Signal fed back to rollers → adjusts pressure to maintain correct thickness.
Beta chosen: absorbed by the sheet but not by surrounding air; alpha too weak to penetrate any sheet; gamma too penetrating (wouldn't distinguish thicknesses).
SMOKE DETECTORS:
Alpha source (americium-241) ionises air between two electrodes.
Ion current flows — circuit active.
Smoke particles absorb alpha radiation → less ionisation → current drops → alarm triggers.
Alpha chosen: short range — doesn't escape detector casing, safe for household use; ionises air well.
PIPELINE FAULT DETECTION:
Gamma source moved through underground pipe.
Gamma detector on surface detects escaping radiation.
Crack or fault → more gamma escapes → indicates location of leak.
CARBON DATING:
Radioactive ¹⁴C decays with 5730-year half-life.
Compare ¹⁴C/¹²C ratio in sample vs living material.
Calculate age from ratio.
Choosing the Right Type
SELECTION CRITERIA — match the type to the application:
ALPHA:
High ionisation — useful for ionising air (smoke detectors).
Short range — safe for household devices.
NOT useful where penetration is needed.
BETA:
Moderate penetration — can pass through a few mm of material.
Absorbed by aluminium — useful for thickness monitoring of thin sheets.
NOT useful for deep tissue medical imaging.
GAMMA:
High penetration — can pass through the body and through thick materials.
Least ionising per path — causes less immediate local damage.
Useful for: medical imaging, radiotherapy, sterilisation, pipeline testing.
Must be shielded with lead/concrete.
HALF-LIFE SELECTION:
Medical tracers: SHORT half-life (hours–days) — patient exposure minimal.
Carbon dating: half-life comparable to age of sample (thousands of years).
⚠️ Common Mistake
Beta is used for THICKNESS monitoring — not alpha or gamma. Alpha is used for SMOKE detectors — not beta or gamma. Gamma is used for DEEP medical imaging and radiotherapy — because it penetrates to internal organs. Medical tracers need SHORT half-lives to minimise patient dose.
📌 Key Note
Alpha: smoke detectors (ionises air, short range). Beta: thickness gauges (absorbed proportional to thickness). Gamma: radiotherapy, medical tracers, sterilisation, pipeline testing (most penetrating). Medical tracers: gamma + short half-life (Tc-99m, 6 hours). Selection based on penetration, ionisation and half-life.
🎯 Matching Activity — Radiation Uses
Match each use to the radiation type and reason it is chosen. — drag the symbols on the right to match the component names on the left.
Smoke detector
Drop here
Paper thickness gauge
Drop here
Medical tracer (organ imaging)
Drop here
Radiotherapy
Drop here
Alpha — ionises air between electrodes; smoke absorbs alpha → current drops → alarm
Beta — absorbed proportional to thickness; gamma too penetrating; alpha too weak
Gamma — penetrates body to reach detector outside; short half-life minimises dose
Gamma — penetrates to deep tumour; multiple beams cross at tumour to concentrate dose
🔬 Triple Science Only
Uses of nuclear radiation (physics only) — not in Combined Science as a separate detailed topic.
🎯 Test Yourself
Question 1 of 2
1. Why is americium-241 (an alpha emitter) used in smoke detectors rather than a gamma emitter?
2. A medical tracer needs to emit gamma radiation and have a short half-life. Why?
⭐ How Well Do You Understand This Topic?
Be honest with yourself — this helps you know what to revise!
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