Ionization cross-sections (experimental) for protons and α-particles

The observation that characteristic X-radiation can be produced by α-particle bombardment seems to have been first made and reported by Rutherford, Chadwick and others during the years immediately following the discovery of characteristic radiation. Since that time, the subject has been re-examined at intervals. In 1928, Bothe and Franz reported a series of measurements in which various elements were irradiated with α-particles from radioactive decay. Cork (1941) carried out a series of measurements of characteristic radiation produced by deuteron bombardment, using a maximum deuteron energy of 10 MeV. He observed that K X-rays were detectable up to an atomic number of 38. Above this value, no K X-rays were observable, but between atomic numbers of 52 and 78 the L X-rays could be detected. Evidently the L electrons can be ejected from the L shell in circumstances where the K electrons are too strongly bound to be ejected by deuterons of this energy.

For detailed measurements we must wait until the 1950s, when the increasing availability of small accelerators opened up the possibility of experimental work on an absolute basis over a wide range of particle energies. From investigations for example by Messelt (1958) and by Khan and Potter (1964) it became clear that the X-ray yield increased rapidly with proton energy.