Feasibility study of activity measurement of positron emitters based on gamma-gamma coincident detection by two NaI(Tl) detectors

► Model for detection of two annihilation quanta is proposed. ► Two NaI(Tl) detectors with almost 4π geometry was used for coincidence γ detection. ► Absolute activity measurement of 94Nb source with coincidence emission of two γ in cascade is performed. ► Activity measurements of 22Na and of 26Al sources with positron-γ coincidence emission are performed.