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The rate of change for the two instruments is quite different. The front-illuminated device XIS3, even with charge injection turned on, has a centroid decrease that is roughly 5 times larger than ACIS-I3, while the back-illuminated device XIS1, with charge injection, has a centroid decrease that is 9 times larger than ACIS-S3. This difference may be due to the much warmer temperatures on XIS (90C versus -120C), or to the different radiation environment encounter in low and high-Earth orbit.
In addition, the XIS data appear much smoother than the ACIS data. This is due to the much higher and more variable particle background found in high-Earth orbit. These particles act as the primary source of sacrificial charge which can reduce the effective CTI. Due to the details of the charge trap time constants on ACIS (Grant et al. 2004? need to check this), the front-illuminated ACIS-I3 is more susceptible to sacrificial charge than ACIS-S3.
The FWHM of the Mn-Kalpha line as a function of time. Again, the two instruments behave differently. Due to the early damage from passage through the Earth's radiation belts, ACIS-I3 starts with a much higher level of CTI and larger line FWHM. The smaller initial XIS1 FWHM, as compared for ACIS-S3, is due to improvements in manufacturing back-illuminated devices. The XIS FWHM increase with time, while the ACIS FWHM are almost unchanging. Utilizing charge injection reduces the FWHM and the rate of FWHM increase.
The FWHM of a Mn-Kalpha line as a function of the line centroid. While CTI and FWHM should go hand-in-hand, the relationship is clearly complicated, possibly device- and environment-specific.
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