The interaction of α particles with low atomic number elements has been studied in an attempt to determine the inner‐shell ionization cross section. These cross sections were determined by measuring the characteristic x rays that were produced when a thin aluminum target was bombarded by α particles from a 48‐MBq224Cm α source. The x rays were detected with a Si(Li) x‐ray detector whose resolution was 190 eV at 5.9 keV. The efficiency of the detector was determined from standard radioactive sources. The energies of the α particles were varied by placing thin Mylar absorbers between the α source and the target. The results are favorably compared to a current ionization theory given in such a form that an undergraduate physics major can do the calculations. This experiment is intended as an undergraduate laboratory experiment in the atomic or nuclear area.