During the interference Correction of the 226Ra 186.2 keV line by the 235U 185.7 keV some authors (F. De Corte et al, 2005) use the formula as given in attachment:
Where, Ig, is the gamma-ray intensities,
εp, is the peak detection efficiencies,
COI, is the correction factors for true coincidence summing
Coincidence summing is called for when your detector is not able to tell two separate gamma-emissions (in quick succession) apart from each other, thus revealing one (apparent) larger energy that is the sum of the two above-mentioned emissions. This phenomenon can be either 'true' (depending on decay scheme, since the source emits a cascade of gamma rays) or 'random' (when two different nuclei emit radiations that are closer in time than detector response time).
Coincidence summing is usually minimized by the implementation of: 1) fast-response detection; 2) large separation between source and detector (if compatible with activity level); 3) sequences of single sources rather than multi-emitters; 4) Compton-suppression systems (anti-coincidence detector ensembles surrounding main detector).
Coincidence summing can be mathematically accounted for, but slightly-more-than-trivial geometries usually require Monte Carlo approaches.
Take a look here:
http://ita.arpalombardia.it/ita/console/files/download/72/05-11nov_Sima_Coincidence_summing_theory.pdf
It's not free. May I have a copy of this article Mohamed Fayez-Hassan
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