Nuclear Physics A379 (1982) 221-238 0 North-Holland Publishing Company DESCRIPTION OF ODD-A NUCLEI IN THE Pt REGION IN THE INTERACITNG BOSON-FERMION MODEL R. BIJKER and A. E. L. DIEPERINK Kert(wisch Versneller Instituut. Rijksunivwrsiteit Groningen. 9747 A A Groningen. The Netherlands Received 9 November 1981 Ahatraet: Properties of unique parity states in odd-proton (,,lr. 79A~) and odd-neutron nuclei (,sPt) are investigated in the framework of the intemcting boaon-fermion approximation model. The core (boson)-particle (fermion) interaction is represented by a quadrupolequadrupole interaction and an exchange term, which takes into account the effects of the Pauli exclusion principle. The even-even core nucleus is d,escribed in terms of the IBA-I hamiltonian. The change in the proper- ties of the corresponding odd-A nuclei can be interpreted in terms of a transition of the core hamil- tonian between the O(6) and SU(3) limiting cases. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPO 1. Introdltctioo Recently it has been shown that collective properties in even-even nuclei can be described successfully in the framework of the interacting boson approximation (IBA) model of Arima and Iachello ’ - ‘). In this model the many-body states are classified according to the totally symmetric representations [NJ of the group SU(6), where N is the number of bosons. In three special cases this hamiltonian can be expressed in terms of generators of a subgroup of SU(6), namely the SU(5) group ‘), the SU(3) group ‘) and the O(6) group 3). These extreme cases correspond4) for N + co to the (anharmonic) vibrator, the axially symmetric deformed rotor and the y-unstable nucleus of the geometrical model “), respectively. Recently the IBA model has been extended to describe odd-even nuclei by coupling a single fermion to the even-even core 6*7). In this approach, which will be referred to as an interacting boson-fermion approximation (IBFA) model, the core is de scribed by the IBA hamiltonian. In the present version no distinction is made between proton and neutron boson degrees of freedom. It has been shown that the interaction between the core and the odd particle can in a good approximation be described by two terms, (i) a quadrupolequadrupole interaction, and (ii) an exchange force. The latter has been introduced in order to take into account the effect of the antisymmetrization between the odd particle and the core nucleus. Whenever the core hamiltonian possesses one of the dynamical symmetries, the corresponding odd-A energy spectra exhibit simple features, which have been shown*) to be analogous to those of (i) the particle-vibration modelg), 221 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONML May lwz