GCSE-readyRadiation in medicine and nuclear power
Radioactive isotopes are used to treat cancer and to see inside the body. The choice of isotope depends on its radiation and its half-life.
Treating tumours
Internal radiotherapy: a radioactive source is placed inside the body, in or near the tumour (for example small implants), or taken as a liquid that collects in one organ (iodine-131 collects in the thyroid). Beta or alpha emitters are used, because their radiation is absorbed within a short distance, damaging the tumour but little else.
PET scanning
1. The patient is injected with a tracer containing an isotope that emits positrons (beta-plus), such as fluorine-18 attached to a glucose-like molecule.
2. The tracer collects in active tissues (such as tumours, which use a lot of glucose).
3. Each positron meets an electron and they annihilate, giving out two gamma rays in opposite directions, which are detected.
PET isotopes have short half-lives (fluorine-18: about 110 minutes), so they must be made close to the hospital (in a cyclotron) and used quickly.
Half-life and danger; nuclear power
Nuclear power: produces no carbon dioxide while generating and gives a reliable supply from little fuel. But it produces radioactive waste that must be stored safely for a very long time, accidents can release radioactive material, and building and decommissioning power stations is very expensive.
A hospital uses fluorine-18 (half-life about 110 minutes) for PET scans. It takes 220 minutes to transport from a distant supplier. What fraction of the activity would be left?
- 220 ÷ 110 = 2 half-lives
- After 1 half-life: 1/2; after 2: 1/4
Answer: 1/4 of the activity, which is why PET isotopes are made close to the hospital.
External radiotherapy aims gamma beams from different directions; internal radiotherapy places alpha or beta emitters at the tumour. PET uses positron-emitting tracers with short half-lives, so they are made nearby. Nuclear power has no CO2 emissions when generating but produces long-lived radioactive waste.
- Power
- The rate of energy transfer, or work done per second. Unit watt (W).
- Isotopes
- Atoms of the same element with the same number of protons but different numbers of neutrons.
- Half-life
- The time taken for half the unstable nuclei in a sample to decay, or for the activity to halve.
- Activity
- The number of nuclei that decay each second in a radioactive sample. Unit becquerel (Bq).
- Time
- 1 hour = 3600 s. Convert minutes and hours to seconds before using P = E ÷ t or Q = I × t.
- Contamination
- When radioactive material gets onto or into an object or person, so they are exposed to radiation for longer.
All GCSE and International GCSE Physics key terms and definitions →
Try two quick questions
See if it has sunk in. Answers are marked straight away.
2 more questions
Try each one, then tap to check.
A tracer has a half-life of 6 hours. What fraction of its activity remains after 24 hours? Give your answer as a fraction in its simplest form.
Show the answer
Answer: 1/16
24 ÷ 6 = 4 half-lives. (1/2)4 = 1/16.
Why is external radiotherapy usually given in several small doses over weeks?
- To make the source less radioactive
- Healthy cells have time to recover between doses
- The tumour grows back faster
- Gamma rays get stronger each week
Show the answer
Answer: Healthy cells have time to recover between doses
Smaller doses allow healthy tissue to repair.
Check you have got it
Answer 6 quick questions with instant marking. If you get one wrong, GCSE-ready shows you why and gives you another go. It is free, and you do not need an account.
- Pearson Edexcel GCSE Physics (1PH0) specification
- Health Canada product monograph: Fludeoxyglucose (18F) Injection
Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.
What do exam command words like explain and evaluate mean? →