The energy distribution of the alpha particles emitted from a source is in general complex. Only under particular circumstances, as in the case of very thin sources measured at large distances from the detector, can the energy distribution be approximated theoretically. In this work, we used the well-known code SRIM to simulate the interaction of alpha particles within a thin radioactive source and within the entrance window of a typical Si semiconductor detector. We considered several thin alpha particle sources measured at a large source-to-detector distance (small solid angle), in order to compare the distributions obtained by simulation with those determined by the theoretical model applied to this case. The study was performed for a variety of realistic alpha particle sources: UF4, UO2, U3O8, Gd2O3, and BaSO4, considering as alpha emitters 235U, 233U, 148Gd and 226Ra. For all these cases, we obtained the distributions due to the source and due to the entrance window of the detector, and also the final distribution given by the convolution of these two distributions. All the energy distributions obtained by simulation showed, in general, good agreement with the predictions given by the theoretical model, which includes corrections for Bohr straggling.