Time evolution of pion emission in heavy ion collisions at SIS energies

Using a magnetic spectrometer pions, kaons and protons were detected in mas s-symmetric heavy ion reactions from C+C to Au+Au and at incident energies between 0.6 and 2.0 A.GeV. The center-of-mass pion spectra deviate from a B oltzmann distribution for all collision systems. Results are presented indi cating that high-energy pions are emitted at an early stage of the collisio n. This is based on (i) a comparison of pi(+) and pi(-) spectra and (ii) th e shielding of pions by spectator matter in peripheral collisions.