Temperatures of exploding nuclei

Breakup temperatures in central collisions of ${}^{197}\mathrm{Au}{+}^{197}\mathrm{Au}$ at bombarding energies $E/A\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}50$ to 200 MeV were determined with two methods. Isotope temperatures, deduced from double ratios of hydrogen, helium, and lithium isotopic yields, increase monotonically with bombarding energy from 5 to 12 MeV, in qualitative agreement with a scenario of chemical freeze-out after adiabatic expansion. Excited-state temperatures, derived from yield ratios of states in ${}^{4}\mathrm{He}$, ${}^{5,6}\mathrm{Li}$, and ${}^{8}\mathrm{Be}$, are about 5 MeV, independent of the projectile energy, and seem to reflect the internal temperature of fragments at their final separation from the system.