Break-up of the N=50 core in ₃₉⁸⁹Y₅₀

The semi-magic nucleus ₃₉ ⁸⁹Y₅₀ has been
studied in the framework of the spherical shell model. Relative to a hypothetical
⁶⁶Ni core the expanded configuration space (0f_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 bveen considered. The results, both for the excitation
energies and electromagnetic transition probabilities, suggest for the
positive-parity states above 4.5 MeV with 17/2 <= I <=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 states above
7.2 MeV with 23/2 <= I <=31/2 are generated by the additional excitation
of one proton to the 0g_9/2 orbital resulting in a parity alteration.
The strength of the electric dipole transitions connecting the levels of
the two proposed high-spin structures might be explained by small contributions
of the neutron 0h_11/2 excitation to the involved wave functions.