Isotopically resolved yields of intermediate-mass fragments from ${}_{}{}^{12}{}_{}{}^{}\mathrm{-}$ and ${}_{}{}^{18}{}_{}{}^{}\mathrm{induced}$ reactions on ${}_{}{}^{58}{}_{}{}^{}$,64Ni, ${}_{}{}^{\mathrm{nat}}{}_{}{}^{}\mathrm{Ag}$, and ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$ at E/A=84 MeV were measured. The systematic variation of the yield ratios with projectile and target shows that these fragments are produced by a mechanism that involves the whole target nucleus. It is demonstrated that, within a statistical interpretation, the isotope-yield ratios are a measure of the entropy and determine a relation between temperature and density at breakup.

• Received 25 November 1986

DOI:https://doi.org/10.1103/PhysRevLett.58.1829