The PlotXNFF program is supplied to plot the form factors for x-rays, neutrons, electrons and magnetic scattering for neutrons using the scattering factor tables supplied with GSAS-II. To access it, use the Calculate/“Run PlotXNFF” menu command. This will open an empty window. The next step is to select an element that will be used in the computation. Use the File/“Pick Element” menu command, which opens a periodic table. Click on an element in the table. That element is immediately selected and the plots are updated to show the scattering lengths and the form factors for the database contents. As an example, the element Ti is selected. The empty window remains empty, but a new plot window is opened and there are four plots available, of which three are shown in Figs. 19.8, 19.9 and 19.10.
Neutron form factors are usually independent of wavelength, but a few isotopes have neutron resonances, with an example shown in Fig. 19.11. GSAS-II will attempt to compensate for the change in scattering length as a function of wavelength provided that the wavelength band used for time-focussing is narrow. If not, one is probably best discarding all neutrons collected at resonance wavelengths. In any case, knowing where the resonances occur is important for experiment planning.
