Beyond the N=50 shell closure: High-spin excitations of $^{87}$Kr and ground-state spin of $^{87}$Br
Résumé
The Kr-87 nucleus has been produced as fission fragment in the fusion reaction O-18+Pb-208 at 85 MeV bombarding energy and studied with the Euroball IV array. High-spin states of this neutron-rich isotope have been identified for the first time. Its level scheme has been obtained up to 6.3 MeV excitation energy and spin I similar to 23/2 (h) over bar. Its structure is interpreted by analogy with those of the heavier isotones. The proposed configurations involve both proton and neutron excitations from several sub-shells located close to the Fermi levels, particularly nu d(5/2), pi p(3/2)f(5/2) and pi g(9/2). Moreover, a revised spin value of 5/2(-) for the Br-87 ground state is proposed.