First direct mass measurements of nuclides around $Z=100$ with a Multireflection Time-of-Flight Mass Spectrograph

Abstract

The masses of ${}^{246}$Es, ${}^{251}$Fm and the transfermium nuclei ${}^{249-252}$Md, and ${}^{254}$No, produced by hot- and cold-fusion reactions, in the vicinity of the deformed $N=152$ neutron shell closure, have been directly measured using a multireflection time-of-flight mass spectrograph. The masses of ${}^{246}$Es and ${}^{249,250,252}$Md were measured for the first time. Using the masses of ${}^{249,250}$Md as anchor points for $\hat{I}\pm$ decay chains, the masses of heavier nuclei, up to ${}^{261}$Bh and ${}^{266}$Mt, were determined. These new masses were compared with theoretical global mass models and demonstrated to be in good agreement with macroscopic-microscopic models in this region. The empirical shell gap parameter $\hat{I}{´}_{2n}$ derived from three isotopic masses was updated with the new masses and corroborate the existence of the deformed $N=152$ neutron shell closure for Md and Lr.