  The neutron emission in the fragmentation of stable and radioactive Sn and La
projectiles of 600 MeV per nucleon has been studied with the Large Neutron
Detector LAND coupled to the ALADIN forward spectrometer at SIS. A
cluster-recognition algorithm is used to identify individual particles within
the hit distributions registered with LAND. The obtained momentum distributions
are extrapolated over the full phase space occupied by the neutrons from the
projectile-spectator source. The mean multiplicities of spectator neutrons
reach values of up to 12 and depend strongly on the isotopic composition of the
projectile. An effective source temperature of T approx. 3 - 4 MeV is deduced
from the transverse momentum distributions.
  For the interpretation of the data, calculations with the Statistical
Multifragmentation Model for a properly chosen ensemble of excited sources were
performed. The possible modification of the liquid-drop parameters of the
fragment description in the hot environment is studied, and a significant
reduction of the symmetry-term coefficient is found necessary to simultaneously
reproduce the neutron multiplicities and the mean neutron-to-proton ratios
<N>/Z of Z <= 10 fragments. Because of the similarity of the freeze-out
conditions with those encountered in supernova scenarios, this is of
astrophysical interest.
