Inclusive proton spectra from reactions induced by ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$, and ${}_{}{}^{58}{}_{}{}^{}\mathrm{Ni}$ projectiles have been decomposed by use of a refined multisource analysis. Yields from centrallike collisions have been extracted and compared with Boltzmann-master-equation predictions. The comparison yields the initial number of degrees of freedom, $n_0$. The excitation energy per initial degree of freedom, $\frac{E^{*}}{n_0}$, is found to be essentially independent of the colliding masses, depending only on the per-nucleon energy of the projectile. An empirical relation connecting $\frac{E^{*}}{n_0}$ with the available incident energy is given.

• Received 28 July 1987

DOI:https://doi.org/10.1103/PhysRevLett.60.193