Radiation effects on the Gaia CCDs after 30 months at L2

TL;DR

This study assesses the radiation effects on Gaia's CCDs over 30 months at L2, comparing in-flight measurements with pre-flight predictions, and finds lower than expected damage due to solar activity and cosmic rays.

Contribution

It provides an in-flight analysis of radiation-induced damage on Gaia CCDs, validating and comparing with pre-flight models, and explores factors influencing radiation effects at L2.

Findings

Radiation damage is less than pre-flight predictions.

Solar activity has been relatively low, reducing damage.

Correlations found between solar events and CTI changes.

Abstract

Since the launch of ESA's Gaia satellite in December 2013, the 106 large-format scientific CCDs onboard have been operating at L2. Due to a combination of the high-precision measurement requirements of the mission and the predicted proton environment at L2, the effect of non-ionizing radiation damage on the detectors was early identified pre-launch as potentially imposing a major limitation on the scientific value of the data. In this paper we compare pre-flight radiation-induced Charge Transfer Inefficiency (CTI) predictions against in-flight measurements, focusing especially on charge injection diagnostics, as well as correlating these CTI diagnostic results with solar proton event data. We show that L2-directed solar activity has been relatively low since launch, and radiation damage (so far) is less than originally expected. Despite this, there are clear cases of correlation between earth-directed solar coronal mass ejection events and abrupt changes in CTI diagnostics over time. These sudden jumps are lying on top of a rather constant increase in CTI which we show is primarily due to the continuous bombardment of the devices by high-energy Galactic Cosmic Rays. We examine the possible reasons for the lower than expected levels of CTI as well as examining the effect of controlled payload heating events on the CTI diagnostics. Radiation-induced CTI in the CCD serial registers and effects of ionizing radiation are also correspondingly lower than expected, however these topics are not examined here in detail.