Radioactive contamination and background radiation

SPEC 8463 • TOPIC 4.4.2.4 / 4.4.3.1 • PAPER 1

Contamination & Background Radiation

Two terms students consistently confuse — and where Paper 1 hands out the easiest marks if you can keep them straight.

Why This Lesson Wins Marks

Two definitions. One distinction. Examiners ask about it almost every year — and the mark scheme rewards precise wording, not approximate. Get the words right and you bank 2–5 marks every paper.

18%

Scored full marks Jun 2023 Q5.5 (2 marks)

2024

"Hazmat suit" rejected for gamma

2-5

Marks per paper on this topic

What You'll Master

By the end of this lesson you'll be able to: define irradiation and contamination using mark-scheme language; list at least four sources of background radiation; and pick the right precaution for any radiation safety question without falling into the "hazmat suit" trap.

Definition 1: Irradiation

Irradiation

The exposure of an object to ionising radiation. The object does not become radioactive itself.

Picture standing in a beam of gamma radiation from an external source. Radiation hits you — but the moment you step out of the beam (or the source is removed), the danger stops. You haven't absorbed any radioactive atoms; you've just been hit by radiation.

Mark-Scheme Wording

Jun 2023 mark scheme accepts "exposure" or "absorption of radiation". Crucially, it must include the idea that the object is exposed to radiation from an outside source — not that the object emits anything itself.

Definition 2: Contamination

Contamination

The unwanted presence of radioactive atoms (i.e. radioactive material) on or in an object. The contaminated object emits radiation.

Picture a speck of radioactive dust on your skin — or inhaled into your lungs. The atoms are now part of you. They will continue emitting radiation until they have all decayed away, or until the contaminant is physically removed.

The Word That Scores

Mark scheme requires the word "atoms" or "material". Vague answers like "contamination is when radiation is on you" fail — radiation is energy/particles being emitted, not the atoms themselves. The atoms are the source.

The Distinction in One Line

If you remember nothing else from this lesson, remember this contrast:

Irradiation

Radiation has hit you.

The radiation is external. Move away → exposure stops. The object never becomes radioactive.

Contamination

Radioactive material is on or in you.

The atoms are with you. They emit radiation until they decay away or are removed.

Memory Hook

Irradiation = "hIT" (radiation hit me). Contamination = "Carry" (I'm carrying the radioactive atoms). Different problem, different solution.

Definition 3: Background Radiation

Background Radiation

The low-level ionising radiation that is always present in the environment.

You are being irradiated right now. So is your phone, the wall, the air. Background radiation is the baseline level of ionising radiation that surrounds us at all times — it's what every detector picks up before you bring a source near it.

Why It Matters in Calculations

When measuring the activity of a source, you must subtract the background count rate from your reading to get the true activity of the source. This is a recurring exam-question pattern in Paper 1.

Sources: Natural

Most background radiation is natural — it has always existed and we cannot remove it.

SourceWhere it comes from

Radon gasReleased from rocks and soil — especially granite. Dominates background in many regions (Cornwall, parts of Scotland). Often the largest single contributor.

Cosmic raysHigh-energy particles from space, mostly from the Sun and beyond. More intense at high altitude — pilots get higher doses.

Food & drinkNatural isotopes like potassium-40 in bananas, leafy vegetables, and many foods.

The human bodyYou contain trace radioactive isotopes (K-40, C-14) — you irradiate yourself, and the people next to you.

Sources: Artificial (Man-Made)

A smaller fraction of background comes from human activity. Medical exposure is the biggest artificial contribution.

SourceDetail

Medical proceduresX-rays, CT scans, radiotherapy, radioactive tracers used in diagnosis. Largest artificial source for most people.

Nuclear power industryRoutine emissions and waste from power stations and reprocessing plants.

Nuclear weapons falloutResidual contamination from past atmospheric weapons tests (peaked in the 1950s–60s).

Nuclear accidentsNamed events: Chernobyl, Fukushima, Three Mile Island.

Mark-Scheme Note

Jun 2020 Q5.1 accepted: medical X-rays, radiotherapy, nuclear weapons testing, or a named nuclear disaster (Chernobyl / Fukushima / Three Mile Island). "Nuclear waste" alone was ignored — not specific enough.

Radiation Dose

Background radiation level and individual dose vary with occupation (radiographers, nuclear workers, pilots) and location (Cornwall > East Anglia because of underlying granite and radon).

Unit of Radiation Dose

Radiation dose is measured in sieverts (Sv).

1000 millisieverts (mSv) = 1 sievert (Sv)

Spec Note

The spec states "Students will not need to recall the unit of radiation dose" — but you do need to recognise it in questions and understand that bigger Sv = bigger dose = bigger biological risk. Don't confuse with becquerels (Bq), which measure activity (decays per second), not dose.

Reducing Exposure: T-D-S-C

Four levers reduce the dose received from a radioactive source. Every safety question wants you to name and explain at least two of them.

LeverWhat to doWhy it works

TimeMinimise time near the sourceDose is proportional to exposure time

DistanceStand further awayIntensity falls rapidly with distance (inverse-square)

ShieldingLead for gamma, aluminium for beta, paper for alphaMaterial absorbs/blocks the radiation

ContainmentStore in sealed, lead-lined containersPrevents both irradiation and contamination escape

Handling Radioactive Sources

When AQA asks "describe one precaution", they want a specific physics-based procedure — not generic safety advice. The mark scheme accepts:

Long-handled tongs / forceps — never bare hands (increases distance)

Lead-lined storage containers when not in use (containment + shielding)

Point the source away from people while handling

Trefoil hazard symbol labelling and restricted-access storage

Wear a dosimeter / film badge to monitor cumulative dose

Lead aprons when working with gamma sources (e.g. radiographers)

Pattern Recognition

Strong answers name the equipment AND state which lever it operates (time, distance, shielding, containment). "Use tongs to increase distance from the source" scores; "use tongs" alone is borderline.

Examiner Alert: The Hazmat Trap

Jun 2024 Q9.2 / Q9.3

Students were asked about radiation safety. A common wrong answer: "wear a hazmat suit" or "wear PPE". The mark scheme rejected these for gamma irradiation. Generic protective clothing is not enough.

Hazmat suits and PPE prevent contamination — they stop radioactive particles getting on your skin or being inhaled. They do not stop gamma irradiation — gamma photons pass straight through cloth, plastic, and skin.

For Contamination

Gloves, hazmat suit, mask

Block particles getting onto or into you.

For Gamma Irradiation

Lead shielding + distance + time

Only mass and distance stop gamma. PPE doesn't.

Alpha vs Gamma: Which Hazard?

The spec says: "the type of radiation emitted affects the level of hazard". The danger of a radiation type flips depending on whether the source is outside or inside your body.

External (Irradiation)Internal (Contamination)

AlphaLow risk — stopped by skin / a few cm of air. Can't reach internal tissue.Highest risk — strongly ionising, deposits all its energy in a tiny volume of living cells.

BetaModerate — penetrates skin a few mm, can reach surface tissue.Moderate — less ionising than alpha, but more penetrating.

GammaHighest risk — passes through the whole body, depositing dose throughout.Lower per atom — weakly ionising, and most photons escape the body without depositing energy.

PEE Worked Example

"Explain why a radiographer leaves the room when an X-ray of a patient is taken."3 marks

P

The radiographer leaves the room to reduce their exposure to ionising radiation (and so reduce their accumulated dose).

E

Each X-ray carries a small risk of ionising cells and damaging DNA, which can cause mutations or cancer.

E

Although one X-ray gives a low dose, the radiographer performs many X-rays per day — accumulated exposure becomes significant. Leaving the room increases distance and reduces their dose effectively to zero.

PEE Worked Example

"Explain why gamma radiation is a serious irradiation hazard but not so much a contamination hazard."4 marks

P

Gamma is dangerous as an irradiation hazard because it is highly penetrating.

E

From an external source, gamma passes through skin, clothing, and even some shielding, depositing dose throughout the whole body.

E

As a contamination hazard, gamma is less ionising per particle than alpha, so each interaction causes less cellular damage.

E

Inside the body, most gamma photons escape the body without being absorbed, depositing very little energy — whereas alpha contamination is concentrated in surrounding tissue and far more harmful.

Common Errors That Cost Marks

✗ Weak

"Hazmat suit / PPE" for any radiation question.

✓ Strong

Named procedure: tongs, lead-lined container, lead shielding, distance, minimise time.

✗ Weak

"Contamination is when radiation is on you."

✓ Strong

Contamination is the unwanted presence of radioactive atoms / material on or in an object.

✗ Weak

"All background radiation comes from radon."

✓ Strong

Radon is the largest natural contributor in many regions; also cosmic rays, food, and the body. Medical is the largest artificial source.

✗ Weak

"Ionises cells." (Jun 2023 Q5.6 — not enough.)

✓ Strong

Cancer / tumours, DNA mutation, damages or kills cells, radiation sickness. Be specific.

Your Turn

A laboratory researcher handles a sealed radioactive source as part of an experiment.

Describe the difference between irradiation and contamination, with reference to this scenario.2 marks

This is a recurring 2-mark Paper 1 pattern. Examiner reports show fewer than 1 in 5 students score full marks — almost always because only one of the two terms is properly contrasted.

1

Irradiation is the exposure of the researcher to radiation from the source (allow "absorption of radiation" for "exposure"). The researcher does not become radioactive.

2

Contamination would be the (unwanted) presence of radioactive material / atoms on or in the researcher — for example if the source leaked and particles landed on skin or were inhaled.

Both marks need contrasting statements. A common 1-mark answer describes only one term and lets the reader infer the other — the mark scheme requires both stated explicitly.

Checkpoint

You should now be able to:

✓

Define irradiation, contamination, and background radiation using mark-scheme language word-for-word.

✓

Distinguish irradiation from contamination with two contrasting statements in any safety question.

✓

Name 4+ sources of background radiation — splitting natural (radon, cosmic, food, body) from artificial (medical, nuclear industry, fallout, accidents).

✓

Recommend specific precautions — tongs, lead-lined containers, lead shielding, distance, time — and avoid the "hazmat suit" trap for gamma.

✓

Compare alpha vs gamma as external vs internal hazards — explain why the danger flips depending on whether the source is outside or inside the body.

✓

Explain why radiation studies must be peer-reviewed — so other scientists can check and replicate the findings before they shape safety policy.

Next Lesson

Lesson 4.06 — matching half-lives to uses of radioactivity. Why a 6-hour half-life is perfect for a medical tracer but useless for a smoke detector.

Contamination & Background Radiation

Why This Lesson Wins Marks

Definition 1: Irradiation

Definition 2: Contamination

The Distinction in One Line

Irradiation

Contamination

Definition 3: Background Radiation

Sources: Natural

Sources: Artificial (Man-Made)

Radiation Dose

Reducing Exposure: T-D-S-C

Handling Radioactive Sources

Examiner Alert: The Hazmat Trap

For Contamination

Gloves, hazmat suit, mask

For Gamma Irradiation

Lead shielding + distance + time

Alpha vs Gamma: Which Hazard?

Sharing the Science: Peer Review

PEE Worked Example

PEE Worked Example

Common Errors That Cost Marks

Your Turn

Checkpoint