First high-precision direct determination of the atomic mass of a\n superheavy nuclide

Abstract

We present the first direct measurement of the atomic mass of a superheavy\nnuclide. Atoms of $^{257}$Db ($Z$=105) were produced online at the RIKEN\nNishina Center for Accelerator-Based Science using the fusion-evaporation\nreaction $^{208}$Pb($^{51}$V, 2n)$^{257}$Db. The gas-filled recoil ion\nseparator GARIS-II was used to suppress both the unreacted primary beam and\nsome transfer products, prior to delivering the energetic beam of $^{257}$Db\nions to a helium gas-filled ion stopping cell wherein they were thermalized.\nThermalized $^{257}$Db$^{3+}$ ions were then transferred to a multi-reflection\ntime-of-flight mass spectrograph for mass analysis. An alpha particle detector\nembedded in the ion time-of-flight detector allowed disambiguation of the rare\n$^{257}$Db$^{3+}$ time-of-flight detection events from background by means of\ncorrelation with characteristic $\\alpha$-decays. The extreme sensitivity of\nthis technique allowed a precision atomic mass determination from 11 events.\nThe mass excess was determined to be\n$100\\,063(231)_\\textrm{stat}(132)_\\textrm{sys}$~keV/c$^2$. Comparing to several\nmass models, we show the technique can be used to unambiguously determine the\natomic number as $Z$=105 and should allow similar evaluations for heavier\nspecies in future work.\n