S4.11.3 For the adult
Half-life
A sample gives out 800 counts each minute. After 20 minutes it gives out 400. How long until it is down to 0?
What this chapter teaches
Half-life is the time for half of the undecayed nuclei in a sample to decay, or for the count rate to fall to half. After each half-life, half of what is left remains, so the amount never reaches exactly zero. The decay of one nucleus is random, and the halving describes a large number. A coin game models it.
Three questions to ask
There is no one right wording. Listen for the idea.
- A sample has a half-life of 10 minutes. How much is left after 30 minutes?
- Why is a sample not safe just because 2 half-lives have passed?
- How do you read a half-life from a decay curve?
After the chapter, the learner can
- Find the half-life from a decay graph
- Model random decay by tossing coins and removing the heads
What they copy into their notebook
Half-life: the time for half of the undecayed nuclei, or the count rate, to halve. After 1, 2, 3 half-lives: one half, one quarter, one eighth. Never exactly 0. Take away background first. One nucleus is random.