Neutron radiography

Definition

• 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.

Neutron imaging

• 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.

Process

• 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.

Moderation

• 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.

Moderation

• 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.

Moderation

• 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.

Collimation

• 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.

Collimation

• 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.

Object

• The object is placed in the neutron beam, as close as possible to the image-recording device.

Conversion

• 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.

Conversion

• 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.

Image recording

• 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.

Neutron radiography

• 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.

Neutron radiography

• The direct method is performed with the film present in the beamline, so neutrons 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.

Neutron radiography

• 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

• 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.

Neutrons

• In some rare cases, radiography is done with neutrons. This type of radiography is called neutron radiography (NR, Nray, N-ray) or neutron imaging. Neutron radiography provides different images than X-rays, because neutrons can pass with ease through lead and steel but are stopped by plastics, water and oils. Neutron sources include radioactive (241Am/Be and Cf) sources, electrically driven D-T reactions in vacuum tubes and conventional critical nuclear reactors. It might be possible to use a neutron amplifier to increase the neutron flux.[2]

Neutrons

• Since the amount of radiation emerging from the opposite side of the material can be detected and measured, variations in this amount (or intensity) of radiation are used to determine thickness or composition of material. Penetrating radiations are those restricted to that part of the electromagnetic spectrum of wavelength less than about 10 nanometers.

Principle

• Neutron Radiography is an imaging technique which provides images similar to X-ray radiography. The difference between neutron and X-ray interaction mechanisms produce significantly different and often complementary information. While X-ray attenuation is directly dependent on atomic number, neutrons are efficiently attenuated by only a few specific elements.

Principle

• For example, organic materials or water are clearly visible in neutron radiographs because of their high hydrogen content, while many structural materials such as aluminium or steel are nearly transparent. The next table shows how most materials behave when placed in the path of a neutron beam.

Principle

• Neutron Radiography is an imaging technique which provides images similar to X-ray radiography. Neutron interactions with matter can be divided into scattering and absorption. Neutrons are able to detect elements containing hydrogen atoms through metallic containers. The information provided by spatial and temporal beam attenuation is recorded on magnetic media via analogic or digital signals.