Structure in the Energy Spectra from Inelastic Heavy-Ion Reactions

Broad structures have been observed in the incompletely relaxed part of the energy spectra of fragments emitted in the symmetrical reactions ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ + ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ at 284 MeV and ${}_{}{}^{63}{}_{}{}^{}\mathrm{Cu}$ + ${}_{}{}^{63}{}_{}{}^{}\mathrm{Cu}$ at 450 MeV. Excitation energies of these structures, angular distributions, and ($Z, N$) distributions leading to multiplicities for nucleon emission have been measured. They can be interpreted as being due to a predominantly direct reaction process, possibly proceeding through the excitation of highly collective high-energy modes in the region of giant resonances.