Measuring Radiation Damage: dpa
Displacements per atom (dpa) is the common yardstick for radiation damage; it lets exposures from different sources be compared on a physical basis.
What dpa counts
When a neutron strikes a material, it can knock atoms out of their lattice positions. Displacements per atom (dpa) estimates, on average, how many times each atom has been displaced over an exposure. It is the standard yardstick for radiation damage because it ties the abstract idea of 'dose' to a physical event in the material.
Why dpa needs context
The same dpa produced by different neutron spectra is not always equivalent, because higher-energy neutrons also produce more gas (helium and hydrogen) inside the material through transmutation. Two exposures with equal dpa but different gas production can leave a material in very different states, so dpa is reported alongside gas-production ratios for fusion-relevant work.
- dpa: average number of times each atom is displaced.
- Higher-energy neutrons produce more gas per dpa.
- Gas-to-dpa ratio distinguishes fusion from fission exposures.
- Fusion qualification tracks both dpa and gas production.
Using dpa honestly
Reporting damage in context
Because equal dpa can leave a material in different states depending on how much gas accompanied the displacements, honest reporting pairs dpa with the gas-production ratio. This context is what lets a result from one source be interpreted correctly against another, and it is especially important for fusion-relevant work, where the gas-to-displacement ratio is high. Treating dpa as a complete description of damage, without the accompanying gas information, can flatter a material tested in a softer spectrum. Reporting both quantities is part of representing fusion-spectrum irradiation results faithfully rather than optimistically.
For the breeder's neutron services, dpa and gas-production characteristics are computed for its 14 MeV spectrum at design stage. Measured accumulation rates on real samples are FOAK-era results. Reporting both dpa and gas ratios is part of representing damage honestly.