A neutron radiograph is similar to an x-ray radiograph, in that a penetrating radiation is passed through an object and an image is recorded on exposed film. The density of the exposed film varies proportionally with the amount of radiation deposited on the film. Differing amounts of attenuation of the incident beam of radiation produce an image on the film. While x-rays are preferentially attenuated by materials with high atomic numbers, such as iron, copper, or lead, neutrons are preferentially attenuated by materials having low atomic numbers such as hydrogen, carbon, or beryllium. Water and other hydrogenous materials will slow and stop neutrons but are relatively poor absorbers for x-rays.
Neutrons normally collide with small nuclei and are slowed down before absorption.
Because neutrons are neutral in charge and do not directly affect film, a converter foil which emits charged particles after irradiation by a neutron is used for neutron radiography to activate the film. The direct method uses a high neutron cross section, short half life gadolinium foil to expose x-ray film. The x-ray film is placed emulsion side toward the foil. Upon absorbing a neutron, the foil decays with a very short half life giving off an internal transition electron. This electron is the primary radiation mechanism for darkening the x-ray film.