Modelling charge transfer inefficiency in Gaia CCDs with in-flight and on-ground data

Abstract

The European Space Agency's Gaia spacecraft was launched in 2013 with the aim of making the largest and most precise map of the Milky Way by taking measurements of almost one billion astronomical objects. It has a focal plane that consists of 106 Charge-Coupled Devices (CCDs), custom designed by Teledyne e2v to help fulfil its objectives. These detectors make measurements of positions, velocities, parallaxes, and other physical properties of any objects, with a sufficiently bright enough magnitude, that pass through their field of view. Operating in space means that the Gaia CCDs have been subjected to radiation damage, both ionizing and non-ionizing in nature, in orbit from predominantly solar radiation. This radiation-induced damage leads to the formation of trap defects in the CCD silicon lattice which can trap electrons during readout leading to the increase of charge transfer inefficiency (CTI) and a reduction in the quality of the returned science data. From previous analysis of in-flight data, the degradation of the CCDs, measured from an increase in CTI, has been calculated to be less that that predicted from pre-flight models and on-ground tests. In this study, in-flight and on-ground data is modelled so that the trap landscapes can be further investigated. This was achieved using a charge transfer model, the Charge Distortion Model (CDM), integrated in the Pyxel detector simulation toolkit. Other simulations, namely C3TM, are used in conjunction with the results from Pyxel to obtain a more thorough understanding of the trap landscape causing the observed CTI effects.