Neutron-proton ratio of collective excitations - art. no. 031303

Using a harmonic oscillator model, it is shown that the isovector giant qua drupole resonance carries little isoscalar quadrupole strength while the is oscalar giant quadrupole resonance contains an isovector strength with B(IV )/B(IS)approximate to((N-Z)/A)(2), when a consistency condition is fulfille d. Taking into account the consistency between the one-particle excitation spectra and the vibrational fields is of vital importance in the proper est imate of the ratio of the contribution by neutrons to that by protons to co llective excitations of nuclei with N not equal Z. The ratio may not be pro perly estimated in the models such as traditional shell models, in which th e Coulomb potential (or interaction) is simulated by using the parameters o mega(p) < omega(n) for nuclei with a neutron excess. [S0556-2813(99)50609-2 ].