The impact of radiation on the CCDs serial readout efficiency has been monitored throughout the course of the Gaia mission with a set of recurrent, dedicated serial CTI calibration tests. These engineering activities, repeated every three to four months, comprise the injection of blocks of consecutive lines with charge at several intensity levels, regulated by the CCD injection voltage setting. At the lowest setting, the blocks are filled with approximately 103 electrons/pixel. During the activity, the voltage is raised a total of five times, reaching an injection level of up to a few hundred thousands of electrons per pixel. The full procedure results in five sets of data of increasing charge injection flux, each one of the five sets consisting of 70 blocks of 225 consecuArticle number, page 3 tive charge injected lines, followed by 200 lines with no injected charge.

In a line, the injected charge value is not exactly the same in all pixels, but the charge pattern repeats line by line. Moreover, within a block, in spite of a constant commanded injection level, the actual signal falls slightly (a phenomenon known colloquially as ‘injection droop’). When these engineering data are read out in the serial direction and the output node, the flux level of the final 12 pixels of the injected lines is recorded, along with the signal of 20 postscan samples (24 for AF1 CCDs, the first strip of seven detectors of the AF devices). The post-scan pixels are ‘virtual’ pixels, read out by over-clocking the serial register beyond the last image section column. Their signal follows an exponential decay (a trail), formed by deferred (or released) charge from previously trapped electrons, superposed on the electronic bias level of the video chain. The cumulative amount of signal in the postscan samples, after subtraction of the CCD electronic offsets (the electronic bias), is an indicator of the density of traps in the serial register, and therefore of the amount of total irradiation and damage of the devices that is affecting the serial transfer process. A total of 26 serial calibration activities has been carried out since the launch of Gaia. In Figure 2 we show as an example the measured deferred charge levels in the post-scan pixels from three epochs of serial calibration activities, at each of the five charge signal levels, to illustrate the data input into the subsequent analyses. These measurements reveal a number of important aspects in regard to the damage. The shape of the charge trails in the post-scan pixels is reminiscent of a sum of exponential decays, regulated by the release timescales with which trapped charge is emitted from defect sites. Only specific trap species with an emission time of the same order as, or longer than, the serial clock period contribute to the observed trails. Faster traps will release the charge just captured within the same pixel, while the slower traps will release their charge on longer timescales compared to the CCDs serial readout frequency. These data also display the effect of signal size on the amount of charge losses during readout. Larger amounts of injected charge occupy a bigger physical volume in the pixels, and can be affected by a higher number of traps, and lose more electrons during the charge transfers. Finally, the charge trails grow larger at later times during the mission. The integral under their curves corresponds to the total amount of charge released by traps after a line is readout into the first 20 following pixels, and the increase over time is a consequence of the accumulation of damage. A dedicated serial CTI model has been developed to process the serial calibration activities. The model is physically motivated, with the goal of extracting as much information as possible on the nature of the damage caused by space irradiation at L2. The model, named CtiPixel, and its calibration based on the engineering test data are presented in the following Section.