷Db ($Z=105$) were produced online at the RIKEN Nishina Center for Accelerator-Based Science using the fusion-evaporation reaction ²⁰⁸Pb(⁵¹V, 2n)²⁵⁷Db. 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 ²⁵⁷Db ions to a helium gas-filled ion stopping cell wherein they were thermalized. Thermalized ²⁵⁷Db³⁺ 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 ²⁵⁷Db³⁺ time-of-flight detection events from background by means of correlation with characteristic α decays. The extreme sensitivity of this technique allowed a precision atomic mass determination from 11 events. The mass excess was determined to be 1000.16em0ex063(231)ₛₜₐₜ(132)sys0.16em0exkeV/c². 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.