Understanding Neutron Radiography

■ Neutron-Gamma Response
One important property of direct exposure methods is the relative response of
the detector to neutron and gamma or X-radiation. A transfer neutron
radiograph has essentially no response to gamma rays. For direct exposures,
however, the film is in the beam and will offer some response to gamma
radiation. Depending on what information is sought from such a radiograph,
the gamma image will probably reduce the contrast obtained with a true
neutron radiograph. In most cases a gamma-ray component in a neutron
radiograph is something to be avoided. Of the direct exposure methods just
discussed, neutron scintillator techniques employing a ZnS phosphor (with an
alpha emitter) provide the best neutron-gamma response ratio. Typically for
these scintillators, an exposure of 10 4 n/cm 2 will yield about the same film
density as an exposure of 1 mR of Co-60 gamma radiation; the exact ratio is
dependent on the film and other variables. The metal conversion screen
methods typically have more response to gamma rays by a factor of ten. The
glass scintillators and the rare earth scintillators provide a relative neutronamma
response between these two extremes.
Charlie Chong/ Fion Zhang