  Isotopic effects in projectile fragmentation at relativistic energies have
been studied with the ALADIN forward spectrometer at SIS. Stable and
radioactive Sn and La beams with an incident energy of 600 MeV per nucleon have
been used in order to explore a wide range of isotopic compositions. Chemical
freeze-out temperatures are found to be nearly invariant with respect to the
A/Z ratio of the produced spectator sources, consistent with predictions for
expanded systems. Consequences for the proposed interpretation of chemical
breakup temperatures as representing the limiting temperatures predicted by
microscopic models are discussed.

Nuclear Experiment