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The authors report here experimental results relating X-ray interaction location and event splitting. The X-ray interaction location can be localized at subpixel scale using the mesh technique. The authors found that the center of gravity of the split event is well-correlated with the X-ray interaction location. They analyzed the data using two models for the charge cloud shape: one is the rectangular model and the other is the Gaussian model. Although the authors could not distinguish between these models, they measured a root mean square charge cloud size of 1 to 2 [[micro]meter] for X rays of Y-L (1.9 keV), Ag-L (3.0 keV), and Ti-K (4.5 keV). When the X rays enter near the pixel boundary, the charge splits into adjacent pixels, allowing determination of the X-ray interaction location with an accuracy of 1.5 to 2.2 [[micro]meter]. The authors, therefore, expect that the X-ray CCD can function as an X-ray imager with subpixel resolution, which will be especially useful in applications involving very high spatial resolution optics. Index Terms - Charge-coupled-device, charge cloud shape, mesh experiment, subpixel resolution.

We present here a measurement at sub-pixel resolution of the X-ray detection efficiency of the CCD detector on board the ASCA satellite. A mesh placed in front of the CCD restricted the interaction position of the X-ray inside the CCD pixel. We clearly show that all the primary charge is collected in one pixel when the X-ray interaction is sufficiently far from the pixel boundary. Conversely, some photo-ionized charge is collected in neighboring pixels when the X-ray interacts near the pixel boundary. We also obtained the thickness of various elements gate structure, and measure channel stop dimensions. From these data, we can obtain more accurate estimates of the X-ray responsivity of the CCD.

A charge cloud produced by an X-ray photon in a charge coupled device (CCD) has a finite size. The size can be measured using a mesh experiment: the X-ray interaction position is restricted with subpixel precision by using the small mesh holes. This enables us to measure the charge cloud shape produced by an X-ray photon in the CCD. We measured charge cloud size for A1-K X-rays and for Mo-L X-rays. At both energies, we find the charge cloud shapes to be elongated perpendicular to the charge transfer direction. We confirmed the symmetric shape of the mesh hole both by SEM photography and by an experimental technique, and conclude that the elongation of the charge cloud shape is not an artifact of the experimental setup. We then applied an image restoration technique to directly measure the actual hole shape from the X-ray data. The inferred hole size is bigger than the geometrical shape due to diffraction of the incident X-rays by the mesh holes. We conclude that the distortion of the charge cloud shape is due to the asymmetry of the electric field in the CCD.