First high-precision direct determination of the atomic mass of a superheavy nuclide

We present the first direct measurement of the atomic mass of a superheavy nuclide. Atoms of ${}^{257}\mathrm{Db}$ ($Z=105$) were produced online at the RIKEN Nishina Center for Accelerator-Based Science using the fusion-evaporation reaction ${}^{208}\mathrm{Pb}({}^{51}\mathrm{V}$, 2n)${}^{257}\mathrm{Db}$. The gas-filled recoil ion separator GARIS-II was used to suppress both the unreacted primary beam and some transfer products, prior to delivering the energetic beam of ${}^{257}\mathrm{Db}$ ions to a helium gas-filled ion stopping cell wherein they were thermalized. Thermalized ${}^{257}\mathrm{Db}^{3+}$ ions were then transferred to a multireflection time-of-flight mass spectrograph for mass analysis. An alpha particle detector embedded in the ion time-of-flight detector allowed disambiguation of the rare ${}^{257}\mathrm{Db}^{3+}$ time-of-flight detection events from background by means of correlation with characteristic $\alpha$ decays. The extreme sensitivity of this technique allowed a precision atomic mass determination from 11 events. The mass excess was determined to be $100063{\left(231\right)}_{\text{stat}}{\left(132\right)}_{\text{sys}}\mathrm{keV}/{\mathrm{c}}^2$. Comparing to several mass models, we show the technique can be used to unambiguously determine the atomic number as $Z=105$ and should allow similar evaluations for heavier species in future work.

Figure 1

Sketch (not to scale) of the apparatus used in the measurement. Dubnium atoms are produced via fusion-evaporation reactions. Ions of the fusion-evaporation products are separated from the primary beam using the gas-filled recoil ion separator GARIS-II. The ions are stopped in the helium gas cell and subsequently stored in an RF ion trap before being sent to a multireflection time-of-flight mass spectrograph (MRTOF-MS) for analysis. The ion detector at the end of the MRTOF-MS can detect ion implantation and subsequent $\alpha$ decay.

Figure 2

Distribution of ToF-correlated $\alpha$-decay events in terms of detected $\alpha$-decay energy ($E_{\alpha }$) and decay time. For nuclides in the decay chain of ${}^{257}\mathrm{Db}$, the probability distributions in terms of decay time [43] are shown at right; two curves are given for ${}^{257}\mathrm{Db}$ and ${}^{253}\mathrm{Lr}$ as they each exhibit isomerism. The detector response function for each $\alpha$ decay is shown at top, overlaying the $\alpha$ singles spectrum with correlated event candidates denoted by colored marks. The multiple $\alpha$ decay channels, which exist for ${}^{253}\mathrm{Lr}$ and ${}^{257}\mathrm{Db}$ are shown.

Figure 3

Apparent $A/q$ evaluated for each ToF single near the expected position of ${}^{257}\mathrm{Db}^{3+}$. The data are plotted in terms of deviation from the $A/q$ for ${}^{257}\mathrm{Db}^{3+}$ as determined from AME16. Statistical uncertainties are only evaluated for $\alpha$-decay correlated ToF events.

Figure 4

Mass excess determined for each $\alpha$-decay correlated ToF event in this work compared to mass excess ranges for $A$ = 257 isobars as determined by various mass models [50, 51, 52, 53, 54, 55, 56, 57] (blue hash) along with values from AME16 [42] (green hash). The red box designates nuclides whose production would be physically possible in the ${}^{208}\mathrm{Pb}({}^{51}\mathrm{V}$, X) reaction.