Z. Phys. A - Atomic Nuclei 330, 365-369 (1988) Zeitschrift fiJr Physik A Atomic Nuclei 9 Springer-Verlag 1988 Nuclear Structure of 97y in the Interacting Boson-Fermion Model S. Brant ~*, K. Sistemieh ~, V. Paar 2, and G. Lhersonneau 3 Institut ffir Kernphysik, Kernforschungsanlage Jfilich, Federal Republic of Germany 2 Prirodoslovno-matemati~ki fakultet, University of Zagreb, Yugoslavia** 3 Institut fiir Kernchemie, Universitfit Mainz, Federal Republic of Germany Received April 7, 1988 Unique-parity states of 97y are studied in the framework of the interacting boson-fermion model (IBFM). Spin assignments for the levels of the unique-parity yrast band are pro- posed on the basis of nuclear systematics and the IBFM band pattern associated with 96 coupling of a particle-type quasiparticle (g9/2) to the SU(5) core 38Srs8. The good fit of the experimental data with the IBFM calculations confirms the single particle character of this immediate neighbour of the doubly sub-magic nucleus 96Zr. PACS: 21.60.Fw; 23.20.Lv; 27.60. +j 1. Introduction Neutron-rich nuclei in the region around A = 100 ex- hibit a rapid change from shell model structure to a pronounced collective rotational pattern. Thus, the high-spin states in the Y isotopes in the neighbour- hood of 96Zr exhibit rather pure shell-model configu- rations, while already 99y has the properties of a sym- metric rotor [1] and 98y seems to contain shape coex- istence with a spherical ground state and a rotational band based on the 495 keV level [2]. Recently, the nucleus 97y with one proton hole and two neutron particles beyond 96Zr has been in- vestigated [3, 4]. A particularly interesting feature of this nucleus is a family of levels which are probably associated 1-4] with the g9/2 proton configuration. These levels have been disclosed through the discov- ery of a high-spin isomer at 3523 keV in 97y which is populated in fission and which is interpreted as a three-quasiparticle configuration with a g9/2 quasi- proton involved. In this paper we investigate these states within the framework of the interacting boson- fermion model (IBFM). * Alexander yon Humboldt fellow, on leave of absence from Uni- versity of Zagreb, Yugoslavia ** Assisted by Internationales Biiro, KFA Jtilich, Federal Republic of Germany 2. Coupling Rules for Unique Parity Bands The investigations of the unique-parity band structure in odd-A nuclei have resulted in the following rules for the pattern of yrast bands [5-7] : i) A strongly-coupled band pattern (E(j+I)<E(j+2), EU+3)<E(j+4 ) .... ) arises if an odd particle j is coupled to an oblate core, or a hole j- 1 to a prolate core. The same type of band pattern arises also if the core is 7 soft, neither prolate nor oblate. ii) A decoupled band pattern (E(j+2)<E(]+I), EU+4)<E(j+3), ...) is observed if an odd particle j is coupled to a prolate core, or a hole j-a to an oblate core. The same rules apply to the case of a unique- parity quasiparticle state y coupled to the core, de- pending on whether f is of particle-type 2 2 (uj >vj) or hole-type (u~ < v~). These rules are rather general, and appear in dif- ferent models which, in a proper way, account for the coupling of the quasiparticle to collective modes. In terms of these rules we can discuss also the limiting cases of unique-parity bands in IBFM. The coupling of a f= gg/z quasiparticle to the har- monic SU(5) boson core has been investigated by Scholten in an illustrative IBFM calculation [8-1. For