Neutron radiography is the process by which film is exposed by first passing neutrons through an object to produce a visible image of the materials that make up the object. It is primarily used in scientific investigations.
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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 a photographic 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.
Around 1960, Harold Berger (US) and John 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 US and France, and eventually in many other countries including Canada, Japan, South Africa, Germany, and Switzerland.
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, and these attenuation properties distinguish neutron and X-ray images. Attenuation of X-rays is proportional to density – denser materials stop more X-rays – whereas neutron absorption is not. Some light materials such as boron strongly absorb neutrons while many commonly used metals allow most neutrons to pass through them.
Neutron imaging requires a source of neutrons, a collimator to shape the emitted neutrons into a fairly unidirectional beam, an object to be imaged, and some method of recording the image.
Generally the neutron source is a nuclear reactor, which can provide a high neutron density (flux). Some work with isotope sources of neutrons has been completed (largely spontaneous fission of californium-252, but also Am-Be isotope sources, among others), these offer mobility and lower costs, but at the expense of much lower neutron densities and thus lower image quality. Additionally, accelerator sources of neutrons are available, including accelerators with spallation targets.
After neutrons are produced, they need to be slowed down ("moderated"), to the speed desired for imaging. This can be achieved with water, polyethylene, or graphite to produce thermal neutrons. In the moderator the neutrons collide with the atomic nuclei and slow down. Eventually the speed of these neutrons will achieve some distribution based on the temperature 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 cold neutrons. Generally, faster neutrons are more penetrating, but some interesting deviations from this trend exist and can sometimes be utilized in neutron imaging. Usually an imaging system is designed and set up to produce thermal or cold neutrons.
In some situations, selection of only a specific energy of neutrons may be desired. This is achieved by scattering neutrons from a crystal or chopping the neutron beam to separate neutrons based on their speed, but this generally produces very low neutron intensities and leads to very long exposures.
In the moderator, neutrons travel in many different directions, whereas they should be collimated to produce a good image. 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 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.
The object is placed in the neutron beam, as close as possible to the image-recording device.
Neutrons are difficult to measure directly, and need to be converted into some other form of radiation. Some form of conversion screen generally is employed to perform this task, though some image capture methods incorporate conversion materials directly into the image recorder. Often this takes the form of a thin layer of gadolinium, a very strong absorber of thermal neutrons. A 25-micrometer-thick layer of gadolinium is sufficient to absorb 90% of the thermal neutrons incident on it. In some situations, other elements such as boron, indium, gold, or dysprosium may be used or materials such as LiF scintillation screens where the conversion screen absorbs neutrons and emits visible light.
A variety of methods are commonly employed to produce images with neutrons. Until recently, neutron imaging was generally recorded on X-ray film, but a variety of digital methods are now available.
Note: The term neutron radiography is often misapplied to all neutron imaging methods.
Neutron radiography is the process of producing a neutron image that is recorded on film. This is generally the highest resolution form of neutron imaging though digital methods with ideal setups are recently achieving comparable results. The most frequently used approach uses a gadolinium conversion screen to convert neutrons into high-energy electrons, which expose a single-emulsion X-ray film.
The direct method is performed with the film present in the beamline, so neutron are absorbed by the conversion screen which promptly emits some form of radiation exposing the film. The indirect method does not have a film directly in the beamline. The conversion screen absorbs neutrons, but some time delay exists prior to the release of radiation. Following recording the image on the conversion screen, the conversion screen is put in close contact with a film for hours to produce an image on the film. The indirect method has significant advantages when dealing with radioactive objects, or imaging systems with high gamma contamination, otherwise the direct method is generally preferred.
Neutron radiography is a commercially available service, widely used in the aerospace industry for the testing of turbine blades in airplane engines, components for space programs, high-reliability explosives, and to a lesser extent in other industry to identify problems during product development cycles.
Track etch is a largely obsolete method. A conversion screen converts neutron to alpha particles that produce damage tracks in a piece of cellulose. An acid bath is then used to etch the cellulose, to produce a piece of cellulose whose thickness varies with neutron exposure.
Several processes exist for taking digital neutron images with thermal neutrons. These imaging methods are widely used in academic circles, in part because they avoid the need for film processors and dark rooms. Additionally film images can be digitized through the use of transmission scanners.
A CCD camera is an imaging system very similar to digital cameras. They can record real-time images (generally with low resolution) that is useful for studying two-phase fluid flow in opaque pipes, hydrogen bubble formation in fuel cells, and lubricant movement in engines. This imaging system, in conjunction with a rotary table, can take a large number of images at different angles which can be reconstructed into a three-dimensional image (neutron tomography).
Neutrons pass through the object to be imaged, then a scintillation screen converts the neutrons to visible light. This light then passes through some optics (intended to minimize the camera's exposure to ionizing radiation), then the image is captured by the CCD camera. Images can be displayed on a TV screen. Generally averaging of numerous images is required to produce a reasonable still image.
X-ray image plates can be used in conjunction with a plate scanner to produce neutron images much as X-ray images are produced with the system. The neutrons still need to be converted into some other form of radiation to be captured by the image plate. Fuji produced neutron-sensitive image plates that contained a converter material in the plate and offered better resolution than is possible with an external conversion material. Image plates offer a process that is very similar to film imaging, but the image is recorded on a reusable image plate that is read and cleared after imaging. These systems only produce still images. Using a conversion screen and an X-ray image plate, comparable exposure times are required to produce an image with lower resolution than film imaging. Image plates with embedded conversion material produce better images than external conversion, but worse than film.
The use of microchannel plates is an emerging method that produces a digital detector array with very small pixel sizes. The device has small (micrometer) channels through it. Its source side is coated with a material (generally gadolinium or boron) which absorbs neutrons and converts them into ionizing radiation that frees electrons. A large voltage is applied across the device, causing the freed electrons to be amplified as they are accelerated through the small channels and then detected by a digital detector array.