Half-Life & Risk (AQA GCSE Physics): Revision Note
Exam code: 8463
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
Examiner Tips and Tricks
Here are some common misconceptions; make sure that you don't have them.
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
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