Uses of Radioactive Isotopes — Atomic Structure | Physics with Kate
Topic 06

🔮 Uses of Radioactive Isotopes 🔮

How the half-life of an isotope determines its use — from medical tracers to smoke detectors and thickness monitors.

✦ Key Applications

🎬
🏥
Medical Tracers
Short half-life + gamma emitter. Injected/swallowed, detected outside the body. Activity falls quickly = less harm. Gamma passes through tissue.
🎯
Cancer Treatment
Gamma radiation directed at tumours. Beams focused from multiple angles to minimise damage to healthy tissue.
🔥
Smoke Detectors
Alpha source, long half-life. Alpha ionises air → current flows. Smoke absorbs alpha → current drops → alarm triggers.
📏
Thickness Monitors
Beta source, long half-life. Beta passes through foil. Thicker foil = lower count. Rollers auto-adjust.

✦ How a Smoke Detector Works

🩰 Smoke Detector — Alpha Source
Normal (No Smoke) Am-241 α source + − + − Detector Current flows ✓ Ions carry charge → circuit complete With Smoke Am-241 α source smoke particles ✕ ✕ Detector Current drops → ALARM! 🔔 Alpha absorbed by smoke particles Alpha used because it is easily absorbed — even by smoke
Americium-241 emits alpha particles that ionise the air. Smoke absorbs the alpha radiation, reducing the current and triggering the alarm.

✦ How a Thickness Monitor Works

🩰 Thickness Monitor — Beta Source
Beta source Foil roller roller ✕ Detector Control computer Adjusts roller pressure Too thick: count ▼ → reduce roller pressure Too thin: count ▲ → increase roller pressure
Beta radiation passes through metal foil. The detector measures the count rate — if thickness changes, the computer adjusts the rollers automatically.

✦ Medical Tracer

🩰 Using Tc-99m as a Medical Tracer
Inject Tc-99m t½ = 6 hrs γ emitter Travels through body to target organ Detect γ rays exit body gamma camera Image of organ diagnosis
Tc-99m is injected, travels to the target organ, emits gamma rays detected outside the body, creating an image for diagnosis.

✦ Why Half-Life Matters

Matching Half-Life to Use

  • Medical tracers: Short half-life — activity drops quickly, reducing harm to patient
  • Monitors & detectors: Long half-life — source doesn't need frequent replacement, gives constant activity
  • Too short = source decays before use; too long for medical = prolonged patient exposure

Radiation Type Must Match Too

  • Alpha for smoke detectors — easily absorbed by smoke particles
  • Beta for thickness monitors — partially absorbed by thin metal
  • Gamma for medical tracers — penetrates tissue so can be detected outside the body
🩰 ✦ 🩰 ✦ 🩰
Question 1 [1 mark]
Which is a common use of gamma radiation in medicine?

A. Detecting water leaks    B. Treating cancer    C. Sterilising food    D. Detecting metal fractures
✦ Answer
B — Treating cancer. Gamma radiation can penetrate through the body to reach deep tumours.
Question 2 [2 marks]
Explain why alpha radiation is used in smoke detectors rather than beta or gamma.
✦ Answer
Alpha radiation is easily absorbed by smoke particles [1], causing a detectable change in current. Beta and gamma would pass through the smoke and the alarm would not trigger [1].
Question 3 [2 marks]
Put these steps of a smoke alarm in the correct order:

A. Radiation reaches detector   B. Smoke absorbs radiation   C. Alarm sounds   D. Change in signal activates alarm   E. Detector signal falls
✦ Answer
A → B → E → D → C [2]. Normally radiation reaches detector. Smoke absorbs alpha. Signal falls. Change activates circuit. Alarm sounds.
Question 4 [3 marks]
Tc-99m has a half-life of 6 hours and an initial activity of 400 Bq.

(a) What is the activity after 6 hours? [1]
(b) Why are short half-life isotopes used for medical tracers? [2]
✦ Answer
(a) 200 Bq — one half-life has passed, so activity halves [1].
(b) Activity falls to a safe level quickly [1], reducing the time the patient is exposed to radiation and minimising harm to the patient [1].
Question 5 [3 marks]
Explain how nuclear radiation is used to detect leaks in underground pipes.
✦ Answer
A gamma-emitting tracer is added to the liquid in the pipe [1]. A detector is moved along the ground above the pipe. At a leak, the tracer escapes and gives a higher reading above that point [1]. Gamma is used because it can penetrate through the ground to the surface [1].
Question 6 — Diagram Question [4 marks]
The diagram below shows the inside of a smoke detector. Use the diagram to answer the following questions.
X + − + − + − Y current Component X Component Y
A diagram of the inside of a smoke detector showing components X and Y.
✦ Answer
(a) X is the radioactive source (Americium-241) which emits alpha radiation [1].
(b) Y is the radiation detector which measures the current [1].
(c) The alpha radiation ionises the air between X and Y, creating charged particles (ions) that carry a current [1].
(d) When smoke enters, it absorbs the alpha particles, reducing the current and triggering the alarm [1].
Question 7 — Diagram Question [4 marks]
The diagram shows a system used to monitor the thickness of aluminium foil in a factory. Study the diagram and answer the questions below.
Source type = ? t½ = ? Al foil Detector count = ? Computer adjusts
A thickness monitoring system used in a factory to produce aluminium foil of constant thickness.
✦ Answer
(a) The source should emit beta radiation [1] — alpha would be fully absorbed even by thin foil, and gamma would pass through regardless of thickness.
(b) The source should have a long half-life [1] — so it doesn't need frequent replacement and gives a constant, reliable reading over time.
(c) If the foil becomes thicker, the detector count rate decreases (more beta absorbed) [1].
(d) The computer would then decrease the roller pressure to make the foil thinner again [1].
Question 8 — Matching [3 marks]
The table below shows three applications and three types of radiation. Match each application to the correct radiation type and explain your reasoning for one choice.
Application 1. Smoke detector 2. Medical tracer 3. Thickness monitor Radiation Type A. Beta (β) B. Alpha (α) C. Gamma (γ) ? ? ?
Match each application to the correct type of radiation and explain one choice.
✦ Answer
1 → B (Alpha) [1] — easily absorbed by smoke particles, causing the alarm to trigger.
2 → C (Gamma) [1] — can penetrate through body tissue so it can be detected outside the body.
3 → A (Beta) [1] — partially absorbed by thin metal foil, so changes in thickness produce measurable differences in count rate.
← Half-Life ↑ All Topics Next: Background Radiation →
Physics with Kate · AQA GCSE Physics 8463 · Paper 1 · Topic 4.4.3

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