\section{Reaction initial state.}

\hspace{1.0em} The GEANT4 $\gamma$-nucleon and $\gamma$-nucleus 
interaction model is capable to predict 
 final states (produced 
hadrons 
which belong to the pseudoscalar meson nonet and the baryon (antibaryon) 
octet) of reactions on nucleon and nuclear targets. The break-up simulation of 
the residual 
excited target nucleus is performed with help of the nucleus deexcitation
models. 
 The recommended bombarding energy in 
the photon--nucleon or 
  photon--nucleus interaction has to be more than 
 20 MeV in the laboratory frame.
 
