Studying Space Radiation Damage in Detectors for Gaia and Euclid

Over the last decade, with the advent of '2D-stitching' techniques, the CCDs have become larger, toward wafer-scale CCDs. In many applications requiring area coverage, the preference is to mosaic many such large area detectors into an array. ESA's Gaia satellite, which is currently being built and is due for launch in 2012, has >100 such large area CCDs, each having area 4.5x6 cm2. This large array represents the largest focal plane array of CCDs both in-space and on the ground. However, in space, radiation damage, particularly from solar protons, produces traps in the silicon which affects the charge transfer efficiency of the devices. In Gaia, this in-turn affects the astrometric and photometric accuracy of the mission. Euclid is a mission concept being proposed for ESA's Cosmic Visions programme, with a similar large array of large CCDs, to study distributions of dark energy through observation of galactic micro-lensing. Whilst the e2v CCD203 4kx4k detector is baselined, ESA is currently funding the development of a new CCD design for the mission, and new detectors should become available later in 2011, around the start of this proposed studentship. In this mission concept, the degradation to CTE will create image distortions which again will affect the scientific return of the mission. Other future missions will undoubtedy be proposed using similar large CCDs where radiation damage is equally important to the science of the mission. In the CEI laboratory we have established an optical cryogenic detector test facility, capable of spot projection onto a detector inside the vacuum chamber, where the detector can be operated at representative operating temperatures of these missions. The system can operate in the time delay integration mode (TDI) of Gaia, or the staring mode of Euclid. During the 3 years, the student will explore radiation damage aspects in these large devices. e2v have already agreed to provide spare Gaia CCDs and the new Euclid CCDs toward the work. These devices will be irradiated with protons to representative fluence levels for space, and carefully and systematically explored for the impact of the damage on the instrument science performance. The work will establish models of the damage, at the microscopic level, where the 3D charge storage within the pixel is explored, to the macroscopic where the effects of trapping are simulated by Monte Carlo technique incorporating the device readout modes. Recent results from Gaia CCD testing have indicated that the individual stitched-zones within the device behave differently, as thought for small signals the device could be treated almost as a number of CCDs. Exploration of this non-uniformity effect in these large CCDs will be explored toward promoting an understanding of the reasons for the behaviour. By 2011, Astrium will conduct its final radiation tests of the Gaia CCDs, and the experimental programme here will represent the only experimental test facility operating Gaia CCDs for the period up to and beyond launch. Furthermore, during the studentship, the Gaia spacecraft will be launched by ESA, and will return data which can be used by the student to verify the work of the laboratory programme. By the end of the studentship we would anticipate that a much greater understanding is obtained into the use and limitations of these large CCDs in Gaia, Euclid and future space missions. The student should also have made a significant contribution to the fundamental understanding of such technologies in space, and conducted work to the benefit of both of these high-priority missions.