Animated Solution for Physics - Atoms and Nuclei: Fast neutrons can easily be slowed down by

Reduction

Imagine you are inside the core of a nuclear reactor. Uranium-235 atoms are splitting apart, releasing a tremendous amount of energy and spitting out fast-moving neutrons. These fast neutrons are zipping around at incredible speeds. However, there is a catch: to sustain the nuclear chain reaction, the surrounding Uranium-235 atoms need to capture these neutrons. The problem is that fast neutrons are moving too fast to be easily captured. They need to be slowed down, or "thermalized." This crucial process of slowing down neutrons is known as moderation.

How do you slow down a subatomic particle that has no electric charge? You cannot use electric or magnetic fields because neutrons are completely neutral. The only way to slow them down is through brute force: making them crash into other atoms!

This brings us to the physics of elastic collisions. Think of a game of billiards. If you shoot a cue ball (which is light) at a heavy bowling ball, the cue ball will just bounce off and retain almost all of its kinetic energy. It changes direction, but it doesn't slow down much. However, if you shoot the cue ball at another billiard ball of the exact same mass, the cue ball can transfer almost all of its kinetic energy to the target ball, coming to a dead stop.

Mathematically, the fractional energy lost by a neutron of mass $m$ colliding with a target nucleus of mass $M$ is given by:

To maximize the energy loss $\Delta E$, the mass of the target nucleus $M$ must be as close as possible to the mass of the neutron $m$.

Let's evaluate the options provided in the question.

What about lead shielding or heavy nuclei? Lead is incredibly massive ($M\gg m$). If a neutron hits a lead nucleus, the denominator in our energy transfer equation becomes huge, meaning the energy transferred is practically zero. The neutron just bounces around, staying fast. Lead is great for stopping radiation from escaping (shielding), but terrible for slowing down neutrons.

What about applying a strong electric field? As we established earlier, neutrons carry zero electric charge. An electric field will exert absolutely zero force on them ($F_e=qE=0$).

This leaves us with heavy water. Normal water ($H_{2}O$) is a decent moderator because it contains Hydrogen, whose nucleus is just a single proton (mass $\approx m$). However, normal Hydrogen has a bad habit of absorbing neutrons to become Deuterium, which removes neutrons from the chain reaction.

Heavy water ($D_{2}O$) is made of Deuterium (${}_1^{2}H$). A Deuterium nucleus consists of one proton and one neutron, giving it a mass $M\approx 2m$. Because its mass is very comparable to that of a single neutron, an elastic collision between a fast neutron and a Deuterium nucleus results in a massive transfer of kinetic energy. The neutron slows down significantly with just a few collisions, and Deuterium rarely absorbs extra neutrons.

Therefore, passing fast neutrons through heavy water is an incredibly efficient way to slow them down, making it one of the best moderators used in nuclear reactors today!