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 – Background Radiation Sources

In everyday life we are surrounded by tiny amounts of ionising radiation. The main contributors to the background radiation we experience are:

1. Radon gas in the air 🌬️
2. Rocks and buildings 🏠
3. Food and drink 🍽️
4. Cosmic rays from space 🌌

Understanding these sources helps you answer exam questions about natural background radiation and its measurement.

(a) Radon Gas (in the air)

Radon is a colourless, odorless noble gas produced by the decay of uranium in soil and rock. It can seep into buildings through cracks and foundations. Once inside, it decays into short‑lived radioactive daughters that emit alpha particles.

• Typical indoor concentration: 10–200 Bq m⁻³ (depends on geology and ventilation).
• Analogy: Think of radon as invisible “smoke” that can linger in a room like a faint perfume.
• Key point for exams: Radon’s decay chain includes isotopes such as \$^{218}\text{Po}\$ and \$^{214}\text{Pb}\$, which are the main sources of indoor alpha and beta radiation.

(b) Rocks and Buildings

Naturally occurring radioactive materials (NORM) in building materials (concrete, bricks, granite) and in the surrounding earth emit gamma rays. The amount depends on the type of stone and the construction materials used.

• Concrete can contain \$^{40}\text{K}\$, \$^{238}\text{U}\$, and \$^{232}\text{Th}\$, contributing 0.1–1 µSv h⁻¹ to indoor exposure.
• Analogy: Imagine each stone as a tiny lantern that flickers with gamma light; the more lanterns, the brighter the glow.
• Exam focus: Gamma radiation from building materials is usually the second largest indoor source after radon.

(c) Food and Drink

Certain foods contain naturally occurring radionuclides, mainly \$^{40}\text{K}\$, \$^{226}\text{Ra}\$, and \$^{137}\text{Cs}\$ (the latter from past nuclear events). The dose from food is usually small but measurable.

• Typical annual dose: 0.1–0.3 mSv from food.
• Examples: Bananas (high \$^{40}\text{K}\$), milk (contains \$^{226}\text{Ra}\$), and canned vegetables (may contain \$^{137}\text{Cs}\$).
• Analogy: Eating a banana is like swallowing a tiny “radioactive pill” that adds a few micro‑sieverts to your yearly dose.
• Exam tip: Remember the “banana equivalent dose” when discussing dietary sources.

(d) Cosmic Rays

High‑energy particles from space (mostly protons) collide with the Earth’s atmosphere, producing secondary particles such as muons, neutrons, and gamma rays. These reach the surface and contribute to background radiation.

• Typical surface dose: 0.3–0.5 µSv h⁻¹, higher at high altitudes and latitudes.
• Analogy: Think of cosmic rays as invisible “space bullets” that constantly hit the Earth like a gentle rain.
• Exam focus: Cosmic rays are the dominant source of outdoor background radiation and increase with altitude.