Fast neutrons can easily be slowed down by

Options

A. (a) the use of lead shielding
B. (b) passing them through heavy water
C. (c) elastic collisions with heavy nuclei
D. (d) applying a strong electric field

Explanation

To solve the question "Fast neutrons can easily be slowed down by," we will analyze the process of slowing down fast neutrons and identify the correct method.

### Step-by-Step Solution:

1. **Understanding Fast Neutrons**:
Fast neutrons are neutrons that have high kinetic energy. They are produced during nuclear reactions, such as fission in heavy nuclei like uranium-235.

**Hint**: Fast neutrons are characterized by their high energy and speed.

2. **Need for Slowing Down Neutrons**:
In nuclear reactions, fast neutrons can cause further fission events. However, to sustain a controlled nuclear reaction (like in a nuclear reactor), these fast neutrons need to be slowed down to thermal energies.

**Hint**: Slowing down fast neutrons is essential for controlling nuclear reactions.

3. **Methods to Slow Down Neutrons**:
There are several materials that can slow down fast neutrons. These materials are known as moderators. Common moderators include:
- Water (H2O)
- Heavy Water (D2O)
- Graphite
- Beryllium

**Hint**: Moderators are materials that reduce the energy of fast neutrons.

4. **Role of Heavy Water**:
Heavy water (D2O) is particularly effective in slowing down fast neutrons because it contains deuterium, which has a similar mass to neutrons. When fast neutrons collide with deuterium nuclei, they lose energy and slow down.

**Hint**: Heavy water is a common moderator used in nuclear reactors.

5. **Conclusion**:
The best answer to the question is that fast neutrons can easily be slowed down by passing them through heavy water.

**Final Answer**: Fast neutrons can easily be slowed down by heavy water.