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Split isobaric analog state in 55Ni: case of strong isospin mixing.

Study of β+ decay of the exotic Tz=-3/2 nucleus 55Cu, via delayed γ rays, has revealed a strongly isospin mixed doublet (4599-4579 keV) in 55Ni, which represents the fragmented and previously unknown isobaric analog of the ground state of 55Cu. The observed small log ft values to both states in the doublet confirm the superallowed Fermi β decay. The near degeneracy of a pair of 3/2- levels in 55Ni 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 55Cu at 57(3) ms was also obtained.