The Michigan State University 4π array has been used to select central-impact-parameter events from the reaction ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$${+}^{51}$V at incident energies from 35 to 85 MeV/nucleon. The event shape in momentum space is an observable which is shown to be sensitive to the dynamics of the fragmentation process. A comparison of the experimental event-shape distribution to sequential- and simultaneous-decay predictions suggests that a transition in the breakup process may have occurred. At 35 MeV/nucleon, a sequential-decay simulation reproduces the data. For the higher energies, the experimental distributions fall between the two contrasting predictions.

• Received 14 March 1990

DOI:https://doi.org/10.1103/PhysRevLett.64.2246