Isotopic scaling and the symmetry energy in spectator fragmentation

Isotopic effects in the fragmentation of excited target residues following collisions of 12C on 112;124Sn atincident energies of 300 and 600 MeV per nucleon were studied with the INDRA 4p detector. Themeasured yield ratios for light particles and fragments with atomic number Z < 5 obey the exponentiallaw of isotopic scaling. The deduced scaling parameters decrease strongly with increasing centrality tovalues smaller than 50% of those obtained for the peripheral event groups. Symmetry-term coefficients,deduced from these data within the statistical description of isotopic scaling, are near g = 25 MeV forperipheral and g<15 MeV for central collisions.