Where are the x-ray event grades formed inside the pixel of the charge-coupled device? The behavior of the primary charge cloud inside the charge-coupled device

We report the use of an improved technique to measure the X-ray detection efficiency of a charge-coupled device (CCD) with a subpixel resolution. This technique makes use of a parallel X-ray beam and metal mesh placed closely on the CCD. The mesh has many circular holes spaced at distance of 4 times the CCD pixel size (multi-pitch mesh). We could identify the interaction position of X-rays both for single events and for split pixel events. By using this method, we demonstrated how various types of X-ray events are formed inside the CCD. We have already shown that the two-pixel split events were formed near the pixel boundary. We clearly showed that the three-pixel events were formed when the interaction position was close to the pixel corner,whereas the four-pixel split events were formed when the interaction position was much closer to the pixel corner. We found that the center of gravity of split events could represent the interaction position with an uncertainty of 0.13 pixel size.