E2 excitation strength in {sup 55}Ni: Coupling of the {sup 56}Ni 2{sub 1}{sup +} collective core vibration to the f{sub 7/2} odd neutron hole

The collectivity of the odd-mass nucleus {sup 55}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{sub 1}{sup +}({sup 56}Ni)x{nu}f{sub 7/2}{sup -1} at the same energy. By similarity with the mirror nucleus {sup 55}Co, transition probabilities were calculated assuming J{sup {pi}}=9/2{sup -} and J{sup {pi}}=11/2{sup -} 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.