Three paths for intermediate-mass fragment formation at a near-onset excitation energy of 1.3 MeV/nucleon

Intermediate mass fragments of 4≤Z≤17 have been studied from the reaction 640 MeV ${}_{}{}^{86}{}_{}{}^{}\mathrm{o}$/rKr${+}^{63}$Cu. Inclusive energy and angular distribution measurements have been made as well as coincidence measurements with fissionlike fragments of 18≤Z≤40. The coincidence configuration was such that two-body exit channels were excluded; nevertheless, heavy fragments were recorded both in plane and out of plane with respect to light fragments of 4≤Z≤10. Three mechanisms have been identified for intermediate mass fragment production. The dominant pathway for all intermediate mass fragments is two-body breakup of fission or evaporationlike character. A second pathway for Z≤10 is intermediate mass fragment ejection from the composite nucleus followed by sequential fission. The third path is simultaneous ternary breakup for 4≤Z≤9 that resembles ternary alpha-accompanied fission at low energies.