Physics · Higher
Radioactivity
Types of radiation, half-life, contamination vs irradiation, uses and safety.
Unstable nuclei can emit radiation as they decay. You need the properties of alpha, beta and gamma, what half-life means, and the difference between contamination and irradiation.
Types of radiation
• **Alpha (α)** – helium nucleus (2p + 2n); highly ionising; weakly penetrating (stopped by paper/skin)
• **Beta (β)** – high-speed electron; moderately ionising and penetrating (stopped by thin aluminium)
• **Gamma (γ)** – electromagnetic wave; weakly ionising; highly penetrating (reduced by thick lead/concrete)
Alpha and beta are deflected by electric/magnetic fields; gamma is not.
Half-life
**Half-life** – time taken for the number of undecayed nuclei (or activity) to halve.
After 1 half-life → ½ remaining
After 2 half-lives → ¼ remaining
After 3 half-lives → ⅛ remaining
Activity is measured in becquerels (Bq) – decays per second.
Worked example
A sample has activity 800 Bq and half-life 4 hours. Activity after 12 hours?
Solution
12 hours = 3 half-lives 800 → 400 → 200 → **100 Bq**
Contamination and irradiation
• **Irradiation** – exposure to radiation from an outside source (object does not become radioactive)
• **Contamination** – unwanted radioactive material on or in an object (continues to emit radiation)
Uses: sterilising equipment, tracing leaks, cancer treatment, dating rocks/archaeology (carbon-14).
Safety: minimise time, maximise distance, use shielding.
Practice questions
Try these without looking at the answers first.
Q1.Which radiation is most penetrating?
Show answer
Gamma
Q2.What is half-life?
Show answer
Time for activity / undecayed nuclei to halve
Q3.Does irradiation make an object radioactive?
Show answer
No