The SNS uses cold hydrogen-rich moderators to cool the neutrons to a few tens of degrees above absolute zero.
Although this is quite cold, the fraction of neutrons with energy low enough to be trapped (equivalent temperature ~ 1 milliKelvin) is vanishingly small, so we rely on a superthermal production mechanism.
The energy/momentum curve of collective oscillations in superfluid helium (phonons and rotons) overlaps the energy-momentum curve of neutrons (E = p2/2m). This allows neutrons of a specific energy (~1 meV, corresponding to a deBroglie wavelength of ~8.9 Å) to lose effectively all their energy in a single collision.
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