Knowledge Organiser: Background Radiation
Key Terms
- Background radiation — always-present low-level radiation
- Count rate — decays detected per unit time
- Corrected count rate — measured minus background
- Radon — radioactive gas from uranium in rocks
- Cosmic rays — high-energy particles from stars
- Dose: the risk of harm to the body from radiation, in millisieverts (mSv)
- The unit is the sievert (Sv); 1000 mSv = 1 Sv. You do not need to recall it.
UK Background Radiation Sources
- Radon gas: about 47% (natural)
- Medical X-rays and scans: about 16% (artificial)
- Rocks, soil and buildings (gamma): about 14% (natural)
- Cosmic rays: about 13% (natural)
- Food and drink: about 10% (natural)
- Nuclear power, fallout and other: less than 1% (artificial)
Correcting for Background
- Remove all sources and measure count rate
- Take multiple readings and average
- Corrected rate = Measured − Background
- Background is random — readings vary naturally
Factors Affecting Dose
- Granite rock areas: higher radon levels
- High altitude: more cosmic rays
- Medical scans: significant additional dose
- UK average: 2.6 mSv per year
- 84% natural, 16% artificial
Key Equations
- Corrected count rate = measured count rate − background count rate
- Activity (Bq) = number of nuclear decays per second
- Dose (mSv) measures biological effect — depends on type and amount of radiation
- UK average background dose ≈ 2.6 mSv per year
Common Mistakes
- Not subtracting background radiation: All experimental radiation measurements must have background count rate subtracted — failing to do this gives artificially high readings
- Saying nuclear power is the main source of background radiation: In the UK, radon gas (from rocks) is the largest source (about 47%); nuclear power contributes less than 0.1% of background dose
- Thinking background radiation is always dangerous: Background radiation is low-level and natural — it has always existed; the risk only increases significantly with high doses or long-term elevated exposure
- Confusing activity and dose: Activity (Bq) measures decays per second at the source; dose (mSv) measures the biological effect on the body — the same activity gives different doses for different radiation types
- Forgetting to measure background count before experiments: In any radioactivity practical, always measure background count rate first (with source removed) before taking readings with the source present
Memory Aid
- Radon gas (natural)
- Medical X-rays and scans (artificial)
- Rocks and buildings, gamma radiation (natural)
- Cosmic rays from space (natural)
- Food and drink (natural)