Inelastic scattering of ${}^{40}$Ca on ${}^{40}$Ca at 50 MeV/nucleon has been measured in coincidence with light charged particles detected over a large angular domain. The giant resonance region of the inelastic spectrum is shown to be dominated by the isoscalar giant quadrupole resonance. The characteristics of the emitted protons allow us to separate four reaction mechanisms contributing to the high excitation energy region of the inelastic spectrum (E${}^{*}>$ 30 MeV): pickup breakup, knockout, inelastic scattering to high-lying target states, and a new mechanism responsible for the emission of fast-moving protons. A sizable direct decay branch towards hole states of ${}^{39}$K is measured for the giant quadrupole resonance in ${}^{40}$Ca. Evidence is given for the excitation of the two-phonon giant quadrupole resonance, characterized by its direct decay scheme. The energy and width of this two-phonon state are in agreement with a harmonic picture for collective nuclear excitations.

• Received 26 March 1997

DOI:https://doi.org/10.1103/PhysRevC.56.3187