Evidence for Thermal Equilibration in Multifragmentation Reactions Probed with Bremsstrahlung Photons

The production of nuclear bremsstrahlung photons ( ${E}_{\ensuremath{\gamma}}>30\mathrm{MeV}$) has been studied in inclusive and exclusive measurements in four heavy-ion reactions at $60A\mathrm{MeV}$. The measured photon spectra, angular distributions, and multiplicities indicate that a significant fraction of the hard photons is emitted in secondary nucleon-nucleon collisions from a thermally equilibrated system. The observation of the thermal component in multifragment ${}^{36}\mathrm{Ar}{+}^{197}\mathrm{Au}$ reactions suggests that the breakup of the thermalized source produced in this system occurs on a rather long time scale.