Application of the boson-expansion method to even Se and Ru isotopes

Abstract

The expansion coefficients of a fourth-order collective Hamiltonian for the low-lying quadrupole vibrations are derived from the microscopic fermion Hamilton operator by a modified Marumori boson-expansion method. Their dependence on the phonon structure, on the parameters of the two-body (surface delta) interaction and on the single-particle energies is numerically investigated. For the isotopes $sup 76$Se, $sup 78$Se, and $sup 100$Ru, $sup 102$Ru the results are compared with coefficients that are obtained from phenomenological fits to low-lying levels. Quadrupole moments and B (E2) values are calculated in lowest order.