Natural and man-made sources; radon, cosmic, medical, nuclear industry; correcting for background
AQA GCSE Physics 4.4
🌍 Explain what background radiation is and where it comes from
☢️ Identify and describe the main natural sources of background radiation
🏥 Identify and describe man-made sources of background radiation
📊 State and explain the typical percentages of each source in the UK
🔬 Correct a measured count rate to account for background radiation
⚠️ Discuss factors that affect how much background radiation a person receives
What is Background Radiation?
Background radiation is the low-level ionising radiation that is present everywhere in the environment at all times, even when no radioactive source is being deliberately used in an experiment.
Background radiation comes from natural sources (rocks, the ground, the atmosphere, cosmic rays) and a smaller proportion from man-made (artificial) sources such as medical procedures and the nuclear industry.
It is constantly bombarding everything on Earth. We cannot avoid it completely — it is a natural feature of our environment. However, the level of background radiation varies depending on where you live, your occupation, and your lifestyle.
In the UK, the average background radiation dose is approximately 2.7 millisieverts (mSv) per year, though this varies considerably from person to person and from place to place.
Because background radiation is always present, it must always be accounted for when carrying out experiments with radioactive sources. Failing to subtract background radiation will give readings that are artificially too high.
Natural Sources of Background Radiation
The majority of background radiation (roughly 85% in the UK) comes from natural sources. The main natural sources are:
1. Radon Gas (the largest single source — ~50%)
Radon is a naturally occurring radioactive gas produced by the radioactive decay of uranium and thorium found in rocks (especially granite). It seeps up through the ground and can accumulate in buildings, particularly in basements and ground floors.
Radon decays by emitting alpha radiation. When radon gas is inhaled, alpha particles are emitted directly inside the lungs, which is particularly dangerous as alpha radiation is the most ionising type. The level of radon exposure depends strongly on the geology of the local area — Cornwall and parts of Devon in the UK have significantly higher radon levels due to underlying granite rock.
2. Gamma Radiation from Rocks and Buildings (~15%)
Radioactive elements such as uranium, thorium, and potassium-40 are found naturally in rocks and soil. These emit gamma radiation which can pass through building materials and irradiate the people inside. Buildings made of granite or other igneous rocks can have higher levels than those made of brick or wood.
3. Cosmic Radiation (~10%)
Cosmic radiation consists of high-energy particles (mainly protons and helium nuclei) arriving from outer space. The Earth's atmosphere and magnetic field absorbs much of this, but some reaches the ground.
People who live at high altitudes (e.g. in the mountains or on high plateaus) receive more cosmic radiation because there is less atmosphere above them to absorb it. Airline pilots and frequent flyers receive significantly higher doses than people at sea level.
4. Food and Drink (~10%)
Natural radioactive elements, particularly potassium-40 and carbon-14, are absorbed by plants and animals and therefore enter our bodies through food and drink. Brazil nuts are famously high in natural radioactivity due to radium absorbed from the soil.
Natural Source
Approximate % (UK)
Type of Radiation
Radon gas (ground/rocks)
~50%
Alpha (mainly)
Gamma from ground/buildings
~15%
Gamma
Cosmic rays
~10%
Various
Food and drink
~10%
Beta, Gamma
Man-Made (Artificial) Sources of Background Radiation
Approximately 15% of background radiation in the UK comes from man-made sources. The two most significant are:
1. Medical Sources (~14%)
The largest man-made contribution comes from medical uses of radiation. This includes:
X-ray imaging — used for bone fractures, chest X-rays, dental X-rays
CT (computed tomography) scans — produce much higher doses than simple X-rays
Nuclear medicine — radioactive tracers (e.g. technetium-99m) injected into the body for diagnostic scans
Radiotherapy — high doses of gamma radiation targeted at cancer tumours
The dose received from medical sources varies enormously between individuals depending on how many procedures they have had.
2. Nuclear Industry (~1%)
The nuclear power and weapons industry contributes a very small fraction of background radiation to the general public. This includes:
Low-level radioactive waste disposal
Routine discharges from nuclear power stations (very tightly regulated)
Fallout from historical nuclear weapons testing in the atmosphere (mostly 1950s–1960s)
Despite public concern, the nuclear industry contributes less than 1% of the average person's background radiation dose in the UK.
Man-Made Source
Approximate % (UK)
Medical (X-rays, scans, tracers)
~14%
Nuclear industry / fallout
~1%
Correcting for Background Radiation
When performing experiments with radioactive sources, we measure the count rate — the number of radioactive decays detected per second (counts per second, cps) or per minute (counts per minute, cpm).
Because background radiation is always present, any detector will register counts even with no source present. This background count rate must be subtracted from any measurement to find the true count rate from the source alone.
Corrected count rate (true count rate from source)
counts per second (cps) or counts per minute (cpm)
Cmeasured
Count rate measured with source present
cps or cpm
Cbackground
Background count rate (measured with no source)
cps or cpm
How to Measure Background Radiation
Before starting any experiment with a radioactive source:
Remove (or shield) all radioactive sources from the area
Measure the count rate over a long period of time (e.g. 5–10 minutes) and calculate the average count rate per minute or per second
This average value is your background count rate
Subtract it from all subsequent measurements taken with the source present
Measuring background over a long time gives a more reliable average because radioactive decay is random — short measurements may give misleadingly high or low values due to statistical variation.
Why the Corrected Count Rate Matters
If you do NOT correct for background radiation, your results will be inaccurate. For example, if a source gives a count rate that is only slightly above background, failing to subtract background could make it appear much more radioactive than it really is. This is especially important in experiments investigating the half-life of a source, where small errors in count rate will produce large errors in the calculated half-life.
Factors Affecting Background Radiation Dose
Different people receive different doses of background radiation depending on several factors:
Factor
Effect on Dose
Location (geology)
Areas with granite bedrock (e.g. Cornwall) have higher radon levels
Altitude
Higher altitude → less atmosphere → more cosmic radiation
Granite buildings emit more gamma than timber buildings
Diet
High radium/potassium-40 foods increase internal dose
The single biggest factor affecting most people's background radiation dose is radon gas — particularly for people living in areas with granite rock.
Example 1: A student measures the background count rate in their laboratory before an experiment. They record 240 counts in 5 minutes. They then place a radioactive source near the Geiger-Müller tube and measure 870 counts in 5 minutes. Calculate the corrected count rate from the source in counts per minute (cpm).
Example 2: A Geiger counter measures a count rate of 320 cpm when a radioactive source is present. The background count rate has been measured as 32 cpm. What is the corrected count rate, and what percentage of the total measured count rate is due to background radiation?
2Calculate the percentage due to background:
Percentage = (Background ÷ Measured total) × 100
Percentage = (32 ÷ 320) × 100 = 10%
✅ Corrected count rate = 288 cpm; Background contributes 10% of the total measured count rate.
Example 3: A student records the following counts to measure background radiation over 6 separate 1-minute intervals: 22, 18, 25, 20, 17, 24 counts. Calculate the mean background count rate per minute and explain why they measured over multiple intervals rather than just once.
2Calculate the mean count rate:
Mean background count rate = 126 ÷ 6 = 21 counts per minute
3Explain why multiple readings are needed:
Radioactive decay is random — the number of decays detected in any given minute varies unpredictably. Taking multiple readings and finding the mean reduces the effect of this random variation and gives a more reliable, accurate estimate of the true background count rate.
✅ Mean background count rate = 21 cpm. Multiple readings are needed because radioactive decay is a random process, so a single short measurement could be unrepresentatively high or low.
Example 4: A physics teacher lives in Cornwall near a granite-rich area. Explain, with reference to specific sources, why she is likely to receive a higher annual background radiation dose than a person living in central London.
1Identify the key source difference — radon:
Granite rock contains naturally occurring uranium and thorium. These decay and produce radon gas, which seeps up through the ground. Cornwall has high concentrations of granite, so radon levels in buildings there are significantly higher than in London (which has less granite bedrock).
2Explain why radon is particularly hazardous:
Radon emits alpha particles when it decays. When inhaled, alpha radiation is absorbed by lung tissue, causing significant ionisation and potentially increasing cancer risk. This means higher radon exposure leads to a greater effective radiation dose.
3Mention gamma from rock/buildings:
Granite also emits gamma radiation directly from radioactive isotopes in the rock. Buildings constructed with local granite materials will expose occupants to slightly higher gamma doses as well.
✅ The Cornwall teacher receives a higher dose mainly because of elevated radon gas levels (from granite geology) which emit alpha radiation inside the lungs, plus higher gamma radiation from granite rock and buildings.
Question 1: Which source contributes the largest proportion of background radiation in the UK?
Question 2: A Geiger-Müller tube measures 560 counts in 4 minutes when a source is present. The background count rate is 15 cpm. What is the corrected count rate?
Enter your answer in counts per minute (cpm):
Question 3: Why does an airline pilot receive a higher annual dose of background radiation than a person who works at sea level?
Question 4: A student records a background count of 180 counts over 10 minutes. In an experiment, they record 95 counts in 2 minutes with a source. What is the corrected count rate in cpm?
Question 5: What type of radiation does radon gas mainly emit, and why is this particularly dangerous when radon is inhaled?
Challenge 1: A scientist measures the following count rates (in cpm) at different distances from a radioactive source. The background count rate has already been determined as 22 cpm.
Distance (cm)
Measured count rate (cpm)
5
322
10
122
20
47
40
29
(a) Calculate the corrected count rate at each distance.
(b) At 40 cm, the corrected count rate is very small. What does this tell you about the type of radiation the source might be emitting? Explain your reasoning.
(c) Suggest one source of error in this experiment and explain how it could be reduced.
(b) The corrected count rate drops to just 7 cpm at 40 cm, which is very close to zero. This suggests the source could be emitting alpha radiation, which has a very short range in air (only a few centimetres), or possibly beta radiation, which has a limited range. The rapid drop with distance is consistent with a weakly penetrating radiation type that is quickly absorbed by air.
(c) A source of error is the random nature of radioactive decay — the count rate fluctuates randomly, so a single reading at each distance may be unrepresentative. This could be reduced by taking multiple readings at each distance and calculating the mean, or by counting for a longer time period at each distance to get a more reliable average.
Challenge 2: A student investigates the half-life of a radioactive isotope. Their raw count rate data at time zero is 480 cpm, and at 30 minutes is 90 cpm. The background count rate in the laboratory is 30 cpm. The student forgets to correct for background. Calculate the half-life the student would incorrectly obtain, and then calculate the correct half-life. Compare your answers and explain the significance of correcting for background radiation.
Without background correction (incorrect method):
At t = 0: 480 cpm; At t = 30 min: 90 cpm
Number of half-lives: 480 → 240 → 120 → 60 ... but 90 is between 60 and 120.
More precisely: 480 × (½)ⁿ = 90 → (½)ⁿ = 90/480 = 0.1875
n = log(0.1875) / log(0.5) = 2.415 half-lives in 30 minutes Incorrect half-life ≈ 30 ÷ 2.415 ≈ 12.4 minutes
With background correction (correct method):
Corrected at t = 0: 480 − 30 = 450 cpm
Corrected at t = 30 min: 90 − 30 = 60 cpm
(½)ⁿ = 60/450 = 0.1333
n = log(0.1333) / log(0.5) = 2.906 half-lives in 30 minutes Correct half-life ≈ 30 ÷ 2.906 ≈ 10.3 minutes
Significance: Failing to correct for background gives a half-life that is too long (12.4 min vs 10.3 min) because the background counts make the source appear less radioactive than it really is at later times. This demonstrates why background correction is essential for accurate experimental results.
Challenge 3: A journalist writes: "Nuclear power stations are the main cause of radiation exposure for people living near them." Evaluate this claim using your knowledge of background radiation sources and their relative contributions to the average annual dose in the UK.
The claim is not supported by the evidence. Here is a full evaluation:
Against the claim: In the UK, the nuclear industry (including nuclear power stations) contributes less than 1% of the average person's annual background radiation dose. Routine discharges from nuclear power stations are strictly regulated and kept as low as reasonably practicable (ALARP). This contribution is negligible compared to natural sources.
What actually dominates: The majority of background radiation (~85%) comes from natural sources. Radon gas alone accounts for approximately 50% of the average dose — far more than the nuclear industry. Medical sources (X-rays, CT scans, nuclear medicine tracers) account for about 14%, which is also much more than the nuclear industry.
For people living near a nuclear power station: While there may be a very slight increase in dose above the national average due to local discharges, this increase is tiny and falls well within safe limits set by regulators. Studies have not consistently shown significant health effects from living near properly operating nuclear power stations.
Conclusion: The journalist's claim is misleading. Nuclear power stations are not the main cause of radiation exposure for nearby residents. Natural sources — especially radon — dominate background radiation exposure for almost everyone in the UK.
Challenge 4 (Extended): A geologist working in Cornwall measures a background count rate of 54 cpm in their home near granite outcrops. A physicist working in London measures a background count rate of 21 cpm in their home. (a) Suggest and explain two reasons for this difference. (b) The geologist seals all the gaps under doors and around floorboards in their house. Predict and explain the likely effect on the background count rate inside the house. (c) Suggest one measure that could reduce the risk from indoor radon without sealing the house.
(a) Two reasons for higher count rate in Cornwall:
1. Higher radon levels — Cornwall has extensive granite bedrock which contains higher concentrations of uranium. Uranium decays to produce radon gas, which seeps into buildings. The geologist's home is exposed to more radon seeping up from the ground, which significantly increases background count rate.
2. Gamma radiation from granite building materials — if the house is built from local granite, the walls themselves emit gamma radiation from radioactive isotopes (uranium, thorium, potassium-40) within the stone, contributing to a higher background reading.
(b) Effect of sealing gaps: Sealing gaps would likely increase the background count rate inside the house initially, then keep it high or even make it higher over time. This is because sealing the gaps prevents radon gas that has already entered from escaping and prevents fresh air from diluting it. Radon would accumulate to higher concentrations inside. This is the opposite of the desired effect and shows that simply sealing a building can make radon exposure worse if ventilation is reduced.
(c) Reducing radon risk without sealing: One measure is to improve ventilation — installing fans or opening windows regularly allows radon gas that seeps in to be diluted and dispersed to the outside air before it can accumulate to dangerous levels. Another option is to install a radon sump (a pump beneath the floor that extracts radon-rich air from under the house and vents it outside before it enters the building).