  The ACIS front-illuminated CCDs onboard the Chandra X-ray Observatory were
damaged in the extreme environment of the Earth's radiation belts, resulting in
enhanced charge transfer inefficiency (CTI). This produces a row dependence in
gain, event grade, and energy resolution. We model the CTI as a function of
input photon energy, including the effects of de-trapping (charge trailing),
shielding within an event (charge in the leading pixels of the 3X3 event island
protect the rest of the island by filling traps), and non-uniform spatial
distribution of traps. This technique cannot fully recover the degraded energy
resolution, but it reduces the position dependence of gain and grade
distributions. By correcting the grade distributions as well as the event
amplitudes, we can improve the instrument's quantum efficiency. We outline our
model for CTI correction and discuss how the corrector can improve
astrophysical results derived from ACIS data.
