AQA GCSE Physics

Revision Notes

4.2.1 Radioactive Decay

Radioactive Decay

Unstable Nuclei

  • Some atomic nuclei are unstable
  • This is because of an imbalance in the forces within the nucleus
    • Forces exist between the particles in the nucleus
  • Carbon-14 is an isotope of carbon which is unstable
    • It has two extra neutrons compared to stable carbon-12

Unstable nucleus, downloadable IGCSE & GCSE Physics revision notes

Carbon-12 is stable, whereas carbon-14 is unstable. This is because carbon-14 has two extra neutrons


  • Some isotopes are unstable because of their large size or because they have too many or too few neutrons


  • Unstable nuclei can emit radiation to become more stable
    • Radiation can be in the form of a high energy particle or wave

Radioactive decay, downloadable IGCSE & GCSE Physics revision notes

Unstable nuclei decay by emitting high energy particles or waves

  • As the radiation moves away from the nucleus, it takes some energy with it
    • This reduces the overall energy of the nucleus
    • This makes the nucleus more stable
  • The process of emitting radiation is called radioactive decay
  • Radioactive decay is a random process
    • This means it is not possible to know exactly when a particular nucleus will decay

Worked Example

Which of the following statements is not true?

A    Isotopes can be unstable because they have too many or too few neutrons

B    The process of emitting particles or waves of energy from an unstable nucleus is called radioactive decay

C    Scientists can predict when a nucleus will decay

D    Radiation refers to the particles or waves emitted from a decaying nucleus


    • Answer A is true. The number of neutrons in a nucleus determines the stability
    • Answer B is true. This is a suitable description of radioactive decay
    • Answer D is true. Radiation is about emissions. It is different to radioactive particles
    • Answer C is not true
    • Radioactive decay is a random process
    • It is not possible to predict precisely when a particular nucleus will decay

Exam Tip

The terms unstable, random and decay have very particular meanings in this topic. Remember to use them correctly when answering questions!


  • Objects containing radioactive nuclei are called sources of radiation
  • Sources of radiation decay at different rates which are defined by their activity
  • The activity is defined as

The rate at which the unstable nuclei from a source of radiation decays

  • Activity is measured in Becquerels
    • The symbol for Becquerels is Bq
  • 1 Becquerel is equal to 1 nucleus in the source decaying in 1 second

Worked Example

A source of radiation has an activity of 2000 Bq. How many unstable atoms decay in 2 minutes?

Step 1: Determine the activity

    • The activity of the source is 2000 Bq
    • This means 2000 nuclei decay every second

Step 2: Determine the time period in seconds

    • The time period is 2 minutes
    • Each minute has 60 seconds
    • The time period in seconds is:

2 × 60 = 120 seconds

Step 3: Multiply the activity by the time period

Activity (Bq) × Time period (s) = 2000 × 120 = 240 000

    • Therefore, 240 000 unstable nuclei decay in 2 minutes

Detecting Radiation

  • Radiation that is emitted from an unstable nucleus can be detected in different ways
    • For example, photographic film changes colour when exposed to radiation
  • A Geiger-Muller tube is a device used to detect radiation

Geiger-Counter, IGCSE & GCSE Physics revision notes

This Geiger-Muller Tube is connected to a Geiger Counter. This a common way of detecting radiation and measuring a count-rate

  • Within the Geiger-Muller tube, ions are created by radiation passing through it
  • The Geiger-Muller tube can be connected to a Geiger counter
  • This counts the ions created in the Geiger-Muller tube
  • Count-rate is the number of decays recorded each second by a detector

Worked Example

A Geiger-Muller tube is used to detect radiation in a particular location. If it counts 16,000 decays in 1 hour, what is the count rate?

Step 1: Identify the different variables

    • The number of decays is 16 000
    • The time is 1 hour

Step 2: Determine the time period in seconds

    • 1 hour is equal to 60 minutes, and 1 minute is equal to 60 seconds

Time period = 1 × 60 × 60 = 3600 seconds

Step 3: Divide the total counts by the time period in seconds

Counts ÷ Time period = 16 000 ÷ 3600 = 4.5

    • Therefore, there are 4.5 decays per second

Exam Tip

Do not confuse activity and count rate.

Activity is the rate at which unstable nuclei decay, whereas count rate is the rate at which radioactive emissions are detected.

Author: Katie

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

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