OCR GCSE Physics A: Radiation hazards, medical uses, fission and fusion (J249) – Revision Notes

These notes condense statements P6.2a to P6.2e of the OCR GCSE (9-1) Gateway Science Physics A specification (J249), Version 5.0 (July 2026), for first assessment in 2018. Only P6.2a is assessed in both tiers. P6.2b to P6.2e are bold in the specification and are tested only in the Higher Tier papers, so they are marked “Higher Tier only”. For full explanations use the study guide, then test yourself with the practice questions.

Hub: OCR GCSE Physics. Checklist: printable checklist. Earlier radioactivity content: half-life and penetrating power. Find weak spots with the free diagnostics.

Key definitions

Term	Meaning
Irradiation	Exposure to radiation from a source outside the body or object. The object does not become radioactive.
Contamination	Radioactive atoms where they are not wanted, on the surface of an object or person or inside them.
Half-life	How long a sample takes to lose half its undecayed nuclei, which also halves its activity.
Tracer	A small amount of radioactive isotope swallowed or injected so that a detector outside the body can follow it.
Radiotherapy	Using radiation to destroy or control unwanted tissue such as a tumour.
Nuclear fission	A heavy, unstable nucleus breaks into smaller nuclei.
Chain reaction	Neutrons from one fission cause further fissions.
Nuclear fusion	Light nuclei combine into a heavier nucleus.

Contamination and irradiation (P6.2a, both tiers)

	Irradiation	Contamination
What reaches you	Radiation	Radioactive atoms
Ends when	Source removed or shielded	Atoms decay or are cleaned away
Reduce the risk	Shielding, distance, short time, tongs	Gloves, protective clothing, no eating or drinking, cleaning

Which is worse? It depends on the radiation.

• Alpha: small hazard outside the body (stopped by skin and a few centimetres of air); serious inside the body, because all its energy goes into nearby living cells.
• Beta: moderate hazard from outside, and a hazard inside.
• Gamma: penetrating, so outside irradiation is a real hazard. Weakly ionising, so many gamma rays from inside the body pass out.

Method for a classification question

1. Ask: have radioactive atoms got onto or into the object or person?
2. If yes, it is contamination. If only radiation arrived, it is irradiation.
3. Give the reason in a short phrase.

Hazards and half-life (P6.2b, Higher Tier only)

	Short half-life	Long half-life
Activity from the same number of nuclei	Higher	Lower
How long the hazard lasts	Short	Very long
Storage	Short-term	Long-term, secure
Use	Medical tracers	Steady sources such as smoke detectors

Why shorter means higher activity. Half the nuclei decay in one half-life. A shorter half-life squeezes the same number of decays into less time, so the rate is higher. A half-life ten times shorter gives roughly ten times the activity at first.

Method: time for activity to reach a safe level

1. Divide the starting activity by the safe activity.
2. Write the answer as a power of 2, so 2ⁿ.
3. n is the number of half-lives. Multiply by the half-life.

Reminder: a spill of 960 Bq with a safe level of 120 Bq: 960 ÷ 120 = 8 = 2³, so three half-lives. With a half-life of 2 hours the wait is 6 hours.

Medical uses (P6.2c, Higher Tier only)

Use	Radiation	Key properties	Why
Imaging with a tracer	Gamma	Half-life of hours	Gamma passes out of the body to the detector. The activity then falls away quickly.
External radiotherapy	Gamma beams	Penetrating	Reaches a tumour from outside. Beams from several directions limit damage to healthy tissue.
Internal treatment	Isotope taken up or placed near the target	Radiation absorbed close by	Energy is deposited in the tissue to be destroyed.

Why not an alpha tracer? Alpha particles cannot leave the body, so the detector would not see them, and they would ionise cells inside.

Why not a very long half-life tracer? The patient would stay radioactive for far too long.

Why not a very short half-life tracer? It may decay before the scan can be done.

Radiotherapy also damages healthy cells, so side effects are weighed against the benefit.

Practical link in the specification: research medical tracers and radiotherapy.

Fission (P6.2d, Higher Tier only)

1. A uranium-235 nucleus usually first absorbs a neutron.
2. The nucleus becomes unstable and splits into two smaller nuclei.
3. Two or three neutrons and gamma radiation are released.
4. The fission products fly apart with kinetic energy, which is transferred to other particles as thermal energy.
5. The released neutrons can cause more fissions: a chain reaction.

Average new fissions per fission	Result
Below 1	Dies away
Exactly 1	Steady energy release
Above 1	Rapid, uncontrolled growth

Counting a chain. If each fission leads on average to 2 more, generation n has 2ⁿ fissions. Generation 6 has 64. The total through generation 5 is 1 + 2 + 4 + 8 + 16 + 32 = 63.

An example equation: ²³⁵U + ¹n → ¹⁴¹Ba + ⁹²Kr + 3 ¹n. Check nucleon number: 235 + 1 = 236, and 141 + 92 + 3 = 236. Check proton number: 92 + 0 = 92, and 56 + 36 = 92.

Fusion (P6.2e, Higher Tier only)

• Two light nuclei combine into one heavier nucleus, for example isotopes of hydrogen forming helium.
• Nuclei repel because both are positive, so very high temperature and pressure are needed to bring them close enough.
• The product has slightly less mass than the starting nuclei. Some mass may be converted into energy carried by radiation.
• This is the process that releases energy in stars.

Example: ²H + ³H → ⁴He + ¹n. Nucleon number: 2 + 3 = 5 and 4 + 1 = 5. Proton number: 1 + 1 = 2 and 2 + 0 = 2.

Must-know distinction

	Fission	Fusion
Nuclei	One large nucleus splits	Two light nuclei join
Trigger	Absorbing a neutron	Extreme temperature and pressure
Neutrons	Released, and keep the chain going	A neutron may be released, as in the example

Worked reminders

Safe level. A source reads 2000 Bq and the safe level is 125 Bq, with a half-life of 4 hours. 2000 ÷ 125 = 16 = 2⁴, so four half-lives: 4 × 4 = 16 hours.

Tracer scan. An injected tracer has half-life 6 hours. After 18 hours there have been 3 half-lives, so the activity is one eighth of the start, a ratio of 8 : 1.

Smoke detector (suggested in the specification). The source needs a steady output for years, which a long half-life gives. Its radiation should be easily stopped, so people nearby are not irradiated.

Precautions list. Irradiation: shield, keep distance, limit time, use tongs. Contamination: gloves, protective clothing, no eating or drinking, clean up spills. Give the precaution that matches the risk.

Quick self-test

1. Define irradiation.
2. Define contamination.
3. Explain why an alpha emitter is more dangerous as a contaminant than as an irradiating source.
4. (Higher Tier only) A source has activity 960 Bq and half-life 5 days. Find the activity after 15 days.
5. (Higher Tier only) Give two reasons why a gamma emitter with a half-life of a few hours suits a medical tracer.
6. (Higher Tier only) Explain why waste with a half-life of thousands of years needs long-term storage.
7. (Higher Tier only) A tracer has activity 600 MBq and half-life 2.0 hours. Find the activity after 6.0 hours.
8. (Higher Tier only) State what must happen to an average fission for the chain reaction to be steady.
9. (Higher Tier only) Why does fusion need a very high temperature?
10. (Higher Tier only) Show that ²H + ³H → ⁴He + ¹n balances for nucleon number.

Answers

1. Exposure to radiation from a source outside the body, with no radioactive atoms transferred.
2. Radioactive atoms on or inside an object or person.
3. Inside the body there is no skin to stop the alpha particles, so all their energy ionises nearby cells.
4. 15 ÷ 5 = 3 half-lives. 960 ÷ 8 = 120 Bq.
5. Gamma passes out of the body to the detector. The short half-life means the activity falls quickly after the scan, so exposure is brief.
6. The activity falls so slowly that the material stays hazardous for a very long time.
7. 6.0 ÷ 2.0 = 3 half-lives. 600 ÷ 8 = 75 MBq.
8. On average, each fission must cause exactly one further fission.
9. The nuclei are both positive and repel. They must move very fast to get close enough to join.
10. 2 + 3 = 5 on the left and 4 + 1 = 5 on the right.

Where marks are usually lost

• Saying an irradiated object becomes radioactive.
• Writing “radiation” for contamination, when the atoms themselves are on or in the object.
• Stating that alpha is harmless without saying it depends on whether it is outside or inside the body.
• Saying a long half-life is safer, without separating lower activity from a longer hazard.
• Choosing an alpha or a very long-lived source as a tracer.
• Describing fusion as splitting and fission as joining.
• Omitting the neutron that triggers fission, or the neutrons released that continue the chain.
• Not showing the power of 2 or the number of half-lives in a decay calculation.

Official syllabus

OCR, GCSE (9-1) Gateway Science Physics A (J249) specification, Version 5.0 (July 2026), for first assessment in 2018, topic P6.2 Uses and hazards.