Radioactive Decay (OCR A Level 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

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

ANSWER:  C

• • 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

• 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