  The X-ray telescope on board the Swift satellite for gamma-ray burst
astronomy has been exposed to the radiation of the space environment since
launch in November 2004. Radiation causes damage to the detector, with the
generation of dark current and charge trapping sites that result in the
degradation of the spectral resolution and an increase of the instrumental
background. The Swift team has a dedicated calibration program with the goal of
recovering a significant proportion of the lost spectroscopic performance.
Calibration observations of supernova remnants with strong emission lines are
analysed to map the detector charge traps and to derive position-dependent
corrections to the measured photon energies. We have achieved a substantial
recovery in the XRT resolution by implementing these corrections in an updated
version of the Swift XRT gain file and in corresponding improvements to the
Swift XRT HEAsoft software. We provide illustrations of the impact of the
enhanced energy resolution, and show that we have recovered most of the
spectral resolution lost since launch.
