Internal Compton effect

Absorption means diminution of coherent x-ray intensity in the crystal through inelastic processes such as atomic absorption and fluorescence, photoelectron emission, and Compton effect extinction means intensity diminution due to loss through diffraction by fortuitously oriented mosaic blocks. The simple extinction expression due to Darwin, given in Eq. (18), is only a rough approximation more accurate treatments will be mentioned in what follows. In Eq. (17) the absorption factor is expressed in terms of the linear absorption coefficient /inn (calculated from tabulated values of the elemental atomic or mass absorption coefficients, updated values of which will appear in Vol. IV of International Tables,2 the path length f, of the incident ray from the crystal surface to the point of diffraction r, and the path length t2 of the diffracted ray from that point to the crystal surface. 

Thomson and Rayleigh scattering are the classical equivalents of the Compton effect with the exception that no (internal) energy is transferred to the atoms, they are neither ionized nor excited, just the direction of the photon is changed. 

In practice a sample is counted normally as described for the channels ratio method and then counted a second time with a source of y-rays (usually Cs or Ba) positioned in the center of the counting chamber just below the sample vial (see Figure 3-19). Any quenching that occurs has the same effect on the efficiency and spectrum of the Compton electrons as it does on those of the sample j8 particles. Since this technique has incorporated the procedures of both internal standardization and channels ratio correction methods, it is no surprise that the data obtained must be treated as described above for both of these techniques. The first or normal count rate obtained in each channel arises only from the sample and may be represented as follows ... 

Using the Compton line of the tube as an internal standard reference sometimes enables a thorough correction of the matrix effects. This is illustrated by the analysis of trace elements Ni and V in petroleum products where the sulphur content is variable. Whilst, in the absence of sulphur, the matrix is extremely light and thus not particularly absorbent, an increase in sulphur content leads to a noticeable absorption of the Ni and V signals. The first curve in Figure 4.11 is plotted from net intensity levels measured on vanadium, whereas, in the lower curve the net V K. intensity levels have been replaced by the intensity ratios ... 

Compton, D. M. T., Schoen, A. H. (eds.) Proc. Second Intern. Conf. on the Mossbauer Effect, Saclay, France 1961. New York Wiley 1962. 

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