- It has been seen that slow
neutrons are very efficient at inducing nuclear reactions - hence fast
neutrons sometimes need to be moderated.
- Slow neutrons are thus a product of many elastic
collisions with nuclei. In all cases the conservation of
S (mass × velocity)before
= S (mass × velocity)after
- Therefore if you take the mass of a neutron to be 1, and the mass of
the target nucleus = A:
||= original neutron energy, and
||= neutron energy after collision
- The average change in the 'ln' of energy is called x
and is proportional to the slowing down power and
- Each neutron has a unique history and overall we can only deduce the
average behaviour, hence only the average cosines (cos
q) of the angles scattered. The
average scattering parameter arises as the average loss of energy is
proportional to the pre-collision energy.
(Greek 'Xi') - As the neutrons go slower, the energy loss per
collision becomes smaller.
(A = atomic mass)
This shows why light elements are used as moderators (e.g. H20).
For hydrogen, A=1 and a=0, so the neutron loses
all its energy.
Slowing down power (SDP) - A large x
is of little importance unless there is a high probability of scattering
reactions taking place.
- SDP = x × SS
where SS = macroscopic
scattering cross section of the moderator for epithermal neutrons.
- SS = N0 × sS
where N0 = atoms per unit volume and sS
= microscopic cross section.
Moderator ratio (MR) - Compromise between too
much moderation and good slowing down power.