  The neutron emission in projectile fragmentation at relativistic energies was
studied with the Large-Area-Neutron-Detector LAND coupled to the ALADIN forward
spectrometer at the GSI Schwerionen-Synchrotron (SIS). Stable 124Sn and
radioactive 107Sn and 124La beams with an incident energy of 600 MeV/nucleon
were used to explore the N/Z dependence of the identified neutron source. A
cluster-recognition algorithm is applied for identifying 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 about 11 and depend strongly on the
isotopic composition of the projectile. An effective source temperature of T
\approx 2-5 MeV, monotonically increasing with decreasing impact parameter, is
deduced from the transverse momentum distributions. For the interpretation of
the data, calculations with the statistical multifragmentation model were
performed. The variety of excited projectile spectators assumed to decay
statistically is represented by an ensemble of excited sources with parameters
determined previously from the fragment production observed in the same
experiments. The obtained agreement is very satisfactory for more peripheral
collisions where, according to the model, neutrons are mainly emitted during
the secondary decays of excited fragments. The neutron multiplicity in more
central collisions is underestimated, indicating that other sources besides the
modeled statistical breakup contribute to the observed neutron yield. The
choice made for the symmetry-term coefficient of the liquid-drop description of
produced fragments has a weak effect on the predicted neutron multiplicities.
