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A neutron source is any device that emits
neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the atomic nucleus, nuclei of atoms. Since protons and ...
s, irrespective of the mechanism used to produce the neutrons. Neutron sources are used in physics, engineering, medicine, nuclear weapons, petroleum exploration, biology, chemistry, and nuclear power. Neutron source variables include the energy of the neutrons emitted by the source, the rate of neutrons emitted by the source, the size of the source, the cost of owning and maintaining the source, and government regulations related to the source.


Small devices


Spontaneous fission Spontaneous fission (SF) is a form of radioactive decay that is found only in very heavy chemical elements. The nuclear binding energy of the elements reaches its maximum at an atomic mass number of about 56 (e.g., iron-56); spontaneous breakd ...
(SF)

Some
isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers ( mass num ...
s undergo SF with emission of neutrons. The most common spontaneous fission source is the isotope
californium Californium is a radioactive chemical element with the symbol Cf and atomic number 98. The element was first synthesized in 1950 at Lawrence Berkeley National Laboratory (then the University of California Radiation Laboratory), by bombarding c ...
-252. 252Cf and all other SF neutron sources are made by irradiating
uranium Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...
or a transuranic element in a nuclear reactor, where neutrons are absorbed in the starting material and its subsequent reaction products, transmuting the starting material into the SF isotope. 252Cf neutron sources are typically 1/4" to 1/2" in diameter and 1" to 2" in length. A typical 252Cf neutron source emits 107 to 109 neutrons per second when new; but with a half-life of 2.6 years, neutron output drops by half in 2.6 years. A typical 252Cf neutron source costs $15,000 to $20,000.


Radioisotopes which alpha decay; mixed with a light element

Neutrons are produced when
alpha particle Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay, but may also be prod ...
s hit any of several light isotopes including isotopes of beryllium, carbon, or oxygen. Thus, one can make a neutron source by mixing an alpha-emitter such as
radium Radium is a chemical element with the symbol Ra and atomic number 88. It is the sixth element in group 2 of the periodic table, also known as the alkaline earth metals. Pure radium is silvery-white, but it readily reacts with nitrogen (rat ...
, polonium, or
americium Americium is a synthetic radioactive chemical element with the symbol Am and atomic number 95. It is a transuranic member of the actinide series, in the periodic table located under the lanthanide element europium, and thus by analogy was n ...
with a low-atomic-weight isotope, usually by blending powders of the two materials. Alpha neutron sources typically produce ~106–108 neutrons per second. An alpha-beryllium neutron source may produce about 30 neutrons per 106 alpha particles. The useful lifetime for such sources depends on the half-life of the radioisotope. The size and cost of these neutron sources are comparable to spontaneous fission sources. Usual combinations of materials are
plutonium Plutonium is a radioactive chemical element with the symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exh ...
-
beryllium Beryllium is a chemical element with the symbol Be and atomic number 4. It is a steel-gray, strong, lightweight and brittle alkaline earth metal. It is a divalent element that occurs naturally only in combination with other elements to for ...
(PuBe),
americium Americium is a synthetic radioactive chemical element with the symbol Am and atomic number 95. It is a transuranic member of the actinide series, in the periodic table located under the lanthanide element europium, and thus by analogy was n ...
-beryllium (AmBe), or americium-
lithium Lithium (from el, λίθος, lithos, lit=stone) is a chemical element with the symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard conditions, it is the least dense metal and the least dense soli ...
(AmLi).


Radioisotopes which decay with high-energy photons co-located with beryllium or deuterium

Gamma radiation with an energy exceeding the neutron binding energy of a nucleus can eject a neutron (
photoneutron Photodisintegration (also called phototransmutation, or a photonuclear reaction) is a nuclear process in which an atomic nucleus absorbs a high-energy gamma ray, enters an excited state, and immediately decays by emitting a subatomic particle. The ...
). Two example reactions are: *9 Be + >1.7 MeV photon → 1 neutron + 2 4He *2 H (
deuterium Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two stable isotopes of hydrogen (the other being protium, or hydrogen-1). The nucleus of a deuterium atom, called a deuteron, contains one proton and one ...
) + >2.26 MeV photon → 1 neutron + 1H


Sealed-tube neutron generators

Some accelerator-based neutron generators induce fusion between beams of
deuterium Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two stable isotopes of hydrogen (the other being protium, or hydrogen-1). The nucleus of a deuterium atom, called a deuteron, contains one proton and one ...
and/or tritium ions and metal hydride targets which also contain these isotopes.


Medium-sized devices


Plasma focus and plasma pinch devices

The dense plasma focus neutron source produces controlled nuclear fusion by creating a dense plasma within which heats ionized
deuterium Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two stable isotopes of hydrogen (the other being protium, or hydrogen-1). The nucleus of a deuterium atom, called a deuteron, contains one proton and one ...
and/or tritium gas to temperatures sufficient for creating fusion.


Inertial electrostatic confinement

Inertial electrostatic confinement devices such as the Farnsworth-Hirsch fusor use an
electric field An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field ...
to heat a plasma to fusion conditions and produce neutrons. Various applications from a hobby enthusiast scene up to commercial applications have developed, mostly in the US.


Light ion accelerators

Traditional particle accelerators with hydrogen (H), deuterium (D), or tritium (T) ion sources may be used to produce neutrons using targets of deuterium, tritium, lithium, beryllium, and other low-Z materials. Typically these accelerators operate with energies in the > 1 MeV range.


High-energy

bremsstrahlung ''Bremsstrahlung'' (), from "to brake" and "radiation"; i.e., "braking radiation" or "deceleration radiation", is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typical ...
photoneutron/photofission systems

Neutrons are produced when photons above the nuclear binding energy of a substance are incident on that substance, causing it to undergo giant dipole resonance after which it either emits a neutron (
photoneutron Photodisintegration (also called phototransmutation, or a photonuclear reaction) is a nuclear process in which an atomic nucleus absorbs a high-energy gamma ray, enters an excited state, and immediately decays by emitting a subatomic particle. The ...
) or undergoes fission (
photofission Photofission is a process in which a nucleus, after absorbing a gamma ray, undergoes nuclear fission and splits into two or more fragments. The reaction was discovered in 1940 by a small team of engineers and scientists operating the Westingho ...
). The number of neutrons released by each fission event is dependent on the substance. Typically photons begin to produce neutrons on interaction with normal matter at energies of about 7 to 40 MeV, which means that
radiotherapy Radiation therapy or radiotherapy, often abbreviated RT, RTx, or XRT, is a therapy using ionizing radiation, generally provided as part of cancer treatment to control or kill malignant cells and normally delivered by a linear accelerator. Rad ...
facilities using megavoltage X-rays also produce neutrons, and some require neutron shielding. In addition, electrons of energy over about 50 MeV may induce giant dipole resonance in nuclides by a mechanism which is the inverse of internal conversion, and thus produce neutrons by a mechanism similar to that of photoneutrons.


Large devices


Nuclear fission reactors

Nuclear fission Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei. The fission process often produces gamma photons, and releases a very large amount of energy even by the energetic standards of radio ...
within a reactor, produces many neutrons and can be used for a variety of purposes including power generation and experiments. Research reactors are often specially designed to allow placement of material samples into a high-neutron-flux environment.


Nuclear fusion systems

Nuclear fusion, the fusing of heavy isotopes of hydrogen, also has the potential to produces large numbers of neutrons. Small scale fusion systems exist for (plasma) research purposes at many universities and laboratories around the world. A small number of large scale fusion experiments also exist including the
National Ignition Facility The National Ignition Facility (NIF) is a laser-based inertial confinement fusion (ICF) research device, located at Lawrence Livermore National Laboratory in Livermore, California, United States. NIF's mission is to achieve fusion ignition w ...
in the US, JET in the UK, and soon the
ITER ITER (initially the International Thermonuclear Experimental Reactor, ''iter'' meaning "the way" or "the path" in Latin) is an international nuclear fusion research and engineering megaproject aimed at creating energy by replicating, on Ear ...
experiment currently under construction in France. None are yet used as neutron sources. Inertial confinement fusion has the potential to produce orders of magnitude more neutrons than
spallation Spallation is a process in which fragments of material (spall) are ejected from a body due to impact or stress. In the context of impact mechanics it describes ejection of material from a target during impact by a projectile. In planetary p ...
. This could be useful for neutron radiography which can be used to locate hydrogen atoms in structures, resolve atomic thermal motion and study collective excitation of nuclei more effectively than
X-rays An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nbs ...
.


High-energy particle accelerators

A
spallation Spallation is a process in which fragments of material (spall) are ejected from a body due to impact or stress. In the context of impact mechanics it describes ejection of material from a target during impact by a projectile. In planetary p ...
source is a high-flux source in which
proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...
s that have been accelerated to high energies hit a target, prompting emission of neutrons. The world's strongest neutron sources tend to be spallation based as high flux fission reactors have an upper bound of neutrons produced. As of 2022, the most powerful neutron source in the world is the
Spallation Neutron Source The Spallation Neutron Source (SNS) is an accelerator-based neutron source facility in the U.S. that provides the most intense pulsed neutron beams in the world for scientific research and industrial development.In 2007, SNS was entered into th ...
in
Oak Ridge, Tennessee Oak Ridge is a city in Anderson County, Tennessee, Anderson and Roane County, Tennessee, Roane counties in the East Tennessee, eastern part of the U.S. state of Tennessee, about west of downtown Knoxville, Tennessee, Knoxville. Oak Ridge's popu ...
,{{cite web , title=SUF Spallation Neutron Source (S... {{! U.S. DOE Office of Science (SC) , url=https://science.osti.gov/bes/suf/User-Facilities/Neutron-Scattering-Facilities/SNS#:~:text=The%20Spallation%20Neutron%20Source%20(SNS,a%20power%20of%201%20MW. , website=science.osti.gov , access-date=19 October 2022 , date=29 April 2022 with the
European Spallation Source The European Spallation Source ERIC (ESS) is a multi-disciplinary research facility based on the world's most powerful pulsed neutron source. It is currently under construction in Lund, Sweden. The ESS Data Management and Software Centre (DMS ...
in
Lund Lund (, , ) is a city in the southern Swedish province of Scania, across the Öresund strait from Copenhagen. The town had 91,940 inhabitants out of a municipal total of 121,510 . It is the seat of Lund Municipality, Scania County. The Öre ...
, Sweden under construction to become the world's strongest intermediate duration pulsed neutron source. Subcritical nuclear fission reactors are proposed to use spallation neutron sources and can be used both for
nuclear transmutation Nuclear transmutation is the conversion of one chemical element or an isotope into another chemical element. Nuclear transmutation occurs in any process where the number of protons or neutrons in the nucleus of an atom is changed. A transmutatio ...
(e.g. production of medical radionuclides or
synthesis of precious metals The synthesis of precious metals involves the use of either nuclear reactors or particle accelerators to produce these elements. Precious metals occurring as fission products Ruthenium, rhodium Ruthenium and rhodium are precious metals produ ...
) and for power generation as the energy required to produce one spallation neutron (~30 MeV at current technology levels) is almost an order of magnitude lower than the energy released by fission (~200 MeV for most fissile actinides).


Neutron flux

For most applications, higher neutron flux is better (since it reduces the time needed to do the experiment, acquire the image, etc.). Amateur fusion devices, like a fusor, generate only about 300 000 neutrons per second. Commercial fusor devices can generate on the order of 109 neutrons per second, hence a usable flux of less than 105 n/(cm² s). Large neutron beams around the world achieve much greater flux. Reactor-based sources now produce 1015 n/(cm² s), and spallation sources generate > 1017 n/(cm² s).


See also

* Neutron emission * Neutron generator, commercial devices * Neutron temperature ('fast' or 'slow') *
Startup neutron source Startup neutron source is a neutron source used for stable and reliable initiation of nuclear chain reaction in nuclear reactors, when they are loaded with fresh nuclear fuel, whose neutron flux from spontaneous fission is insufficient for a reliab ...
* Zetatron * A Subcritical nuclear reactor relies on an "external" neutron source


References


External links


Neutronsources.orgScience and Innovation with Neutrons in Europe in 2020 (SINE2020)
Source Nuclear technology