Full text Author(s): Viktor Jahnke Total Authors: 1 Document type: Master's Dissertation Press: São Paulo. Institution: Universidade de São Paulo (USP). Instituto de Física (IF/SBI) Defense date: 2012-10-22 Examining board members: Vito Roberto Vanin; Johnny Ferraz Dias; Elisabeth Mateus Yoshimura Advisor: Vito Roberto Vanin Abstract The cross sections for the ionization of the K shells of Au and Bi atoms by electrons with energies from 100 keV down to the respective thresholds (80 and 90 keV) were measured. To this end, thin films of Au and Bi, evaporated on very thin C backings, were positioned in a vacuum chamber perpendicularly to the electron beam delivered by the the microtron injector at the Universidade de São Paulo, Brazil. The x-rays emitted at an angle of 120 degrees relative to the beam direction were observed with a planar HPGe detector, whose full-energy peak efficiency curve had been determined using radioactive sources of known activities. For each beam energy, the experimental x-ray production cross section was determined using the area of the characteristic $K\\alpha_{2,1}$ peaks, the area of the spectrum in an energy interval close to the bremsstrahlung tip and the theoretical doubly-differential bremsstrahlung cross section. Tabulated fluorescence yields and x-ray emission rates were employed to convert x-ray production to ionization cross sections. The obtained experimental values for the K shells of the studied atoms are slightly larger than that of recent calculations carried out within the distorted-wave Born approximation. In the case of Au, the few measurements reported in the literature at energies close to the threshold differ by almost a factor of 4 at 100 keV, and our cross sections lie between those older data. On the other hand, the present experiment provides the first cross section measurements for the K shell ionization of Bi below 2 MeV. (AU) FAPESP's process: 10/03159-2 - Electron impact inner-shell atomic ionization cross-section measurement Grantee: Viktor Jahnke Support type: Scholarships in Brazil - Master