$ \ce{^{A}_{Z}X} + n \longrightarrow \ce{^{A}_{(Z-1)}X} + p $ The **(n-p) reaction**, or **(n,p) reaction**, is the reaction where a neutron enters a nucleus and a proton leaves the nucleus simultaneously. Most (n,p) reactions have threshold neutron energies such that the reaction cannot take place due to a charged particle in the exit channel. To overcome this Coulomb barrier, the emitted proton requires more energy.