Multilayer Diffraction Research Articles

Calculated Performance Of A Wolter Type I X-Ray Telescope Coated By Multilayers

Deposition of multilayered diffraction coatings on the reflecting surfaces of a Wolter Type I x-ray telescope offers the potential of using Bragg reflection to achieve enhanced effective area in selected high energy bandpasses. The structure of a gold-carbon multilayer has been optimized for study of 1.8 A iron-line emission with an x-ray telescope designed for observations from Spacelab. The response of this mirror assembly has been calculated as a function of x-ray energy, and the resulting effective area is enhanced by a factor of ten and peaks at 160 cm2 with a bandpass of 0.4 keV FWHM. This enhanced response is sufficient to allow study of the angular distribution of iron-line emission from the 20 brightest cluster x-ray sources during a seven day Spacelab mission. The possibility of achieving x-ray imaging in narrow bandpasses at even higher energies has been investigated. We conclude that it should be possible to achieve effective areas of approximately 5 cm2 in bandpasses -1 keV wide for energies in the 15 to 25 keV range, a spectral region of much importance during the impulsive phase of solar flares.

Bragg diffractors with graded-thickness multilayers

Possible designs, characteristics and uses of multilayer diffraction elements in which the layer thickness is non-uniform are discussed.

High-efficiency low-energy x-ray spectroscopy in the 100–500-eV region

The lead myristate multilayer analyzer has provided a basis for a relatively simple and efficient spectroscopy for the low-energy x-ray emissions in the 20–80-Å region (where conventional crystal spectroscopy and grazing incidence grating spectroscopy are generally inefficient). The percent reflectivity, the integrated coefficient of reflection, and the Bragg diffraction width of the lead myristate analyzer have been measured and found to be consistent with the predictions of a simple theoretical model for multilayer diffraction. This multilayer spectroscopy at large Bragg angles has a high efficiency (high instrument transmission) as compared to grazing incidence grating spectroscopy in this 20–80-Å region. However, the resolution is limited to that set by the diffraction width of the lead myristate analyzer of about 1 eV. Because the collimator-crystal broadening function can be precisely defined, a simple and effective deconvolution procedure can be applied with this multilayer spectroscopy to bring the resolution into the sub-electron-volt region. To demonstrate the efficiency of lead myristate spectroscopy in the 20–80-Å region, spectra were measured and analyzed from x-ray excited fluorescent sources which are characteristically of low intensity. (X-ray excitation yields a minimum of background spectra and of radiation damage.) These include the L2,3 atomic spectrum of argon and the C-K molecular spectrum of CO2, both in the gas phase, and the Cl-L2,3 and O-K spectra from solid lithium perchlorate. Many samples undergo appreciable radiation-induced chemical change during the exposure time that is required for measurement—even with an optimally fast spectrograph and with fluorescent excitation. A method has been developed to evaluate and to correct for radiation damage by distributing the exposure over an effectively large sample volume either by gas flow or by rotating through multiple samples during measurement. Several spectral scans were made on the LiClO4 using six samples. The total exposure time for each data point in each scan was recorded which permitted an extrapolation into a ’’zero’’ exposure spectrum. Finally, Fe-L2,3/O-K spectrum (from Fe2O3) in the 17–25-Å region is presented to illustrate the effectiveness of the lead myristate analyzer in third-order diffraction. For this multilayer, the third-order diffraction efficiency is one-third that of the first order and is nearly twice that of the second order for this wavelength region.

Optical simulation of electron diffraction of thin crystals

An optical crystal (a multilayer diffraction plate) which optically simulates electron diffraction by thin crystals has been developed. The lattice was constructed with appropriate lattice constants to allow a direct optical simulation of electron diffraction by mica. The resulting optical-diffraction patterns agree with theory and vividly illustrate Laue zones. Optical-diffraction patterns for different crystal orientations are directly compared to electron-diffraction patterns for the same orientations of a thin mica crystal. The breadths of Laue zones diminish as a crystal becomes thicker, reducing the diffraction pattern to the Laue spots described by Bragg’s law.