Photon interferometry at SIS and GANIL energies

The measurement of photon correlations provides an important tool to obtain information on the space-time aspects of the early stages of relativistic heavy ion collisions. We consider the bremsstrahlung emission of ''hard'' photons (Eγ ≥ 25 MeV) at low and intermediate energies. To this end, we derive expressions more general then previous results by D. Neuhauser which were limited to the case of a system that has no ''memory'' of the collision axis (e.g., a system in thermal equilibrium). As a consequence of polarization correlations it turns out that for a purely chaotic source the intercept of the correlation function of photons can exceed significantly the value of 1.5 obtained by Neuhauser. This value depends on the dynamics of the collision, in particular on the amount of stopping and the contribution of Fermi motion. We compare our results with GANIL data on photon interferometry in Xe + Au at 44 AMeV. We expect that the formalism presented here can be applied up to SIS energies. (orig.)