Electrode level Monte Carlo model of radiation damage effects on astronomical CCDs

Current optical space telescopes rely upon silicon charge-coupled devices (CCDs) to detect and image the incoming photons. The performance of a CCD detector depends on its ability to transfer electrons through the silicon efficiently, so that the signal from every pixel may be read out through a single amplifier. This process of electron transfer is highly susceptible to the effects of solar proton damage (or non-ionizing radiation damage). This is because charged particles passing through the CCD displace silicon atoms, introducing energy levels into the semiconductor band gap which act as localized electron traps. The reduction in charge transfer efficiency (CTE) leads to signal loss and image smearing. The European Space Agency's astrometric Gaia mission will make extensive use of CCDs to create the most complete and accurate stereoscopic map to date of the Milky Way. In the context of the Gaia mission CTE is referred to with the complementary quantity charge transfer inefficiency (CTI = 1-CTE). CTI is an extremely important issue that threatens Gaia's performances: the CCDs are very large so that the electrons need to be transferred a long way; the focal plane is also very large and difficult to shield; the mission will operate at second Lagrange point where the direct solar protons are highly energetic (penetrating) and the science requirements on image quality are very stringent. In order to tackle this issue, in depth experimental studies and modelling efforts are being conducted to explore the possible consequences and to mitigate the anticipated effects of radiation damage. We present here a detailed Monte Carlo model that has been developed to simulate the operation of a damaged CCD at the pixel electrode level. This model implements a new approach to both the charge density distribution within a pixel and the charge capture and release probabilities, which allows the reproduction of CTI effects on a variety of measurements for a large signal level range in particular for signals of the order of a few electrons.