Describe natural and man-made sources of background radiation, how dose varies with occupation and location, and that radiation dose is measured in sieverts (Sv).
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GCSE Physics Revision
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GCSE Physics revision
Background radiation
Hazards and uses of radioactive emissions and of background radiation
Your specification
AQA student objectives
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Apply the specified scientific knowledge of background radiation to a relevant example.
Analyse an unfamiliar example of background radiation using the specified scientific ideas.
Revision summary
Key knowledge
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What is Irradiation?
- Irradiation is the process by which an object or person is exposed to radiation of any type, including both ionising and non-ionising radiation.
- Ionising radiation includes alpha particles, beta particles, gamma rays, and X-rays, whilst non-ionising radiation includes ultraviolet and microwave radiation.
- Being irradiated does not make a person radioactive or capable of emitting radiation to others.
What is Contamination?
- Contamination occurs when radioactive particles are deposited onto or inside an object or person, for example radioactive material on your skin.
- The real danger of contamination is that the radioactive material will decay and irradiate the person from close range, increasing their exposure to radiation.
- A contaminated person could potentially harm others if they are carrying a radioactive source on or inside their body.
Ionising Radiation and Living Cells
- Ionising radiation is generally the most dangerous type because it can penetrate living cells and interact with molecules inside them.
- Radiation can ionise DNA within cells, causing mutations that may lead to uncontrolled cell division and the development of cancer.
- Ultraviolet radiation can also cause cancer despite not always being classified as fully ionising radiation.
How the Type of Radiation Affects Harm
- When ranking radiation from most to least harmful, alpha is the most harmful overall, followed by beta, gamma, X-rays, ultraviolet, and then radio waves at the least harmful end.
- For external sources outside the body, alpha radiation is the least dangerous because it is stopped by a short distance of air and cannot penetrate the skin.
- Beta and gamma radiation are the most dangerous from external sources because they can penetrate the skin and reach internal tissues.
- For internal sources — such as inhaled, ingested, or contaminating particles — alpha radiation becomes the most dangerous as it can directly irradiate cells at close range.
The Role of Dosage
- The dosage of radiation received depends on three key factors: the distance from the source, the duration of exposure, and the radioactivity of the substance.
- Increasing distance from a radioactive source significantly reduces the amount of radiation received.
- The longer a person is exposed to a radioactive source, the greater the total dosage and therefore the greater the risk of harm.
Safety Precautions Around Radioactive Sources
- Protective clothing such as gloves and overalls should be worn when handling radioactive materials to reduce the risk of contamination.
- Radioactive sources should be handled using tools such as tongs rather than bare hands to minimise direct exposure.
- Radioactive materials should be stored in a lead-lined box whenever possible to absorb radiation and protect those nearby.
Key Differences: Irradiation vs Contamination
- Irradiation is temporary — once the source is removed, exposure stops immediately, whereas contamination continues to irradiate a person until the radioactive material is removed.
- Contamination poses an ongoing risk because the radioactive source remains in contact with or inside the body, continuously emitting radiation.
- Understanding the distinction between irradiation and contamination is essential for assessing and managing radiation risks safely.
Natural Sources of Background Radiation
- Radon gas, found naturally in the air, is the single largest source of background radiation, accounting for approximately 50% of the total dose.
- Radioactive rocks and the buildings constructed from them contribute to background radiation through the materials we live and work in.
- Cosmic rays are high-energy particles and radiation that travel from outer space and constantly bombard the Earth's surface.
- Food and drink, such as bananas, contain trace amounts of naturally occurring radioactive isotopes that contribute to background radiation.
Man-Made Sources of Background Radiation
- Medical procedures such as X-rays are a man-made source of background radiation, though they contribute only a small percentage of the total dose.
- Nuclear weapons testing, nuclear accidents (such as Chernobyl in 1986 and Fukushima in 2011), and nuclear power stations all contribute to man-made background radiation.
- All man-made sources combined account for only a very small fraction (around 4%) of the total background radiation dose received.
Chernobyl and Fukushima: Notable Nuclear Accidents
- The Chernobyl disaster occurred in 1986 in what is now Ukraine (then part of the USSR) and was one of the worst nuclear accidents in history.
- The Fukushima disaster occurred in Japan in 2011 following a tsunami, and both events released radioactive material that contributed to background radiation levels globally.
Measuring Radiation Dose: The Millisievert
- The dose of radiation received by a person is measured in sieverts (Sv), where one sievert equals 1,000 millisieverts (mSv).
- The average background radiation dose in the UK is approximately 2.5 millisieverts (mSv) per year per person.
- The dose an individual receives varies depending on their location, where they live, and the type of job they do.
- When Does Radiation Become Dangerous?
- A dose of approximately 100 mSv is considered the minimum level at which a detectable increase in the risk of cancer can be observed.
- Most people's annual background dose of around 2.5 mSv is well below the 100 mSv threshold, meaning everyday background radiation does not pose a significant cancer risk.
- Workers in industries such as nuclear power or medical imaging may receive higher doses, but these are carefully monitored to remain well below dangerous levels.
Detecting Radiation: The Geiger-Müller Tube
- A Geiger-Müller (GM) tube is a device used to detect ionising radiation and measure count rate, typically in counts per minute (cpm) or counts per second.
- Even with no radioactive source present, a GM tube will still detect a small amount of radiation due to background radiation — a typical value might be around 15 counts per minute.
Correcting for Background Radiation in Experiments
- When measuring the activity of a radioactive source, the background count rate must always be subtracted to find the true count rate from the source alone.
- The corrected count rate is calculated using: corrected count rate = text{count rate with source} - background count rate For example, if a source gives 51 counts per minute and the background count is 15 counts per minute, the corrected count rate is 51 - 15 = 36 counts per minute.
Factors Affecting an Individual's Background Radiation Dose
- A person's geographical location affects their dose, as some areas have higher concentrations of radon gas or radioactive rocks in the ground.
- Occupation plays a significant role — jobs involving X-rays, nuclear power, or other radiation sources can increase a person's annual dose above the average 2.5 mSv.