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 ${}^{257}$Db ($Z$=105) were produced online at the RIKEN Nishina Center for Accelerator-Based Science using the fusion-evaporation reaction ${}^{208}$Pb(${}^{51}$V, 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 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 ${}^{257}$Db${}^{3+}$ time-of-flight detection events from background by means of correlation with characteristic $\hat{I}\pm$-decays. The extreme sensitivity of this technique allowed a precision atomic mass determination from 11 events. The mass excess was determined to be $100063(231{)}_{\text{stat}}(132{)}_{\text{sys}}$~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.

Submitted to PRC as Rapid Comm., 5 pages, 4 figures