The aim of this paper is to contribute to analytical characterization of the spatial dependency of the mean energy of fast neutrons produced in non-dispersive media by sealed-off neutron generating tubes (NGT) having axial symmetry. Neutrons are generated in most of NGT by accelerating a mixture of hydrogen isotope ions (deuterons and tritons) onto plane self-targets. Deuterons and tritons are implanted in the mentioned targets and become target nuclei for the subsequent incoming ions. d-T, t-D and d-D fast neutrons are generated. Analytical expressions for the neutron mean energy in these three cases are deduced, taking into account the exothermic character of the reactions. Due consideration is paid to collision kinetics, depending upon mass and kinetic energy of particles involved in the nuclear reaction, nuclear reaction energy, etc.