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![[Figure 2]](ge5117fig2mag.jpg)
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Figure 2
(b) A schematic representation of the essential elements of a rudimentary neutron diffractometer, upstream of the sample position (which would be located to the right of the beam exit slit of width W defined by a pair of opaque masks as indicated by the black rectangular blocks). A mosaic crystal (e.g. pyrolytic graphite) directs incident neutrons (by energy-selective Bragg reflection), originating from a temporally and spatially extended incoherent source, through a pair of slits, resulting in a quasi-monochromatic beam being incident on a sample. This beam is a collection of individual neutrons, each with an associated wave packet (in the drawing, ΔrT indicates a measure of the packet transverse spatial width). The jth individual wave packet corresponds to one specific neutron that is a member of a collection of N similar neutrons. Each packet has a mean wavevector kM. (a) Another schematic illustrating how the beam is characterized by a distribution of packet mean wavevectors that define a geometrical angular divergence related to W/L = tan(ΔθBEAM). As is also shown, each packet is itself composed of a coherent distribution of basis states with corresponding eigenvector directions associated with a distribution characterized by a width ΔθWP (where the subscript `WP' indicates wave packet). The resultant picture is one in which both coherent and incoherent distributions of wavevectors corresponding to the individual neutron packets and beam, respectively, coexist. The monochromator and pair of apertures together define both the individual and collective properties of the packets and beam, respectively, as described in the main text. |
![[Figure 2]](ge5117fig2.jpg)
 | JOURNAL OF APPLIED CRYSTALLOGRAPHY |
ISSN: 1600-5767
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