Half-life
The time taken for half of the unstable nuclei in a sample to decay. It is also the time for the count rate to fall to half.
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Analogy
Like a cake that halves each time you eat half of what is left: it never quite reaches zero.
Example
Half-life = 6 hours.Start: 80 gAfter 6 h: 40 gAfter 12 h: 20 gAfter 18 h: 10 g
In practice
Doctors choose tracers with short half-lives so they vanish from the body quickly.
Practice
Flashcards
- What is half-life?The time for half the unstable nuclei in a sample to decay.
Interview questions
- A radioactive source has a half-life of 3 hours. A sample starts with 64 g of the isotope. How much is left after 12 hours?
- 12 hours is 4 half-lives
- Halve four times: 32, 16, 8, 4
- 4 g of the isotope remains
Questions people ask
How do I do a half-life calculation?
Work out how many half-lives have passed by dividing the total time by the half-life. Then halve the starting amount once for each half-life. A 80 g sample with a half-life of 6 hours leaves 80 / 2 / 2 / 2 = 10 g after 18 hours, which is three half-lives.
Does a radioactive sample ever disappear completely?
In theory the number of unstable nuclei keeps halving and never reaches exactly zero. In practice, after many half-lives so few nuclei remain that the sample counts as safe. After ten half-lives only about one thousandth of the original sample is left.