Absolute measurement of oxygen edge structure in the quantum efficiency of X-ray CCDs

As part of the program to calibrate X-ray CCD detectors for the AXAF mission we have performed high-spectral resolution, absolute measurements of CCD quantum efficiency using the synchrotron radiation and grating monochromator facilities of the PTB radiometry laboratory at BESSY. We present here detailed measurements of the absolute quantum efficiency in the spectral region surrounding the oxygen K absorption edge for both front- and back-illuminated MIT Lincoln Laboratory CCDs. The absolute scale of the measurements is established by reference to an Electrical Substitution Radiometer. Near-edge absorption structure produces variations as large as 40% in the detection efficiency of the front-illuminated detectors. We find structure at energies immediately below the K absorption edge which we attribute to resonant absorption in the oxide layers of the CCD gate structure. The amplitude of edge structures in the back-illuminated devices is, as expected, much smaller. Finally, we demonstrate that edge structure can be reliably detected in CCD response using an electron impact source and a grating monochromator