Modelling electron distributions within ESA's Gaia satellite CCD pixels
  to mitigate radiation damage
  The Gaia satellite is a high-precision astrometry, photometry and
spectroscopic ESA cornerstone mission, currently scheduled for launch in 2012.
Its primary science drivers are the composition, formation and evolution of the
Galaxy. Gaia will achieve its unprecedented positional accuracy requirements
with detailed calibration and correction for radiation damage. At L2, protons
cause displacement damage in the silicon of CCDs. The resulting traps capture
and emit electrons from passing charge packets in the CCD pixel, distorting the
image PSF and biasing its centroid. Microscopic models of Gaia's CCDs are being
developed to simulate this effect. The key to calculating the probability of an
electron being captured by a trap is the 3D electron density within each CCD
pixel. However, this has not been physically modelled for the Gaia CCD pixels.
In Seabroke, Holland & Cropper (2008), the first paper of this series, we
motivated the need for such specialised 3D device modelling and outlined how
its future results will fit into Gaia's overall radiation calibration strategy.
In this paper, the second of the series, we present our first results using
Silvaco's physics-based, engineering software: the ATLAS device simulation
framework. Inputting a doping profile, pixel geometry and materials into ATLAS
and comparing the results to other simulations reveals that ATLAS has a free
parameter, fixed oxide charge, that needs to be calibrated. ATLAS is
successfully benchmarked against other simulations and measurements of a test
device, identifying how to use it to model Gaia pixels and highlighting the
effect of different doping approximations.
