📖 In-Depth Theory
Medical Uses
CANCER TREATMENT (RADIOTHERAPY):
Gamma rays from cobalt-60 focused on tumour from multiple directions.
Crossing beams concentrate dose at tumour — minimise dose to surrounding healthy tissue.
Kills cancer cells by damaging DNA.
Gamma used because it penetrates deeply enough to reach internal tumours.
MEDICAL TRACERS:
A gamma-emitting radioisotope injected into patient.
Gamma camera detects distribution of the tracer in the body.
Reveals function of organs (not just structure like X-rays).
Technetium-99m: short half-life (~6 hours) — activity falls rapidly → low patient dose.
Iodine-123: accumulates in thyroid → used to diagnose thyroid disorders.
Gamma used: penetrates body to reach detector outside; shorter range alpha/beta wouldn't exit.
STERILISATION OF MEDICAL EQUIPMENT:
Gamma radiation from cobalt-60 kills bacteria and viruses on medical instruments.
Equipment sealed in packaging — gamma penetrates to sterilise contents.
No heat needed — suitable for heat-sensitive equipment.
Also used to sterilise food (extending shelf life).
Industrial Uses
PAPER/SHEET THICKNESS MONITORING:
Beta source placed above a moving sheet (paper, metal foil).
Beta detector below measures transmitted intensity.
More beta absorbed → thicker sheet.
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.
Industrial sources: LONGER half-life (years) — source doesn't need replacing frequently.
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.