Moderating a free neutron
Principle
Moderation is the process of reducing the kinetic energy of a neutron.
The initial kinetic energy of a neutron released from a nucleus is generally of the order of a few MeV.
To reduce it to a few meV (~10-3 eV), the neutron must undergo collisions with light nuclei.
The energy transfer depends on the mass ratio A between the target nucleus and the neutron.
(ΔE / E) = f(A, θ)
The kinetic energy of the neutron after collision is minimal for θ = 180°.
Efinal = Einitial × ((A − 1)/(A + 1))²
Progressive reduction of neutron energy through successive collisions.
The closer the nucleus mass is to the neutron mass, the more efficient the energy reduction is during each collision.
To moderate a neutron, it must undergo several tens of successive collisions.