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

Abstract

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 ($Z=29\ensuremath{-}83$) and of incident electron energies ($E=150\ensuremath{-}900$ 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 $\frac{L\ensuremath{\beta}}{L\ensuremath{\alpha}}$, $\frac{L\ensuremath{\gamma}}{L\ensuremath{\alpha}}$, and $\frac{K}{L}$ intensity ratios are also given. A search for nuclear $\ensuremath{\gamma}$ rays produced by high-energy electron scattering was also undertaken in this work. No nuclear $\ensuremath{\gamma}$-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 $\ensuremath{\gamma}$ line was observed. This line was associated with positron annihilation and was apparently produced in the target.