Radioactive Decay (OCR A Level Physics)

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

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

Radiation

• 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

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