| English | Half-life |
|---|---|
| Bahasa Melayu | Setengah hayat |
| 中文 | 半衰期 |
Definition
The time taken for half the radioactive nuclei in a sample to decay.
In exam terms
The half-life is the time taken for half the radioactive nuclei in a sample to decay, or equivalently for the activity to fall to half its value. It is measured in units of time, usually seconds (s), and each isotope has its own fixed half-life.
Because decay is random, half-life is a statistical average, not the exact time for any one nucleus.
A correct sentence: "After two half-lives, only one quarter of the original nuclei remain undecayed." For example, from 80 g of a source after 3 half-lives: 80 g → 40 g → 20 g → 10 g remaining.
Do not confuse it with…
Do not confuse half-life with the total time to decay completely. A sample never fully decays to zero in a fixed time; each half-life removes only half of what remains.
Also distinguish it from activity: activity is the number of decays per second (Bq), whereas half-life is the time for that activity to halve.
In Paper 2 you may be asked to define half-life, determine it from a decay graph, or calculate the remaining mass or activity after a whole number of half-lives.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)