In nuclear physics, separation energy is the energy needed to remove one nucleon (or other specified particle or particles) from an atomic nucleus.
The lowest separation energy among stable nuclides is 1.67 MeV, to remove a neutron from beryllium-9.
The energy can be added to the nucleus by an incident high-energy gamma ray.
It is the sum of multiple separation energies, which should add to the same total regardless of the order of assembly or disassembly.
The reaction leads to photoionization, photodissociation, the photoelectric effect, photovoltaics, etc.