Types of Nuclear Reactions
4 min read
There are many different types of interactions between neutrons and atomic nuclei. In the nuclear power industry, one of the key interactions is the absorption of a neutron by a fissile nucleus and its subsequent fission. However, an encounter between a neutron and a nucleus can have several different outcomes.
Video: No interaction.
No Interaction
If a neutron passes near a nucleus but remains more than about 2 × 10−15 m from its surface, it is not significantly affected by the short-range nuclear force and no nuclear interaction occurs.
Video: Elastic scattering.
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Elastic Scattering
An incident neutron can scatter from a nucleus in accordance with the laws of conservation of energy and momentum and then continue in a different direction. Some of its kinetic energy is transferred to the recoiling nucleus, but the nucleus is not excited and no nuclear transformation occurs. This is referred to as elastic scattering. We can visualise this interaction as the collision of a ping-pong ball (the neutron) with a bowling ball (the nucleus). The amount of energy transferred depends strongly on the mass of the target nucleus. Collisions with light nuclei, particularly hydrogen, are therefore very effective in slowing down neutrons.
The probability that a neutron will undergo a particular interaction with a nucleus is characterised by its microscopic cross-section. Nuclear cross-sections depend strongly on neutron energy and are commonly expressed in barns, where 1 barn = 10−28 m2.
Video: Inelastic scattering.
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Inelastic Scattering
In inelastic scattering, an incident neutron transfers part of its kinetic energy to the nucleus, leaving it in an excited state. The scattered neutron therefore emerges with lower energy. The excited nucleus subsequently releases its excess energy, usually by emitting one or more gamma rays. At sufficiently high neutron energies, other nuclear reactions involving the emission of particles may also become possible.
Video: Radiative capture.
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Radiative Capture
During radiative capture, a target nucleus absorbs an incident neutron and forms an excited compound nucleus. The excess energy is then released by the emission of one or more gamma rays. This reaction is commonly denoted (n,γ). An important example in nuclear reactors is the capture of a neutron by 238U, which produces 239U and a gamma ray. Materials containing strong neutron absorbers, such as boron or cadmium, are also widely used in reactors to control the neutron population.
Video: Fission.
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Fission
Neutron-induced fission can occur when a fissile nucleus absorbs a neutron and forms an excited compound nucleus. The nucleus deforms and may eventually split into two lighter nuclei, known as fission fragments. Fission is a common outcome when a 235U nucleus absorbs a thermal neutron with an energy of about 0.025 eV. The fission fragments move apart at speeds of roughly 10,000 km/s and gradually lose their kinetic energy through interactions with surrounding atoms. At the same time, typically two or three fast neutrons are emitted, allowing a fission chain reaction to develop. Other fissile nuclides, such as 239Pu and 233U, can undergo neutron-induced fission in a similar way.
When 238U captures a neutron, it becomes 239U. This subsequently undergoes two beta decays, first forming 239Np and then fissile 239Pu.
