Extended · Practice questions · Uses of radioactivity

Choose, and justify.

Six original Cambridge-style questions on uses of radioactivity: picking the right type and half-life for smoke alarms, gauging, sterilisation, tracers, treatment and dating.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

What the examiner wants

Reason for the type, reason for the half-life.

Type: alpha (short range, safe), beta (thickness gauging), gamma (through body or packaging).
Half-life: short to decay away fast, long to last for years.
Always justify both choices.

Application

[3 marks]

A smoke alarm contains a small radioactive source. State the type of radiation used and give two reasons why it is suitable.

Alpha radiation. ✓
It ionises the air over a short gap and is blocked by smoke, and it is safe because the casing stops it. ✓
It has a long half-life, so it lasts for the life of the alarm. ✓

Application

[3 marks]

A factory checks the thickness of metal foil using a radioactive source and detector. Explain why a beta source is chosen rather than alpha or gamma.

Alpha would be completely stopped by the foil, giving no reading. ✓
Gamma would pass straight through with almost no change, so it would not detect thickness changes. ✓
Beta is partly absorbed, so the amount getting through depends on the thickness. ✓

Application

[2 marks]

Medical instruments are sterilised using a gamma source while still sealed in their packaging. Explain why gamma is suitable.

Gamma is very penetrating, so it passes through the packaging. ✓
It kills the bacteria inside without opening the package. ✓

Application

[3 marks]

A tracer is injected into a patient so that doctors can follow it with a detector outside the body. State the type of radiation and the kind of half-life needed, with reasons.

Gamma radiation, because it can escape the body to reach the detector. ✓
A short half-life. ✓
So the source decays away quickly and the patient receives only a small dose. ✓

Application

[2 marks]

Explain why a source used to treat a tumour deep inside the body is a gamma emitter rather than an alpha emitter.

Gamma is penetrating, so it can reach the tumour deep inside the body. ✓
Alpha would be stopped by the skin and never reach it. ✓

Application

[2 marks]

Carbon-14 is used to find the age of old wood. Explain why a source with a very long half-life is needed for dating.

The material may be thousands of years old. ✓
A long half-life means enough of the source is still left to measure after that time. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.