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Nuclear processes

Radioactive decay processes Alpha decay Beta decay Cluster decay Double beta decay Double electron capture Electron capture Gamma radiation Internal conversion Isomeric transition Neutron emission Positron emission Proton emission Spontaneous fission Nucleosynthesis Nuclear fusion Proton-proton chain reaction CNO cycle Triple-alpha process Carbon burning process Neon burning process Oxygen burning process Silicon burning process Neutron capture The R-process The S-process Proton capture: The P-process The Rp-process Spallation

Neutron capture is a kind of nuclear reaction in which an atomic nucleus collides with one or more neutrons and they merge to form a heavier nucleus. Since neutrons have no electric charge, they can enter a nucleus more easily than charged particles which are repelled by electrostatic repulsion.

Neutron capture plays an important role in the cosmic nucleosynthesis of heavy elements. In stars, it can proceed in two ways - as a rapid process (an r-process) or a slow process (an s-process). By neutron capture, nuclei of masses greater than 56 can be formed that could not be formed by thermonuclear reactions, i.e., by nuclear fusion.

Contents 1 Neutron capture at small neutron flux 2 Neutron capture at high neutron flux 3 Uses 4 See also

At small neutron flux, as in a nuclear reactor, a single neutron is captured by a nucleus. For example, when natural gold (¹⁹⁷Au) is irradiated by neutrons, the isotope ¹⁹⁸Au is formed in a highly excited state which then quickly decays to the ground state of ¹⁹⁸Au by the emission of γ rays. In this process, the mass number increases by one. In terms of a formula, this is written ¹⁹⁷Au(n,γ)¹⁹⁸Au. If thermal neutrons are used, this is called thermal capture.

The isotope ¹⁹⁸Au is a beta emitter that decays into the mercury isotope ¹⁹⁸Hg (see decay scheme). In this process, the atomic number (the number of protons in the nucleus) rises by one.

The s-process mentioned above happens in the same way, but inside of stars.

The r-process happens inside stars if the neutron flux density is so high that the atomic nucleus has no time to decay via beta emission in between neutron captures. The mass number therefore rises by a large amount while the atomic number (i.e., the element) stays the same. Only afterwards, the highly unstable nuclei decay via many β^- decays to stable or unstable nuclei of high atomic number.

Neutron capture can be used to remotely detect the chemical composition of materials. This is because different elements release different characteristic radiation when they absorb neutrons. This makes it useful in many fields related to mineral exploration and security.

• Beta decay
• Gamma ray
• List of particles
• Neutron
• Neutron emission
• Neutron radiation
• Radioactivity
• Rays: α — β — γ — δ — ε
• p-process (proton capture)

http://ie.lbl.gov/ng.html Thermal Neutron Capture Data