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Neutron source
From Wikipedia, the free encyclopedia
Neutron source is a general term referring to a variety devices that emit neutrons, irrespective of the mechanism used to produce the neutrons. Depending upon variables including 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, these devices find use in a diverse array of applications in areas of physics, engineering, medicine, nuclear weapons, petroleum exploration, biology, chemistry, nuclear power and other industries.
There are several kinds of neutron sources:
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- Spontaneous Fission
- Certain isotopes undergo spontaneous fission with emission of neutrons. The most commonly used spontaneous fission source is the radioactive isotope californium-252. Cf-252 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. Cf-252 neutron sources are typically 1/4" to 1/2" in diameter and 1" to 2" in length. When purchased new a typical CF-252 neutron sources emit 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 Cf-252 neutron source is from $15,000 to $20,000.
- Alpha Reaction
- 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 intermixing a radioisotope that emits alpha particles such as radium or polonium with a low atomic weight isotope, usually in the form of a mixture of powders of the two materials. Sources based upon this reaction are comparable in size and cost with spontaeous fission neutron sources. 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 price of these neutron sources is also comparable to spontaneous fission sources. Usual combinations of materials are plutonium-beryllium (PuBe), americium-beryllium (AmBe), or americium-lithium (AmLi).
- Sealed Tube Neutron Generator
- Some accelerator-based neutron generators exist that work by inducing fusion between beams of deuterium and/or tritium ions and metal hydride targets which also contain these isotopes.
- Photofission
- Neutrons are produced when gamma rays above the nuclear binding energy of a substance are incident on that substance, causing it to fission. The number of neutrons released by each fission event is dependent on the substance.
- Photoneutron
- Gamma radiation with an energy exceeding the neutron binding energy of a nucleus can eject a neutron. Two examples and their decay products:
- Beryllium 9 + 1.7Mev gamma ray > 1 neutron + 2 Helium 4
- Mercury 198 + 6.8Mev gamma ray > 1 neutron + Mercury 197(half-life 2.7 days > Gold 197)
- Plasma Focus and Plasma Pinch
- The plasma focus neutron source (see Plasma focus, not to be confused with the so-called Farnsworth-Hirsch fusor) produces controlled nuclear fusion by creating a dense plasma within which ionized deuterium and/or tritium gas is heated to temperatures sufficient for creating fusion.
- Nuclear fission in a reactor produces neutrons which can be used for experiments. This (and not the study of nuclear fission itself) is the purpose of nuclear research reactors.
- 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.
For most applications, a higher neutron flux is always 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 useable 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).