Question

A good moderator should
A) be a gas
B) have an appetite for neutrons
C) be lighter in mass number
D) be heavier in mass number

Answer 1

A moderator is a medium that reduces the speed of fast neutrons, produced during the fission reaction. This slowing or moderation of the neutrons allows them to be more easily absorbed by fissile nuclei, creating more fission events. Therefore, a good moderator must be able to slow down the neutrons effectively.

Complete step by step answer:
Materials used for moderation need a very specific set of properties.
First, it is to be noted that a moderator cannot absorb neutrons by itself. This means that the moderator needs to have a low neutron absorption cross-section. Therefore, option (b) is canceled out.
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. So, option (a) is neglected as gas cannot provide a greater cross-section at the atomic level.
Therefore, they need to be lighter in mass numbers so they can absorb neutrons to some extent. Hence, option (c) is the correct answer.
The moderator must have low atomic mass and not higher, therefore, option (d) is incorrect.

So, the correct answer is “Option C”.

Note:
Some other properties that a good moderator must have are,
1. They should have low atomic mass
2. The size of the atom in moderator must be of the almost same size as the the prompt neutron
3. They should have a good scattering property
4. They should have poor absorption of mass