What is background radiation, where does it come from, and how is radioactivity detected?
Background radiation and detection: the meaning and sources of background radiation from Earth and space, and detecting radioactivity with photographic film and a Geiger-Muller tube.
A focused answer to Edexcel GCSE Physics 6.12 to 6.14, covering what background radiation is, its main sources from the Earth (rocks, radon) and from space (cosmic rays), and how radioactivity is measured and detected using photographic film and a Geiger-Muller tube.
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What this dot point is asking
Edexcel statements 6.12 to 6.14 want you to explain what background radiation is, to describe its origins from the Earth and from space, and to describe methods for measuring and detecting radioactivity, limited to photographic film and a Geiger-Muller tube.
What background radiation is
Background radiation is normal and unavoidable, and at the levels most people experience it is not harmful. Because it is always present, any measurement of a radioactive source actually includes the background, which is why experiments must account for it. The level varies from place to place, mainly because of differences in the local rocks.
Sources from Earth and space
The majority of background radiation is natural. Radon, a radioactive gas seeping from rocks (especially granite), is the dominant source in many regions, which is why some homes are tested for it. Cosmic rays increase with altitude, so air crew and people living high up receive a little more. Knowing both an Earth source (radon or rocks) and a space source (cosmic rays) is what examiners look for.
Detecting radioactivity
These are the only two detection methods Edexcel requires. The film badge gives a cumulative record of dose over time (useful for safety monitoring), while the GM tube gives a live count rate (useful for measuring activity). The GM tube's reading is a count rate (counts per second or per minute), which relates to the source's activity once the background is removed.
Correcting for background
How Edexcel examines this
This dot point is examined on both tiers, usually as a "what is background radiation and where does it come from" question and a detection-and-correction question. The mark scheme for sources rewards naming both natural (radon and rocks, cosmic rays, food and the body) and artificial (medical, nuclear) origins, with radon often credited as the largest source. For detection, examiners reward naming photographic film (fogging) and a Geiger-Muller tube (counting ionising particles), and crucially the reason the background count must be measured and subtracted: it is always present, so the true source count is the measured count minus the background. A frequent omission is the background subtraction, so include it whenever a count-rate calculation appears. You may also be asked why the background level differs between places (mainly the local rocks and radon) or why it rises with altitude (more cosmic rays). Keep the two detectors distinct: film for cumulative dose, GM tube for live count rate.
Try this
Q1. Name one source of background radiation from space. [1 mark]
- Cue. Cosmic rays (from space or the Sun).
Q2. A GM tube reads counts per minute; the background is counts per minute. State the corrected count rate. [1 mark]
- Cue. counts per minute.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20204 marksExplain what is meant by background radiation, and describe its main sources.Show worked answer →
Background radiation is the low-level ionising radiation that is present everywhere around us all the time, from natural and some artificial sources (1 mark). Natural sources include radioactive rocks and soil (especially radon gas released from the ground), cosmic rays from space (and the Sun), and radioactive materials in food, drink and our own bodies (2 marks for two or more sources). Artificial sources include medical X-rays and nuclear power or weapons fallout, which make up a smaller fraction (1 mark). Markers reward defining background radiation as ever-present low-level radiation and naming sources from both the Earth (radon, rocks) and space (cosmic rays).
Edexcel 20223 marksDescribe two methods that can be used to detect or measure radioactivity, and explain why the background count must be measured in an experiment.Show worked answer →
Radioactivity can be detected using photographic film, which darkens (becomes fogged) when exposed to ionising radiation, as used in film badges to monitor exposure (1 mark). It can also be detected with a Geiger-Muller (GM) tube connected to a counter, which clicks or counts each time radiation enters and ionises the gas inside (1 mark). The background count must be measured and subtracted because background radiation is always present, so the true count rate from the source is the measured count minus the background count (1 mark). Markers reward naming photographic film and a GM tube, and explaining that background must be subtracted to find the source's true activity.
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Sources & how we know this
- Pearson Edexcel GCSE (9-1) Physics (1PH0) specification — Pearson (2016)