Understanding Neutron Radiography

3) Fast Neutrons
The other energy range of interest for neutron radiography is fast neutrons,
energies above 10 keV. Neutron cross sections at these energies tend to be
low so that detectors generally require greater neutron exposures than do
thermal neutron radiographs. The detection approaches are similar to those
used for thermal neutrons, but different conversion materials are normally
used. Direct exposure has been obtained with fluorescent screens made for
X-radiography; the screens are combined with light-sensitive X-ray films. The
X-ray phosphor screens work primarily because neutron interactions with
hydrogen, in the plastic binder or in the plastic or paper backing, yield protons
that stimulate light from the phosphor. Fast X-ray phosphor screens and film
provide neutron radiographs of Me V energy neutrons with total exposures on
the order of 10 7 to 10 8 n/cm 2 . X-ray film without screens also responds to fast
neutrons but requires exposures of 4 X 10 8 n/cm 2 to more than 10 9 n/cm 2 .
Charlie Chong/ Fion Zhang