Neutron imaging
Encyclopedia
Neutron Imaging is the process of making a 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-ray
's are attenuated based on a materials density. Denser materials will stop more x-rays. With neutrons, a materials likelihood of attenuation a neutron
is not related to 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.
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 (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 sixties, mostly in the USA and France
, and eventually in many other countries including Canada
, Japan
, South Africa
, Germany
, and Switzerland
.
, where a large numbers of neutrons per unit area (flux) is available. Some work with isotopes 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 accelerators with spallation targets and these can be suitable sources for neutron imaging.
), 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 (an isotope of Hydrogen
), 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 a imaging system is designed and setup 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.
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.
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 neutron are absrobed 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 (generally 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 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.
Neutrons pass through the object to be imaged, then a scintillation screen converts the neutrons to visible light. This light then pass 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 quality still image.
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 neutron still need to be converted into some other form of radiation to be captured by the image plate. For a short time period, 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 (static). 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 imbedded conversion material produce better images than external conversion, but currently do not produce as good of images as film.
or boron
). The neutron absorbing material absorbs neutrons and converts them into ionizing radiation that free electrons. A large voltage is applied across the device, causing the freed electrons to be amplified as they are accelerated through the small channels then detected by a digital detector array.
, Canada
, X-R-I | N-Ray Services, LLC in North Carolina
, Aerotest Operations Inc. in California
(Now Closed), and McClellan Nuclear Research Center in California
. Several (other) university research reactors also have some capabilities with neutron imaging, but they do not generally perform production volume work.
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
images, but since the image is based on neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
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-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
's are attenuated based on a materials density. Denser materials will stop more x-rays. With neutrons, a materials likelihood of attenuation a neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
is not related to density. Some light materials such as boron
Boron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
will absorb neutrons while hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
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.
Brief History of Neutron Imaging
The neutronNeutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
was discovered by James Chadwick
James Chadwick
Sir James Chadwick CH FRS was an English Nobel laureate in physics awarded for his discovery of the neutron....
in 1932. The first demonstration of neutron radiography was made by Hartmut Kallmann
Hartmut Kallmann
Harmut Kallmann was a German physicist. He is known for his work on the scintillation counter for the detection of gamma rays.-Biography:...
and E. Kuhn in the late nineteen thirties; they discovered that upon bombardment with neutrons, some materials emitted radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
that could expose film
Film
A film, also called a movie or motion picture, is a series of still or moving images. It is produced by recording photographic images with cameras, or by creating images using animation techniques or visual effects...
. 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
John Barton
John Barton may refer to:* John Barton , English theatre director and founding member of the Royal Shakespeare Company* John Barton , engineer noted for his engravings using his Ruling Engine...
(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 sixties, mostly in the USA and France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
, and eventually in many other countries including Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
, Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
, South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
, Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
, and Switzerland
Switzerland
Switzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....
.
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 nuclear reactorNuclear reactor
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...
, where a large numbers of neutrons per unit area (flux) is available. Some work with isotopes sources of neutrons has been completed (largely spontaneous fission
Fission
Fission is a splitting of something into two parts.Fission may refer to:*In physics, nuclear fission is a process where a large atomic nucleus is split into two smaller particles....
of Californium-252, but also Am
AM
Am or am may refer to:* Americium, a chemical element with symbol Am* Attometre , a unit of length * A minor , a minor chord in music* am, a form of the verb to be used as a Copula...
-Be
BE
BE, B.E., Be or be may refer to:*The verb to be *Bachelor of Engineering, undergraduate academic degree*Buddhist Era of the Thai solar calendar*Bahá'í Era, the date notation of the Bahá'í calendar...
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 accelerators with spallation targets and these can be suitable sources for neutron imaging.
Moderation
After neutrons are produced, they need to be slowed down (decrease in kinetic energyKinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
), 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 (an isotope of Hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
), 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 a imaging system is designed and setup 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 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 gadoliniumGadolinium
Gadolinium is a chemical element with the symbol Gd and atomic number 64. It is a silvery-white, malleable and ductile rare-earth metal. It is found in nature only in combined form. Gadolinium was first detected spectroscopically in 1880 by de Marignac who separated its oxide and is credited with...
is sufficient to absorb 90% of the thermal neutrons incident on it. In some situations, other elements such as boron
Boron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
, indium
Indium
Indium is a chemical element with the symbol In and atomic number 49. This rare, very soft, malleable and easily fusible post-transition metal is chemically similar to gallium and thallium, and shows the intermediate properties between these two...
, gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
, or dysprosium
Dysprosium
Dysprosium is a chemical element with the symbol Dy and atomic number 66. It is a rare earth element with a metallic silver luster. Dysprosium is never found in nature as a free element, though it is found in various minerals, such as xenotime...
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)
Note: The term “Neutron Radiography” is often misapplied to refer 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, that expose a single emulsion x-ray film.
The direct method is performed with the film present in the beamline, so neutron are absrobed 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 (generally 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 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.
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. A 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 process 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.CCD Camera (DR System)
A CCD camera is a imaging system very similar to digital cameras. They allow real time images (generally with low resolution) 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 (neutron tomography).Neutrons pass through the object to be imaged, then a scintillation screen converts the neutrons to visible light. This light then pass 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 quality still image.
Image Plates (CR System)
X-rayX-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
image plates can be used in conjunction with a plate scanner to produce neutron images much as x-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
images are produced with the system. The neutron still need to be converted into some other form of radiation to be captured by the image plate. For a short time period, 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 (static). Using a conversion screen and an x-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
image plate, comparable exposure times are required to produce an image with lower resolution than film imaging. Image plates with imbedded conversion material produce better images than external conversion, but currently do not produce as good of images as film.
Flat Panel Silicon Detectors (DR system)
A digital technique similar to CCD imaging. Neutron exposure leads to short lifetimes of the detectors that has resulted in other digital techniques becoming preferred approaches.Micro Channel Plates (DR system)
A emerging method that produces a digital detector array with very small pixel sizes. The device has small (micrometer) channels through it, with the source size coated with a neutron absorbing material (generally gadoliniumGadolinium
Gadolinium is a chemical element with the symbol Gd and atomic number 64. It is a silvery-white, malleable and ductile rare-earth metal. It is found in nature only in combined form. Gadolinium was first detected spectroscopically in 1880 by de Marignac who separated its oxide and is credited with...
or boron
Boron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
). The neutron absorbing material absorbs neutrons and converts them into ionizing radiation that free electrons. A large voltage is applied across the device, causing the freed electrons to be amplified as they are accelerated through the small channels then detected by a digital detector array.
Service Providers
The principle providers of this service in North America are Nray Services Inc. in OntarioOntario
Ontario is a province of Canada, located in east-central Canada. It is Canada's most populous province and second largest in total area. It is home to the nation's most populous city, Toronto, and the nation's capital, Ottawa....
, Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
, X-R-I | N-Ray Services, LLC in North Carolina
North Carolina
North Carolina is a state located in the southeastern United States. The state borders South Carolina and Georgia to the south, Tennessee to the west and Virginia to the north. North Carolina contains 100 counties. Its capital is Raleigh, and its largest city is Charlotte...
, Aerotest Operations Inc. in California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
(Now Closed), and McClellan Nuclear Research Center in California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
. Several (other) university research reactors also have some capabilities with neutron imaging, but they do not generally perform production volume work.