Study of ${\beta }^{+}$ decay of the exotic $T_z=-3/2$ nucleus ${}^{55}\mathrm{Cu}$, via delayed $\gamma$ rays, has revealed a strongly isospin mixed doublet (4599–4579 keV) in ${}^{55}\mathrm{Ni}$, which represents the fragmented and previously unknown isobaric analog of the ground state of ${}^{55}\mathrm{Cu}$. The observed small $\log ft$ values to both states in the doublet confirm the superallowed Fermi $\beta$ decay. The near degeneracy of a pair of $3/2^{-}$ levels in ${}^{55}\mathrm{Ni}$ results in the strong isospin mixing. The isospin mixing matrix element between the $T=3/2$ and $T=1/2$ levels is inferred from the experiment to be 9(1) keV, which agrees well with the matrix element of the charge symmetry breaking shell model Hamiltonian of Ormand and Brown. A precise value of the half-life of ${}^{55}\mathrm{Cu}$ at 57(3) ms was also obtained.

• Received 28 August 2013

DOI:https://doi.org/10.1103/PhysRevLett.111.262501