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

Detection of two and more γ-rays in coincidence by two NaI(Tl) detectors with almost 4π geometry allows absolute characterization of radionuclides emitting coincidence gammas. The method is a generalization of the Eldridge-Crowther method developed originally for x-rays and low energy γ-rays. This method is applied to the case of (94)Nb decay with two coincident gamma-rays emitted in one cascade. The application of this method for the case of coincident positron-gamma emission ((22)Na and (26)Al sources) meets some difficulties. In these decays, two 511 keV gamma quanta produced in positron annihilation are strongly correlated. Despite the fact that the third gamma emitted in (22)Na and (26)Al decays is not correlated with two annihilation quanta, the number of independent observables for (22)Na and (26)Al decays is less than the number of unknowns. The small parameter ω(00), the probability that both annihilation quanta escape detection in both NaI(Tl) detectors, cannot be determined. However, if this parameter is defined from experimental data for one source with known activity ((22)Na), the activity of the other source ((26)Al) can be calculated from experimental data for (26)Al decay.