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We have implemented a Monte Carlo algorithm to model and predict the response of various kinds of CCDs to X-ray photons and minimally-ionizing particles and have applied this model to the CCDs in the Chandra X-ray Observatory's Advanced CCD Imaging Spectrometer. This algorithm draws on empirical results and predicts the response of all basic types of X-ray CCD devices. It relies on new solutions of the diffusion equation, including recombination, to predict the radial charge cloud distribution in field-free regions of CCDs. By adjusting the size of the charge clouds, we can reproduce the event grade distribution seen in calibration data. Using a model of the channel stops developed here and an insightful treatment of the insulating layer under the gate structure developed at MIT, we are able to reproduce all notable features in ACIS calibration spectra.The simulator is used to reproduce ground and flight calibration data from ACIS, thus confirming its fidelity. It can then be used for a variety of calibration tasks, such as generating spectral response matrices for spectral fitting of astrophysical sources, quantum efficiency estimation, and modeling of photon pile-up.

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The flight AXAF CCD Imaging Spectrometer (ACTS) and the High-Resolution Mirror Assembly (HRMA) telescope were extensively calibrated at NASA MSFC's X-Ray Calibration Facility (XRCF). We present results based on HRMA/ACIS characteristics that were obtained with the following tests:(a) Point-Spread-Function (PSF) tests measuring the core and wings of the PSF on-axis and at several off-axis positions, at the point of ideal focus.(b) Effective-Area (EA) tests measuring the total effective collecting area over the PSF at many energies. We investigate the dependence of the effective area and energy response of ACTS/HRMA as a function of several ACTS parameters.(c) Count-rate-linearity (pile-up) tests measuring the effect of increasing the mean incident rate of photons per pixel on the PSF, the EA, source centroid, and photon detection.The goal of the calibration is to provide accurate estimates of the in-orbit performance of the ACIS/HRMA instrument and to enable translation of in-orbit measurements to absolute values of the incident x-ray flux and physical models of the source emission.

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