ACIS UV/optical blocking filter calibration at the Natl. Synchrotron Light Source

Measurements of the transmission properties of the AXAF CCD imaging spectrometer (ACIS) UV/optical blocking filters were performed at the National Synchrotron Light Source at Brookhaven Laboratories. The X-ray transmissions of two Al:Si/LEXAN/Al:Si three layer filters were measured between 260 and 3000 eV. The main purpose of the calibration was to determine a model transmission function with an accuracy of better than 1 percent. We present results from fits of model transmission functions to the measured x-ray transmission data. Detailed fine energy scans above the Al-K and C-K absorption edges revealed the presence of fine oscillations of the x-ray transmission. These features are most likely extended x-ray absorption fine structures (EXAFS). The amplitude of the EXAFS oscillations above the Al absorption edge is about 5 percent of the mean value of the x-ray transmission. 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. Since the ACIS filters will be at a much lower temperature on orbit we provide a prediction of the EXAFS component for the expected on orbit temperature of the ACIS filters.