Neutron imaging is the process of making an image with neutrons. The resulting image is based on the neutron attenuation properties of the imaged object. The resulting images have much in common with industrial X-ray images, but since the image is based on neutron attenuating properties instead of X-ray attenuation properties, some things easily visible with neutron imaging may be very challenging or impossible to see with X-ray imaging techniques (and vice versa). X-rays are attenuated based on a material's density. Denser materials will stop more X-rays. With neutrons, a material's likelihood of attenuation of neutrons is not related to its density. Some light materials such as boron will absorb neutrons while hydrogen will generally scatter neutrons, and many commonly used metals allow most neutrons to pass through them. This can make neutron imaging better suited in many instances than X-ray imaging; for example, looking at O-ring position and integrity inside of metal components, such as the segments joints of a Solid Rocket Booster.
History The neutron was discovered by James Chadwick in 1932. The first demonstration of neutron radiography was made by Hartmut Kallmann and E. Kuhn in the late 1930s. They discovered that upon bombardment with neutrons, some materials emitted radiation that could expose film. The discovery remained a curiosity until 1946 when low quality radiographs were made by Peters. The first neutron radiographs of reasonable quality were made by J. Thewlis (UK) in 1955. Circa 1960, Harold Berger (US) and John P. Barton (UK) began evaluating neutrons for investigating irradiated reactor fuel. Subsequently, a number of research facilities were developed. The first commercial facilities came on-line in the late 1960s, mostly in the United States and France, and eventually in other countries including Canada, Japan, South Africa, Germany, and Switzerland.
Process To produce a neutron image, a source of neutrons, a collimator to shape the emitted neutrons into a fairly mono-directional beam, an object to be imaged, and some method of recording the image are required.
Neutron sources Generally the neutron source is a research reactor, where a large number of neutrons per unit area (flux) is available. Some work with isotope sources of neutrons has been completed (largely spontaneous fission of Californium-252, but also Am-Be isotope sources, and others). These offer decreased capital costs and increased mobility, but at the expense of much lower neutron intensities and significantly lower image quality. Additionally, accelerator sources of neutrons have increased in availability, including large accelerators with spallation targets and these can be suitable sources for neutron imaging. Portable accelerator based neutron generators utilizing the neutron yielding fusion reactions of deuterium-deuterium or deuterium-tritium.
Moderation After neutrons are produced, they need to be slowed down (decrease in kinetic energy), to the speed desired for imaging. This can take the form of some length of water, polyethylene, or graphite at room temperature to produce thermal neutrons. In the moderator the neutrons will collide with the nucleus of atoms and so slow down. Eventually the speed of these neutrons will achieve some distribution based on the temperature (amount of kinetic energy) of the moderator. If higher energy neutrons are desired, a graphite moderator can be heated to produce neutrons of higher energy (termed epithermal neutrons). For lower energy neutrons, a cold moderator such as liquid deuterium, can be used to produce low energy neutrons (cold neutron). If no or less moderator is present, high energy neutrons (termed fast neutrons), can be produced. The higher the temperature of the moderator, the higher the resulting kinetic energy of the neutrons is and the faster the neutrons will travel. Generally, faster neutrons will be more penetrating, but some interesting deviations from this trend exist and can sometimes be utilized in neutron imaging. Generally an imaging system is designed and set up to produce only a single energy of neutrons, with most imaging systems producing thermal or cold neutrons. In some situations, selection of only a specific energy of neutrons may be desired. To isolate a specific energy of neutrons, scattering of neutrons from a crystal or chopping the neutron beam to separate neutrons based on their speed are options, but this generally produces very low neutron intensities and leads to very long exposures. Generally this is only carried out for research applications. This discussion focuses on thermal neutron imaging, though much of this information applies to cold and epithermal imaging as well. Fast neutron imaging is an area of interest for homeland security applications, but is not commercially available currently and generally not described here.
Collimation In the moderator, neutrons will be traveling in many different directions. To produce a good image, neutrons need to be traveling in a fairly uniform direction (generally slightly divergent). To accomplish this, an aperture (an opening that will allow neutrons to pass through it surrounded by neutron absorbing materials), limits the neutrons entering the collimator. Some length of collimator with neutron absorption materials (e.g. boron) then absorbs neutrons that are not traveling the length of the collimator in the desired direction. A tradeoff exists between image quality, and exposure time. A shorter collimation system or larger aperture will produce a more intense neutron beam but the neutrons will be traveling at a wider variety of angles, while a longer collimator or a smaller aperture will produce more uniformity in the direction of travel of the neutrons, but significantly fewer neutrons will be present and a longer exposure time will result.
Object The object is placed in the neutron beam. Given increased geometric unsharpness from those found with X-ray systems, the object generally needs to be positioned as close to the image recording device as possible.
Detection A variety of methods are commonly employed to detect and record neutron images. Until recently, neutron imaging was generally recorded on X-ray film, but a variety of digital methods are now available.
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