Spectroscopy of ${}^{155}\mathrm{Yb}$: Structure evolution in the $N=85$ isotones

High-spin states in ${}^{155}\mathrm{Yb}$ have been studied via the ${}^{144}\mathrm{Sm}({}^{16}\mathrm{O},5n){}^{155}\mathrm{Yb}$ reaction at a beam energy of 118 MeV. One negative-parity and one positive-parity cascade built on the $\nu f_{7/2}$ and $\nu i_{13/2}$ states, respectively, are established for the first time. The structures observed in ${}^{155}\mathrm{Yb}$ are compared with those in the neighboring $N=85$ isotones and with semiempirical shell-model (SESM) calculations. According to adiabatic and configuration-fixed constrained triaxial covariant density functional theory (CDFT) calculations, a coexistence of prolate and oblate shapes is predicted to exist in ${}^{155}\mathrm{Yb}$.

Figure 1

Coincident $\gamma$-ray spectra gated on the (a) 815.1 keV and (b) 688.1 keV transitions. The peaks marked with stars are known contaminants from other nuclei, and the peaks marked with triangles are not included in the partial level scheme of Fig. 2.

Figure 2

Partial level scheme of ${}^{155}\mathrm{Yb}$. New levels are indicated by red lines.

Figure 3

Systematics of known excitations associated with the (a) $\nu f_{7/2}$, $\nu h_{9/2}$, and $\nu i_{13/2}$ states, in the $N=85$ even-$Z$ isotones; (b) $\nu f_{7/2}$, $\nu h_{9/2}$, and $\nu i_{13/2}$ intrinsic states in the $N=87$ even-$Z$ isotones; (c) $3^{-}$ excitations in the $N=84$ even-even isotones; (d) $3^{-}$ excitations in the $N=86$ even-even isotones. Data are taken from Refs. [7, 10, 11, 12, 13, 14, 15, 16, 17, 18, 23, 24, 25] and the present work.

Figure 4

The potential-energy surface in the $\beta \text{−}\gamma$ plane ($0^{\circ }\le \gamma \le {60}^{\circ }$) for ${}^{155}\mathrm{Yb}$ calculated by constrained triaxial CDFT with a newly proposed point-coupling energy density functional PC-PK1. The energy separation between each contour line is 0.2 MeV.

Figure 5

Experimental and theoretical systematics of excited states in ${}^{154,155}\mathrm{Yb}$.