The history of neutron radiography with thermal neutrons can be traced to the discovery of neutrons in 1932. A neutron is an uncharged particle with a mass slightly greater than that of a proton. An important property of a neutron is that it is electrically neutral, resulting in negligible electrostatic interaction with the atoms electrons. Neutron radiography can inspect large thicknesses of heavy metals and discriminate between neighboring elements such as boron and carbon.
The basic principle underlying this technique is the penetrating nature of neutron radiation and its differential absorption by the material to obtain details of the internal structure. The object to be examined is placed in a collimated neutron beam. Neutrons on passing through the object are differentially absorbed. This absorption depends on atomic number, thickness of materials, homogeneity and composition. The geometric pattern of the transmitted neutron intensity is recorded using a suitable detector and visualized.
Neutron sources available for radiography fall into three classes namely (1) nuclear reactors (2) particle accelerators and (3) radioisotopes. A majority of practical neutron radiography has been done using nuclear reactors as the source because nuclear reactors are prolific sources of neutrons even when operating at low or medium power levels. In case of accelerators nuclear reactions are used to produce neutrons from accelerated charged particles. Neutrons are obtained from radioisotopes by making use of either the (a, n) reaction or (?, n) reaction.
Thermal neutrons are used for performing all neutron radiography work. Whether it is from a reactor, accelerator or isotope primary neutrons are high-energy neutrons. Their average energy is moderated to thermal range using a moderator such as water or hydrogenous materials.
Detection of Neutrons
Neutrons are not ionizing radiation and hence, have no effect on conventional films used in industrial radiography. Hence detection systems for neutrons consist of a converter screen, which absorbs neutrons and converts them into a form of secondary radiation. The converter screens are often metallic foils. The emissions from these foils can either be charged particles or electromagnetic radiation, which produce an image on the film/screen. The technique used for imaging can be classified as direct or indirect.
Direct technique: A foil of gadolinium is used before the film. Gadolinium atoms in the foil absorb a neutron and promptly emit an electron as the secondary radiation. A scintillator screen consisting of a mixture of lithium-6 and zinc sulphide can also be used. On observing a neutron a lithium atom emits an alpha particle and this strikes the zinc sulphide screen, which in turn emits a proton. In this method the film is in contact with the converter during neutron exposure.
Indirect technique: This is also referred to as transfer technique. This method relies on the buildup of radioactivity in the foil produced by neutron absorption. In this way, an activation image is formed on the foil and this is subsequently transferred to a photographic film in contact, thereby allowing decay radiations from the foil to produce a latent image on the film. This technique is much slower compared to the direct one.
The following are applications for neutron radiographic techniques.
In the nuclear field neutron radiography has been extensively used for post irradiation metallurgical examination of nuclear fuel elements, control rods, irradiation rigs for differentiation of isotopes like uranium 235 and uranium 238.
In the non-nuclear field, this technique is widely used for the inspection of explosive devices, cooling passages in turbine blades, foreign materials in electronic relays and packages.
Recent applications of neutron radiography are in the study of multiphase flow measurements in thermal hydraulics using real time image intensifier based systems.
Neutron radiography is applied in aerospace industry to detect hidden corrosion damage in multi layered structures made of aluminum alloys.