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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 ¼ 152 neutron shell closure, have been directly measured using a multireflection time-of-flight mass spectrograph. 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 δ 2n derived from three isotopic masses was updated with the new masses and corroborates the existence of the deformed N ¼ 152 neutron shell closure for Md and Lr. DOI: 10.1103/PhysRevLett.120.152501 Precision mass measurements of unstable nuclei, providing a direct measure of the nuclear binding energy, are invaluable for the study of nuclear shell evolution and collective effects, such as deformations, far from stability [1,2]. For transfermium nuclei and the yet poorly investigated region towards the superheavy nuclei (SHN), where proton repulsion becomes a generally dominant feature, the description of nuclear lifetimes depends crucially on shell stabilization effects mainly driven by deformed shells [3][4][5]. Theoretical studies with increasing particle numbers investigate the so-called "island of stability" [6], where features like the continuing decrease of energy gaps [7] and the emergence of shape coexistence [8] have a crucial impact on the predicted position and localization of stability regions and the corresponding lifetimes of the nuclei. Although the first experimental evidence for SHN has reached the region of the predicted subshell closure at N ¼ 162 [9-11], the deformed shell closure at N ¼ 152 for transfermium nuclei (see, e.g., [12]) and, as recently pointed out, weaker shell effects in the vicinity [13], still represents the cutting edge for thorough experimental investigations. The transfermium nuclei, however, can be produced only online, in heavy-ion fusion and nucleon transfer reactions, and consequently only low yields are available for study, necessitating highly efficient techniques. Direct mass measurements of transfermium nuclei have so far been performed for only six nuclei-four isotopes of nobelium and two isotopes of lawrencium-with the Penning trap mass spectrometer SHIPTRAP [14,15].In this Letter, we report the first implementation of a multireflection time-of-flight mass spectrograph (MRTOF MS) for transfermium nuclei as shown in Fig. 1, including new mass measurements of 246 Es, 251 Fm, [249][250][251][252]

International audienceUsing a multireflection time-of-flight mass spectrograph located after a gas cell coupled with the gas-filled recoil ion separator GARIS-II, the masses of several α-decaying heavy nuclei were directly and precisely measured. The nuclei were produced via fusion-evaporation reactions and separated from projectilelike and targetlike particles using GARIS-II before being stopped in a helium-filled gas cell. Time-of-flight spectra for three isobar chains, Fr204-Rn204-At204-Po204, Fr205-Rn205-At205-Po205-Bi205, and Fr206-Rn206-At206, were observed. Precision atomic mass values were determined for Fr204–206, Rn204,205, and At204,205. Identifications of Bi205, Po204,205, Rn206, and At206 were made with N≲10 detected ions, representing the next step toward use of mass spectrometry to identify exceedingly low-yield species such as superheavy element ions