displacements per atom (dpa) is the standard measure of 📝neutron damage in materials, counting how many times, on average, each atom in a solid has been knocked out of its crystal lattice site by irradiation.
The unit exists because raw neutron counts mislead. Neutron fluence — the time-integrated 📝neutron flux — says how many neutrons passed through a material, but not what they did to it: a fast neutron displaces vastly more atoms than a slow one, and different materials absorb the same bombardment differently. Displacements per atom folds fluence, energy spectrum, and material response into a single damage dose, which makes doses comparable across facilities with different spectra. A dose of 1 dpa means every atom has, on average, been displaced once; most displaced atoms fall back into lattice sites, and it is the small surviving population of defects that hardens and embrittles the material.
The measure has a known blind spot for fusion. Structural components near a fusion plasma must survive doses measured in tens of displacements per atom, but the 14 MeV neutrons of the 📝D-T reaction also transmute nuclei and generate helium inside the metal — and two exposures with identical dpa but different helium production damage a material very differently. Fusion materials work therefore tracks the helium-to-dpa ratio alongside the dose itself, one reason fission reactor data alone cannot qualify materials for fusion service.