The Fusion Neutron Spectrum

Fusion neutrons peak near 14 MeV, a harder spectrum than fission produces, which changes how they interact with and damage materials.

Harder than fission

A neutron source is characterized by its energy spectrum. Fission reactors produce a broad spectrum peaking well below a few MeV. Deuterium-tritium fusion produces neutrons sharply peaked near 14 MeV. This 'harder' spectrum matters because the damage a neutron does depends strongly on its energy.

Why the spectrum changes the physics

Higher-energy neutrons open reaction channels — such as certain transmutation and gas-producing reactions — that lower-energy neutrons cannot. They also displace atoms differently. Testing a fusion material in a fission spectrum can therefore misrepresent how it will behave, which is the core reason fusion-spectrum sources are valued for qualification.

• Fusion neutrons peak near 14 MeV; fission neutrons are far softer.
• Higher energy opens gas-producing and transmutation reactions.
• Spectrum fidelity is essential for representative materials tests.

Spectrum fidelity in testing

The practical import of the harder spectrum is that a test is only as representative as the neutrons it uses. Because higher-energy neutrons displace atoms differently and produce more gas through transmutation, exposing a candidate fusion material to a softer fission spectrum can misstate how it will actually behave in service. Achieving spectrum fidelity — testing with neutrons whose energies match the intended environment — is therefore central to trustworthy qualification. A domestic 14 MeV source provides that fidelity, which is the specific capability the breeder is studied to add for materials programs at home.

The breeder's 14 MeV flux provides a fusion-relevant spectrum domestically. This capability is a computed design attribute of a machine at simulation stage; measured spectra and fluxes are FOAK-era results.