X-ray Measurement of the subpixel structure of the XMM EPIC MOS CCD

We report here the results of a mesh experiment to measure the subpixel structure of the EPIC MOS CCDs on board the XMM X-ray observatory. The pixel size is 40$\mu$m square while the mesh hole spacing is 48$\mu$m, a combination quite different from our standard mesh experiment. We have verified that this combination functions properly and have analyzed the CCD structure with sub-pixel resolution. The EPIC MOS CCD has an open electrode structure to improve detection efficiency at low energies. We obtained the distribution of various grades of X-ray events inside the pixel. A horizontally split two-pixel event is generated near the channel stop which forms a straight vertical pixel boundary whereas a vertically split two-pixel event is generated where the potential due to the thinned gate structure forms a wavy horizontal pixel boundary. Therefore, the effective pixel shape is not a square but is distorted. The distribution of X-ray events clearly shows that the two etched regions in each pixel, separated by the bridging finger of the enlarged (open) electrode. We measured the difference in X-ray transmission between the conventional and open regions of the pixel using O-K and Cu-L X-ray emission lines, and found it to be consistent with an electrode thickness comprising $0.2\pm0.1\mu$m of Si and $0.6\pm0.2\mu$m of SiO2.