Half-life
- Radioactive decay is random, so we can't say when one nucleus will decay, but we can predict what happens to a large number.
- The half-life of a radioactive isotope is the time it takes for the number of unstable nuclei in a sample to halve.
- It is also the time it takes for the count rate (or activity) from a sample to fall to half its starting level.
Finding half-life from a graph
- Plot count rate against time. Read off the starting count rate, find half of it, and read across to the curve and down to the time axis.
- Check by halving again: the time for each halving is the same.
Half-life calculations
- After 1 half-life, 1/2 is left; after 2 half-lives, 1/4; after 3, 1/8; after 4, 1/16.
- Example: a sample with an activity of 800 Bq and a half-life of 2 hours will have an activity of 800 → 400 → 200 → 100 Bq after 6 hours.
- (Higher) The net decline can be given as a ratio. After 3 half-lives, the activity is 1/8 of the original, so it has fallen by 7/8.
Why half-life matters
- Isotopes with a short half-life become safe quickly but are very active at first.
- Isotopes with a long half-life stay radioactive for a long time, so waste must be stored safely for many years.
Key terms
- Half-life
- The time for the number of unstable nuclei, or the count rate, to halve.
- Random
- Impossible to predict for a single event.
- Count rate
- The number of decays detected per second.
- Activity
- The number of decays per second from a source, in becquerels.
- Background radiation
- Radiation that is around us all the time, which should be subtracted from readings.