Kinetic energies of light fragments ( $A\le 10$) from the decay of target spectators in ${}^{197}\mathrm{Au}{+}^{197}\mathrm{Au}$ collisions at 1000 MeV per nucleon have been measured with high-resolution telescopes at backward angles. Except for protons and apart from the observed evaporation components, the kinetic-energy spectra exhibit slope temperatures of about 17 MeV, independent of the particle species, but not corresponding to the thermal or chemical degrees of freedom at breakup. It is suggested that these slope temperatures may reflect the intrinsic Fermi motion and thus the bulk density of the spectator system at the instant of becoming unstable.

• Received 15 December 1999

DOI:https://doi.org/10.1103/PhysRevLett.84.4557