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 str ucture of the EPIC MOS CCDs on board the XMM X-pay observatory. The pixel s ize 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 t his 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 tw o-pixel event is generated near the channel stop which forms a straight ver tical pixel boundary whereas a vertically split two-pixel event is generate d where the potential due to the thinned gate structure forms a wavy horizo ntal pixel boundary. Therefore, the effective pixel shape is not a square b ut is distorted. The distribution of X-ray events clearly shows that the tw o etched regions in each pixel, separated by the bridging finger of the enl arged (open) electrode. We measured the difference in X-ray transmission be tween the conventional and open regions of the pixel using O-K and Cu-L X-r ay emission lines, and found it to be consistent with an electrode thicknes s comprising 0.2 +/- 0.1 mu m of Si and 0.6 +/- 0.2 mu m of SiO2. (C) 1999 Elsevier Science B.V. All rights reserved.