Lorentz and polarisation factors

The same theory allows almost exact prediction of spot size and shape around a film. Even for 3corr=0 a very definite spot shape can be predicted around a film due to the exact collimation of the SR beam. 

On the right-hand side of relation (6.3) are the L, P and A factors. The Lorentz factor, L, takes account of the relative time each reflection spends in the reflecting position. It depends on the precise diffraction geometry used. For example, in the rotation method 

In the case of an unpolarised incident beam (such as a conventional X-ray source beam without monochromator) the polarisation factor is given by 

The form of this correction can be understood in the following way. For any reflecting plane it is possible to resolve the incident beam into states of polarisation parallel and perpendicular to the plane whereby 

After reflection I is attenuated by cos2 26hke so that the overall reflected beam intensity 

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The conditions for the data collections are summarized in ref. The intensities of selected reflections [I > 2cr(I)] were corrected for Lorentz and polarisation factors but not for absorption. 

The Lorentz and polarisation factors are usually combined, for powder X-ray diffraction, into a single correction term ... 

Before going further, it may be noted that the flipping ratio does not depend either on the Lorentz factor or on absorption in the sample. Certain instrumental parameters such as the polarisation of the neutron beam for the two spin states, the half wavelength contamination of the neutron beam and the dead-time detector can readily be taken into account when analysing the data. On the other hand, the extinction which may occur in the scattering process is not so easy to assess, but must also be included [14]. Sometimes, it is even possible to determine the magnetisation density of twinned crystals [15]. 

The remaining factors in equation (6.1) have to be calculated for each reflection separately, i.e. LPA, the Lorentz, polarisation and absorption corrections respectively. The following relationship will now be considered in more detail ... 

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