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Half-life and uses of radiation
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Radioactive decay is random, but large samples behave predictably. Half-life tells you how quickly.
Half-life
The half-life is the time for the activity (or the number of unstable nuclei) to fall to half.
After 1 half-life: 12 left. After 2: 14. After 3: 18.
After 1 half-life: 12 left. After 2: 14. After 3: 18.
Calculating
A sample has activity 800 Bq and a half-life of 5 hours.
After 5 h: 400. After 10 h: 200. After 15 h: 100 Bq.
After 5 h: 400. After 10 h: 200. After 15 h: 100 Bq.
Uses and safety
Uses: smoke alarms (alpha), thickness control in paper factories (beta), sterilising and cancer treatment (gamma), medical tracers (short half-life gamma emitters).
Irradiation: exposure to radiation; the object does not become radioactive.
Contamination: radioactive material gets on or in an object. Reduce risk with distance, shielding and short exposure times.
Irradiation: exposure to radiation; the object does not become radioactive.
Contamination: radioactive material gets on or in an object. Reduce risk with distance, shielding and short exposure times.
A sample has 1200 undecayed nuclei and a half-life of 3 days. How many are left after 9 days?
- 9 ÷ 3 = 3 half-lives
- 1200 → 600 → 300 → 150
Answer: 150
Half-life is the time for activity to halve. Count the half-lives and halve each time. Alpha for smoke alarms, beta for thickness monitoring, gamma for sterilising and tracers. Contamination is worse than irradiation.
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