Compton modified radiation

Radiation so scattered is called Compton modified radiation, and, besides having its wavelength increased, it has the important characteristic that its phase has no fixed relation to the phase of the incident beam. For this reason it is also known as incoherent radiation. It cannot take part in diffraction because its phase is only randomly related to that of the incident beam and cannot therefore produce any interference effects. Compton modified scattering cannot be prevented, however, and it has the undesirable effect of darkening the background of diffraction patterns. 

The beam of secondary radiation issuing from the sample consists largely of fluorescent radiation, but there are some other weak components present as well. These are coherent scattered radiation, coherent diffracted radiation, and incoherent (Compton modified) radiation. These components appear as a background on which the spectral lines are superimposed. This background is normally low (see Fig. 15-3), but it may become rather high if the sample contains a large proportion of elements of low atomic number, because the sample will then emit a large amount of Compton modified radiation. 

In order to determine the natural structure of the Compton modified line it is necessary to first minimize an experimental cause for the breadth of the line which is ordinarily superposed upon the natural breadth so as to mask the latter. This cause is the unavoidable inhomogeneity of scattering angle. The x-radiation incident upon the scattering material... 

Placement of the monochromator in the diffracted beam has the advantage of suppressing background radiation originating in the specimen, such as fluorescent radiation and incoherent (Compton modified) scattered radiation. For example, if a steel specimen or any iron-rich material is examined with copper radiation in an ordinary diffractometer, the background due to fluorescent Fe K radiation will be unacceptably high. But if a monochromator is added and oriented to reflect only Cu Aa, the background is reduced practically to zero, because the fluoresced Fe Kol and Fe K(i do not enter the counter. A monochromator may therefore... 

These effects, however, are all very weak and are masked by the other forms of diffuse scattering which are always present. As a result, the details shown in Fig. 13-9 are never observed in an ordinary powder pattern made with filtered radiation. To disclose these details and so learn something about the structure of the solid solution, it is necessary to use strictly monochromatic radiation and, preferably, single-crystal specimens, and to make allowances for the other forms of diffuse scattering, chiefly temperature-diffuse and Compton modified, that are always present. 

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