The (n,p) reaction is a type of nuclear reaction where a neutron enters a nucleus, and simultaneously, a proton leaves the nucleus. For instance, when sulfur-32 (S-32) is bombarded with neutrons, it undergoes an (n,p) nuclear reaction, resulting in the formation of phosphorus-32 (P-32).^[1]

Similarly, nitrogen-14 (N-14) can undergo an (n,p) nuclear reaction to produce carbon-14 (C-14). This reaction, 14N (n,p) 14C, occurs continuously in the Earth’s atmosphere, leading to equilibrium levels of the radionuclide carbon-14.

Most (n,p) reactions have threshold neutron energies below which the reaction cannot occur because the charged particle in the exit channel requires energy (usually more than a MeV) to overcome the Coulomb barrier experienced by the emitted proton. However, the 14N (n,p) 14C nuclear reaction is an exception to this rule. It is exothermic, meaning it can occur at all incident neutron energies. This reaction is primarily responsible for the radiation dose delivered to the human body by thermal neutrons. Thermal neutrons, absorbed by nitrogen (N-14) in proteins, emit a proton, which then deposits its kinetic energy over a very short distance in the body tissue, thereby contributing to the radiation dose.