Background Radiation: Sources and Measurement
CSEC Physics: The Invisible Radiation Around Us
Essential Understanding: Background radiation is the ever-present ionising radiation that surrounds us every day, coming from natural sources in our environment, from space, and even from within our own bodies. Understanding background radiation is essential for accurate radioactive measurements and appreciating our natural radiation exposure.
What is Background Radiation?
Background Radiation Defined
Definition: Background radiation is the ever-present ionising radiation that exists in our environment at all times, originating from natural sources that have been present since before human existence.
Key Characteristics:
- Cannot be eliminated or reduced to zero
- Comes from multiple natural sources
- Present everywhere on Earth and in space
- Affects all radioactive measurements
- Exposure is constant throughout our lives
Typical Value: Approximately 2.4 millisieverts (mSv) per year globally
Why Background Matters in Experiments
The Problem: When measuring radioactivity from a source, the Geiger counter or detector picks up BOTH the radiation from your source AND the background radiation.
The Solution: Subtract the background count rate to get the true activity of your sample.
Formula:
Example: If your source measures 85 Bq and background is 5 Bq, the true activity is 80 Bq.
🌟 Did You Know?
Even in “clean” areas with no nuclear facilities or mining, background radiation never drops to zero. This is because cosmic rays from space constantly bombard the Earth, and radioactive elements like uranium, thorium, and potassium-40 are naturally present everywhere in the Earth’s crust, air, water, and even in the foods we eat!
The Three Major Sources of Background Radiation
1. Cosmic Rays
Source: High-energy particles from outer space, primarily from the Sun and distant stars and galaxies.
Composition:
- Protons (about 90%)
- Alpha particles (about 9%)
- Electrons and other particles (about 1%)
Factors Affecting Exposure:
- Altitude: Higher at mountains, lower at sea level
- Latitude: Higher near the poles (Earth’s magnetic field is weaker)
- Time: Increases during solar flares
Contribution: About 10% of total background exposure
2. Terrestrial Radiation
Source: Radioactive materials naturally present in the Earth’s crust, including uranium, thorium, and their decay products.
Key Radionuclides:
- Uranium-238: Found in rocks and soil
- Thorium-232: Common in granite and basalt
- Radon-222: Noble gas produced from uranium decay
- Potassium-40: Present in rocks and minerals
Factors Affecting Exposure:
- Local geology (granite areas have higher levels)
- Building materials (brick and stone homes)
- Indoor accumulation (poor ventilation)
Contribution: About 50% of total background exposure
3. Internal Radiation
Source: Radioactive isotopes that are naturally present within our bodies, incorporated through food, water, and air.
Key Radionuclides:
- Potassium-40 (K-40): Most significant internal source
- Carbon-14 (C-14): Incorporated into all organic tissue
- Rubidium-87 (Rb-87): Present in body tissues
- Polonium-210 (Po-210): Found in respiratory tract
How We Get These:
- Bananas contain potassium including K-40
- All organic food contains carbon including C-14
- Water and air contain trace radioactive elements
Contribution: About 40% of total background exposure
Background Radiation Breakdown by Source
Visualizing Background Radiation Sources
Objective: Understand the relative contributions of different sources to our total background radiation exposure.
From outer space
Rocks, soil, radon
In our bodies (K-40, C-14)
Man-Made Sources of Radiation
While background radiation comes from natural sources, humans also create additional radiation exposure through various activities and medical procedures. These are not part of background radiation but are worth understanding for context.
Medical Sources
Common Procedures:
- X-rays (dental, chest, CT scans)
- CT scans (highest medical dose)
- Nuclear medicine (radioactive tracers)
- Radiation therapy for cancer
Average Dose: About 0.6 mSv per year from medical procedures (varies widely)
Note: These are NOT background radiation but ADD to our total exposure.
Other Man-Made Sources
Additional Exposures:
- Air travel (increased cosmic ray exposure at altitude)
- Smoking (polonium-210 in tobacco)
- Building materials (some contain radioactive elements)
- Coal power plants (fly ash contains radioactive materials)
Note: Nuclear power plants contribute very little to public exposure (less than 0.1% of total).
Measuring and Accounting for Background Radiation
How to Measure Background Radiation
Correcting for Background
Worked Example:
A Geiger counter reads 85 counts per minute near a radioactive source. When the source is removed, the background reading is 5 counts per minute.
True source activity = 85 – 5 = 80 counts per minute
Background Correction Simulator
Objective: Practice calculating corrected count rates by subtracting background radiation.
Total Reading
85 Bq
Source + Background
Background
5 Bq
Environmental only
Corrected Reading
80 Bq
Source only
Formula: Corrected Reading = Total Reading – Background Reading
Click “New Problem” to practice more calculations!
CSEC Examination Mastery Tip
Identifying Background Sources: When asked to identify three sources of background radiation, remember:
- COSMIC: ☀️ From space (the sky above us)
- TERRESTRIAL: 🌍 From the Earth (rocks, soil, radon gas)
- INTERNAL: 🫀 From within us (potassium-40 in our bodies)
Memory Trick: “Cosmic comes from Above, Terrestrial from Below, Internal is Within!”
Remember: Medical X-rays and nuclear power are NOT background radiation – they are additional man-made sources that ADD to our total exposure.
CSEC Practice Arena
Test Your Understanding
1. Cosmic rays – high-energy particles from space
2. Terrestrial radiation – radioactive materials in the Earth’s crust
3. Internal radiation – radioactive isotopes like potassium-40 in our bodies
True Activity = Total Reading – Background
True Activity = 120 – 8 = 112 counts per minute
Chapter Summary
Three Major Sources
- Cosmic Rays: From space (10% of exposure)
- Terrestrial: From Earth (50% of exposure)
- Internal: From our bodies (40% of exposure)
Key Formulas
- Corrected Reading = Total – Background
- Typical background: ~5-10 Bq depending on location
- Total annual dose: ~2.4 mSv per year
Remember!
Background radiation is natural and unavoidable – it surrounds us every moment of every day.