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Session 55 - Space Instrumentation.
Display session, Wednesday, June 12
Tripp Commons,
The 0.26 to 3 keV X-ray transmission of the AXAF/ACIS UV/Optical blocking filters was measured at the National Synchrotron Light Source (NSLS) at Brookhaven. The main objective of the calibration was to determine an X-ray transmission function with an uncertainty of less than 1%. The ACIS UV/Optical Imager and Spectrometer blocking filters consist of LEXAN,a polycarbonate plastic with a chemical composition of C_16H_14O_3, with a layer of aluminum coated on each side to provide optical light blocking.
The X-ray transmission data can not be well fitted, especially at fine energy scales, with models that incorporate Henke absorption coefficients alone. Residuals between the X-ray transmission data and models that utilize the Henke absorption coefficients are as high as 50% and are mostly due to extended X-ray absorption fine structure (EXAFS) that appears above the C, O and Al absorption edges. We provide an analytic expression for the transmission function of the ACIS UV/Optical blocking filters which combines an X-ray transmission derived from Henke mass attenuation coefficients with a component to characterize the EXAFS contribution.
EXAFS theory predicts a temperature dependence on the amplitude of the EXAFS oscillations. This dependence arises from the fact that thermal vibrations of the atoms in a solid produce a phase mismatch of the backscattered electron wave function. We derive a simple expression to predict the on-orbit X-ray transmission from the X-ray transmission measured on ground.
Finally we present several examples where the presence of EXAFS in the X-ray transmission of a filter may significantly influence the interpretation of spectral data originating from celestial X-ray sources.
