Heavy atoms absorption edge

When the X-ray wavelength is near the heavy-atom absorption edge, a fraction of the radiation is absorbed by the heavy atom and re-emitted with altered phase. The effect of this anomalous scattering on a given structure factor fhp in the heavy-atom data is depicted in vector diagrams as consisting of two perpendicular contributions, one real (AFr) and the other imaginary (AFj). 

In Fig. 6.11, F p represents a structure factor for the heavy atom derivative measured at wavelength where anomalous scattering does not occur. F p is the same structure factor measured at a second X-ray wavelength X2 near the absorption edge of the heavy atom, so anomalous scattering alters the heavy-atom contribution to this structure factor. The vectors representing... 

The general rule for choosing the P-filter is to use a material, which is rich in a chemical element, one atomic number less than the anode material in the periodic table. This assures proper location of the K absorption edge, i.e. between the Kai and Kp lines. For heavy anode materials (e.g. Mo), this rule can be extended to two atomic numbers below the element of the anode. A list of p-filter elements, suitable for the most commonly used anode materials, is found in Table 2.1. Thus, for a Cu anode, a foil made from Ni will work best as the P-filter, while for Mo radiation both Nb and Zr are good P-filters. The former material (Zr) is more often used in practice because its K absorption edge is closer to the wavelength of the Kai line. 

The scope for utilising anomalous dispersion is considerable in conjunction with the synchrotron. SR has a high intensity over the X-ray wavelength range of interest and can access the absorption edges of (a) commonly occurring metals in metallo-proteins, (b) metals which are used in heavy atom derivatives, (c) selenium in specially grown selenoproteins (see section 9.8) and (d) bromine in brominated nucleotides. 

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