When neutron activation analysis of voluminous samples is performed using the absolute method, the detector absolute efficiency for γ-ray emitting distributed sources must be known. In this study, a Monte-Carlo program was developed to include the calculation of the effective solid angle subtended by a collimated detector from irregularly shaped voluminous samples. The program can cope with different sample shapes and geometries provided that the sample covers the view of the detector. Data suh as the source and detector dimensions, the source-detector distance, the detector view at a certain distance, the thickness and the composition of any intervening materials, the γ-ray energies of interest and the corresponding attenuation coefficients for each material are required. The method adopted for calculting the detector absolute efficiency of the voluminous sample in a certain geometry takes into account the effect of γ-rays being emitted from different positions within the sample and also considers their attenuation in the sample material as well as any intervening materials between the sample and the detector and is compared with a reference point source. The calculations were verified experimentally using a distributed source of 75 mm diameter and 100 mm thickness and two semiconductor detectors. The difference between the calculated and measured absolute efficiencies did not exceed 4%.