Development of CCDs for REXIS on OSIRIS-REx

The Regolith x-ray Imaging Spectrometer (REXIS) is a coded-aperture soft x-ray imaging instrument on the OSIRIS-REx spacecraft to be launched in 2016. The spacecraft will fly to and orbit the near-Earth asteroid Bennu, while REXIS maps the elemental distribution on the asteroid using x-ray fluorescence. The detector consists of a 2×2 array of backilluminated 1k×1k frame transfer CCDs with a flight heritage to Suzaku and Chandra. The back surface has a thin p[superscript +]-doped layer deposited by molecular-beam epitaxy (MBE) for maximum quantum efficiency and energy resolution at low x-ray energies. The CCDs also feature an integrated optical-blocking filter (OBF) to suppress visible and near-infrared light. The OBF is an aluminum film deposited directly on the CCD back surface and is mechanically more robust and less absorptive of x-rays than the conventional free-standing aluminum-coated polymer films. The CCDs have charge transfer inefficiencies of less than 10[superscript -6], and dark current of 1e-/pixel/second at the REXIS operating temperature of –60 °C. The resulting spectral resolution is 115 eV at 2 KeV. The extinction ratio of the filter is ~10[superscript 12] at 625 nm.United States. National Aeronautics and Space Administration. Strategic Astrophysics Technology Program (Grant NNX12AF22G)United States. National Aeronautics and Space Administration (Contract NNG12FD70C)United States. National Aeronautics and Space Administration (IPR NNG12FC01I)United States. National Aeronautics and Space Administration. Strategic Astrophysics Technology Program (IPR NNH12AU04I)United States. Air Force (Contract FA8721-05-C-0002