The Random Nature of Decay (Edexcel GCSE Physics: Combined Science)

• It cannot be predicted when a particular unstable nucleus will decay
• Radioactive decay is a random process, this 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 probability of a certain nucleus decaying in a given time period

• This is done by the half-life which enables the activity of a very large number of nuclei to be predicted during the decay process
  • During one half-life a particular nucleus will have a 50% chance of decaying
  • This means that, on average, 50% of nuclei will decay during one half-life
  • As a result, the number of nuclei remaining after each successive half-life will on average halve
  • However, even after a large number of half-lives, there is still a small probability that a particular nucleus will not have decayed
  • As a result, the number of nuclei remaining will never quite fall to zero

• A researcher might take some readings of background radiation
• If the researcher reset the counter to zero, waited one minute and then took the count
  reading and repeated the procedure, they might obtain results such as:

32    11    25    16    28

• The readings don’t appear to follow a particular trend
  • This happens because of the randomness of radioactive decay

Dice Analogy

• An analogy is a way of understanding an idea by using a different but similar situation
• Rolling dice is a good analogy of radioactive decay because it is similar to the random nature of radioactive decay

A dice roll is a random process because you don’t know when you will roll a particular value. However, you can determine the probability of a particular result

• Imagine someone rolling a dice and trying to get a ‘6’
  • Each time they roll, they do not know what the result will be
  • But they know there is a 1/6 probability that it will be a 6

• If they were to roll the dice 1000 times, it would be very likely that they would roll a 6 at least once

• 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