First high-precision direct determination of the atomic mass of a superheavy nuclide
Abstract
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 multi-reflection 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 100\,063(231)stat(132)sys~keV/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.