Measurements Of Surface Scattering From Mirrored Surfaces Using A Triple Axis X-Ray Spectrometer

Abstract

Measurements of surface scattering from mirrored surfaces using a triple axis x-ray spectrometerFinn E. Christensen, K.P. Singh and H.W. Schnopper Danish Space Research Institute, DK-2800 Lyngby, Denmark *) on leave from: Tata Institute of Fundamental Research, Bombay 400005, India.AbstractA triple axis X-ray spectrometer has been used to test the smoothness of mirrored surfaces. A perfect channel-cut Si or Ge crystal extracts Fe-ka radiation from a conventional X-ray tube. The radiation is incident on the test surface and the specularly reflected and scattered radiation from the surface is analyzed by another perfect channel-cut Si or Ge crystal. The channel-cut crystals provide an essentially tailless probe of the scattered radiation. The test surfaces in this study include three standard flats from the EXOSAT program, the AXAF-program and the ROSAT program, respectively, and test foils made in connection with the construction of a high throughput thin foil reflector telescope for the ESA X-ray Spectroscopy Mission XMM.IntroductionOne of the essential ingredients in the coming ESA X-ray Spectroscopy Mission XMM is a high throughput thin foil reflector telescope. The design goal of 30 Half Energy Width (HEW) for the point spread function (PSF) at Fe-Ka must be achieved by the accurate positioning of Au, Pt or Ni coated foils in a conical approximation to a Wolter I telescope .Deviations from the perfect conical shape and/or scattering off the specular direction from the foils will degrade the telescope performance. In this paper we will present measurements at Fe-Ka of the surface scattering of X-rays from a test flat from the German ROSAT program and a stretched electroformed Ni-foil and compare these to earlier measurements performed on standard flats from the EXOSAT program, the AXAF program and two test foils. We will also deduce from the EXOSAT and the AXAF data the bandwidth limited r.m.s. surface roughness a and the r.m.s. slope error m and the mean surface wavelength d of the roughness .Experimental set-upThe experimental arrangement is shown in Figure 1 and is described in detail in Reference 2. The types of test flats and certain characteristics of the experimental set up used for scattering measurements are shown in Table 1. The essential feature is the sharp resolution function obtained by letting the X-rays be reflected several times in the channel-cut crystals.