Dispersion

Abstract

The basic function of the spectrometer is to provide a means of isolating a selected wavelength from the polychromatic beam of characteristic radiation excited in the sample, in order that individual intensity measurements can be made. Although this is normally achieved by making use of the specific diffracting property of large single crystals, this is not by any means the only way of selecting a specific wavelength range and other methods which have been employed include the use of diffraction gratings,¹⁾ balanced filters^2-3) and energy resolution in the form of pulse height selection. The usual wavelength range of the conventional X-ray spectrometer is between 0.2 to 15 Å and over this region the single crystal is certainly the most efficient and versatile means of dispersion, particularly in combination with pulse height selection for the removal of harmonic overlap (See Chapter 4). However, the recent successful attempts to extend the operating range of the X-ray spectrometer into the soft X-ray and vacuum ultra-violet region have provided greater incentive for a more detailed study of the use of gratings in this region as well as the exclusive use of pulse height selection. Since measurements in the soft X-ray region invariably require some modification to the commercially available spectrometer, usually by way of modification to the source of primary radiation and the detector, it is the intention to first consider dispersion in the conventional wavelength range and then to discuss the soft X-ray region as a separate topic.