Key Stage 4
Meaning
A half life is the time it takes for half of the unstable isotopes in a radioactive sample to decay.
About Half Life
- Every isotope has a unique half life.
- The half life of an isotope depends on how stable the isotope is.
- The half life of an isotope can be determined by measuring the emission of ionising radiation. The time it takes for the count rate of ionising radiation to halve, is the same as its half life.
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| This diagram shows the number of Fermium-252 atoms halves each day, so the half life is 1 day. |
- The half life of an isotope can be found from plotting a graph of the amount or isotope or the count rate against the time. The half life is the time these take to halve from their initial value.
- The count rate of a sample of unstable isotope can be predicted given its initial count rate, its half life and the time it has been given.
| Time /s | 0 | 10 | 20 | 30 | 40 | 50 | 60 | 70 | 80 |
| Count rate /Bq | 1000 | 707 | 500 | 353 | 250 | 176 | 125 | 88 | 62 |
| Time /min | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| Count rate /Bq | 5000 | 3968 | 3150 | 2500 | 1984 | 1575 | 1250 | 992 | 788 |