  The QCD gauge field is modeled as an ensemble of statistically independent
selfdual and antiselfdual regions. This model is motivated from instanton
physics. The scale anomaly then allows to relate the topological susceptibility
to the gluon condensate. With the help of Wittens formula for m_eta' and an
estimate of the suppression of the gluon condensate due to light quarks the
mass of the eta' can be related to f_pi and the physical gluon condensate. We
get the quite satisfactory value m_eta'=884+-116 MeV. Using the physical eta'
mass as an input it is in princple possible to get information about the
interaction between instantons and anti-instantons.
