  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. The primary effect of the damage was to increase the charge
transfer inefficiency (CTI) of the eight front illuminated CCDs by more than
two orders of magnitude. The ACIS instrument team is continuing to study the
properties of the damage with an emphasis on developing techniques to mitigate
CTI and spectral resolution degradation. We will discuss the characteristics of
the damage, the detector and the particle background and how they conspire to
degrade the instrument performance. We have developed a model for ACIS CTI
which can be used to correct each event and regain some of the lost
performance. The correction uses a map of the electron trap distribution, a
parameterization of the energy dependent charge loss and the fraction of the
lost charge re-emitted into the trailing pixel to correct the pixels in the
event island. This model has been implemented in the standard Chandra data
processing pipeline. Some of the correction algorithm was inspired by the
earlier work on ACIS CTI correction by Townsley et al. (2000; 2002). The
details of the CTI model and how each parameter improves performance will be
discussed, as well as the limitations and the possibilities for future
improvement.
