It has been shown that two main effects contribute to the tailing in the shapes of α-peaks measured with silicon detectors: the ionization loss in the source and the response of the detector. We studied the relative influence of both factors in measurements with small source-to-detector distances. The computer simulation code ‘the stopping and range of ions in matter’ has been used to obtain the energy spectra of a 226Ra source as a function of the source thickness. Results of the simulation are compared with experimental measurements.