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

This guide follows the OCR GCSE (9-1) Gateway Science Physics A specification (J249), Version 5.0 (July 2026), for first assessment in 2018. It covers statements P6.2a to P6.2e in topic P6.2, Uses and hazards. P6.2a is assessed in both Foundation and Higher Tier papers. P6.2b, P6.2c, P6.2d and P6.2e are printed in bold in the specification and are tested only in the Higher Tier papers, so they are labelled “Higher Tier only” below.

Related pages: revision notes, practice questions, half-life and penetrating power, nuclear structure and decay equations, the OCR GCSE Physics hub, the printable checklist and the free diagnostics.

What this unit covers

Statement	What you must be able to do	Tier
P6.2a	Recall the differences between contamination and irradiation effects and compare the hazards of the two	Both tiers
P6.2b	Explain why the hazards of radioactive material differ according to the half-life involved	Higher Tier only
P6.2c	Describe the uses of nuclear radiations for exploring internal organs and for controlling or destroying unwanted tissue	Higher Tier only
P6.2d	Recall that some nuclei are unstable and may split, and relate this to emitted radiation, energy transfer to other particles and chain reactions	Higher Tier only
P6.2e	Describe nuclear fusion	Higher Tier only

The specification adds two notes. For fission you need the term nuclear fission, and the unstable nucleus must usually first absorb a neutron. For fusion you need to know that some mass can be converted into energy carried by radiation.

Contamination and irradiation (P6.2a)

A radioactive source emits radiation. Harm depends on whether the source itself, or only its radiation, reaches you.

Irradiation is exposure to radiation from a source outside the body or object. The radioactive atoms stay in the source. An irradiated object does not become radioactive. Exposure stops when the source is removed, switched off or shielded, or when you move away.

Contamination means radioactive atoms are where they are not wanted, on the surface of an object or person or inside them. The atoms themselves have got onto skin, clothes, surfaces, or into food, water or air that is then taken into the body. The exposure continues, and goes wherever the person goes, until the atoms decay or are removed.

Comparing the hazards

Alpha particles are strongly ionising but are stopped by skin or a few centimetres of air. From outside the body, an alpha source is a small hazard. If alpha-emitting atoms are swallowed or breathed in, there is no skin in the way. The alpha particles deposit all their energy in living tissue, next to the cells they ionise. Contamination by an alpha emitter is therefore far more hazardous than irradiation by the same source.

Gamma rays are weakly ionising but very penetrating. They pass through the body from an outside source, so irradiation by gamma rays is a real hazard. Many gamma rays from a contaminating atom inside the body pass straight out.

To reduce irradiation, use shielding, tongs, short times and large distances. To reduce contamination, wear gloves and protective clothing, do not eat or drink where sources are handled, and clean away any spilt material.

Practical link. The specification suggests using a spark chamber as a different type of activity counter.

Worked example 1: classifying situations

State whether each is contamination or irradiation, and give a reason.

(a) A worker’s gloves pick up radioactive powder from a spilt sample. (b) A sealed source in a metal case sits on a shelf, and a person stands one metre from it.

1. In (a), radioactive atoms have got onto the gloves. This is contamination, because the source material itself is on the gloves.
2. In (b), the source is sealed, so no atoms leave the case. Only radiation reaches the person. This is irradiation.

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

Half-life is how long a sample takes to lose half of its undecayed nuclei, which also halves its activity. Two samples with different half-lives are hazardous in different ways.

Short half-life. The nuclei decay quickly, so for the same number of nuclei the activity is high at first and the radiation is intense. The sample soon loses most of its activity, so the hazard fades in a short time.

Long half-life. The nuclei decay slowly, so the activity from a given number of nuclei is lower. However, the activity falls so slowly that the material stays radioactive for a very long time. Waste with a long half-life needs secure storage for a long time.

To see why a short half-life means higher activity, compare two samples with the same number of undecayed nuclei. Half of each sample decays in one half-life. If sample X has a half-life ten times shorter than sample Y, the same number of decays happens in one tenth of the time. X therefore gives out radiation at roughly ten times the rate, at first.

Also remember the type of radiation. A long-lived alpha emitter is a minor hazard outside the body but a serious one once inside it. A very short half-life can also make a source hard to deliver before it decays.

Practical link. The specification suggests discussing smoke detectors as an everyday use of radioactive sources. A detector needs a source that gives a steady output for years, so a long half-life is suitable. The radiation chosen should be easily stopped, so that people near the detector are not irradiated.

Worked example 2: how long until a spill is safe

A spill has an activity of 3200 Bq. The activity is considered safe at 100 Bq or below. The half-life of the isotope is 3 days. How long is it before the area is safe?

1. Find the ratio: 3200 ÷ 100 = 32.
2. 32 = 2⁵, so five half-lives are needed.
3. Check by halving: 3200 → 1600 → 800 → 400 → 200 → 100.
4. Time = 5 × 3 days = 15 days.

If the half-life were 3 years, the same working would give 15 years.

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

Radiation is useful as well as harmful. In medicine it is used in two ways: to look inside the body, and to control or destroy unwanted tissue. The specification suggests researching radioactive tracers and radiotherapy.

Exploring internal organs: tracers

A tracer is a small amount of a radioactive isotope that is swallowed or injected. It travels in the body, or is taken up by a particular organ. A detector outside the body, such as a gamma camera, picks up the radiation and builds a picture of where the tracer has gone.

A good tracer has these properties:

• It emits gamma rays, which are penetrating enough to pass out of the body to the detector. Alpha particles would be absorbed inside the body, would never reach the detector and would damage tissue.
• It has a short half-life, of the order of hours. It lasts long enough for the scan, but its activity falls quickly afterwards, so the patient is exposed to radiation for a short time only.
• Its half-life is not so short that it decays before the scan can be done.

One tracer in common use, technetium-99m, emits gamma rays and has a half-life of about six hours.

Controlling or destroying unwanted tissue: radiotherapy

Ionising radiation damages cells. If enough radiation is absorbed by cancer cells, they are killed or stop dividing. In radiotherapy a beam of gamma rays is aimed at the tumour. Beams are directed from several angles so that the tumour receives a high total amount of radiation while healthy tissue along each path receives much less. Radioactive sources can also be placed inside or next to the tumour, or a radioactive isotope such as iodine-131 can be given that is taken up by the thyroid gland, so the radiation is delivered to the target.

Radiotherapy has side effects, because healthy cells are also damaged, so the risk is weighed against the benefit.

Worked example 3: tracer activity

A patient is injected with a tracer of activity 480 MBq. The half-life is 6.0 hours. Find the activity of the tracer in the patient after 24 hours, if none has been removed from the body.

1. Number of half-lives = 24 ÷ 6.0 = 4.
2. Activity after 4 half-lives = 480 ÷ 2⁴ = 480 ÷ 16.
3. Activity = 30 MBq.

The ratio of original to final activity is 16 : 1. In practice the body also removes some tracer, so the real activity would be lower.

Nuclear fission (P6.2d, Higher Tier only)

Nuclear fission happens when a heavy, unstable nucleus breaks into two smaller nuclei. Most nuclei do not split easily. For fission to occur, the unstable nucleus must usually first absorb a neutron. A common example is uranium-235, which absorbs a neutron and becomes an unstable nucleus that then splits.

What emerges from a fission event:

• Two smaller nuclei, the fission products, which fly apart with a lot of kinetic energy.
• Two or three neutrons.
• Gamma radiation.

The energy is released as kinetic energy of the fission products. Collisions transfer it to surrounding particles, which raises the temperature of the material.

Chain reactions

The neutrons released can be absorbed by other uranium-235 nuclei. Each of those splits, releasing more neutrons, and so on. This is a chain reaction.

Average number of new fissions caused by each fission	Result
Less than 1	The reaction dies away
Exactly 1	Steady rate of energy release
More than 1	The rate increases rapidly and uncontrollably

Worked example 4: growth of a chain reaction

Each fission releases three neutrons. On average, two of them cause another fission. One fission starts the chain (generation 0). Find the number of fissions in generation 10, and the total up to and including generation 10.

1. Each generation doubles the number: generation n has 2ⁿ fissions.
2. Generation 10: 2¹⁰ = 1024 fissions.
3. Total = 1 + 2 + 4 + … + 1024 = 2¹¹ − 1 = 2047 fissions.

Doubling every generation becomes enormous quickly, which is why a chain reaction must be controlled by keeping the average at 1.

Nuclear fusion (P6.2e, Higher Tier only)

In nuclear fusion, two light nuclei combine into one heavier nucleus. This is how stars release energy.

Nuclei are positively charged, so they repel each other. They fuse only if they move fast enough to get very close, which needs very high temperature and pressure. This is why fusion is hard to achieve on Earth.

The mass of the nucleus formed is slightly less than the total mass of the nuclei that joined. The specification notes that some mass may be converted into energy carried by radiation, and this is the source of the energy released.

Worked example 5: checking a fusion equation

An isotope of hydrogen with mass number 2 fuses with an isotope of hydrogen with mass number 3, producing a helium nucleus (atomic number 2, mass number 4) and a neutron. Show that nucleon number and charge are conserved.

1. Nucleon number before: 2 + 3 = 5. After: 4 + 1 = 5.
2. Proton number before: 1 + 1 = 2. After: 2 + 0 = 2.
3. Both totals match, so the equation balances.

²H + ³H → ⁴He + ¹n

Common errors

• Writing that an irradiated object becomes radioactive. It does not.
• Saying a long half-life means a source is safer. It gives lower activity per nucleus, but the hazard lasts much longer.
• Choosing an alpha emitter as a medical tracer. Alpha particles cannot leave the body.
• Describing fission as nuclei joining, or fusion as nuclei splitting.

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.