Calibrating the Regolith X-ray Imaging Spectrometer (REXIS)

This paper describes the onboard calibration process of REXIS (the Regolith X-ray Imaging Spectrometer), an instrument on OSIRIS-REx. OSIRIS-REx, scheduled to be launched in 2016, is a planetary mission intending to return a regolith sample from a near Earth asteroid called Bennu. REXIS, a student-led collaboration between Harvard and MIT, is a soft X-ray (0.5-7.5 keV) coded-aperture telescope with four X-ray CCDs and a gold coated stainless steel mask. REXIS will measure the surface elemental abundance of Bennu by capturing X-ray fluorescence (XRF) lines induced by solar X-ray flux. For the energy calbration of the XRF lines, REXIS will use a set of onboard 55Fe radioactive sources (which generate 5.89 keV lines) placed alongside the X-ray CCDs. To ensure that the activity level of the 55Fe sources is adequate for in-flight calibration, we have measured and analyzed the CCD response to various 55Fe activity levels for the REXIS engineering model (EM). We also characterized the spectral resolution, the gain, and the uniformity of the EM CCDs through spectral line fitting