Problem
Problem
Fast neutrons can easily be slowed down by (JEE 1994) a. the use of lead shielding b. passing them through heavy water c. elastic collisions with heavy nuclei d. applying a strong electric fieldIntuitive explanation
The key insight is understanding which material best removes kinetic energy from a neutron through elastic collisions. When a neutron collides head-on with a nucleus, it transfers the most energy when the nucleus is light—ideally comparable to the neutron's own mass. A heavy nucleus barely budges and reflects the neutron; a very light nucleus (like hydrogen in water or paraffin) acts like a perfect elastic collision partner, absorbing significant momentum with each hit. This is why moderators work: repeated collisions with light nuclei gradually thermalize fast neutrons.
Intuitive explanation
The key insight is understanding which material best removes kinetic energy from a neutron through elastic collisions. When a neutron collides head-on with a nucleus, it transfers the most energy when the nucleus is light—ideally comparable to the neutron's own mass. A heavy nucleus barely budges and reflects the neutron; a very light nucleus (like hydrogen in water or paraffin) acts like a perfect elastic collision partner, absorbing significant momentum with each hit. This is why moderators work: repeated collisions with light nuclei gradually thermalize fast neutrons.