Know the sources that make a significant contribution to background radiation including: (a) radon gas (in the air) (b) rocks and buildings (c) food and drink (d) cosmic rays

5.2.1 Detection of Radioactivity

Objective

Know the sources that make a significant contribution to background radiation, including:

• Radon gas (in the air)
• Rocks and buildings
• Food and drink
• Cosmic rays

What is Background Radiation?

Background radiation is the ionising radiation that is present everywhere in the environment, even in the absence of any artificial sources. It is measured in microsieverts per year (\$\mu\$Sv yr⁻¹) and contributes to the natural dose received by all people.

Major Natural Sources

1. Radon Gas (in the Air)

Radon (\$^{222}\$Rn) is a colourless, odourless noble gas produced by the decay of uranium in soil and rocks. It can accumulate in buildings, especially in basements and poorly ventilated rooms.

• Typical indoor radon concentration: 20–200 Bq m⁻³.
• Average annual dose from radon: about 1.2 mSv yr⁻¹ (≈ 1200 µSv yr⁻¹).
• Health impact: the leading cause of natural‑radiation‑related lung cancer.

2. Rocks and Buildings

Many rocks contain naturally occurring radionuclides such as uranium (\$^{238}\$U), thorium (\$^{232}\$Th) and potassium‑40 (\$^{40}\$K). These emit gamma rays that can penetrate walls and reach occupants.

• Granite, basalt and some building materials have higher radionuclide content.
• Typical contribution to the annual dose: 0.2–0.5 mSv yr⁻¹ (200–500 µSv yr⁻¹).

3. Food and Drink

Plants absorb radionuclides from soil and water; animals then ingest them. The main contributors are potassium‑40 and trace amounts of uranium and thorium series isotopes.

• Average dose from ingestion: 0.3 mSv yr⁻¹ (≈ 300 µSv yr⁻¹).
• Bananas are often cited because they contain about 0.1 µSv per banana (the “banana equivalent dose”).

4. Cosmic Rays

High‑energy particles from outer space interact with the Earth’s atmosphere, producing secondary particles (muons, neutrons, etc.) that reach the surface.

• Dose depends on altitude and latitude.
• Typical sea‑level dose: 0.04 mSv yr⁻¹ (≈ 40 µSv yr⁻¹).
• At 2 km altitude (e.g., aircraft cruising height) the dose can be 5–10 times higher.

Summary of Typical Annual Doses

Key Points for Examination

1. Identify which of the four sources contributes the largest portion of the natural background dose (radon gas).
2. Explain why radon levels are higher in basements and poorly ventilated rooms.
3. State how altitude affects the cosmic‑ray contribution.
4. Describe how building materials can increase exposure to gamma radiation.
5. Recall the typical dose ranges for each source to compare their relative importance.