Radioactive Decay (OCR A Level Physics)

Revision Note

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Katie M


Katie M



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!

Simulating the Random Nature of Radioactive Decay

  • Radioactive decay is defined as:

The spontaneous disintegration of a nucleus to form a more stable nucleus, resulting in the emission of an alpha, beta or gamma particle

  • Radioactive decay is a random process, which means that:
    • There is an equal probability of any nucleus decaying
    • It cannot be known which particular nucleus will decay next
    • It cannot be known at what time a particular nucleus will decay
    • The rate of decay is unaffected by the surrounding conditions
    • It is only possible to estimate the proportion of nuclei decaying in a given time period

  • Radioactive decay is a spontaneous process, which means that: 
    • The decay of nuclei is not affected by the presence of other nuclei in the sample
    • External factors such as pressure do not have an effect on the decay


  • The random nature of radioactive decay can be demonstrated by observing the count rate of a Geiger-Muller (GM) tube
    • When a GM tube is placed near a radioactive source, the counts are found to be irregular and cannot be predicted
    • Each count represents a decay of an unstable nucleus
    • These fluctuations in count rate on the GM tube provide evidence for the randomness of radioactive decay

Radioactivity Fluctuations, downloadable AS & A Level Physics revision notes

The variation of count rate over time of a sample radioactive gas. The fluctuations show the randomness of radioactive decay

Simulating this Random Nature

  • The random nature of unstable nuclei can be simulated in many ways: 
    • Rolling lots of dice
    • Flipping coins
    • Making popcorn 

Rolling lots of dice

  • Each die represents unstable undecayed nuclei in a sample
  • Roll the dice and remove all the dice that land with a six facing up
    • The dice with a six have now decayed into stable nuclei of a different element
    • They are no longer part of the sample
  • Repeat this process again
  • Keep repeating
  • The number of dice that are removed each time is completely random


Flipping Coins

  • Start with 10 coins in a bag
  • One coin represents one nucleus in the sample
  • Shake the bag and tip out the coins
  • When the coin lands on a tail
    • that nucleus has decayed into a stable nucleus
    • it is no longer part of the sample
  • The coins that land on a head is still unstable and have not yet decayed


Making Popcorn

  • Each popcorn kernel represents one undecayed nucleus in the sample
  • When the popcorn is cooked in a microwave each pop represents a single decay
  • At the start, there are lots of un popped kernels and the popping rate is high
  • As the amount of un popped kernels decreases, so does the popping rate


Exam Tip

It is important to understand how each of these simulations work as you may be asked questions about them in your examination.

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Katie M

Author: Katie M

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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