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Managing radiation degradation of CCDs on the Chandra X-ray Observatory III

Soon after the start of science operations of the Chandra X-ray Observatory, it became apparent that weakly penetrating (0.1-0.5 MeV) protons in the Earth's radiation belt were causing an unexpectedly rapid increase in the charge-transfer inefficiency of Chandra's front-illuminated CCDs. Fortunately, the Chandra team developed, implemented, and maintains a radiation-protection program that successfully reduced the rate of degradation of the CCDs' performance to acceptable levels. Since implementing this program, the average rate of increase of the charge-transfer inefficiency has slowed to 3.2×10-6/y (2.3%/y) for the front-illuminated CCDs and 1.0×10-6/y (5.8%/y) for the back-illuminated CCDs. This paper reviews the Chandra radiation-management program, reports the current status, and describes changes planned or implemented since the previous paper on this topic.