Gaia serial CTI modelling and radiation damage study

TL;DR

This study characterizes serial CTI in Gaia CCDs using a pixel-based forward-model (CtiPixel) grounded in Shockley–Read–Hall trap physics. It calibrates the model with 26 dedicated serial CTI measurements taken periodically during the mission, revealing the emergence of multiple radiation-induced trap species with longer emission times and a general linear increase in trap density over time, punctuated by step changes after major solar events and an end-of-life annealing procedure. The authors demonstrate that a three-trap-species model with per-species temperature evolution provides significantly better fits than an initial two-trap setup, and interpret the trap populations in terms of manufacturing defects plus radiation-induced defects, including a possible transitional defect state. While promising for interpreting serial CTI signatures, they note limitations for applying CtiPixel directly to science data due to scene complexity, computational demands, and incomplete post-scan data, but the work offers key physical insights and a pathway for improving radiation-damage mitigation in Gaia-like missions.

Abstract

During the course of its mission, ESA's Gaia spacecraft has generated a map of the stars of the Galaxy of exquisite detail. While in its L2 orbit, the satellite has been exposed to high energy cosmic rays and solar particles, that caused permanent damage to its CCDs. The main effect of radiation damage on Gaia data is the distortion of its images and spectra, caused by the CCDs charge transfer inefficiency (CTI) during the readout process, that, if not taken into account, can result in inaccurate measurements of a star's location and flux. In this work, the impact of CTI in the serial readout direction, larger than in the parallel due to the presence of CCDs manufacturing defects, has been analysed and modelled. A pixel-based, physically motivated CTI model, CtiPixel, has been developed to characterise the damage in Gaia CCDs. The model has been calibrated using dedicated serial CTI diagnostic data, taken every 3-4 months over the course of the mission. The model is shown to be a good representation of the observed signatures of CTI in the calibration datasets, and its parameters reveal significant insights into the nature of the CCD defects generated by space irradiation. The evolution of the damage in the serial direction shows a general small linear increase over time, with sudden step changes after strong solar flares and coronal mass ejections directed towards Earth. The serial CTI showed a further step increase as a consequence of the engineering CCD annealing experiment carried out after the completion of Gaia science observations.

Figures (16)

• Figure 1: Evolution of the measured charge trails intensity in the parallel direction averaged over all Astrometric Field devices, covering the entire course of the Gaia mission. The trails are an empirical measure of the amount of radiation accumulated in orbit. The gradual increase in damage is caused by Cosmic Rays, while strong solar flares directed towards Earth are responsible for the step changes in the damage evolution. Time intervals affected by temperature disturbances of the focal plan, due for example to decontamination activities, or instances of incomplete telemetry have been removed from the plot for clarity.
• Figure 2: Example charge trails from three epochs of Gaia in--orbit serial CTI calibration activities. The mean level of the charge injection in the image section CCD columns is shown before the start of the post-scan measurements illustrating the 2 orders--of--magnitude range in the commanded blocks. A small increase in serial CTI is observed from early to late epochs in the mission time line, particularly for the release signature at longer timescales. These data come from device AF5 on row 5 near the middle of the focal plane array.
• Figure 3: Time evolution in the Gaussian--scaled median absolute deviation in unit--weight residual, with time measured as On--Board Mission Time for initial 2--trap species calibrations fitting every device and epoch independently at an assumed constant focal plane operating temperature. The top panel is for all blue (AF and BP) devices, the bottom panel for RP devices.
• Figure 4: Gaia Focal Plane Array temperature variation over the course of the ten year mission at three different positions: (blue) row 1 SM; (red) row 4 AF5; (green) row 7 RVS. The large variations seen later in the timeline are caused by a series of end-of-life engineering tests.
• Figure 5: Calibration data (orange points with measured uncertainties), final models (blue lines) and residuals for 3 trap species CtiPixel models for two device/epochs: a typical calibration for row 5, strip 8 (=AF5) near the start of the mission at OBMT 2536 revolutions (upper panels); and a poor example for row 3, strip 12 (=AF9) towards the end of the mission. Calibration data consists of seven individual postscan trails, measured over a large range in injected signal level, and having a length of 20 pixels each; they are shown concatenated in these plots for convenience.