Radioactivity
Unstable nuclei, radiation types, and real-world applications — the complete guide to Cambridge IGCSE Physics Topic 5.2
Background Radiation: The Invisible Backdrop
Every second of every day, you're being exposed to radiation. Not from a nuclear power plant or a medical procedure — from the world around you. This is background radiation, and it's something you need to understand.
The air around you contains countless radioactive particles, just as it contains oxygen molecules. You can't see them, but they're everywhere — in rocks, in food, in the sky.
What is Background Radiation?
Background radiation is the constant, low-level radiation that exists in the environment at all times. It comes from two main sources:
Natural sources (about 82% of total):
- Radon gas – The biggest contributor. Released from uranium in rocks, it seeps into homes. It's colourless, odourless, and dangerous in large quantities.
- Rocks and building materials – Uranium and thorium occur naturally in granite, concrete, and brick. They decay slowly but constantly.
- Food and drink – Potassium-40 in bananas, carbon-14 in all living things. The amounts are tiny and harmless.
- Cosmic rays – High-energy particles from space hit Earth's atmosphere, creating secondary gamma radiation that reaches the ground.
Man-made sources (about 18%):
- Medical X-rays and CT scans
- Nuclear weapons testing (historical; levels are now very low)
- Nuclear accidents (rare, but the impact is serious)
- Nuclear power plant operations
Why Background Radiation Varies
The count rate of background radiation is not uniform everywhere. It depends on geology (granite regions have more), altitude (cosmic rays are stronger at height), and building materials. This is why scientists always measure background radiation first before measuring a source — they need to subtract it out.
Corrected count rate = Total count rate − Background count rate
Detecting Background Radiation
A Geiger–Müller tube (or Geiger counter) is the standard tool. Every time it detects radiation, it makes a clicking sound and registers a count. The count rate is measured in counts per second (counts/s) or counts per minute (counts/min).
The further a detector is from a radioactive source, the lower the count rate, because radiation spreads out as it travels. This is the inverse square law at work.
Practice Question 1
A Geiger counter detects 48,000 decays in 10 minutes when placed next to a radioactive source. What is the count rate in counts per second?