Measurement of the charge cloud shape produced by an X-ray photon inside the CCD using a mesh experiment

We present here the direct measurement of the charge cloud shape in the charge-coupled device (CCD) produced by an X-ray photon using a mesh technique. This technique makes use of a parallel X-ray beam and a metal mesh placed just above the CCD. The CCD we used has a 12 μm square pixel while the mesh has many circular holes with 3.4 μm in diameter spaced at 4 times that of the CCD pixel size (multi pitch mesh). We employed the Al-K X-rays. By using this technique, we could unambiguously identify the interaction position of X-rays. When the X-ray enters near the pixel boundary, the charge cloud splits into two or more pixels. We obtain how much charge is collected in one pixel according to the interaction position of Al-K X-rays. By analyzing the split events for various part of the pixel, we can reconstruct the charge cloud shape in detail. The charge cloud shape we obtained is well expressed with an asymmetric Gaussian function (the horizontal width is 1.7 μm and the vertical width is 0.9 μm). The asymmetry comes from the electric field in the CCD.