Possible bound nuclei beyond the two-neutron drip line in the 50≤Z≤70 region

Abstract

We present the first direct measurement of the atomic mass of a superheavy nuclide. Atoms of Db257 (Z=105) were produced online at the RIKEN Nishina Center for Accelerator-Based Science using the fusion-evaporation reaction Pb208(V51, 2n)Db257. 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 Db257 ions to a helium gas-filled ion stopping cell wherein they were thermalized. Thermalized Db3+257 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 Db3+257 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 100063(231)stat(132)syskeV/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. © 2021 American Physical Society.