BREAK-UP OF THE N=50 CORE IN Y-89(39)50

The semi-magic nucleus Y-89(39)50 has been studied in the framework of the spherical shell model. Relative to a hypothetical Ni-66 core the expanded configuration space (Of(5/2),1p(3/2),1p(1/2),0g(9/2)) and (1p (1/2),0g(9/2),1d(5/2),0g(7/2)) for protons and neutrons, respectively, has been considered. The results, both for the excitation energies an d electromagnetic transition probabilities, suggest for the positive-p arity states above 4.5 MeV with 17/2 less than or equal to I less than or equal to 23/2 significant admixtures of the neutron [(0g(9/2))(-1) (1d(5/2))] excitation coupled to the odd proton occupying the 0g(9/2) orbital. The calculations predict that the experimentally observed sta tes above 7.2 MeV with 23/2 less than or equal to I less than or equal to 31/2 are generated by the additional excitation of one proton to t he 0g(9/2) orbital resulting in a parity alteration. The strength of t he electric dipole transitions connecting the levels of the two propos ed high-spin structures might be explained by small contributions of t he neutron 0h(11/2) excitation to the involved wave functions.