Ground-state configuration of the $N=157$ nucleus ${}^{259}$No

The ground-state configuration of the $N=157$ nucleus ${}^{259}$No has been identified through $\alpha$-$\gamma$ coincidence and $\alpha$-singles measurements. Three $\gamma$ transitions were observed for the first time in the $\alpha$ decay of ${}^{259}$No, and its decay scheme was established. The neutron 9/2${}^{+}$[615] configuration was assigned to the ground state of ${}^{259}$No as well as to the 231.4 keV level in ${}^{255}$Fm. Ground-state deformations and neutron single-particle energies in $Z=102$ isotopes were calculated with the macroscopic-microscopic model. The 9/2${}^{+}$[615] orbital was calculated to be the highest among the five orbitals between the $N=152$ and 162 deformed shell gaps, which is consistent with the experimental one-quasiparticle energies in $N=153$ and 155 isotones, but is inconsistent with the present experimental result of the 9/2${}^{+}$[615] ground state at $N=157$. To reproduce the 9/2${}^{+}$[615] ground state at $N=157$, the order of the neutron orbitals should be different between the $N=153$ and 157 isotones.

Figure 1

(a) $\alpha$-singles spectrum observed in the $\alpha$-$\gamma$ coincidence measurement at JAEA using Si detectors placed at a distance with a solid angle of 43$\%$ of 4$\pi$ sr. The inset shows a decay curve of the 7505-keV $\alpha$ peak. (b) $\alpha$-energy spectrum observed in the $\alpha$-singles measurement at RIKEN using Si detectors with a solid angle of 10$\%$ of 4$\pi$ sr.

Figure 2

$\gamma$-ray spectrum observed in coincidence with the 7260–7810 keV $\alpha$ particles which corresponds to the $\alpha$-energy range of ${}^{259}$No. The time window was set at the range of $-$25 to $+$340 ns from the prompt peak.

Figure 3

Two-dimensional plots of $\alpha$-$\gamma$ coincidence events.

Figure 4

$\alpha$-decay schemes for the $N=157$ isotones of (a) ${}^{259}$No proposed in the present work and (b) ${}^{257}$Fm established in Ref. [3].

Figure 5

(a) Neutron single-particle energies calculated for $Z=102$ isotopes. The energies of the five neutron orbitals located between the $N=152$ and 162 deformed shell gaps are plotted relative to the 11/2${}^{-}$[725] one. (b) Ground-state deformation parameters calculated for $Z=102$ isotopes. These calculations are performed in the present work using the macroscopic-microscopic model [24, 25] which is based on the FRLDM and the Strutinsky shell-correction method with the folded-Yukawa single-particle potential and the Lipkin-Nogami pairing model.