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![[Figure 6]](ge5117fig6mag.jpg)
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Figure 6
Diffraction patterns calculated for a model π-phase-shift grating with equal column and trough thicknesses at a 5 Å neutron wavelength. One set of patterns corresponds to coherent contributions from two grating periods (top) and the other set to contributions from four (bottom). The period of the grating is 2.4 µm. Assuming that the grating itself is perfectly uniform, the number of coherently contributing periods then depends on the transverse width of the neutron wavefront over which the phase is of the requisite uniformity. The geometrical angular divergence of the incident neutron beam – which corresponds to a distribution of transverse components of packet mean wavevectors – determines how well the features of the pattern are resolved. Both figures include cases for zero-beam angular divergence (red lines) and two other finite values (3.6 and 7.2′′, green and blue lines, respectively) convoluted with the natural pattern. Note that the general shape of the pattern is preserved, while the widths and magnitudes of the principle and subsidiary reflections are affected by the degree of the geometrical angular divergence of the incident beam. This description in which a distinction can be made between the effect of the intrinsic transverse width of an individual neutron packet and that of the beam geometrical angular divergence is supported by measurements reported here (which are to follow) and those of others (Treimer et al., 2006  ). |
![[Figure 6]](ge5117fig6.jpg)
 | JOURNAL OF APPLIED CRYSTALLOGRAPHY |
ISSN: 1600-5767
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