Abstract
The problem of penetration of γ-rays through wide aperture cylindrical collimators, placed close to a body organ for radioisotope uptake studies, is examined. Values for the penetration factors are numerically computed for axial point sources and compared with those given by the radius-independent formulae, normally used, to establish criteria for the use of these formulae. Penetration factors are also measured experimentally, and within the limitations of the experimental method there is good agreement with the computed values, even for sources close to wide aperture collimators.