Z. Phys. A 359, 19–22 (1997) ZEITSCHRIFT F ¨ UR PHYSIK A c  Springer-Verlag 1997 Microscopic aspects of identical bands in 78 Sr and 78 Rb ∗ A. Petrovici 1,2 , K.W. Schmid 2 , Amand Faessler 2 1 Institute for Physics and Nuclear Engineering, R-76900 Bucharest, Romania 2 Institut f¨ ur Theoretische Physik, Universit¨ at T¨ ubingen, D-72076 T ¨ ubingen, Germany Received: 26 June 1996 / Revised version: 14 March 1997 Communicated by D. Schwalm Abstract. Recent investigations of the shape transition and shape coexistence phenomena dominating the structure of the even–even N  Z nuclei in the A  80 mass region are ex- tended to the odd-odd nucleus 78 Rb. Special attention is paid to the structure of some “identical” bands which have been re- cently identified in 78 Sr and 78 Rb. The ground band of 78 Sr and the yrast as well as excited negative parity bands in 78 Rb are studied within the EXCITED VAMPIR approximation using complex Hartree-Fock-Bogoliubov transformations in a rel- atively large model space. The results are compared with the available experimental data. The emerging picture reveals the role played by the strong quadrupole deformation on the ap- pearance of identical bands as well as the influence of the shape coexistence on their evolution. Predictions for the electromag- netic and alignment properties of the bands are presented. PACS: 21.10.-h; 27.50.+e 1 Introduction The properties of the N  Z nuclei in the A ∼ 80 region have attracted considerable attention in the last decade, both theoretically as well as experimentally. These nuclei have very elongated ground-state shapes with the Sr and Zr isotopes in- terpreted as approaching deformations as large as β 2 ∼ 0.4 [1,2]. Large deformed shell gaps in the single-particle spec- trum stabilize these shapes much like the subshell closures associated with the superdeformed orbitals. Because of these gaps rapid changes in shape are seen when the particle number changes by only two units. Changes in spin manifest them- selves in quite dramatic changes in shape sometimes, too, and many nuclei in this region exibit shape coexistence phenom- ena. In this context the recently identified so called identical bands in 78 Sr and 78 Rb [3] become a challenge for theoretical models. Besides that, the occurence of identical bands itself is still an open question [4].  Work supported by the Institute for Physics and Nuclear Engineering, Bucharest, Romania and the DFG, Germany Recently we obtained a consistent microscopic picture for a couple of doubly even NZ nuclei from Z=36 to Z=42 in- cluding the 78 Sr isotope [1]. This was achieved by a completely microscopic variational approach in which all the essential degrees of freedom are dynamically determined by the cho- sen Hamiltonian. In each investigated nucleus the lowest few states of a given spin and positive parity were approximated within the complex EXCITED VAMPIR approach [5], which like all the complex versions of the various models of the VAMPIR family [6] includes already in the mean field the proton-neutron interaction and unnatural-parity correlations. In the present study now the negative parity bands in the odd- odd nucleus 78 Rb are investigated. The yrast sequence (from spin 5 − to 15 − ) is considered as a candidate for a band be- ing identical to the ground-state band up to spin 12 + in the even-even nucleus 78 Sr. The theoretical spectra are compared with the experimen- tal data presently available. Furthermore predictions for the alignment and electromagnetic properties of the bands are pre- sented. In Sect. 2 we give a rough outline of the theoretical approach, define the model space and give some details of the effective Hamiltonian being used. In Sect. 3 we present then the results obtained for the investigated bands. Some conclu- sions are given in Sect. 4. 2 The theoretical framework As in the earlier calculations for nuclei out of the A ∼ 70 mass region [1,7-9] also in the present work a 40 Ca core is used and as single particle basis states the 1p 1/2 ,1p 3/2 ,0f 5/2 ,0f 7/2 , 1d 5/2 and 0g 9/2 oscillator orbits for both protons and neutrons are taken. The corresponding single-particle energies are (in units of the oscillator energy ¯ hω) 0.040, -0.270, 0.300, -0.560, 0.157 and 0.029 for the proton, and -0.070, -0.332, 0.130, -0.690, 0.079 and -0.043 for the neutron levels, respectiv- ely. For the effective two-body interaction, too, the renormal- ized G-matrix out of [1,8,9] is taken. It consists out of a nuclear matter G-matrix derived from the Bonn One-Boson- Exchange potential [10] modified by two short range (0.707 fm) Gaussians for the isospinT=1 proton-proton and neutron-