Radioactivity and half-life · Year 9
Some nuclei are unstable and decay at random, giving out alpha, beta or gamma radiation. We cannot predict when one nucleus will decay, but a large sample always halves its activity in the same time – its half-life. Radioactivity is used in medicine, smoke detectors and dating ancient objects, but ionising radiation must be handled with care.
Key facts
- Alpha (⁴₂He): stopped by paper, very ionising. Beta (fast electron): stopped by a few mm of aluminium. Gamma (EM wave): reduced by thick lead or concrete.
- Alpha decay: A − 4, Z − 2. Beta decay: A unchanged, Z + 1. Gamma: no change in A or Z.
- Half-life: the time for the activity (or number of undecayed nuclei) to halve.
- After n half-lives, the fraction left is (½)ⁿ; in general A = A₀ × 0.5^(t/(T½)).
- Subtract background radiation before using count rates; decay constant λ = ln 2 ÷ T½.
- Contamination: the source gets on or in you. Irradiation: you are exposed to its radiation.