PARTICLE-HOLE EXCITATIONS IN THE INTERACTING BOSON MODEL .2. THE U(5)-O(6) COUPLING

Particle-hole excitations leading to two coexisting systems in the sam e nucleus are studied in the framework of an extended interacting boso n model (EIBM). We apply the group theoretical concepts of the EIBM, a version of the interacting boson model where a distinction is made be tween particle and hole bosons, to the particular case of two coexisti ng systems in the same nucleus exhibiting the U(5) regular and the O(6 ) intruder configuration. One can allow for such coexisting systems th rough K-symmetry breaking in the ''vertical'' classification of partic le-hole excitations. Furthermore, the ''horizontal'' classification al lows to test the concept of intruder analog multiplets. Besides the de scription of these ''global'' properties, mixing calculations between the coexisting systems illustrate the important effects of ''local'' p erturbations. Detailed applications are given for nuclei in the region of the shell closure Z = 50. (C) 1997 Elsevier Science B.V.