Phonon mechanisms of mixing collective and quasiparticle excitations

A phonon approach with subsequent mapping onto bosons is developed for desc ribing transitional nuclei spectra including the first backbending energy r egion. Quasiparticle random-phase approximation and its modification with m inimum ground state correlations an employed to find B-J phonons (spin J gr eater than or equal to 4) and most collective quadrupole D phonons, respect ively. Phonon functions and matrix elements of B-D interaction an calculate d with factorized quasiparticle forces. Besides the interaction mechanisms known in the interacting boson fermion model (IBFM) a new one is introduced that leads to three into one boson transformation. After mapping onto s-, d-, and b(J)-boson space a boson Hamiltonian comprises IBM1 Hamiltonian, wh ose parameters depend on the presence or absence of b(J) bosons and are det ermined phenomenologically, and b(J)-d interaction with constants calculate d microscopically inside phonon space. This approach applied to energy spec tra and B(E2) values in B-126,B-128 and Ce-130 gives reasonable agreement w ith experiment. [S0556-2813(99)05806-9].