An algorithm for the solid angle calculation applied in alpha-particle counting

An algorithm is presented to calculate the solid angle for a flat source facing an axially symmetric detector set-up with one or more coaxial diaphragms. The calculation is based on numerical integration and its validity was checked by comparing with the results of a Monte Carlo program. Formulas are given for point sources as well as circular and annular sources. The latter is applied in a method to take into account the possible inhomogeneity, non-circular shape, and/or eccentricity of the radioactive source with respect to the symmetry axis of the detector.