# Radioactive Decay(OCR A Level Physics)

## Revision Note

Author

Katie M

Expertise

Physics

#### 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

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

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

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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