Isotopic Scaling and the Symmetry Energy in Spectator Fragmentation

Isotopic effects in the fragmentation of excited target residues following collisions of ${}^{12}$C on ${}^{112,124}$Sn at incident energies of 300 and 600 MeV per nucleon were studied with the INDRA 4$\pi$ detector. The measured yield ratios for light particles and fragments with atomic number $Z\le$ 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 $\gamma =$ 25 MeV for peripheral and $\gamma <$ 15 MeV for central collisions.