The (n-p) reaction, or (n,p) reaction, is an example of a nuclear reaction. It is the reaction which occurs when a neutron enters a nucleus and a proton leaves the nucleus simultaneously.
For example, sulfur-32 (S) undergoes an (n,p) nuclear reaction when bombarded with neutrons, thus forming phosphorus-32 (P).
The nuclide nitrogen-14 (N) can also undergo an (n,p) nuclear reaction to produce carbon-14 (C). This nuclear reaction N (n,p) C continually happens in the Earth's atmosphere, forming equilibrium amounts of the radionuclide C.
Most (n,p) reactions have threshold neutron energies below which the reaction cannot take place as a result of the charged particle in the exit channel requiring energy (usually more than a MeV) to overcome the Coulomb barrier experienced by the emitted proton. The (n,p) nuclear reaction N (n,p) C is an exception to this rule, and is exothermic – it can take place at all incident neutron energies. The N (n,p) C nuclear reaction is responsible for most of the radiation dose delivered to the human body by thermal neutrons – these thermal neutrons are absorbed by the nitrogen N in proteins, causing a proton to be emitted; the emitted proton deposits its kinetic energy over a very short distance in the body tissue, thereby depositing radiation dose.