Direct high-precision mass spectrometry of superheavy elements with SHIPTRAP

Direct mass measurements in the region of the heaviest elements were performed with the Penning-trap mass spectrometer SHIPTRAP at GSI Darmstadt. Utilizing the phase-imaging ion-cyclotron-resonance mass-spectrometry technique, the atomic masses of ${}^{251}\mathrm{No}$ ($Z=102$), ${}^{254}\mathrm{Lr}$ ($Z=103$), and ${}^{257}\mathrm{Rf}$ ($Z=104$) available at rates down to one detected ion per day were determined directly for the first time. The ground-state masses of ${}^{254}\mathrm{No}$ and ${}^{255,256}\mathrm{Lr}$ were improved by more than one order of magnitude. Relative statistical uncertainties as low as $\delta m/m\approx {10}^{-9}$ were achieved. Mass resolving powers of 11 000 000 allowed resolving long-lived low-lying isomeric states from their respective ground states in ${}^{251,254}\mathrm{No}$ and ${}^{254,255}\mathrm{Lr}$. This provided an unambiguous determination of the binding energies for odd-$A$ and odd-odd nuclides previously determined only indirectly from decay spectroscopy.

Figure 1

Part of the nuclear chart, showing direct mass spectrometry carried out with the RIKEN MR-ToF MS (purple [24, 25]), TRIGATRAP (dark gray), and SHIPTRAP [blue [21, 22, 23] and red (this work)]. Black and light gray squares represent stable nuclides and nuclides listed by the Atomic Mass Evaluation of 2020 (AME2020) [15], respectively. The black-dotted lines indicate deformed nuclear shell closures.

Figure 2

Phase images of ${}^{254}\mathrm{No}^{2+}$ ions after a phase-evolution time of $\approx 1188$ ms (dashed red rectangles) for the magnetron motion (a) and the modified cyclotron motion (b). The images are superimposed with phase images of center measurements of ${}^{133}\mathrm{Cs}^{+}$ ions. The individual frequency ratios for the nobelium and lawrencium isotopes of interest are symmetrically scattered around their mean value (c). The error bars correspond to the combined statistical and systematic uncertainty. For further details see text.

Figure 3

Tentative decay and level schemes of the decay chain ${}^{258}\mathrm{Db}-{}^{254}\mathrm{Lr}-{}^{250}\mathrm{Md}$ as proposed by [32] in which the literature $Q$ values are compared with this work. The energies of excited states (dashed black lines) are given in keV and observed $\alpha$ and $\gamma$ decays are represented as arrows. The thick blue dashed lines represent long-lived isomeric states and gray arrows indicate weak transitions. The half-lives for isomeric states are also given.

Figure 4

AME evaluation including our new results. Gray squares indicate nuclides listed in the AME2020 [15]. Red squares represent the directly measured atomic masses of this work using PI-ICR. Orange squares represent nuclides for which the mass precision was improved indirectly by at least 10%. Black dots indicate nuclides for which the mass was determined experimentally for the first time. Dashed lines indicate nuclear shell closures. For further information see text.

Figure 5

Two-neutron shell gap parameter at $N=152$ for $Z=97–105$ (top figure) and two-neutron shell gap parameters for $Z=106,108,110$ as a function of the neutron number (bottom figure) after the incorporation of our results into the AME. Data points are divided into filled (33% contribution from our work) and empty (no contribution). For further details see text.