nature of the radiation damage

The physically motivated representation of radiation damage in CtiPixel is of great advantage in the interpretation of the fits Article number, page 9 of several defects types, with distinct charge release times, only a limited number of post-scan pixels are available to fit the charge trails.

[Figure: Fig. 8: Evolution of the mean trap density with time (measured as On–Board Mission Time (OBMT), in units of the satellite revolution period of 6 hours) for the Blue (AF and BP) and Red (RP) CCDs, for the final 3–trap species calibrations.]

This in turn limits the ability to constrain the fitting parameters, the release times (or trap energy levels) in particular, and can cause degeneracy between the trap parameters and possibly over-fitting, in case too many additional species are added to the model. Even more importantly, the simplifying assumption that the damage is composed of a small number of trap species with distinct energy levels might not be a realistic representation of the dynamical processes involved in the formation of defects in the Gaia space environment, as discussed below. Finally, as already mentioned earlier, the model doesn’t appear to fit the very first trail samples perfectly, an effect that cannot be convincingly explained by an incorrect bias subtraction or by injected charge blooming from neighbouring pixels. The clearest outcome of fitting the serial calibration datasets with CtiPixel is the emergence of new trap species in addition to the manufacturing Oxygen defect (the A-trap) that dominated the serial CTI of the Gaia CCDs at launch. As can be seen in Figure 8, the density of the radiation-induced traps shows a general gradual increase, while small step changes can also be observed shorty after Coronal Mass Ejections directed towards Earth. Another significant step increase in serial CTI was observed in a dedicated calibration dataset collected after an engineering CCD annealing activity, carried out after the final science observations were acquired. This result is discussed in Appendix A. The new species generated by space radiation present higher energy levels and therefore longer emission timescales compared to the A-trap. Their peaks in the distribution of the traps release times can be clearly identified in Figure 11. The intermediate trap species is characterised by an energy level of approximately 0.20 eV below the conduction band and emission time constant of 1.25×10−7 seconds at the CCDs operating temperatures, suggestive of a possible association with the multi-vacancy defect. The slowest species peaks at approximately 6×10−7 seconds; interestingly, its distribution appears to have a tail of longer release time values, extending as long as ∼10−5 seconds. The origin of the wider distribution of characteristic energies and emission values of the third species could be in part attributed to poorly constrained best fit parameters due to the rel-

[Figure: Fig. 6: Residuals (upper panel) and reduced chi–squared goodness–of–fit (lower panel) histograms of 87 out of 88 (i.e. excluding AF9 on row 3) 3 trap species CtiPixel final calibrations, subdivided into blue (AF and BP) and red variant (RP and RVS) samples (see text).]

[Figure: Fig. 7: Time evolution in the Gaussian–scaled median absolute deviation of the unit–weight residual with time measured as OBMT for the final 3–trap species calibrations (cf. the same for initial 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).]