Neutron Moderation App
When a neutron scatters off a moderator with molecular mass 'A', energy is transferred from the incident neutron to the material. The plot below visualises the share of energy as a function of scattering angle for a neutron with an initial energy of 1 MeV.
The logarithmic energy decrement, Ⲝ, is the average change in the natural logarithm of the neutron energy after an interaction. The average loss of energy is proportional to the pre-collision energy. As neutrons slow down, the energy loss per collision is correspondingly smaller. The advantage of using Ⲝ is that it is independent of the current neutron energy:
Ⲝ = ln(E0/E1)
E0 - initial neutron energy
En - final neutron energy
Ⲝ - logarithmic energy decrement
With each collision, the average value of ln(E) decreases by Ⲝ and, after n collisions, the average value of ln(En) is:
ln(En) = ln(E0) - nⲜ
where; Ⲝ = 1 + ((A - 1)2/2A)ln((A - 1)/(A + 1))
Ⲝ is dependens on the atomic mass, A, of the material in which the neutrons are being scattered. For a neutron with an initial energy of 1 MeV, the number of interactions for it to be thermalised (En = 0.025 eV) can be determined using the interactive table below:
| Ⲝ | n |
|---|---|