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Session 29 - High Resolution X-ray Imaging & Spectroscopy with AXAF.
Display session, Tuesday, June 10
South Main Hall,
We discuss the effects of an apparent molecular surface contamination on the reflectance of the Advanced X-ray Astrophysics Facility (AXAF) High Resolution Mirror Assembly (HRMA) witness flats. These flats are used to monitor the iridium coated surfaces of the HRMA and allow us to derive iridium optical constants for each of the HRMA mirror surfaces. Reflectance measurements of representative witness flats have been made at the Advanced Light Source (ALS) synchrotron facility over an energy range of 60 - 940 eV and we have observed a reduction in expected reflectance in the vicinity of 270 - 330 eV. This energy range includes the Carbon k absorption edge as well as the iridium N-IV and N-V absorption edges. The energy range is also an important region since it includes a number of spectral lines of astrophysical importance such as the C-V line.
The observed decrease in mirror reflectance is believed to be the combined result of the presence of an organic thin film, on the mirror surfaces, plus the effects of carbon on the ALS beamline optics. Methods to unfold these two effects from the data to obtain the desired iridium surface optical constants are discussed. Moreover, it appears that the tested mirror surfaces have a maximum level of molecular contamination amounting to an effective carbon thickness of from 5-10 angstroms. The cause of this contamination is still unknown. We discuss the net effect on AXAF effective area of such a contamination level, should it also be present on the HRMA.
