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

The masses of ${}^{246}\mathrm{Es}$, ${}^{251}\mathrm{Fm}$, and the transfermium nuclei ${}^{249-252}\mathrm{Md}$ and ${}^{254}\mathrm{No}$, 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 ${}^{246}\mathrm{Es}$ and ${}^{249,250,252}\mathrm{Md}$ were measured for the first time. Using the masses of ${}^{249,250}\mathrm{Md}$ as anchor points for $\alpha$ decay chains, the masses of heavier nuclei, up to ${}^{261}\mathrm{Bh}$ and ${}^{266}\mathrm{Mt}$, 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 corroborates the existence of the deformed $N=152$ neutron shell closure for Md and Lr.

Figure 1

Nuclear chart above californium ($Z=98$). The squares indicate nuclei synthesized so far. Nuclei whose masses were determined in this work are indicated by the solid (direct) and the left-hatched (indirect) red squares. Similarly, nuclei whose masses were determined by SHIPTRAP measurements are indicated by the solid (direct) and the right-hatched (indirect) black squares.

Figure 2

Schematic view of the experimental setup. Dashed and solid arrows schematically depict the paths of reference and analyte ions, respectively. Curved lines in the cryogenic gas cell represent equipotential lines of the static field that focus ions onto the rf carpet, while arrows depict the nominal trajectories of ions in the static field.

Figure 3

(a) Pictorial representations of the concomitant referencing scheme in the second ion trap system. (b) Slice-by-event analysis method.

Figure 4

TOF spectra in the anticipated vicinity of ${{}^{250}\mathrm{Md}}^{2+}$ at $n=144$ and 145 laps for ${}^{\mathrm{nat}}\mathrm{Tl}$ and ${}^{197}\mathrm{Au}$ targets.

Figure 5

Fitted TOF spectrum of ${{}^{250}\mathrm{Md}}^{2+}$ at $n=145$ laps. The shape parameters of the fitting function were predetermined with a high statistics reference (${}^{133}{\mathrm{Cs}}^{+}$) peak in the same measurement, at the same number of laps.

Figure 6

Deviations between mass values determined in this work and AME16 [33] values. Error bars indicate $1\sigma$ standard uncertainty of our data, while solid lines indicate the uncertainty of AME16. Isotopes designated with a superscript # have extrapolated mass values in AME16.

Figure 7

Plots of empirical shell gap ${\delta }_{2n}$ for Md and Lr isotopes. Data points are divided into three sections to indicate the contributions of the three mass values used in each. White indicates the extrapolated value from AME16, gray indicates the experimental value from AME16, and black indicates the experimental values from this work. Lines indicate results of theoretical models.

Figure 8

Comparison of experimental masses with mass models. Each bar in a model corresponds to an isotope whose mass was determined for the first time in this work: ${}^{249,250,252}\mathrm{Md}$, ${}^{253,254}\mathrm{Lr}$, ${}^{257,258}\mathrm{Db}$, ${}^{261,262}\mathrm{Bh}$, and ${}^{266}\mathrm{Mt}$. Meshed bars indicate experimentally determined mass uncertainties.