Theory of quasi-monochromatic γ lines produced in nuclear reactions due to the virtual processes

A mechanism is proposed and explicit expressions are presented for the amplitude of (γ, xγ') reactions on light nuclei with emission of a quasi-monochromatic γ line, which is not generated by transitions between discrete states of the nuclei, but is the γ radiation from one of virtual processes involved in the mechanism of the (γ, xγ') reaction. A particular case is calculated where such a γ line has an energy of 2.22 MeV, a width of 15 keV and is observed in the (γ, γ') reactions on nuclkei 7Li, 9Be, 14N. The existence of the line is an evidence that the radiative capture of virtual neutron and proton is an element of the (γ, γ')-reaction mechanism