Geant4 simulation of the moderating neutrons spectrum

In this paper we consider the problem of predicting a steady-state neutron spectrum in various media with different compositions and geometries. The analytical calculations of such spectrum are often too complex, if at all possible. We describe a method of Geant4-based Monte Carlo calculation of the steady-state neutron spectrum in a medium containing a fixed neutron source. As an interesting and valuable "by-product" of this method, we obtain the snapshots of the neutron spectrum evolution in time.