Half-Life & Risk
- The half-life is the time it takes for the activity of a radioactive source to decrease to half of its original value
- Different radioactive isotopes can have very different half-lives
- For example:
- Francium-218 has a half-life of only 1 millisecond (0.001 seconds)
- Polonium-210 has a half-life of about 140 days
- Uranium-235 has a half-life of about 700 million years
Short Half-Life Values
- If an isotope has a short half-life, the nuclei will decay very quickly
- This means that the isotope will emit a lot of radiation in a short amount of time
- If only a small amount of the isotope is used, having a short half-life can be advantageous, as the material will quickly lose its radioactivity
- If a large amount is used, however, the levels of radiation emitted could make handling the isotope extremely dangerous
Long Half-Life Values
- If an isotope has a long half-life then a sample of it will decay slowly
- Although it may not emit a lot of radiation, it will remain radioactive for a very long time
- Sources with long half-life values present a risk of contamination for a much longer time
- Radioactive waste with a long half-life is buried underground to prevent it from being released into the environment
Depending on the activity of radioactive waste, it is buried in different ways
When teaching my GCSE students about the associated risks of long and short half-lives, I would set two rocks on my desk, one labelled 'long half-life' and one labelled 'short half-life'. After reassuring the class that it was perfectly safe to enter the room, we would discuss some scenarios related to potential dangers of half-life. For example, which of these two samples might you use for medical tests that involve swallowing a small amount of the sample? If you had to build a house on top of one of these samples, which would be the safest? If you had to put one of the samples in your bag and take it to the Headteacher's office which one would you choose? For each answer a student gave, I would ask them why. This allowed lots of misconceptions to be uncovered.
The most common misconceptions were:
- The mass of the sample decreases with each decay
- When an atom decays, it changes into another element, it doesn't disappear. The mass will decrease slightly for alpha decay, but the change in mass is very small
- Substances with short half-lives emit more radiation per decay than substances with a long half-life
- If all other factors are equal (the type of radiation emitted, the mass of the sample etc) then the amount of radiation released in each decay is equal. For short half-lives, the decays happen in quick succession, so the radiation is released in quick succession. Let's say that Sample A has a half-life of 1 second, and Sample B has a half-life of 1 hour, and each decay in each sample releases 1000 alpha particles. Sample A releases 1000 alpha particles each second, whereas Sample B releases 1000 alpha particles each hour.
