Subpixel spatial resolution of the x-ray CCD based on the charge cloud shape

When an X-ray photon is photoabsorbed in the CCD, it generates a primary charge cloud expanding through the diffusion. A mesh experiment for the X-ray CCD enables us to specify the interaction position of the X-ray photon with subpixel resolution. Furthermore, we can directly measure the charge cloud shape that can be well expressed by Gaussian profile. When an X-ray photon enters the pixel (event pixel), the primary charge is mainly collected into the event pixel. When the X-ray landing position is close enough to the pixel boundary, the primary charge spills over the adjacent pixel forming split events. The X-ray event is sorted by the event pattern, how many pixels does the primary charge split, resulting various grades of the events. We can easily understand that there are three parameters coupled together: the X-ray landing position inside the pixel, the X-ray event pattern and the primary charge cloud shape. We can determine any one of them from the other two parameters. Since we know the charge cloud shape using the mesh experiment, we can calculate the X-ray landing position inside the event pixel using the grade of the event. We applied our method to the Ti- K X-rays for the CCD with 12 micrometer square pixel. Once the primary charge splits into adjacent pixel, we can determine the X-ray landing position with subpixel resolution. Using the three- or four-pixel split event, we obtained the accuracy of the X-ray landing position about 1 micrometer. For two-pixel split event, we obtained the similar position accuracy in the split direction while no improvement perpendicular to it. We will discuss what type of the CCD will be able to achieve the subpixel resolution for all X-ray photons.