Mitigating CCD radiation damage with charge injection: first flight results from Suzaku

The CCD detectors in the X-ray Imaging Spectrometers (XIS) aboard Suzaku have been equipped with a precision charge injection capability. The purposes of this capability are to measure and reduce the detector degradation caused by charged particle radiation encountered on-orbit. Here we report the first results from routine operation of the XIS charge injection function. After 12 months' exposure of the XIS to the on-orbit charged particle environment, charge injection already provided measurable improvements in detector performance: the observed width of the 5.9 keV line from the onboard calibration source was reduced from 205 eV to less than 145 eV. The rate of degradation is also significantly smaller with charge injection, so its benefit will increase as the mission progresses. Measured at 5.9 keV, the radiation-induced rate of gain degradation is reduced by a factor of 4.3 ± 0.1 in the front-illuminated sensors when injecting charge greater than 6 keV equivalent per pixel. The corresponding rate of degradation in spectral resolution is reduced by a factor 6.5 ± 0.3. Injection of a smaller quantity of injected charge in the back-illuminated XIS sensor produces commensurately smaller improvement factors. Excellent uniformity of the injected charge pattern is essential to the effectiveness of charge injection in the XIS.