Understanding Neutron Radiography

1) Epithermal Neutrons
Work with neutrons in the epithermal or resonance energy region
(approximately 0.3 eV to 10 keV energy) has been concentrated mainly on
the lower end of the energy spectrum. Although neutron cross sections tend
to decrease as the neutron energy increases above the thermal energy range,
there are also large resonances that occur in this energy region. Therefore,
detectors such as indium, with a large activation resonance at 1.46 eV, have
been widely used for these neutrons. Gold, with a resonance at 4.9 eV, is
another potentially useful detector. A common detection method has been to
filter the neutron beam with cadmium or gadolinium (to remove thermal
neutrons) and to then use a transfer detection method with a conversion
screen such as indium. This technique has been used to examine fast reactor
fuels because its higher energy provides greater neutron transmission
through enriched U-235 and plutonium materials (see Fig. 2 for an example).
Greater transmission (with less scatter) for radiography ofhydrogenous
objects can be accomplished in a similar manner.
Charlie Chong/ Fion Zhang