The GEANT3 code, a general software for detector simulations widely used in
the experiments of particle physics, is introduced to the Monte Carlo simu
lation of iron emulsion chambers. Some Characteristics of transition curves
of cascade shower and their FWHM (Full Width of Half Maximum) are studied.
The probability of discriminating hadrons from gamma rays using the FWHM d
istributions for the shower events with starting depth Deltat< 6c. u. is di
scussed and an efficient joint criterion of discriminating hadrons from <ga
mma>-rays in iron emulsion chambers is obtained.