First Direct Mass Measurements of Nuclides around Z=100 with a Multireflection Time-of-Flight Mass Spectrograph

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 $\alpha$ 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 $\delta_{2n}$ 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.