Neutron source
A neutron source is a device, used in solid state physics (see neutron diffraction), particle physics and to start nuclear chain reactions, that emits neutrons.
There are several kinds of neutron sources:
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Small devices
- Neutrons can be generated by mixing beryllium and an alpha emitter such as radium or polonium. Neutrons are liberated when beryllium is hit by alpha particles at about 30 neutrons/million alpha particles.
- Certain isotopes undergo spontaneous fission with emission of neutrons. Californium 252 can be used this way as it decays with a conveniently intermediate half-life time of 2.65 years. To produce such an isotope, one needs a high flux nuclear reactor.
- The Farnsworth-Hirsch fusor can produce controlled nuclear fusion (at a net energy loss) and is sold commercially as a neutron source.
- Some particle accelerator-based neutron sources exist that workby inducing fusion between a beam of deuterium ions and a deuterium or tritium target.
Large devices
- 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 accelerator to high energies, hit a target material, prompting the emission of neutrons.
Neutron flux
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 fluence 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).