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We present the first direct measurement of the atomic mass of a superheavynuclide. Atoms of $^{257}$Db ($Z$=105) were produced online at the RIKENNishina Center for Accelerator-Based Science using the fusion-evaporationreaction $^{208}$Pb($^{51}$V, 2n)$^{257}$Db. The gas-filled recoil ionseparator GARIS-II was used to suppress both the unreacted primary beam andsome transfer products, prior to delivering the energetic beam of $^{257}$Dbions to a helium gas-filled ion stopping cell wherein they were thermalized.Thermalized $^{257}$Db$^{3+}$ ions were then transferred to a multi-reflectiontime-of-flight mass spectrograph for mass analysis. An alpha particle detectorembedded in the ion time-of-flight detector allowed disambiguation of the rare$^{257}$Db$^{3+}$ time-of-flight detection events from background by means ofcorrelation with characteristic $\alpha$-decays. The extreme sensitivity ofthis technique allowed a precision atomic mass determination from 11 events.The mass excess was determined to be$100\,063(231)_\textrm{stat}(132)_\textrm{sys}$~keV/c$^2$. Comparing to severalmass models, we show the technique can be used to unambiguously determine theatomic number as $Z$=105 and should allow similar evaluations for heavierspecies in future work.

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