📷 How it works
Photographic film is covered in a chemical that changes when radiation hits it. After the film is developed, the parts that were hit by radiation darken. The more radiation that hit the film, the darker it gets.
Radioactivity » Detecting Radiation and Background Radiation
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 7.9, 7.10
- How photographic film and a Geiger-Müller detector detect ionising radiation
- Where background radiation comes from, on Earth and in space
- Why background radiation is different in different places
- How to correct a count rate for background radiation
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Unlock This CourseYou cannot see or feel radiation
Alpha, beta and gamma radiation are invisible. You cannot see them, smell them or feel them. So how do we know they are there? We use detectors that react to ionising radiation. The spec names two: photographic film and a Geiger-Müller detector.
Key terms:
- Photographic film: film that is darkened by radiation, in the same way that light darkens camera film.
- Geiger-Müller (GM) tube: a gas-filled tube that detects each particle or ray of ionising radiation that enters it.
- Count rate: the number of counts the detector records each second or each minute.
- Background radiation: the low level of ionising radiation that is around us all the time.
Photographic film
👷 Film badges
People who work with radiation, such as in hospitals or nuclear power stations, wear a film badge. It holds a small piece of film in a light-proof case. At the end of the month the film is developed. If it has darkened, the worker has been exposed to radiation.
Film does not click or beep. It gives a record of the total radiation received over a period of time.
The Geiger-Müller detector
A Geiger-Müller detector has two parts: a GM tube and a counter. Here is how it works, step by step.
- The tube is filled with a gas at low pressure.
- When ionising radiation enters the tube, it ionises some of the gas.
- This makes a tiny pulse of electric current in the tube.
- The counter records each pulse as one count. Many counters also click for each count.
The counter can show the total number of counts, or the count rate, such as 30 counts per minute. The faster the clicks, the more radiation is entering the tube.
Alpha particles are stopped by almost anything, so the tube has a very thin window at one end to let them in.
| Detector | What it gives | Example use |
|---|---|---|
| Photographic film | Darkening: a record of total exposure | Film badge worn by a hospital worker |
| Geiger-Müller detector | Counts or a count rate, with clicks | Checking how much radiation a source gives out |
Background radiation
Switch on a Geiger-Müller detector with no radioactive source anywhere near it. It still clicks now and then. That is background radiation. It is around us all the time, and some of it comes from the Earth and some from space.
⛰️ Rocks and buildings
Some rocks contain radioactive substances. Radon is a radioactive gas given off by rocks such as granite. It can seep out of the ground and collect in buildings. Building materials made from rock also give out radiation.
🍎 Food and drink
Plants take in tiny amounts of radioactive substances from the soil and water. These pass into food and drink, and so into our bodies.
🌌 Cosmic rays
Cosmic rays are fast particles that reach the Earth from space. The atmosphere absorbs many of them, but some get through.
There are also small man-made contributions, such as medical uses of radiation. Most background radiation is natural, and the man-made part is small.
No percentages needed
You do not need to remember how much each source contributes. You need to name the sources and explain why the level changes.
Why background radiation varies
- Type of rock. A place built on granite has more radon than a place built on rock with little radioactive material, so its background is higher.
- Building materials. Houses built from different materials can give different readings.
- Height. There is less air above you high up, so more cosmic rays get through. Background is higher on a mountain or in an aircraft.
- Job and medical treatment. People who have medical scans or work with radiation receive extra.
Correcting a count rate for background
When you measure a radioactive source, the detector also picks up background radiation. To find the count rate from the source alone, measure the background first, with the source far away. Then subtract it.
Corrected count rate
corrected count rate = measured count rate − background count rate
Worked example 1
With no source present, a detector records a background count rate of 18 counts per minute. With a source placed near it, the count rate is 138 counts per minute.
Corrected count rate = 138 − 18 = 120 counts per minute.
Worked example 2
A detector records 360 counts in 3 minutes with a source nearby. In 3 minutes with no source it records 45 counts.
Step 1: measured count rate = 360 ÷ 3 = 120 counts per minute.
Step 2: background count rate = 45 ÷ 3 = 15 counts per minute.
Step 3: corrected count rate = 120 − 15 = 105 counts per minute.
Common mistakes
Subtracting totals that were counted over different times. Always turn counts into a count rate first, so both are in the same unit. Another mistake is adding the background instead of subtracting it. Also, do not forget the unit: counts per minute or counts per second.
Exam-style question
A student uses a Geiger-Müller detector to measure the radiation from a rock sample.
(a) State what the counter records each time ionising radiation enters the tube. [1 mark]
(b) With the rock far away, the detector records 80 counts in 4 minutes. State what causes this reading and give one natural source of it. [2 marks]
(c) With the rock near the detector, the count rate is 45 counts per minute. Calculate the corrected count rate from the rock. [2 marks]
(d) A film badge is worn by a hospital worker. Explain how it shows that the worker has been exposed to radiation. [2 marks]
Model answer
(a) A count (one pulse of current). (1)
(b) It is background radiation (1). Any one from: radon gas from rocks such as granite, rocks and building materials, food and drink, cosmic rays. (1)
(c) Background = 80 ÷ 4 = 20 counts per minute (1). Corrected count rate = 45 − 20 = 25 counts per minute (1).
(d) Radiation darkens the film (1). When the film is developed, a darker film shows it has been exposed. (1)
Exam tip
In (c), turn 80 counts in 4 minutes into a rate before you subtract. Writing the background rate as a separate step earns the method mark even if you slip later.