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Neutron Sources

A neutron source is any device that emits neutrons. Neutron sources have many applications. They can be used in research, engineering, medicine, petroleum exploration, biology, chemistry, and nuclear power. Many factors characterize a neutron source:
  • Significance of the source
  • Intensity. The rate of neutrons emitted by the source.
  • Energy distribution of emitted neutrons.
  • Angular distribution of emitted neutrons.
  • Mode of emission. Continuous or pulsed operation.

Classification by significance of the source

  • Large (Significant) neutron sources
    • Nuclear Reactors. Some nuclei can undergo fission on their own spontaneously, but only certain nuclei, like uranium-235, uranium-233, and plutonium-239, can sustain a fission chain reaction. This is because these nuclei release neutrons when they break apart, which can induce the fission of other nuclei. Uranium-235, which exists as 0.7% of naturally occurring uranium, undergoes nuclear fission with thermal neutrons with the production of, on average, 2.4 fast neutrons and the release of ~ 180 MeV of energy per fission. Free neutrons released by each fission play a very important role as a trigger of the reaction, but they can also be used for another purpose. For example, one neutron is required to trigger further fission. Part of free neutrons (let say 0.5 neutrons/fission) is absorbed in other material, but an excess of neutrons (0.9 neutrons/fission) can leave the surface of the reactor core and can be used as a neutron source.
    • Fusion Systems. Nuclear fusion is a nuclear reaction in which two or more atomic nuclei (e.g.,, D+T) collide at very high energy and fuse together. Thy byproduct of DT fusion is a free neutron (see picture). Therefore, nuclear fusion reactions can produce large quantities of neutrons.
    • Spallation Sources. A spallation source is a high-flux neutron source in which protons that have been accelerated to high energies hit a heavy target material, causing the emission of neutrons. The reaction occurs above a certain energy threshold for the incident particle, typically 5 – 15 MeV.
  • Medium neutron sources
    • Bremssstrahlung from Electron Accelerators / Photofission. When slowed down rapidly in a heavy target, Energetic electrons emit intense gamma radiation during the deceleration process. This is known as Bremsstrahlung or braking radiation. The interaction of the gamma radiation with the target produces neutrons via the (γ,n) reaction, or the (γ, fission) reaction when a fissile target is used. e-→Pb → γ→ Pb →(γ,n) and (γ,fission). The Bremsstrahlung γ energy exceeds the binding energy of the “last” neutron in the target. A source strength of 1013 neutrons/second produced in short (i.e., < 5 μs) pulses can be readily realized.
    • Dense plasma focus. The dense plasma focus (DPF) is a device known as an efficient source of neutrons from fusion reactions. The dense plasma focus (DPF) mechanism is based on nuclear fusion of short-lived plasma of deuterium and/or tritium. This device produces a short-lived plasma by electromagnetic compression and acceleration that is called a pinch. This plasma is during the pinch hot and dense enough to cause nuclear fusion and the emission of neutrons.
    • Light ion accelerators. Neutrons can also be produced by particle accelerators using targets of deuterium, tritium, lithium, beryllium, and other low-Z materials. In this case, the target must be bombarded with accelerated hydrogen (H), deuterium (D), or tritium (T) nuclei.
  • Small neutron sources
    • Neutron Generators. Neutrons are produced in the fusion of deuterium and tritium in the following exothermic reaction. 2D + 3T → 4He + n + 17.6 MeV.  The neutron is produced with a kinetic energy of 14.1 MeV. This can be achieved on a small scale in the laboratory with a modest 100 kV accelerator for deuterium atoms bombarding a tritium target. Continuous neutron sources of ~1011 neutrons/second can be achieved relatively simply.
    • Radioisotope source – (α,n) reactions. In certain light isotopes, the ‘last’ neutron in the nucleus is weakly bound and is released when the compound nucleus formed following α-particle bombardment decays. The bombardment of beryllium by α-particles leads to the production of neutrons by the following exothermic reaction: 4He + 9Be→12C + n + 5.7 MeV. This reaction yields a weak source of neutrons with an energy spectrum resembling that from a fission source and is used nowadays in portable neutron sources. Radium, plutonium, or americium can be used as an α-emitter.
    • Radioisotope source – (γ,n) reactions. (γ,n) reactions can also be used for the same purpose. In this type of source, because of the greater range of the γ-ray, the two physical components of the source can be separated, making it possible to ‘switch off’ the reaction if so required by removing the radioactive source from the beryllium. (γ,n) sources produce monoenergetic neutrons, unlike (α,n) sources.  The (γ,n) source uses antimony-124 as the gamma emitter in the following endothermic reaction.

124Sb→124Te + β− + γ

γ + 9Be→8Be + n – 1.66 MeV

    • Radioisotope source – spontaneous fission. Certain isotopes undergo spontaneous fission with the emission of neutrons. The most commonly used spontaneous fission source is the radioactive isotope californium-252. Cf-252 and 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.
Nuclear chain reaction as a neutron source
A nuclear chain reaction occurs when one single nuclear reaction causes an average of one or more subsequent nuclear reactions.

Nuclear fusion reaction as a neutron source

Neutron Source Example Neutron Yield Released Energy

MeV / neutron

(α,n) reactions Radium-Beryllium source 8 x 10-5 / α 6,600,000
(D,T) fusion 400 keV deuterons in neutron generator 4 x 10-5 / deuteron 10,000
Electron Bremsstrahlung

/ Photofission

100 MeV electrons on uranium 5 x 10-2 / electron 2,000
Fission Induced fission of U-235  in thermal reactor 1 / fission 180
Spallation 800 MeV protons on uranium 30 / proton 55
(D,T) fusion Inertial confinement fusion 1 / fusion 18

See previous:

Shielding of Neutrons

See above: