|Science with Neutrons|
A neutron source is a device that emits neutrons. There exist a wide variety of neutron sources of differing mechanism and size, ranging from hand-held radioactive sources and tabletop electrically powered neutron generators, to large neutron research facilities operating research reactors and spallation sources.
Depending upon neutron energy, neutron flux, size of the source, costs, and government regulations, 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.
- Radioisotopes which undergo 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.[clarification needed]
- 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 lithium, 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. 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 also comparable to spontaneous fission sources. Usual combinations of materials are plutonium-beryllium (PuBe), americium-beryllium (AmBe), or americium-lithium (AmLi). The neutron initiators of early nuclear weapons used a device constructed of polonium-beryllium layers separated by nickel and gold, until a neutron pulse was desired. The implosion of the bomb then caused the mechanical mixing of the layers, by swirling them together and breaking the nickel and gold barriers between polonium and beryllium.
- Photoneutron sources, or radioisotopes which decay with high energy photons (gamma decay) co-located with beryllium or deuterium
- Gamma radiation with an energy exceeding the neutron binding energy of a nucleus can eject a neutron, called a photoneutron. Two examples and their decay products are:
- Sealed tube neutron generators
- Some particle accelerator-based neutron generators exist that work by inducing nuclear fusion between beams of deuterium and/or tritium ions and metal hydride targets which also contain these isotopes; a typical example is made with a glass, or ceramic envelope with cylindrical symmetry, to facilitate ion beam control. Presently, the smallest known, accelerator based, neutron source is the Neutristor, which is made in the shape of a computer chip, using solid state technology.
- Plasma focus and plasma pinch devices
- The plasma focus neutron source (see dense 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.
- 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 voltages in the > 1 MeV range,
- High energy photoneutron/photofission systems
- Neutrons (photoneutrons) 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 (photodisintegration) 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 megavoltage photon radiotherapy facilities may produce neutron radiation as well, and require special shielding for it. In addition, fast electrons of energy over about 50 MeV may also induce giant dipole resonance in nuclides by a mechanism which is the inverse of internal conversion, and electron linacs can be used directly as sources of "virtual photons" that produce bombardment neutrons by a mechanism similar to that of photoneutrons.
Modern neutron research facilities operate either fission reactor or a spallation source.
- Nuclear fission reactors
- Nuclear fission which takes place within a nuclear reactor produces very large quantities of neutrons. In nuclear power reactors, the neutrons are no more than an unavoidable byproduct. In contrast, research reactors are primarily operated to produce neutron beams. The Institut Laue-Langevin source in Grenoble in France is the major fission source in Europe, the High Flux Isotope Reactor at ORNL is the equivalent in the US. Reactor neutron sources tend to require highly-enriched uranium fuel, making their construction somewhat controversial.
- Spallation at 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. Examples are the Swiss neutron source SINQ, the British ISIS neutron source, and the U.S. Spallation Neutron Source; the China spallation neutron source is under construction in Dongguan
- Nuclear fusion systems
- Nuclear fusion, the combining of the heavy isotopes of hydrogen, also has the potential to produce large quantities of neutrons. Small scale fusion systems exist for research purposes at many universities and laboratories around the world. A small number of large scale nuclear fusion systems also exist including the National Ignition Facility in the USA, JET in the UK, and soon the recently started ITER experiment in France.
Neutron flux density
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 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).
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- Rev. Sci. Instr., Vol. 31, No. 3, pg. 241-248, March 1960