Measurement of Cross Sections for X-Ray Production by High-Energy Electrons

Using high-resolution semiconductor detectors, a study was made of the large K-ionization cross sections of atoms over a wide range of atomic numbers (Z=29-83) and of incident electron energies (E=150-900 MeV). For the higher-Z elements ($Z>60$), the L x-ray cross sections were also measured over the incident electron energy range. Ratios of the relative intensities of various distinct lines were determined and compared to recent low-energy work. The results of the K-ionization experiment are compared to the recent relativistic calculations of Kolbenstvedt. The experimental K-ionization cross sections for each incident electron energy appear to fit a power-law dependence on the atomic number. The Kolbenstvedt theory also exhibits a power-law dependence and is in good agreement with this experiment. All differential x-ray cross sections were found to be isotropic. The L x-ray cross sections measured for four high-Z elements increase with atomic number and incident electron energy. The LβLα, LγLα, and K/L intensity ratios are also given. A search for nuclear γ rays produced by high-energy electron scattering was also undertaken in this work. No nuclear γ-ray lines were detected. Recent theoretical calculations show that the expected cross sections are below the minimum observable levels in these experiments. However, a 511-keV γ line was observed. This line was associated with positron annihilation and was apparently produced in the target.