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C. E. Grant, B. LaMarr, M. W. Bautz (MIT)
Soon after launch, the Advanced CCD Imaging Spectrometer (ACIS), one of the focal plane instruments on the Chandra X-ray Observatory, suffered radiation damage from exposure to soft protons during passages through the Earth's radiation belts. Since mid-September 1999, ACIS has been protected during radiation belt passages and there is a continuing effort to prevent any further damage particularly from coronal mass ejections or other geomagnetic activity. Monitoring the charge-transfer inefficiency (CTI) of the CCDs using observations of the ACIS external calibration source can be an effective tool to detect any change in performance, however this measure and the instrument performance are also dependent on the total amount of charge deposited onto the CCD, which is dominated by cosmic rays. An increasing rate of cosmic ray hits will reduce the number of electron trap vacancies and thus decrease the measured CTI. We report on efforts by the ACIS Instrument team to understand and quantify the affect of the cosmic ray background on the measured CTI of the ACIS CCDs. We also report on attempts to remove the variation due to the background component from ACIS CTI measurements in order to monitor the health of the instrument.
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