Radioactive decay and half-life Edexcel International GCSE Science (Double Award) revision
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In plain words
Some nuclei are unstable and throw out radiation to become more stable. There are three kinds: alpha (stopped by paper), beta (stopped by thin aluminium) and gamma (needs thick lead). Nobody can say when one particular nucleus will decay, but a large number of them follow a reliable pattern: the half-life is the time it takes for half of them to go.
Three things to know
- Alpha (a helium nucleus) is the most ionising and is stopped by paper. Beta (a fast electron) is stopped by a few millimetres of aluminium. Gamma (an electromagnetic wave) is the least ionising and needs thick lead to reduce it.
- Half-life is the time for half the unstable nuclei in a sample to decay (or for the count rate to halve). Decay is random.
- Background radiation is always present, from rocks, cosmic rays, radon gas and medical sources.
Worked example
A sample has a count rate of 240 counts per second and a half-life of 5 minutes. What is the count rate after 15 minutes?
- 15 minutes is 3 half-lives.
- Halve it three times: 240 → 120 → 60 → 30.
- = 30 counts per second.
Tips and tricks
- Half-life questions: count the half-lives, then keep halving. Draw the arrows.
- If a background count is given, take it off the readings first.
It lands in your notebook with its questions as flashcards.
Radioactive decay and half-life: 6 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
Which radiation is the most ionising?
Alpha particles are heavy and carry a 2+ charge.
What is a beta particle?
It is emitted when a neutron in the nucleus changes into a proton.
A sample has a half-life of 3 days. What fraction is left undecayed after 9 days?
Three half-lives: ½ × ½ × ½.
Which material is needed to absorb most gamma rays?
Gamma rays are very penetrating.
Which is a source of background radiation?
Rocks contain radioactive elements and release radon gas.
What happens to a nucleus when it emits an alpha particle?
An alpha particle is 2 protons and 2 neutrons.
Quiz
6 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
4 questions, 10 marks. Write your answers on paper, then check them.
Radioactive decay and half-life
Edexcel International GCSE Science (Double Award) 4SD0 · 10 marks · papermunch.org
Name ______________________________ Date ______________
A source has a count rate of 800 counts per minute. Its half-life is 2 hours. Calculate the count rate after 6 hours.[3]
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100 counts per minute. Three half-lives: 800 → 400 → 200 → 100.
State which type of radiation is stopped by a sheet of paper, and which is the most penetrating.[2]
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Alpha is stopped by paper. Gamma is the most penetrating.
The count rate from a sample falls from 600 to 75 counts per second in 12 days. Calculate the half-life.[3]
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4 days. 600 → 300 → 150 → 75 is three half-lives; 12 ÷ 3.
Give two precautions for handling a radioactive source safely.[2]
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Any two: handle it with tongs, keep it at a distance, keep the exposure time short, store it in a lead-lined box, never point it at anyone.
Answers: Radioactive decay and half-life
- 1. 100 counts per minute. Three half-lives: 800 → 400 → 200 → 100.
- 2. Alpha is stopped by paper. Gamma is the most penetrating.
- 3. 4 days. 600 → 300 → 150 → 75 is three half-lives; 12 ÷ 3.
- 4. Any two: handle it with tongs, keep it at a distance, keep the exposure time short, store it in a lead-lined box, never point it at anyone.