Neutron Radiography

the production of an image of a specimen as a result of the action on a photosensitive layer of the secondary radiation that is generated when a specimen is irradiated by neutrons. Neutron radiography is used primarily in the investigation of metals, alloys, and minerals to ascertain the presence and distribution of various impurities in the specimen (seeFLAW DETECTION). The nuclei become radioactive as a result of neutron capture.

The neutron radiography method is based on the different probabilities of capture of a neutron by different atomic nuclei. If a neutron-irradiated specimen, usually a thin sheet, is placed against a photographic film, regions with different degrees of darkening are produced on the developed photograph; this procedure is called neutron photography. The darker areas correspond to nuclei that absorb neutrons more strongly. The presence and distribution of certain impurities in the specimen can be determined not only from the secondary radiation but also from the attenuation of the primary neutron flux as a result of neutron absorption by impurity nuclei. A foil made of an element that becomes beta active on exposure to neutrons, such as silver, dysprosium, or indium, is placed between the specimen and the photographic layer. In this case, the lighter areas correspond to stronger absorption of neutrons.