Machine background

From time to time you will certainly perform test snapshots2 that are only accumulated in the memory of the detector before you start the real business. If everything looks fine - do not store the test image from the detector memory. Instead repeat the snapshot using the command that directly stores the data in a file. Storage from memory will most probably not dump the correct environmental data to the data files - and if afterwards you want to use the data as, e.g., the machine background and you find out that the exposure and the primary beam intensity have not been stored, you may have a problem. 

If the synchrotron is operated in discontinuous mode, the storage ring will be refilled two or three times every day. The interval between two consecutive refills is called a synchrotron radiation run. The parasitic scattering (machine background) should at least be recorded once within each synchrotron radiation run. You might consider... 

Figure 8.2. WAXS curves from semicrystalline and amorphous poly(ethylene terephthalate) (PET). Separation of the observed intensity into crystalline, amorphous, and machine background (laboratory goniometer Philips PW 1078, symmetrical-reflection geometry)... 

A simple phenomenological method can be used to describe changing crystallinity from WAXS data of isotropic materials. It is based on the computation of areas in Fig. 8.2. First we search the border between first-order and second-order amorphous halo. For PET this is at 29 37° (vertical line in the plot). Then we integrate the area between the amorphous halo and the machine background. Let us call the area Iam. Finally we integrate the area between the crystalline reflections and the amorphous halo, call it Icr, and compute a crystallinity index... 

It is assumed that the machine background has separately been measured and properly been subtracted under consideration of the absorption factor (Sect. 7.6). In particular it is not allowed to take the diffuse scattering background of a molten or amorphous sample for the machine background. 

Practical Value. The presented analytical expressions are very useful, predominantly for the analysis of the scattering from weakly distorted nanostructures. Because of their detailed SAXS curves, direct fits to the measured data return highly significant results (cf. Sect. 8.8.3). Nevertheless, some important corrections have to be applied [84], They comprise deviations from the ideal multiphase structure as well as thorough consideration of the setup geometry and machine background correction (cf. Sect. 8.8). 

Flaws in engineering design and setup. If one intends to eliminate the machine background from a measured pattern, it is necessary to extract the exposure intervals, primary beam intensities, /q, and attenuated intensities, from complex data sets which are different at different beamlines. Moreover, the sequence of the stored numbers changes as a function of the wiring during setup. Thus, if background correction and normalization are not done by the beamline staff, some users will not be able to carry out these steps. 

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