  Total cross sections for the quasi-free pn --> pn eta reaction in the range
from the kinematical threshold up to 20 MeV excess energy have been determined.
At threshold they exceed corresponding cross sections for the pp --> pp eta
reaction by a factor of about three in contrast to the factor of six
established for higher excess energies. To large extent, the observed decrease
of the ratio sigma(pn --> pn eta)/sigma(pp --> pp eta) towards threshold may be
assigned to the different energy dependence of the proton-proton and
proton-neutron final state interactions.
  The experiment has been conducted using a proton beam of the cooler
synchrotron COSY and a cluster jet deuteron target. The proton-neutron
reactions were tagged by the spectator proton whose momentum was measured for
each event. Protons and neutron outgoing from the pn --> pn eta reaction have
been registered by means of the COSY-11 facility, an apparatus dedicated for
threshold meson production.
