A nuclear reaction is a process when two atomic nuclei or subatomic particles interact to produce one or more new particles or gamma rays. Thus, a nuclear reaction must cause a transformation of at least one nuclide to another. Sometimes if a nucleus interacts with another nucleus or particle without changing the nature of any nuclide, the process is referred to as a nuclear scattering rather than a nuclear reaction. Perhaps the most notable nuclear reactions are the nuclear fusion reactions of light elements that power the energy production of stars and the Sun. Natural nuclear reactions also occur in the interaction between cosmic rays and matter.
Types of Nuclear Reactions
Although the number of possible nuclear reactions is enormous, nuclear reactions can be sorted by type. Most nuclear reactions are accompanied by gamma emissions. Some examples are:
- Elastic scattering. In elastic scattering, the kinetic energy of a particle is conserved in the center-of-mass frame, but its direction of propagation is modified. There is no energy transferred into nuclear excitation in an elastic scattering reaction. It is a crucial reaction for neutron moderators in nuclear reactors. To be an effective moderator, the probability of an elastic reaction between the neutron and the nucleus must be high.
1H (n, n) 1H
- Inelastic scattering. In inelastic scattering, the particle is absorbed and then re-emitted. The difference of kinetic energies is saved in an excited nuclide. An inelastic scattering plays an important role in slowing down neutrons, especially at high energies and by heavy nuclei.
238U (n, n’) 238U*
- Capture reaction. The capture reaction is one of the two possible absorption reactions that may occur. Capture reactions result in the loss of a neutron coupled with the production of one or more gamma rays. The resulting nucleus may also undergo a subsequent decay, such as beta decay in this example, which is a very important reaction in nuclear fuel.
238U (n, ˠ) 239U
- Transfer Reaction. The absorption of a particle accompanied by the emission of one or more particles is called the transfer reaction. These reactions are common in particle accelerators and astrophysics.
4He (α, p) 7Li
- Fission reactions. Nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts (lighter nuclei). The fission process often produces free neutrons and photons (in the form of gamma rays) and releases a large amount of energy.
235U (n, 3n) fission products
- Fusion reactions. Occur when two or more atomic nuclei collide at a very high speed and join to form a new type of atomic nucleus. The fusion reaction of deuterium and tritium is exciting because of its potential of providing energy for the future.
3T (d, n) 4He
- Spallation reactions. Spallation reaction occurs when a particle hits a nucleus with sufficient energy and momentum to knock out several small fragments or smash them into many fragments. Nuclear spallation is one of the processes by which a particle accelerator may be used to produce a beam of neutrons.
209Bi (α,xn)213-xAt
- Nuclear decay (Radioactive decay). Occurs when an unstable atom loses energy by emitting ionizing radiation. Radioactive decay is a random process at the level of single atoms, in that, according to quantum theory, it is impossible to predict when a particular atom will decay. There are many types of radioactive decay:
- Alpha radioactivity. Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus. Because of its huge mass (more than 7000 times the mass of the beta particle) and its charge, it heavily ionizes material and has a very short range.
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- Beta radioactivity. Beta particles are high-energy, high-speed electrons or positrons emitted by specific radioactive nuclei such as potassium-40. The beta particles emitted are a form of ionizing radiation, also known as beta rays. The beta particles have a greater range of penetration than alpha particles but still much less than gamma rays. The production of beta particles is termed beta decay.
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- Gamma radioactivity. Gamma rays are electromagnetic radiation of a very high frequency and are therefore high-energy photons. Nuclei decay produces them as they transition from a high-energy state to a lower state known as gamma decay. Most nuclear reactions are accompanied by gamma emissions.
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- Neutron emission. Neutron emission is a type of radioactive decay of nuclei containing excess neutrons (especially fission products), in which a neutron is simply ejected from the nucleus. This type of radiation plays a key role in nuclear reactor control because these neutrons are delayed neutrons.