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T. Dotani, A. Yamashita (ISAS), K. Mukai, T. Yaqoob (GSFC), ASCA SIS Team
Performance of SIS on board the ASCA satellite is gradually degrading due to the accumulation of the radiatin damage. Radiation damage has two direct effects on the CCD: one is to increase the dark current and the other is to decrease of the charge transfer efficiency (CTE). We have studied characteristics of these two effects and constructed empirical models for these two effects. We have also studied how these two effects affect the X-ray data from SIS, and found that the decrease of CTE (1) decreases the gain, (2) degrades the energy resolution, and (3) reduces the quantum detection efficiency at higher/lower energy range. Similarly, increase of the dark current has been found to (1) reduce the quantum detection efficiency, (2) degrade the energy resolution, and (3) change the line profile. Furthermore, these performance changes may be a function of exposure time and position on the chip. We report the current status of SIS calibration.