Cross section for coincident two-photon emission at ±90° in electron-atom collisions

Abstract

The cross section for coincident two-photon emission in the bombardment of thin targets of Al, Cu, Tb, and Au with 70-keV electrons has been measured in the \ifmmode\pm\else\textpm\fi{}90\ifmmode^\circ\else\textdegree\fi{} geometry. The present results for Au are about two orders of magnitude lower than the results of Altman and Quarles [Phys. Rev. A 31, 2744 (1985); Nucl. Instrum. Methods Phys. Res. A 240, 538 (1985)] for 75-keV incident electrons in the same geometry. The present results are generally consistent with the relativistic first Born approximation for double bremsstrahlung. While an explanation for the larger yield observed earlier is still not available, we believe that the increased sensitivity of the present coincidence setup makes the present data more reliable. We have also directly observed a coincident cross-talk effect due to the escape of a Ge K x ray from one detector and its coincident detection by the second detector. This effect leads to a large coincidence yield when an energy window includes the Ge K x ray. The implication of such escape effects in \ifmmode\pm\else\textpm\fi{}90\ifmmode^\circ\else\textdegree\fi{} geometry experiments is discussed.