Question

a. Compare the interactions of protons/alpha particles to electrons with absorber materials? What properties are factors in slowing down and ultimately stopping these particles?

b. Do you expect neutrons to slow down as a result of the same interactions? What type of material do you think could be used to slow down a neutron? Why? Hint: In a nuclear reactor, neutrons produced in the fuel are slowed down in the water. Think about why water is effective at slowing down the neutrons.

Answer 1

b.

Neutron moderators are a type of material in a nuclear reactor that work to slow down the fast neutrons (produced by splitting atoms in fissile compounds like uranium-235), to make them more effective in the fission chain reaction. This slowing or moderation of the neutrons allows them to be more easily absorbed by fissile nuclei, creating more fission events .

Materials used for moderation need to a very specific set of properties. First, a moderator cannot absorb neutrons itself. This means that the moderator should have a low neutron absorption cross-section. However, the moderator should be able to slow down neutrons to an acceptable speed. Thus, in an ideal moderator the neutron scattering cross-section is high.This neutron scattering is a measure of how likely a neutron will interact with an atom of the moderator. If the collisions between neutrons and nuclei are elastic collisions, it implies that the closer in size the nucleus of an atom is to a neutron, the more the neutron will be slowed. For this reason, lighter elements tend to be more efficient moderators. The table below shows that common moderators have a low neutron absorption cross-section but a comparatively large neutron scattering cross-section.