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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.
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