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a flat profile. The final shape of neutron beam profile fi-
nally determine by the shape of aperture. Rectangular
shape has advantageous since its can produce large
beam area as compared with the one produce when
using circular aperture.

In addition to shaping the geometry of thermal neu-
tron beam, the collimator has a function as shielding to
reduce gamma radiation and unwanted neutron from
the source. In order to fullfil this function, materials like
lead or bismuth are composed in the collimator. Lead
is used to eliminate gamma or neutron emitted outside
the optical path shapes by the aperture while bismuth
usually used to reduce gamma radiation in the main
stream within the optical path from the aperture. The
bismuth will make thermal neutron after passing the
aperture clean from gamma. Therefore, thermal neu-
tron to gamma ratio within the optical path from aper-
ture to collimator opening will increase.

Typical design of collimator used for Neutron Ra-
diography and Imaging (NURI) facility at reactor TRIGA
PUSPATI are shown in Figure 5.4 and details geome-
try and the materials used to compose the collimator
is given in Figure 5.4. The beam size is determined by
the 4 cm aperture opening made of 1 mm thick cad-
mium sheet. It is placed after the bismuth rod. Dis-
tance from the aperture to the collimator opening is
about 720 cm. Usage of Ferro-Boron is to minimise
leakage of neutrons and also to reduce gamma in sur-

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