As a result of nuclear resonance investigations of N=50 nuclei [1,2] the question arises to what extent neutron degrees of freedom play a role in the structure of low-spin states in such semi-magic nuclei. Moreover, nothing is known up to now about the response of the N=50 nucleus ⁸⁷Rb to real photons. The first (g,g¢) experiment on ⁸⁷Rb was performed with bremsstrahlung of an endpoint energy of 4.0 MeV delivered by the Dynamitron at Stuttgart. Gamma-rays scattered on a 2.507 g ⁸⁷Rb₂CO₃ target (⁸⁷Rb enriched to 99.2 %) were measured with three HPGe detectors of 100 % relative efficiency. In Fig. 1 a portion of a g-ray spectrum is shown.

Fig. 1 g-ray spectrum of ⁸⁷Rb. Peaks denoted by their g-ray energy belong to ⁸⁷Rb.

Fig. 2 Experimental results for magnetic dipole excitations compared the results of large-scale shell model calculations.

The width G of 20 states could be measured for the first time. For ten levels for the first time g-deexcitation was observed and for five for the first time g-branchings. All observed states are dipole excitations, twelve of them M1 and six E1 excitations. For the 845 and 3837 keV transitions very large B(M1) and B(E1) values have been found, respectively. Shell model calculations [3] for ⁸⁷Rb have been performed. The shell model reproduces the right ordering of the experimentally observed magnetic dipole excitations and gives a fair reproduction of the excitation energies. There is a remarkable agreement of the measured B(M1) values with the calculated values (Fig. 2). The very strong 845 keV transition is the proton p_1/2®p_3/2 spin-flip transition. Dominating proton p_3/2, f_5/2 and p_1/2 single particle character is found for the g.s., the 403 keV and 845 keV states, respectively. The other magnetic dipole excitations are dominated by three protons. Almost all negative parity states contain to some percent contributions of excitations of the N=50 neutron core. Even our large scale shell model calculations considering also the breakup of the N=50 core are not able to describe the electric dipole excitations.

¹ Institut für Strahlenphysik, Universität Stuttgart, 70569 Stuttgart
² Institut für Kernphysik, Universität zu Köln, 50937 Köln

References

[1]J. Reif et al., Nucl. Phys. A 620 (1997) 1

[2]L. Käubler et al., Eur. Phys. J. A 7 (2000) 15

[3]E.A. Stefanova et al., Phys. Rev. C 62 (2000) 054314  IKH 06/21/01 © L. Käubler