PREEQUILIBRIUM EFFECTS IN THE POPULATION OF GIANT-DIPOLE RESONANCES

High-energy gamma rays associated with the decay of the giant dipole r esonance have been measured for two fusion reactions leading to the Sm -140 compound nucleus at an excitation energy of 71 MeV. The observed yield increases with the asymmetry in the ratios of the number of neut rons to protons in the entrance channel. This is interpreted as result ing from giant dipole phonons excited at the moment of collision in an N/Z asymmetric reaction.