Abstract The computer simulation code SRIM (stopping and range of ions in matter) developed by Ziegler and coworkers, has been applied previously to study the transport of a-particles within various materials. In particular, the detection of α-particles in silicon or gas detectors has been investigated. In this work, we apply the code to simulate the propagation through a common type of α-particle source used in spectrometry. The source consists of 226Ra activity in secular equilibrium with its daughters distributed within a barium sulfate matrix. We have calculated the number and energy spectrum of those particles that reach the detector from the source, for several source thicknesses and for various detector distances. As these parameters are changed, the resulting energy resolution and the consequent accuracy of derived activities may be derived. Thus the optimum conditions for a-spectrometry can be determined given the accuracy desired.