A nuclear reaction transforms one or more atomic nuclei through interactions with particles or other nuclei. Total energy, momentum, electric charge and nucleon number constrain ordinary low-energy reaction bookkeeping. Nuclear fusion, alpha capture, neutron capture and photodisintegration are important examples in stellar nucleosynthesis. Weak processes can also change the proton-to-neutron ratio.
A nuclear reaction initiated by absorption or scattering of a photon. Such channels may contribute to attenuation at photon energies above their relevant nuclear thresholds; they are distinct from atomic photoelectric absorption and electron–positron pair production.
An intermediate excited atomic nucleus formed when the reactants' available energy is redistributed among nuclear degrees of freedom before decay. Its resolved levels can appear as resonances. An isolated level's formation and decay into particular channels are described by partial decay widths in the Breit-Wigner resonance cross section; direct reactions and overlapping resonances need not obey that simple one-level description.
The effective area for a specified nuclear reaction channel at given incident energy. Multiplying incident particle flux, target count and this cross section gives an ideal event rate; target losses and detection efficiency require separate corrections.
A reaction coefficient formed from target number densities and microscopic nuclear reaction cross-sections. It has inverse-length units. Multiplying it by the scalar neutron flux gives the corresponding reaction rate per volume, under the stated energy averaging.
A cross section for removal of an incident neutron by absorption, including capture and neutron-induced fission where relevant. Its value depends on nuclide and neutron energy.
The initial minus final rest energy in a nuclear reaction; positive values represent energy available for outgoing kinetic energy in addition to the incident kinetic energy.
The same compound-state energy is measured from different rest-energy thresholds in different entrance channels. Subtracting those thresholds converts the required centre-of-momentum kinetic energies. For a stationary target and nonrelativistic projectile, the laboratory kinetic energy is . A negative converted means that entrance channel is closed at that resonance energy.
