Modeling contamination migration on the Chandra X-ray Observatory

During its first 6 years of operation, the cold (-60°C) optical blocking filter of the Advanced CCD Imaging Spectrometer (ACIS), on board the Chandra X-ray Observatory, has accumulated a contaminating layer that attenuates the low-energy x rays. To assist in assessing the likelihood of successfully baking off the contaminant, members of the Chandra team developed contamination-migration simulation software. The simulation follows deposition onto and (temperature-dependent) vaporization from surfaces comprising a geometric model of the Observatory. A separate thermal analysis, augmented by on-board temperature monitoring, provides temperatures for each surface of a similar geometric model. This paper describes the physical basis for the simulations, the methodologies, and the predicted migration of the contaminant for various bake-out scenarios and assumptions.