The global results of the activation process are genrally dependent of the activated material, type of nuclear reaction, energy of incident particles, etc. by means of some specific parameters (crouss section, flux denisty, etc.). To characterize the above mentioned process, the energy behavior study of the radiation fiels generated by extended sources can be useful. In this paper, the energy spread in nondispersive media of some exoergic nuclear reactions (i.e., d-D, d-T reactions at low energy excitation, levels, etc.) lodged in axi-symmetrically emitting disk-shaped sources, used as radiation sources in activation anlysis, have been investigated analytically. The contribution of the first tems of an angular depending series approximation is underlined. Computationally tractable expressions, in tems of elementary and elliptic integral type functions, are proposed.