Positive-parity rotational band in ${}^{158}\mathrm{Tb}$

Excited states of the odd-odd nucleus ${}^{158}\mathrm{Tb}$ have been studied via the ${}^{154}\mathrm{Sm}({}^7\mathrm{Li},3n){}^{158}\mathrm{Tb}$ reaction at a beam energy of 27 MeV. The new level scheme of ${}^{158}\mathrm{Tb}$ involving a positive parity rotational band is established up to excitation energy $\sim 2.3\mathrm{MeV}$ and spin $\sim 17\hslash$. By comparing with the rotational bands in the neighboring nuclei ${}^{157}\mathrm{Tb}$ and ${}^{157}\mathrm{Gd}$, the configuration was tentatively assigned as $\pi d_{5/2}\otimes \nu i_{13/2}$, and the phenomenon of signature inversion is discussed.

Figure 1

ADO ratios shown as a function of $\gamma$-ray energy for (a) ${}^{157}\mathrm{Tb}$ and (b) ${}^{158}\mathrm{Tb}$. The dotted and dashed lines indicate the typical values observed for the known $\gamma$ rays in this experiment for stretched quadrupole and stretched pure dipole transitions, respectively.

Figure 2

The level scheme of ${}^{158}\mathrm{Tb}$ established in the present work. The $\gamma$-ray transitions with an asterisk are obtained from previous work [7]. The decay of the $7^{-}$ isomer and the 55.0 keV ($4^{+}\to 3^{-}$), 66.1 keV ($7^{-}\to 7^{+}$) transitions, although not observed in this study, are included in the level scheme for completeness.

Figure 3

Background-corrected coincidence spectrum obtained by gating on the (a) 162.5 keV, (b) 228.3 keV, (c) 194.6 keV, and (d) 261.6 keV $\gamma$-ray transitions. Peaks associated with ${}^{158}\mathrm{Tb}$ are labeled with their energies in keV; the 125, 154, 297, and 358 keV $\gamma$ rays obtained by gating on (a) the 162.5 keV $\gamma$-ray transition and (c) the 194.6 keV $\gamma$-ray transition are assigned to ${}^{157}\mathrm{Tb}$ [8].

Figure 4

The experimental Routhians of (a) ${}^{157}\mathrm{Tb}$, (b) ${}^{157}\mathrm{Gd}$, and (c) ${}^{158}\mathrm{Tb}$ relative to ${}^{156}\mathrm{Gd}$. The orbits are labeled with $(\pi ,\alpha )$. Full (open) lines are used for positive (negative) parity and closed (open) symbols for signature $\alpha =1/2(\alpha =-1/2)$. The orbits labeled with ${(+,1/2)}_2$ and ${(+,-1/2)}_2$ in (a) denote the second positive band ($5/2^{+}\left[413\right]$ band, see Ref. [8]) in ${}^{157}\mathrm{Tb}$. The kinks at $\omega \approx 0.3$ MeV in (a) resulted from the alignment of $i_{13/2}$ quasineutrons.

Figure 5

Experimental energy differences $S\left(I\right)$ of the $\pi d_{5/2}\otimes \nu i_{13/2}$ band against spin $I$ for the doubly odd isotopes with $Z=65$. Closed (open) symbols are used for odd (even) spin. Data sources are ${}^{154}\mathrm{Tb}$ [5], ${}^{156}\mathrm{Tb}$ [1, 6], and ${}^{158}\mathrm{Tb}$ from this work. The inset in panel (c) shows a vertically extended plot for the spin from $6\hslash$ to $12\hslash$.