5.2.1 Detection of Radioactivity – Background Radiation Sources
Objective
Students should be able to:
- Identify the four natural sources that give a significant contribution to background radiation.
- Explain why each source is present in the environment.
- State the typical activity (Bq) or dose (µSv yr⁻¹) range for each source.
- Describe briefly how the contribution of each source is measured.
- Compare the relative importance of the four sources.
Units box
| Quantity | Symbol | Unit | Typical conversion (syllabus) |
|---|
| Activity | – | becquerel (Bq) | 1 Bq = 1 decay s⁻¹ |
| Count rate | cps / cpm | counts s⁻¹ or counts min⁻¹ | 1 cps = 60 cpm |
| Effective dose | D | microsievert (µSv) or millisievert (mSv) | 1 mSv = 1000 µSv |
| Dose rate (typical indoor GM tube) | – | µSv h⁻¹ per cpm | ≈ 0.005 µSv h⁻¹ cpm⁻¹ |
What is background radiation?
Background radiation is the ionising radiation that is always present in the natural environment, even when no artificial sources are nearby. It is expressed as an effective dose in microsieverts per year (µSv yr⁻¹). The dose from background radiation forms the natural part of the total annual dose received by every person.
Major natural sources
1. Radon gas (in the air)
- Origin / why it is present: Radon‑222 (⁸²²Rn) is a noble gas produced continuously by the decay of uranium‑238 in soil and rocks. Because radon is a gas it can migrate through the ground, enter buildings and accumulate in enclosed spaces.
- Typical activity: 20–200 Bq m⁻³ in most homes; values > 1 kBq m⁻³ occur in high‑risk areas.
- Typical annual dose: 0.5–10 mSv yr⁻¹ (average ≈ 1.2 mSv yr⁻¹ ≈ 1200 µSv yr⁻¹).
- How it is measured: Use a calibrated radon detector (e.g., an alpha‑track or continuous‑monitoring device). Record the count rate in cpm, subtract the instrument’s background, and apply the manufacturer’s conversion to dose.
2. Rocks and building materials
3. Food and drink (ingestion)
- Origin / why it is present: Plants take up radionuclides from soil and water; animals then ingest them. Potassium‑40 is present in all living tissue, and trace amounts of uranium‑ and thorium‑series nuclides follow the food chain.
- Typical activity (order of magnitude): K‑40 ≈ 0.012 Bq g⁻¹ of fresh food; U/Th ≈ 0.001 Bq g⁻¹.
- Typical annual dose: ≈ 0.3 mSv yr⁻¹ (≈ 300 µSv yr⁻¹).
- How it is measured: Activity of a food sample is measured in a laboratory (gamma spectrometry). The dose is obtained by multiplying the activity by the appropriate ingestion dose coefficient (given in the syllabus tables).
- Banana‑equivalent dose: One banana ≈ 0.1 µSv.
4. Cosmic rays
- Origin / why it is present: High‑energy particles from outer space strike the Earth’s atmosphere, producing secondary particles (muons, neutrons, electrons, photons) that reach the ground.
- Typical flux (order of magnitude): ≈ 1 particle cm⁻² min⁻¹ at sea level, corresponding to a count rate of about 0.2–0.5 cpm with a standard GM tube.
- Typical annual dose: 0.04 mSv yr⁻¹ (≈ 40 µSv yr⁻¹) at sea level.
- How it is measured: Record the count rate of a calibrated GM tube in an open‑air location, subtract the instrument background and use the conversion factor to obtain dose.
- Altitude & latitude effect: Dose increases roughly linearly with height and is higher towards the poles because the atmosphere is thinner and geomagnetic shielding is weaker. At 2 km altitude (commercial aircraft) the dose is ≈ 5–10 times sea‑level values (≈ 0.2–0.4 mSv yr⁻¹).
Typical annual dose contributions (summary)
| Source | Typical annual dose (µSv yr⁻¹) | Typical activity / notes |
|---|
| Radon gas (indoor) | ≈ 1 200 (range 500–10 000) | 20–200 Bq m⁻³ (higher in basements, poorly ventilated rooms) |
| Rocks & building materials | 200–500 | U‑238 ≈ 30 Bq kg⁻¹, Th‑232 ≈ 30 Bq kg⁻¹, K‑40 ≈ 400 Bq kg⁻¹ |
| Food & drink (ingestion) | ≈ 300 (range 200–500) | K‑40 ≈ 0.012 Bq g⁻¹; “banana equivalent” ≈ 0.1 µSv per banana |
| Cosmic rays (sea level) | ≈ 40 (range 30–50) | ≈ 0.2–0.5 cpm with a typical indoor GM tube; increases with altitude/latitude |
Relative importance
Radon gas is the dominant contributor, accounting for roughly 70 % of the total natural background dose. The remaining 30 % is shared roughly equally between rocks & building materials, food & drink, and cosmic rays.
Worked example – converting a radon count rate to dose
Assume a calibrated indoor GM tube records 50 cpm** from radon decay products.
- Convert to counts per hour: 50 cpm × 60 min h⁻¹ = 3 000 cph.
- Apply the dose conversion (0.005 µSv h⁻¹ per cpm):
Dose rate = 50 cpm × 0.005 µSv h⁻¹ cpm⁻¹ = 0.25 µSv h⁻¹.
- Annual dose: 0.25 µSv h⁻¹ × 8 760 h yr⁻¹ ≈ 2 190 µSv yr⁻¹ ≈ 2.2 mSv yr⁻¹.
This value lies within the typical indoor radon dose range (0.5–10 mSv yr⁻¹) and would be considered a moderate exposure.
Key points for examination (AO1 / AO2)
- Radon gas contributes the largest part of the natural background dose (~70 %).
- Explain why radon concentrations are higher in basements and poorly ventilated rooms (gas rises from the ground and is trapped).
- State the typical dose ranges (order of magnitude) for each source to enable comparison.
- Recall that cosmic‑ray dose increases with altitude and latitude.
- Know the standard units (cps, cpm, Bq, µSv yr⁻¹) and the simple conversion used for a typical GM tube.
- Be able to give a one‑sentence reason for the presence of each source (U‑238 decay → radon; primordial radionuclides in rocks; food chain uptake; atmospheric interactions of cosmic particles).
Cross‑reference
- 5.2.2 Radioactive Decay – half‑life and activity
- 5.2.3 Detection of α, β and γ radiation – types of detectors
- 5.2.4 Radiation safety – shielding and dose limits
Suggested diagram
Flow‑chart showing the origin of each natural background source (soil → radon, crust → rocks, biosphere → food, outer space → cosmic rays) together with its typical dose contribution.
1International Commission on Radiological Protection (ICRP) Publication 115, 2010.
2World Health Organization (WHO) – “Ionizing Radiation, Health Effects and Protective Measures”, 2006.