Evidence for Thermal Equilibration in Multifragmentation Reactions probed with Bremsstrahlung Photons

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