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 at incident energies of 300 and 600 MeV per nucleon were studied with the INDRA 4 detector. The measured yield ratios for light particles and fragments with atomic number Z 5 obey the exponential law of isotopic scaling. The deduced scaling parameters decrease strongly with increasing centrality to values 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 25 MeV for peripheral and <15 MeV for central collisions