Distortion of a charge cloud shape inside the CCD produced by an X-ray photon

A charge cloud produced by an X-ray photon in a charge coupled device (CCD) has a finite size. The size can be measured using a mesh experiment: the X-ray interaction position is restricted with subpixel precision by using the small mesh holes. This enables us to measure the charge cloud shape produced by an X-ray photon in the CCD. We measured charge cloud size for Al-K X-rays and for Mo-L X-rays. At both energies, we find the charge cloud shapes to be elongated perpendicular to the charge transfer direction. We confirmed the symmetric shape of the mesh hole both by SEM photography and by an experimental technique, and conclude that the elongation of the charge cloud shape is not an artifact of the experimental setup. We then applied an image restoration technique to directly measure the actual hole shape from the X-ray data. The inferred hole size is bigger than the geometrical shape due to diffraction of the incident X-rays by the mesh holes. We conclude that the distortion of the charge cloud shape is due to the asymmetry of the electric field in the CCD.