Radiation effects on the Gaia CCDs after 30 months at L2

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.

GaiaData Release 1

“…No treatment of micro-meteoroid hits or micro-clanks was included (except for the exclusion of the data from short time intervals affected by large hits) leading to attitude modelling errors which in turn will limit the astrometric accuracy that can be attained (see Lindegren et al 2016, in particular appendix D). The treatment of CTI effects was not included in Gaia DR1, which is justified given the present low levels of radiation damage to the Gaia CCDs (Crowley et al 2016a). We stress that the above description of the data processing for Gaia DR1 is mainly illustrative and not intended as a complete description of all the simplifications that were introduced to enable a timely first Gaia data release.…”

TheGaiamission

“…3. The radiation damage to the CCD detectors is measured through first-pixel response in the along-scan profiles of lines of charge injection and through onboard counters registering the number of rejected detections in the sky mapper; this radiation damage is generally as expected, essentially showing a slow yet persistent degradation, caused by the omnipresent flux of high-energy Galactic cosmic rays (a handful of particles cm −2 s −1 ), combined with a handful of discontinuities corresponding to solar activity and associated proton events (Kohley et al 2014;Crowley et al 2016a). The data also show a clear sign of the well-known anti-correlation between cosmic-ray fluxes and solar activity (e.g.…”

GaiaData Release 1

“…In fact, a study (Hopkinson et al 2010) that compared the effects of trapping between warm (room temperature) and cold (133 K) proton irradiated sections of a Gaia CCD found that the trap densities were generally around a factor of two higher for the cold-irradiated section than for the warm-irradiated section. A detailed analysis of the L2 radiation environment seen by Gaia can be found in Crowley et al (2016).…”