Energy spectra for intermediate mass fragments produced in central Au + Au collisions at $\frac{E}{A}=100\mathrm{}$ MeV indicate a collective expansion at breakup. For the first time, values for this collective expansion energy per nucleon are extracted independently for each charge. Typically, these values are one-third to one-half of the incident kinetic energy per nucleon in the c.m. system, but they decrease with $Z_f$, suggesting that all fragments do not participate equally in the collective expansion.

• Received 29 April 1994

DOI:https://doi.org/10.1103/PhysRevLett.73.3367