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4.2.3 Aperture slit

Neutron beam reflected after monochromator is in gau-
sian form, where the highest neutron flux is within the
peak center. Therefore, the size of neutron beam within
the peak center is shaped by using aperture slit. The
neutron beam within the tales side of aperture slit will
absorbed of the aperture, thus has no influence to the
diffraction process. Due to this reason, the material
of aperture should has high capability in absorb ther-
mal neutron. In this case, cadmium material with 1 mm
thickness is sufficient to make an aperture. The diffrac-
tion of neutron occurs for neutron within the highest
peak only.This is important character function of the
aperture slit.

Another crucial function of aperture slit is it will lim-
its the beam penetrating the object under test, thus de-
termines the gauge volume fraction of interaction with
the crystal planes. The arrangement of aperture slit
will also determine the size of gauge volume. Larger
gauge volume will be more representative and accu-
rate. However, this will reduces thermal neutron flux at
the detector position significantly. Opening of aperture
has a square form with a knive shape of each edge.
Otherwise some reduction in thermal neutron flux oc-
curs due to unsharp edge of aperture opening.

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