The Radiation You Can't Escape
⚛️ The Radiation You Can't Escape
Right now, as you read this, you are being bombarded with radiation. Cosmic rays from distant exploding stars are passing through you. Radon gas, seeping up from uranium-bearing rocks beneath your feet, is dissolving into your lungs. Potassium-40 in your own bones is spontaneously decaying. Your stone walls, your food and drink, even the air around you — all faintly radioactive.
This is background radiation — the unavoidable, low-level radiation that exists everywhere on Earth. We have evolved with it over billions of years, and at normal background levels it presents a very small risk to health. But it is always there, and physicists must account for it whenever they make any measurement of radioactivity.
In the UK, the average person receives about 2.7 millisieverts (mSv) of radiation dose per year from background sources. This varies enormously depending on where you live — people in Cornwall (which sits on granite rock) receive about twice the UK average because granite is rich in uranium and thorium, which produce radon gas.
Figure 1: Sources of background radiation in the UK — radon gas is by far the largest contributor
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🧠 Memory Aids
Sources of Background Radiation — "RCFCGM"
Roughly in order of size in the UK: Radon, Cosmic, Food and drink, Cround (gamma from ground/buildings), Gamma from ground, Medical
Or use the mnemonic: "Really Cold Food Causes Great Medical Bills" — Radon, Cosmic, Food, Cosmic gamma, Ground gamma, Medical, Buildings
Corrected Count Rate Formula
Think of it as: you measured too much (because background was included), so you take away the background: Correct = Measured − Background. "You can't take credit for what was already there."
Radon is the Biggest Source
Remember: R is the first letter, so Radon is #1. It comes from Rocks (both R words). And it gets into buildings because it's a Gas — another G to remember alongside Ground gamma.
Quick Check: Why is it important to measure background radiation before investigating a radioactive source? And why should you take several readings of the background count rate rather than just one?
Background radiation must be measured so it can be subtracted from the total count rate, giving the true count rate from the source alone. Several readings are needed because radioactive decay (including background) is random — individual readings vary. By taking multiple readings and averaging, you get a more reliable value for the background count rate.
Now try it yourself
Quiz · Question 1 of 13
What is background radiation?
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