Isoscalar and isovector quadrupole resonance effects in proton-gamma-gamma angular correlations

The direct-semidirect model is extended to determine the nucleon-gamma-gamma angular correlations when the proton radiative capture to an excited level is followed by the electromagnetic de-excitation of the nucleus. The correlation function is derived for the 142Ce(p,γγ) reaction, relative to two different measure geometries, when electric dipole and quadrupole capture mechanisms are taken into account. Starting from the renormalized effective charge calculus for a real particle-vibration coupling, the model provides useful indications as to the effects produced by the excitation of the isoscalar and isovector giant quadrupole resonances on the geometrical features of the double photon emission. (orig.)