$E2$ excitation strength in ${}^{55}\mathrm{Ni}$: Coupling of the ${}^{56}\mathrm{Ni}{2}_1^{+}$ collective core vibration to the $f_{7∕2}$ odd neutron hole

The collectivity of the odd-mass nucleus ${}^{55}\mathrm{Ni}$ was explored via intermediate-energy Coulomb excitation using a powerful combination of particle and $\gamma$-ray spectroscopy. A $\gamma$-ray at 2879(18) keV was observed and is interpreted to deexcite a member of the core-coupled quintuplet $2_1^{+}\left({}^{56}\mathrm{Ni}\right)\otimes \nu f_{7∕2}^{-1}$ at the same energy. By similarity with the mirror nucleus ${}^{55}\mathrm{Co}$, transition probabilities were calculated assuming $J^{\pi }=9∕2^{-}$ and $J^{\pi }=11∕2^{-}$ for this state. Both assumptions lead to a transition strength higher than predicted by a large-scale shell-model calculation using the GXPF1 effective interaction and exceed the value predicted within a simple weak-coupling approach.

Figure 1

Laboratory-frame (top) and projectile-frame (middle) $\gamma$-ray spectra in coincidence with ${}^{55}\mathrm{Ni}$ particles. The $\gamma$ rays from the deexcitation of an excited state of ${}^{55}\mathrm{Ni}$ are apparent in the middle panel at about 2880 keV. Only the ${}^{197}\mathrm{Au}$ deexcitation $\gamma$ rays detected in the 90° ring of SeGA can be seen in the middle panel, as the $\gamma$ rays are Doppler shifted to approximately 410 keV in the 37° ring. The bottom panel shows an expansion of the projectile-frame spectrum with fits overlayed. The solid black line is the total fit which contains the sum of the simulated response functions for the 37° (solid gray line) and 90° (dashed gray line) rings. The double-exponential background is indicated by the dot-dashed line.

Figure 2

Low-lying $E2$ strength in ${}^{55}\mathrm{Ni}$ calculated within the shell model using the GXPF1 effective interaction and the effective charges $e_p=1.5e$ and $e_n=0.5e$.

Figure 3

Comparison of the ${}^{55}\mathrm{Ni}$ level scheme with the mirror nucleus ${}^{55}\mathrm{Co}$ [16]. We note that the first $2^{+}$ state of ${}^{56}\mathrm{Ni}$ lies at 2700.6(7) keV, supporting the assumption that the observed state(s) could correspond to a member or members of the core-coupled quintuplet of negative parity $2_1^{+}\left({}^{56}\mathrm{Ni}\right)\otimes \nu f_{7∕2}^{-1}$.