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Radioactive decay is a random process where unstable atomic nuclei emit radiation to become more stable. Free GCSE Physics notes and 15 FAQs.
Radioactive Decay: Stability and Activity is part of Atoms and nuclear radiation in GCSE AQA Higher Physics.
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GCSE AQA Higher
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Radioactive decay is a random process where unstable atomic nuclei emit radiation to become more stable. Free GCSE Physics notes and 15 FAQs.
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Use this lesson to stay inside Atoms and nuclear radiation while connecting the detail back to the wider Atomic structure topic.
Read the core explanations from Radioactive Decay: Stability and Activity before testing yourself from memory.
Picture an atomic nucleus as a crowded room full of protons and neutrons. In stable atoms like carbon-12, everyone gets along perfectly. But in unstable atoms like uranium-238, there's too much pushing and shoving — the nucleus becomes unstable and eventually breaks apart.
When an unstable nucleus decays, it's trying to reach a more stable arrangement. Think of it like a wobbly tower of blocks — it will eventually topple to find a more stable position. The nucleus achieves this by ejecting particles or energy as radiation.
**Trap**: Many students think radioactive decay happens at regular intervals like a clock. Actually, it's completely random — you can't predict when any individual nucleus will decay, only the average rate for a large sample.
When we want to know how 'radioactive' a sample is, we measure its activity. This tells us how many nuclei are decaying every second — like counting how many people leave a crowded room each minute.
Activity is measured in becquerel (Bq), named after Henri Becquerel who discovered radioactivity. If a sample has an activity of 500 Bq, it means 500 nuclei decay every second on average.
But here's the catch — we can't directly count every single decay happening inside a sample. Instead, we use radiation detectors like Geiger-Muller tubes to detect the radiation coming out.
Count-rate is what our detector actually measures. It's usually lower than the true activity because not all radiation reaches the detector — some gets absorbed by air or the sample itself.
In exam questions, you'll often need to calculate activity from count-rate data. The key is making sure your units are consistent and showing clear working for method marks.
Worked example: Calculating Activity from Count Data
A Geiger-Muller tube detects 1800 counts in 3 minutes from a radioactive source. Calculate the activity. [3 marks]
Answer: 10 Bq
**Trap**: Students often forget to convert minutes to seconds, giving answers that are 60 times too small. Always check your time units match the required units for activity (Bq = decays per second).
Worked example: Background Radiation Correction
A detector shows 450 counts per minute near a radioactive source. The background count-rate is 30 counts per minute. Calculate the activity of the source. [4 marks]
Answer: 7 Bq
In the lab, we use radiation detectors to measure count-rates safely. The most common detector at GCSE level is the Geiger-Muller tube, which produces a click for each radiation particle it detects.
Real measurements always include background radiation — cosmic rays and natural radioactivity around us. This creates a constant 'noise' that must be subtracted from our readings to find the true activity of our source.
Activity questions are common in GCSE physics papers. Examiners love testing unit conversions, background radiation corrections, and the random nature of decay. Here's how to secure full marks.
For GCSE 'explain' questions about randomness, state that decay is random, say individual decays cannot be predicted, and conclude that only the average behaviour of large samples can be described.
The key idea for GCSE is that radioactive decay is unpredictable for individual atoms, but the overall pattern for large samples can be measured and described.
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-20.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Radioactive decay is a random process where unstable atomic nuclei emit radiation to become more stable. The timing of individual decays cannot be predicted; only the average rate for a large sample can be described.
Radioactivity was discovered by Henri Becquerel, and the unit of activity, the becquerel (Bq), is named after him. One becquerel equals one decay per second.
Count-rate is the number of radioactive decays detected per second by a radiation detector such as a Geiger-Muller tube, measured in counts per second. It is usually lower than the activity of the source.
The becquerel (Bq) is the unit of activity, the rate at which a source of unstable nuclei decays. One becquerel equals one decay per second, so a sample with an activity of 500 Bq has 500 nuclei decaying every second on average.
Activity is the rate at which unstable nuclei in a source decay, measured in becquerels. Count-rate is the number of decays detected per second by a detector such as a Geiger-Muller tube, and it is usually lower than the true activity because not all radiation reaches the detector.
Nuclear instability is the condition where an atomic nucleus has an imbalance of protons and neutrons, making it likely to decay and emit radiation. Uranium-238 is unstable, while carbon-12 is stable.
Count-rate is usually lower than the true activity because not all the radiation reaches the detector; some gets absorbed by the air or by the sample itself. Count-rate is what the detector actually measures.
To correct for background radiation, subtract the background count-rate from the measured count-rate. A detector showing 450 counts per minute with a background of 30 counts per minute gives 420 counts per minute from the source, which is 420 ÷ 60 = 7 counts per second.
Some atomic nuclei are unstable because they have an imbalance of protons and neutrons, making them likely to decay and emit radiation. The nucleus ejects particles or energy as radiation to reach a more stable arrangement.
1800 counts detected in 3 minutes gives an activity of 10 Bq. The time is converted to seconds, 3 × 60 = 180 s, and activity = 1800 ÷ 180 = 10 Bq.
A Geiger-Muller tube is a radiation detector that produces a click for each radiation particle it detects. It is the most common detector at GCSE level and measures the count-rate in counts per second.
Radioactive decay does not happen at regular intervals like a clock; it is completely random. The timing of individual decays cannot be predicted, only the average rate for a large sample.
A source reading 450 counts per minute with a background of 30 counts per minute has an activity of 7 Bq. Subtracting background gives 420 counts per minute, and 420 ÷ 60 = 7 counts per second.
Radioactive decay is explained as random by stating that individual decays cannot be predicted, and that only the average behaviour of large samples can be described. The overall pattern for a large sample can be measured, even though single atoms are unpredictable.
Minutes must be converted to seconds for activity because the becquerel means decays per second. Without this conversion, the answer is 60 times too small.