K-SHELL IONIZATION CROSS-SECTIONS OF AN, PB AND BI INDUCED BY LOW-ENERGY PROTON IMPACT

K X-ray production cross sections induced by proton impact were measur ed on thick targets of Au, Pb and Bi in the energy range 0.5 < E(0) < 1.7 MeV. The yields of characteristic X-rays were normalised to the yi elds of backscattered protons. For proton energies E(0) > 0.8 MeV, the deduced K shell ionization cross sections are uncertain by 12%, these uncertainties being smaller than those of the data published before. Our experimental results are compared with the semiclassical cross sec tions calculated within the united atom binding procedure. Experimenta l values agree well with theoretical ones, their difference being gene rally smaller than our experimental errors. The ionization cross secti ons measured for E(0) < 0.8 MeV are too large to be explained by proto n induced direct ionization. We showed that for thick targets and prot on energies smaller than E(0) < 0.8 MeV, the K-shell ionization is pro duced notably by fluorescence on account of bremsstrahlung photons emi tted in the collisions between protons and target nuclei.