Absolute measurement of activity of a volumetric object by collimated detectors: Solid angle issues

A Monte Carlo method has been developed to determine that part of a radioactive volumetric object, which is common to the fields of view of collimated detectors combined for the absolute measurement of the activity of the object. Furthermore, the method evaluates the solid angle subtended by the detectors at this part of the object and allows an optimised positioning of the detectors in order to achieve a maximum possible solid angle. Photon self-absorption within the object is demonstrated to be an important parameter which, depending on the photon energy, geometry and the material of the object, may affect the size of the volume of intersection and hence the solid angle.