Radioactive contamination — AQA GCSE Physics

Contamination is hazardous because the atoms that have been transferred go on decaying exactly where they have landed. Every decay happens in contact with, or inside, living tissue, so the ionising radiation is absorbed by cells rather than by the air or by clothing. This flips the usual ranking of danger. Outside the body alpha radiation is almost harmless because its low penetrating power means skin stops it, but inside the body that same alpha radiation is absorbed by tissue and organs and, being highly ionising, does a great deal of damage in a small volume. The extra dose raises the chance of damage to cells, DNA and genes, and so of mutations or cancer. The hazard lasts as long as the contaminating atoms keep decaying.

Alpha, beta and gamma differ in two opposite ways and the hazard depends on which one matters in a given situation. Ionising power runs alpha, then beta, then gamma: an alpha particle is large and doubly charged, so it rips electrons from atoms quickly and gives up all its energy within a few centimetres of air. Penetrating power runs the other way: alpha is stopped by paper or skin, beta by a few millimetres of aluminium, and gamma is only reduced by thick lead or concrete. Outside the body a gamma source is the greater hazard because the radiation reaches internal organs. Inside the body an alpha source is far worse, because all that ionisation happens inside a small volume of living tissue.

Irradiation is being exposed to radiation from a source that stays outside the object. The source is separate; nothing is transferred. Radiation travels from it, some is absorbed by the object, and the moment the source is removed or shielded, the exposure stops. That is the crucial practical difference from contamination, where the material has moved onto or into the object and keeps decaying there. Irradiation is used deliberately: gamma sources sterilise surgical instruments and food by killing bacteria, and radiotherapy beams irradiate a tumour to destroy unwanted tissue. The exposure is controlled by time, distance and shielding, which is why sources are kept in lead-lined boxes and handled at arm's length with tongs.

For the alpha, beta and gamma sources used at GCSE, irradiating an object does not make it radioactive. Radiation passing through matter ionises atoms, knocking electrons out of them, but it leaves the nuclei unchanged. Radioactivity comes from unstable nuclei, so if no new unstable nuclei are created the object cannot become a source. As soon as the radioactive source is removed or shielded, the object emits nothing and can be handled safely. This is exactly why gamma irradiation is used to sterilise surgical instruments and food: the radiation kills bacteria as it passes through the sealed packet, then the contents are inert. Contamination is different, because there the source material itself is now sitting on or in the object.

Comparing the two hazards needs three contrasts. First, what is present: irradiation is exposure to radiation, contamination is the unwanted presence of radioactive material on or inside a person or object. Second, duration: irradiation stops when the source is removed or shielded, while contamination keeps delivering a dose for as long as the transferred atoms go on decaying, which for a long half-life can be years. Third, which radiation matters: outside the body gamma is the bigger risk because it penetrates to organs, while contamination makes alpha the bigger risk because it is highly ionising and gets absorbed directly by tissue. That is why gloves are worn even though they cannot stop irradiation: they prevent transfer of material.

Anything used to irradiate an object is itself a radiation source, so the people nearby must be protected. Three controls do the work. Reduce time: return the source to its lead-lined box between measurements rather than leaving it out. Increase distance: handle sources with long tongs, hold them at arm's length and point them away from people, since the radiation spreads out and the dose falls with distance. Add shielding: lead or thick concrete for gamma, a few millimetres of aluminium for beta. Sealed sources and gloves add protection against contamination as well, which matters because gloves prevent transfer of radioactive material even though they do not stop the hands being irradiated. Workers wear film badges to monitor the dose they accumulate.

Findings about how radiation affects people carry real consequences, so they are published rather than kept in-house. Publication lets other scientists peer review the work: they read the method and the data, look for flaws in the design and check whether the conclusions genuinely follow from the results. It also lets other groups repeat the study independently, which tests whether the findings are reproducible rather than a product of one team's equipment, sample or bias. A single result that nobody can reproduce is not treated as established. Publication also promotes further enquiry, prompting new experiments and new explanations. This is how a claim that challenges accepted theory gets tested rather than simply accepted or dismissed.