The principal source of helium is _________.

Explanation

To answer the question, "The principal source of helium is _________," we can break down the information provided in the video transcript step by step.

### Step-by-Step Solution:

1. **Understanding Helium**: Helium is a noble gas with the chemical symbol He and atomic number 2. It is known for being non-reactive and is the second lightest element in the universe.

2. **Sources of Helium**: Helium can be sourced from several places:
- It is produced through the radioactive decay of heavy elements, specifically uranium and thorium.
- Helium can also be extracted from the air by liquefying it, but this method yields only small amounts.
- Another source is natural gas, where helium is present in trace amounts.

3. **Identifying the Principal Source**: Among the sources mentioned, the most significant source of helium is from the radioactive decay of uranium and thorium. These elements are found in certain minerals.

4. **Key Mineral for Helium**: The important mineral that serves as a source of helium is called monazite sand. Monazite sand contains thorium and uranium, which contribute to the production of helium through radioactive decay.

5. **Final Answer**: Therefore, the principal source of helium is **monazite sand**.

### Final Answer:
The principal source of helium is **monazite sand**.