2.3.9 Other

CCDs are susceptible to radiation damage. The most critical type of damage is primarily introduced by low energy protons causing lattice vacancies, which trap electrons which reduces the charge transfer efficiency of the CCD, and may produce other artifacts such as flickering and hot pixels.

The CCDs have been specially treated to implant a special `trough' to reduce the effect of this damage, but this cannot remove it entirely.

The CCD camera includes material specifically designed to act as a shield to reduce the low energy proton flux, which has the effect of extending the lifetime of the CCD and reducing the degradation effects associated with radiation damage.

The best information on the effect of radiation damage on X-ray CCDs comes from the ASCA SIS experiment. ACIS will operate considerably colder (-120 C vs. -63 C) and have shorter frame rates (3.3 seconds vs. 4 to 16 seconds). The lower temperature `freezes' out damage induced traps, by extending the time constant for electron escape from the traps (and hence preventing X-ray produced electrons from falling into already filled traps). The shorter frame time also helps the `freezing out', and decreases flickering pixel effects by reducing the time for charge to flicker into the pixels, and reducing dark current.

Lab testing of radiation damaged CCDs from proton accelerators suggests these two changes will greatly improve the ACIS CCD resistance to radiation damage effects.

One important source of ionizing radiation is the focussed X-ray beam coming from the HRMA. The opinion of the Lincoln Lab CCD team is that radiation damage effects from X-rays start to be of concern at dose levels of 5 krads. The extremely sharp focus of the HRMA causes most of the flux from a point source to be focussed into a very small number of pixels, thus leading to a rapid accumulation of dose. For 1 keV X-rays the dose per photon is 0.005 rads/photon/pixel. A source like Sco X-1 can reach the dose limit (if there is no dither) in a very short time, so operations will need to exercise care to avoid placing extremely bright sources within the field of view of the CCDs for extended periods. Note that the dose accumulates even if the CCD is not turned on, and even if the radiation is absorbed in the gate (and so does not produce a detected event.)