Discovery of New Isotope ${}^{241}\mathrm{U}$ and Systematic High-Precision Atomic Mass Measurements of Neutron-Rich Pa-Pu Nuclei Produced via Multinucleon Transfer Reactions

The new isotope ${}^{241}\mathrm{U}$ was synthesized and systematic atomic mass measurements of nineteen neutron-rich Pa-Pu isotopes were performed in the multinucleon transfer reactions of the ${}^{238}\mathrm{U}+{}^{198}\mathrm{Pt}$ system at the KISS facility. The present experimental results demonstrate the crucial role of the multinucleon transfer reactions for accessing unexplored neutron-rich actinide isotopes toward the $N=152$ shell gap in this region of nuclides.

Figure 1

Microscopic shell correction energy calculated with the FRDM12 [18] for the region spanning $Z=90-102$ and $N=143-156$. The red lines indicate the limits of known isotopes. For nuclides with known mass, the methods of determination—direct and/or indirect—are noted. The hexagons denote the isotopes whose masses were directly measured in this work.

Figure 2

Sketch of the KISS experimental setup. The blue- and yellow-colored areas are filled with Ar and He gases, respectively. Differential pumping systems are located after the doughnut-shaped gas cell as well as before and after the GCCB.

Figure 3

Typical TOF spectra with the red solid lines denote the fitting curves. (a) The reference ion ${}_{55}{}^{133}{\mathrm{Cs}}^{+}$ after 1000 laps in the MRTOF MS. (b) $A/q=120.5$ region after 1001 laps in the MRTOF MS with ${\lambda }_1=387.217\mathrm{nm}$ for the new isotope ${}_{92}{}^{241}\mathrm{U}$. (c) $A/q=120.5$ region after 600 laps with ${\lambda }_1=385.062\mathrm{nm}$ for ${}_{93}{}^{241}\mathrm{Np}$. The absolute TOF values between (b) and (c) are different due to the difference in the number of laps.

Figure 4

Difference of the present masses from those listed in AME2020. The gray shadows show the experimental AME2020 uncertainties, while the red color indicates the extrapolated ones for ${}_{92}{}^{241,242}\mathrm{U}$.

Figure 5

Two-neutron shell gap parameter ${\mathrm{\Delta }}_{2n}$ for (a) ${}_{102}\mathrm{No}$, (b) ${}_{93}\mathrm{Np}$, and (c) ${}_{92}\mathrm{U}$ isotopes. Data points are divided into three segments to indicate which mass data are used for three masses contributing to ${\mathrm{\Delta }}_{2n}(N,Z)$ [upper left: $m(N-2,Z)$, upper right: $m(N+2,Z)$, and bottom: $m(N,Z)$]. Black segment indicates experimental values from this work. White and gray segments show the extrapolated and experimental values from AME2020, respectively. The extrapolated masses have typical uncertainty of $\delta m\sim 200\mathrm{keV}/c^2$. Solid and dashed lines represent the calculations from the various theoretical mass models specified in the top panel, DZ10 [53], FRDM12 [18], WS4RBF [54], HFB32 [55], and KTUY05 [2].