AN O(6)-]SU(3) PARTICLE-HOLE SYMMETRY TRANSITION NEAR THE Z=50 CLOSED-SHELL

Particle-hole excitations near closed shells are incorporated in an ex tended interacting boson model (EIBM) thereby enlarging the region of applicability of this algebraic approach. We study the consequences of classifying various many-particle-hole (mp-nh) excitations into multi plets, according to dynamical symmetries of the EIBM. Applications to the Z = 50 region are presented for the Ru-Cd-Te-Ba nuclei. Here, we s how (i) that the ''global'' smooth change in the intruder multiplet ba nd structure is described by an O(6) --> SU(3) symmetry transition and (ii) how the ''local'' mixing between the intruder structure and the regular anharmonic vibrational states (U(5)) radically changes in the above O(6) --> SU(3) symmetry transition.