Sub-pixel Spatial Resolution Using Backside Illuminated X-ray CCDs

X-ray astronomy CCDs are conventionally operated in short integration modes to allow them simultaneously to detect photons with high efficiency and measure their energy at moderately high resolution. The drawback of operating in this single photon counting mode is that ordinarily information about where, within the pixel, that the X-ray struck is lost. The latest state-of-the-art CCDs being produced for both AXAF and XMM use backside illumination to increase still further the low energy quantum efficiency. We point out that these CCDs also offer the opportunity to reconstruct (down to at least 0.1 pixel) the location of the X-ray interaction in the CCD. This opportunity results from the substantial charge spreading which occurs while the X-ray induced electron cloud traverses the CCD. This results in most interactions producing charge in multiple pixels. By classifying the structure of charge detected over a 3x3 neighborhood, we show (via Monte Carlo simulations) that the original location point can be reconstructed with a mean accuracy of better than 0.1 pixel in both directions. The implications of this technique are very important for the AXAF ACIS experiment. The AXAF HRMA mirrors may have central core resolution as high as 0.2 arcsec, which would be undersampled by the 0.5 arcsec ACIS CCD pixels. By applying this technique, we expect to eliminate completely CCD pixel effects from degrading the HRMA performance.