Radioactive contamination and irradiation

What you'll learn

This revision guide covers the crucial distinction between radioactive contamination and irradiation, a topic that frequently appears in AQA GCSE Physics exams. You'll understand how radioactive materials pose different risks depending on whether they enter the body or remain external. This knowledge is essential for explaining safety procedures in medical, industrial, and research contexts.

Key terms and definitions

Irradiation — exposure to ionising radiation from a source outside the body; the person does not become radioactive themselves

Contamination — unwanted presence of radioactive atoms on or inside an object or organism; the contaminated object will emit radiation as long as the radioactive atoms remain

Ionising radiation — radiation with sufficient energy to remove electrons from atoms, creating ions; includes alpha, beta, and gamma radiation

Half-life — the time taken for the number of radioactive nuclei in a sample to halve, or for the activity to halve

Radioactive decay — the spontaneous, random process by which unstable atomic nuclei emit radiation to become more stable

Background radiation — low-level ionising radiation present in the environment at all times from natural and artificial sources

Activity — the rate at which a radioactive source decays, measured in becquerels (Bq); one becquerel equals one decay per second

Dose — a measure of the biological effect of radiation exposure on living tissue, measured in sieverts (Sv)

Core concepts

Understanding irradiation

Irradiation occurs when a person or object is exposed to ionising radiation from an external source. The radiation may pass through the body or be absorbed, but crucially, the irradiated person or object does not become radioactive.

Key characteristics of irradiation:

• The source remains separate from the person or object being exposed
• Exposure stops immediately when the source is removed or shielded
• No radioactive material transfers to the irradiated object
• Used deliberately in medical treatments (radiotherapy) and food sterilisation

Medical applications of irradiation:

Radiotherapy uses carefully controlled gamma or beta radiation to kill cancer cells. The patient is irradiated for specific durations, but does not become radioactive. Medical staff can treat the patient immediately after exposure without risk. The radiation source (often cobalt-60 or a linear accelerator) remains in a shielded unit.

Industrial applications:

Food irradiation destroys bacteria and parasites, extending shelf life without making the food radioactive. The UK permits irradiation of certain foods, though they must be clearly labelled. This process is particularly useful for spices, herbs, and some fruits.

Understanding contamination

Contamination happens when radioactive atoms are present on or inside an object or organism where they shouldn't be. Unlike irradiation, contaminated objects continue to emit radiation as long as the radioactive material remains.

Key characteristics of contamination:

• Radioactive material is physically present on/in the contaminated object
• The object continues to emit radiation until the material is removed or fully decays
• Can spread to other objects through contact or transfer
• Particularly hazardous if ingested, inhaled, or absorbed through wounds

Internal vs external contamination:

External contamination occurs when radioactive material lands on skin, clothing, or surfaces. It can often be removed by washing or changing clothes. Internal contamination is more serious, occurring when radioactive material enters the body through:

• Inhalation of radioactive dust or gas
• Ingestion of contaminated food or water
• Absorption through cuts or wounds
• Injection (accidental)

Internal contamination is especially dangerous because:

• The radiation source is in direct contact with living tissue
• Removal is difficult or impossible
• Alpha radiation becomes highly hazardous inside the body (though safe externally)
• Exposure continues 24 hours per day until the material decays or is excreted

Comparing hazards: irradiation vs contamination

The relative danger of each depends on the specific situation:

When irradiation is more hazardous:

• High-intensity gamma sources can deliver large doses quickly
• X-ray machines in hospitals pose risks if safety protocols fail
• Nuclear weapons produce intense instantaneous radiation

When contamination is more hazardous:

• Alpha-emitting materials (e.g., polonium-210, radon gas) are highly dangerous if inhaled or ingested
• Long half-life materials (e.g., caesium-137, strontium-90) provide continuous exposure
• Contamination can spread, affecting multiple people and large areas
• Examples include the Chernobyl disaster (1986) where contaminated material spread across Europe

Sources and types of radiation exposure

Alpha (α) radiation:

• Stopped by paper or skin's dead outer layer
• Safe outside the body (cannot penetrate to living cells)
• Extremely dangerous if contamination occurs internally
• Highly ionising, causing maximum damage over short distances
• Example: radon gas (from rocks like granite) can be inhaled

Beta (β) radiation:

• Stopped by aluminium foil or a few millimetres of tissue
• Moderate penetration into the body during irradiation
• Dangerous if contamination occurs, particularly near sensitive organs
• Example: strontium-90 mimics calcium and concentrates in bones

Gamma (γ) radiation:

• Highly penetrating, requiring thick lead or concrete shielding
• Poses significant irradiation risk from external sources
• Same risk whether external or internal (equally penetrating)
• Used in radiotherapy because it reaches deep tumours

Safety precautions and protection measures

Preventing irradiation:

• Shielding — placing absorbing material between the source and people (lead aprons in X-ray departments, concrete bunkers for gamma sources)
• Distance — radiation intensity follows the inverse square law; doubling distance quarters the intensity
• Time — minimising exposure duration reduces total dose received
• Protective equipment — lead-lined rooms, lead glass windows, robotic handling for high-activity sources

Preventing contamination:

• Sealed sources prevent radioactive material escaping
• Protective clothing (gloves, suits, masks) prevents contact with skin
• Fume hoods and ventilation systems remove radioactive gases
• Monitoring equipment detects contamination on surfaces and personnel
• Decontamination procedures (washing, removing clothing) if exposure occurs
• Controlled areas restrict access to trained personnel only

Medical and laboratory protocols:

• Glove boxes for handling contaminated materials
• Regular monitoring with Geiger counters
• Disposal in designated radioactive waste containers
• Film badges or digital dosimeters measure accumulated dose for workers
• Annual dose limits strictly enforced (1 mSv for public, 20 mSv for radiation workers in the UK)

Real-world incidents and their lessons

Litvinenko poisoning (2006, London):

Former Russian agent Alexander Litvinenko was poisoned with polonium-210 in tea. This alpha-emitter caused fatal internal contamination. The case demonstrates why alpha sources are safe to handle externally but lethal internally — traces contaminated locations across London, detectable months later.

Fukushima (2011, Japan):

Earthquake and tsunami damaged cooling systems, leading to meltdowns and hydrogen explosions. Radioactive material (iodine-131, caesium-137) contaminated land and ocean. Evacuation zones established based on contamination levels, not just proximity to the plant. Some areas remain uninhabitable due to persistent contamination.

Medical accidents:

Radiotherapy machines have occasionally delivered excessive doses due to equipment malfunction or human error. The Therac-25 incidents (1985-1987) showed how software errors in medical linear accelerators could cause massive over-irradiation, with fatal consequences.

Worked examples

Example 1: Distinguishing contamination from irradiation

Question: A hospital worker accidentally spills a small amount of radioactive liquid containing technetium-99m on their glove. They immediately remove the glove and wash their hands thoroughly.

(a) Explain whether the worker experienced irradiation, contamination, or both. [2 marks]

(b) The worker's hands show no radioactive material after washing. Explain why they are now safe. [2 marks]

Mark scheme answers:

(a)

• The worker experienced both irradiation and contamination [1 mark]
• They were contaminated because radioactive material was on their skin/glove [1 mark]
• They were also irradiated by radiation from the source while it was nearby

Alternative: The worker was contaminated [1 mark] because radioactive atoms were physically present on the glove/hand [1 mark]

(b)

• Removing the glove and washing removed the radioactive material [1 mark]
• Without the radioactive source present, there is no further radiation exposure/they cannot be irradiated by material that's been removed [1 mark]

Example 2: Comparing radiation hazards

Question: A laboratory has two radioactive sources:

Source A: emits alpha radiation, half-life 20 years Source B: emits gamma radiation, half-life 5 days

(a) Which source would be more hazardous if a worker was standing 2 metres away from it? Explain your answer. [3 marks]

(b) Which source would be more hazardous if a small amount entered the worker's body? Explain your answer. [3 marks]

Mark scheme answers:

(a)

• Source B (gamma) is more hazardous [1 mark]
• Gamma radiation is very penetrating and can reach the worker from 2 metres away [1 mark]
• Alpha radiation would be stopped by air and would not reach the worker [1 mark]

(b)

• Source A (alpha) is more hazardous [1 mark]
• Inside the body, alpha radiation is highly ionising and causes maximum damage to surrounding tissue [1 mark]
• The long half-life means it continues emitting radiation for many years/gamma passes through tissue causing less damage per emission [1 mark]

Example 3: Safety procedures

Question: A food processing company uses a cobalt-60 source (gamma emitter) to sterilise packaged food. The source is kept in a lead-lined chamber.

(a) Explain why the food does not become radioactive during this process. [2 marks]

(b) Suggest two safety measures, other than lead lining, that the company should use to protect workers. [2 marks]

Mark scheme answers:

(a)

• The food is only irradiated/exposed to radiation [1 mark]
• No radioactive material is added to the food/the atoms in the food do not become radioactive [1 mark]

(b) Any two from:

• Keep workers at a distance from the source / use the inverse square law [1 mark]
• Minimise the time workers spend near the source [1 mark]
• Use remote handling equipment/robots [1 mark]
• Monitor workers with dosimeters/film badges [1 mark]
• Restrict access to authorised personnel only [1 mark]
• Use warning signs and designated controlled areas [1 mark]

Common mistakes and how to avoid them

• Confusing irradiation with contamination — Remember: irradiation is exposure from outside (like sunlight on your skin); contamination means radioactive material is actually on or in the object (like getting paint on your clothes). Irradiated objects don't become radioactive; contaminated objects do emit radiation.

• Thinking all radiation types are equally dangerous in all situations — Alpha is harmless outside the body but devastating inside; gamma is dangerous both outside and inside. Always consider both the radiation type AND the exposure scenario.

• Believing radioactive material can be "washed off" once it enters the body — External contamination can often be removed by washing, but internal contamination cannot be cleaned away. This is why preventing inhalation and ingestion is crucial.

• Forgetting that contamination provides continuous exposure — Unlike irradiation which stops when you move away from the source, contamination continues exposing you 24/7 until the material is removed or decays. This makes even weak sources dangerous over time.

• Not recognising that sealed sources prevent contamination — If a source is properly sealed, it can only irradiate; contamination is impossible. This is why medical and industrial sources are encapsulated in metal or glass.

• Assuming distance helps with contamination — The inverse square law applies to irradiation from external sources. If you're contaminated internally, distance from the original source is irrelevant — you're carrying the source with you.

Exam technique for "Radioactive contamination and irradiation"

• Command word awareness — "Explain" questions require you to give reasons (not just descriptions). For example, don't just state "alpha is dangerous inside the body" — explain that it's highly ionising and causes maximum damage over short distances to surrounding tissue.

• Contextual answers — Questions often provide scenarios (medical, industrial, accident). Always relate your answer to the specific context given. If asked about food irradiation, mentioning that the food doesn't become radioactive is essential.

• Comparative questions — When asked to compare contamination and irradiation, make clear statements about both. Use comparative language ("whereas," "in contrast," "however") and address the specific comparison requested in the question.

• Extended response structure — For 6-mark questions, plan your answer to cover: (1) definitions of key terms, (2) how each applies to the scenario, (3) specific examples or mechanisms, (4) safety implications or conclusions. Aim for approximately 2 marks per developed point.

Quick revision summary

Irradiation is exposure to radiation from an external source; the object doesn't become radioactive and exposure stops when the source is removed. Contamination occurs when radioactive material is on or inside an object, which then emits radiation continuously. Alpha radiation is safe externally but extremely dangerous if contamination occurs internally due to high ionisation. Gamma radiation penetrates easily, making it hazardous during irradiation but no more dangerous than other types during internal contamination. Protection involves shielding, distance, time reduction for irradiation, and sealed sources, protective clothing, and containment for contamination prevention.