Tailoring solid angle calculations to the actual radioactivity distribution of planar sources

In the frame of accurate radioactivity measurements by means of alpha-particle counting at defined low solid angles, a procedure is presented that takes into account the sourceʹs actual radioactivity distribution in the solid angle calculation. The method pertains to an axially symmetric set-up in which a thin flat source is placed parallel to the diaphragm(s) and the detector. From a digital autoradiograph, the sourceʹs radial activity distribution is assessed. The source is virtually subdivided into concentric rings and the corresponding solid angle values are weighted with the relative contribution of the annular regions-of-interest to the total radioactivity of the source.