Development of CCDs for REXIS on OSIRIS-REx

Abstract

Abstract-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 + -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 -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 12 at 625 nm.

Regolith X-Ray Imaging Spectrometer (REXIS) Aboard the OSIRIS-REx Asteroid Sample Return Mission

“…Rejecting visible light is also critical to our detector performance; thus the REXIS detectors use a 320-nm-thick aluminum layer optical blocking filter (OBF) directly deposited on the CCD surface. REXIS is the first instrument to fly CCID-41 detectors with a directly deposited optical blocking filter(Ryu et al 2014). To enable the data output, the REXIS CCDs are electronically connected to the Video Board in the electronics box via four flexprint cables.…”