  Isotopic effects in spectator fragmentations following heavy-ion collisions
at relativistic energies are investigated using data from recent exclusive
experiments with SIS beams at GSI. Reactions of 12C on 112,124Sn at incident
energies 300 and 600 MeV per nucleon were studied with the INDRA multidetector
while the fragmentation of stable 124Sn and radioactive 107Sn and 124La
projectiles was studied with the ALADIN spectrometer.
  The global characteristics of the reactions are very similar. This includes
the rise and fall of fragment production and deduced observables as, e.g., the
breakup temperature obtained from double ratios of isotope yields. The mass
distributions depend strongly on the neutron-to-proton ratio of the decaying
system, as expected for a simultaneous statistical breakup. The ratios of
light-isotope yields from neutron-rich and neutron-poor systems follow the law
of isoscaling. The deduced scaling parameters decrease strongly with increasing
centrality to values smaller than 50% of those obtained for the peripheral
event groups. This is not compensated by an equivalent rise of the breakup
temperatures which suggests a reduction of the symmetry term required in a
liquid-drop description of the fragments at freeze-out.
