The analysis of experimental statistics, related to perfect CsI(Tl) crystals without impurities of oxygen radicals, shows that irradiation-induced variations in scintillation light output are mainly originated by the production of lattice defects capable of trapping electronic excitations. Below the nature of such irradiation-produced defects considered. It is shown that in the dose range of the order of 10 krad, where the known mechanisms of irradiation- induced changes are inefficient, the latter are originated by small pores growing under irradiation. Micropores affect scintillation yield even at low concentrations (approximately 10$+$MIN@7$/) due to a large section of exciton trapping exceeding the geometric section. The proposed model of growing micropores permits one to explain the observed does dependences of conversion efficiency and to use them for the control over the concentration of incipient micropores - a structural characteristic important for radiation stability.
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