Calculation of the average solid angle subtended by a photon-emitting source at a collimated detector and the contribution of collimator edge penetration

A Monte Carlo aided method of calculating the average solid angle subtended by a collimated detector from surface and volumetric sources is presented which incorporates the contribution of the collimator edge penetration effect. The validity of the method which utilizes total variance reduction is demonstrated using point reference sources. The results show that the collimator edge penetration effect could be highly significant when measuring photons with energies greater than a few hundred keV and that the method outlined is capable of quantifying the contribution to the calculated average solid angle. Case studies of the possible applications of the method to the field of quantitative photon measurement are presented and discussed.