Free neutron

A free neutron is a hydrogen atom:

Even though it is not a mass number of one.

Production

Various nuclides become more stable by expelling neutrons as a spontaneous fission.

Cosmic radiation interacting the earth's atmosphere continuously generates neutrons that can be detected at the surface.

Nuclear fission reactors naturally produce free neutrons; their role is to sustain the energy-producing neutron capture.

Experimental nuclear fusion reactors produce free neutrons as a waste product. However, it is these neutrons that possess most of the energy, and converting that energy to a useful form has proved a difficult engineering challenge to nuclear physicists. This also explains why this form of energy is likely to create around twice the amount of radioactive waste of a fission reactor, but with a short (50-100 years) decay period (as opposed to the 10,000 years for fission waste). [1] [2]

Thermal neutron

A neutrons arrive at this energy level, provided that they are not absorbed.

In many substances, thermal neutrons have a much larger effective cross-section than faster neutrons, and can therefore be absorbed more easily by any chemical element as a result.

Most fission reactors use a fast breeder reactors, use fission energy neutrons directly.

Cold neutrons

These neutrons are thermal neutrons that have been equilibrated in a very cold substances such as liquid neutron scattering research facilities.

Fission energy neutron

A nuclear fission.

Fast neutrons can be made into thermal neutrons via a process called moderation. This is done with a graphite are used to moderate neutrons.

Fusion neutrons can have higher energies such as 14.1 MeV for D-T fusion, or 2.45 MeV for D-D fusion to ³He. See Nuclear fusion#Criteria and candidates for terrestrial reactions for a list.

Intermediate neutrons

A fission energy neutron that is slowing down is often said to have intermediate energy. There are not many non-elastic reactions in this energy region, so most of what happens is just slowing to thermal speeds before eventual capture. Intermediate energy neutrons are a hazard in reactors owing to the existence of a Doppler broadening.

High-energy neutrons

These neutrons have more energy than fission energy neutrons and generated in accelerators or in the atmosphere from cosmic particles. They can have energies as high as tens of joules per neutron.

See also

• Neutron radiation
• Neutron scattering
• Neutron temperature
• List of particles
• Nuclear reaction
• Thermal reactor
• Dineutron
• Fast neutron
• Ionizing radiation
• Isotope
• Neutron flux
• Neutron star
• Neutronium
• Tetraneutron
• Neutron generator

References

1. ^ [Particle Data Group's Review of Particle Physics 2006]

• Krane, K. S. (1998) Introductory Nuclear Physics
  
• G. L. Squires (1997) Introduction to the Theory of Thermal Neutron Scattering
  
• M. S. Dewey, D. M. Gilliam, J. S. Nico, M. S. Snow and F. E. Wietfeldt NIST Neutron Lifetime Experiment