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A neutron source is any device that emits neutrons, 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

Certain isotopes undergo spontaneous fission with emission of neutrons. The most commonly used spontaneous fission source is the radioactive isotope californium-252. 252Cf and all other spontaneous fission neutron sources are produced by irradiating uranium or another 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. When purchased new a typical 252Cf neutron source emits between 1×107 to 1×109 neutrons per second but, with a half-life of 2.6 years, this neutron output rate drops to half of this original value in 2.6 years. The price of a typical 252Cf neutron source is from $15,000 to$20,000.
Radioisotopes which decay with alpha particles packed in a low-Z elemental matrix

Neutrons are produced when alpha particles impinge upon any of several low-atomic-weight isotopes including isotopes of beryllium, carbon, and oxygen. This nuclear reaction can be used to construct a neutron source by mixing a radioisotope that emits alpha particles such as radium, polonium, or americium with a low-atomic-weight isotope, usually by blending powders of the two materials. Typical emission rates for alpha reaction neutron sources range from 1×106 to 1×108 neutrons per second. As an example, a representative alpha-beryllium neutron source can be expected to produce approximately 30 neutrons for every one million alpha particles. The useful lifetime for these types of sources is highly variable, depending upon the half-life of the radioisotope that emits the alpha particles. The size and cost of these neutron sources are comparable to spontaneous fission sources. Usual combinations of materials are plutonium-beryllium (PuBe), americium-beryllium (AmBe), or americium-lithium (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 (a photoneutron). Two example reactions are:

9Be + >1.7 MeV photon → 1 neutron + 2 4He
2H (deuterium) + >2.26 MeV photon → 1 neutron + 1H

Sealed-tube neutron generators

Some accelerator-based neutron generators induce fusion between beams of deuterium 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 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 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 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) or undergoes fission (photofission). 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 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.[1]
Large devices
Nuclear fission reactors

Nuclear fission which takes place within a reactor produces very large quantities of 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 experiments into a high-neutron flux environment.
Nuclear fusion systems

Nuclear fusion, the combining of the heavy isotopes of hydrogen, also has the potential to produces large quantities 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 nuclear fusion experiments also exist including the National Ignition Facility in the US, JET in the UK, and soon the ITER 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.[2] This could be useful for neutron radiography which can be used to locate hydrogen atoms in structures, resolve atomic thermal motion and study collective excitations of nuclei more effectively than X-rays.
High-energy particle accelerators

A spallation source is a high-flux source in which protons that have been accelerated to high energies hit a target material, prompting the emission of neutrons.
Neutron flux

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

Neutron emission
Neutron generator, commercial devices
Neutron temperature ('fast' or 'slow')
Startup neutron source
Zetatron

References

Giant Dipole Resonance Neutron Yields Produced by Electrons as a Function of Target Material and Thickness

Taylor, Andrew; Dunne, M; Bennington, S; Ansell, S; Gardner, I; Norreys, P; Broome, T; Findlay, D; Nelmes, R (February 2007). "A Route to the Brightest Possible Neutron Source?". Science. 315 (5815): 1092–1095. Bibcode:2007Sci...315.1092T. doi:10.1126/science.1127185. PMID 17322053.