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. 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.
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 nineteen thirties; 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 in 1955.Around 1960, Harold Berger and John P. Barton 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 sixties, mostly in the United States and France, and eventually in many 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 is available. Some work with isotope sources of neutrons has been completed. 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, 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 of the moderator. If higher energy neutrons are desired, a graphite moderator can be heated to produce neutrons of higher energy. For lower energy neutrons, a cold moderator such as liquid deuterium, can be used to produce low energy neutrons. If no or less moderator is present, high energy 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. To accomplish this, an aperture, 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 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.Conversion
Though numerous different image recording methods exist, neutrons are not generally easily measured and need to be converted into some other form of radiation that is more easily detected. 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 for thermal neutrons. A 25 micrometer 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 (film)
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, that expose a single emulsion x-ray film.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 that exposes 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 a period of time, 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 for airplane engines, components for space programs, high reliability explosives, and to a lesser extent in other industry to identify problems during product development cycles.
The term "neutron radiography" is often misapplied to refer to all neutron imaging methods.
Track etch
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.Digital neutron imaging
Several processes for taking digital neutron images with thermal neutrons exists that have different advantages and disadvantages. These imaging methods are widely used in academic circles, in part because they avoid the need for film processors and dark rooms as well as offering a variety of advantages. Additionally film images can be digitized through the use of transmission scanners.Neutron camera (DR System)
A neutron camera is an imaging system based on a digital camera or similar detector array. Neutrons pass through the object to be imaged, then a scintillation screen converts the neutrons into visible light. This light then pass through some optics, then the image is captured by the CCD camera.Neutron cameras allow real time images, which has proved 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 that can be reconstructed into a three-dimensional image.
When coupled with a thin scintillation screen and good optics these systems can produce high resolution images with similar exposure times to film imaging, though the imaging plane typically must be small given the number of pixels on the available CCD camera chips.
Though these systems offer some significant advantages, significant disadvantages exist including dead pixels on the camera, gamma sensitivity of the scintillation screens, limited field of view, and the limited lifetime of the cameras in the high radiation environments.