Anomalous annealing of a high-resistivity CCD irradiated at low temperature

The front-illuminated charge-coupled device (CCD) detectors in the Chandra X-ray Observatory\´s ACIS instrument suffered radiation damage from soft protons focused by the telescope mirror early in the mission. In the course of assessing this damage, the focal plane was temporarily warmed from its normal operating temperature (then -100°C) to +30°C. Following this "bakeout", the radiation-damaged CCDs exhibited significantly greater charge transfer inefficiency (CTI). We performed a laboratory experiment with a sibling of the flight detectors in an attempt to reproduce and better understand this phenomenon. The test CCD was cooled to -100°C, irradiated by 120 keV protons and then warmed to +30°C for 8 hours. As expected, after the initial irradiation, but before detector warmup, a substantial CTI increase was observed. The subsequent warmup itself then produced an additional factor ≈2.5 increase in CTI. Following smaller subsequent irradiations with the detector cold, a "bakeout" for 8 hours at -60°C produced no observable increase in CTI. However, a subsequent bakeout to +30°C for another 8 hours resulted in an additional increase in CTI of roughly 15%. The CTI changes produced by the room temperature bakeout are accompanied by dramatic changes in the de-trapping times of electron traps responsible for the CTI. The distributions of signal amplitudes in the pixels trailing X-ray events indicate that annealing at room temperature can cause large changes of the trap emission times, from which we infer that conversion of trapping defects takes place. The observed phenomena can be explained by the previously suggested mechanism of carbon-related defect transformation. Specifically, the room-temperature annealing may allow carbon interstitials to form metastable complexes with phosphorus and/or carbon substitutional atoms.