Isotopic effects in the fragmentation of excited target residues following collisions of ${}^{12}\mathrm{C}$ on ${}^{112,124}\mathrm{Sn}$ 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 $\gamma =25\mathrm{MeV}$ for peripheral and $\gamma <15\mathrm{MeV}$ for central collisions.

• Received 20 September 2004

DOI:https://doi.org/10.1103/PhysRevLett.94.162701