2–trap calibrations assuming fixed temperature in Figure 3).]

results, as it provides a direct link of the model parameters to the nature of the defects accumulated on Gaia CCDs over the course of the mission. At the same time, there are limitations with our analysis that should not be overlooked. The first one is that, while the results clearly show that the damage is composed Article number, page 10 launch.

[Figure: Fig. 9: Comparison of fitted trap parameters between two independent calibration runs combining every odd or even epoch’s data set from the full time–ordered mission set (see text).]

This apparently surprising result is probably simply due to the degeneracy between the species’ parameters when a third defect type is added to the fits. It is indeed unlikely that the observed trend could alternatively be caused, at least in part, to the physical process of annealing. Thermal annealing, resulting in the rearrangement of defects within the silicon lattice, happens quite quickly at high CCD temperatures, but in the cold environment of the Gaia focal plane any annealing process would be very slow, and therefore only contribute to the rearrangement of a very small fraction of defects. Averaging the trap densities over all devices as shown in Figure 8 risks hiding the finer details of the accumulation of defects in the Gaia devices. To more clearly reveal all aspects of the damage evolution, we present the example of CCD (ROW3, AF6), positioned in the third row and sixth strip of AF devices in the focal plane in Figure 13. The Figure shows the combined number of defects per pixel of the 3 trap species as a function of the number of revolutions since launch, as derived from the CtiPixel best fits. The general trend is dominated by a gradual accumulation of defects caused by the impact of Galactic cosmic rays on the detectors. As in the parallel case (Crowley et al. 2016a) the serial damage also shows step increases in data taken immediately following the strongest solar flares and coronal mass ejections of the last solar cycle. An interesting difference in the measured damage evolution between the two readout directions is observed in the magnitude of the step changes after the major flaring events in September 2017 and in the spring of 2024. The 2017 X8.2 class flare had the largest effect in the
atively low density of this defect, especially early on in the mission. An alternative, plausible explanation is that the observed continuum is instead intrinsic to this type of defect at the time of the measurements, when the newly generated traps had not yet formed a stable bond with impurities or other vacancies in the lattice: in this case, the wide range of derived energy level values reflects their transitional state and the dynamics of the damage accumulation. A similar scenario has been experienced by the VIS CCDs (Euclid Collaboration et al. 2025a) on board ESA’s Euclid mission (Euclid Collaboration et al. 2025b). The analysis of the damage experienced by the VIS CCDs during Euclid’s first year in space, measured using the trap pumping calibration technique — also known as pocket pumping (Janesick 2001; Hall et al. 2014) — showed a continuum of emission values for the newly developed traps (Skottfelt et al. 2024), just as seen in the CtiPixel model fits of Gaia’s damaged CCDs. The clustering of the trap parameters for the distinct species can also be seen in the scatter plot of Figure 12 for the final 3– species fits. In this figure the fitted trap parameters α have been translated into a corresponding capture cross–section for each device at the appropriate operating temperature (see Equation 7). While the model parameters are evidently correlated, the plot illustrates the presence of at least two clearly distinct trap species populations, clustering at specific energy levels and cross section values. The evolution of the measured mean damage of Figure 8, seems to suggest a slight decline in the number density of the A-type manufacturing species, that dominated the defects at
Article number, page 11 this surprising result has been attributed to the sudden temperature variation in the Gaia focal plane recorded in May 2024, that contributed to the changes in measured CTI.

[Figure: Fig. 12: Scatter plot of trap capture cross–sections [cm2] versus energy level [eV] for the 3 trap–species model fits to the entire Gaia mission data set. The red, green and blue dots are relative to the first, second and third trap species in the CtiPixel model fit.]