Radiation damage to charge coupled devices in the space environment

We have investigated the characteristics of radiation damage to charge coupled devices (CCDs) in the space environment. The X-ray astronomy satellite ASCA launched on February 20, 1993 in low Earth orbit carries CCDs specially developed for soft X-ray detection. We have traced the performance of the CCDs for 3 years. We have observed both the gradual decrease of charge transfer efficiency (CTE) and the increase of dark current. These are phenomenologically explained by the increase of charge traps due to irradiation by high energy charged particles. However, some of the effects of the radiation damage in the CCD are quite non-uniform over the chip on various scales. We discuss characteristics of the charge traps and possible origins of the non-uniformity.