The charge cloud shape generated by a Cu-K X-ray photon inside the CCD

An X-ray photon generates a finite size of charge cloud in a charge-coupled device (CCD). It forms various pattern of events, depending on the X-ray interaction position on the CCD. The mesh experiment is the only available method to determine the charge cloud shape, which depends on the incident X-ray energy. We employed a metal mesh of 13μm in thickness with many holes of 2μm diameter periodically spaced of 48μm. The valid energy range depends on the X-ray opacity of the metal. It has been thought to be limited to below 7.0 keV for the gold mesh employed. The new method is introduced to roughly estimate the X-ray interaction position to reduce ‘background’ events so that the energy range can be extended to 8.2 keV. Applying our method to Cu-K X-rays (8.04 keV), we measure the charge cloud size generated by Cu-K X-rays and find it to be bigger than that expected from a diffusion model in the depletion region of the CCD. This is due to the fact that the CCD employed has a depletion region of about 40μm that is much shorter than the attenuation length of Cu-K X-rays in Si.