Does an X-Ray Photon Really Generate a Charge Cloud with an Asymmetric Shape?

Abstract

An X-ray photon produces a finite-sized primary charge cloud inside a charge-coupled device (CCD). Its shape is determined to be asymmetric using a mesh experiment. It is elongated in the direction perpendicular to the charge transfer direction. We checked whether or not the asymmetry of the charge cloud shape was due to the experimental setup. The geometrical shape of the mesh holes is confirmed to be symmetric by a Scanning Electron Microscope (SEM). A new method is introduced to quantitatively measure the effective size of the mesh hole for the X-rays. The geometrical diameter of the mesh hole is 3.4 µm while the effective diameter of the mesh hole is 3.9± 0.3 µm for Al-K X-ray photons (1.5 keV) and 4.0±0.3 µm for Mo-L X-ray photons (2.3 keV), respectively. These values are consistent with that taking into account the diffraction of X-rays. We conclude that the asymmetry of the primary charge cloud generated inside the CCD is due not to the experimental setup but to the asymmetry of the electric field inside the CCD.