Using Radioactive Materials Safely
Irradiation and Contamination
Radioactive materials emit ionising radiation, which can damage living cells. Scientists and regulators distinguish carefully between two different types of exposure, because the health risks are quite different.
Irradiation occurs when a person is exposed to radiation from an external source — for example, standing near a radioactive sample that emits gamma rays. The body receives a dose, but no radioactive material enters it. Crucially, once the source is taken away, the exposure ends straight away.
Contamination is far more dangerous. Here, radioactive material gets onto or inside the body — swallowed, inhaled, or absorbed through the skin. Because the source stays close to body cells, it continues to irradiate them until it is removed or decays away. Alpha emitters are particularly hazardous as contaminants: alpha particles cause intense ionisation over a very short range, so they do serious localised damage to tissue once inside the body, even though they cannot penetrate skin from outside.
Irradiation = external source, exposure stops when source is removed. Contamination = radioactive material in or on the body, exposure continues. Contamination is more hazardous because the source cannot easily be removed.
Radiation and Health Risks
Ionising radiation can kill cells outright or damage DNA within them. Damaged DNA that is not repaired may lead to cancer. The severity of the risk depends on three factors: the type of radiation (its ionising ability), the half-life of the isotope (how long the source remains active), and whether the exposure is irradiation or contamination.
Alpha radiation has the highest ionising ability but the lowest penetrating power — stopped by a few centimetres of air or a sheet of paper. Beta particles are stopped by a few millimetres of aluminium. Gamma rays require several centimetres of lead or thick concrete to be significantly reduced. This means an alpha source is safe to handle externally if kept at arm's length, but catastrophic if inhaled — while a gamma source poses an external irradiation risk even from a distance.
Decisions about using ionising radiation in medicine always involve weighing risks against benefits. Background radiation — from cosmic rays, rocks, food, and medical procedures — means everyone receives a small annual dose. The question regulators ask is not “is there any risk?” but “is the benefit worth the risk?”
For a cancer patient, the benefit of destroying a tumour with targeted radiation may far outweigh the small extra risk of triggering a secondary cancer. A healthy person undergoing a routine scan faces a much smaller benefit, so lower-dose techniques or alternatives (such as ultrasound) are preferred. This risk–benefit reasoning underpins how regulators licence the use of radioactive materials in healthcare and industry.
Medical Uses of Radioactive Materials
Ionising radiation has two broad medical uses: imaging the inside of the body, and destroying unwanted tissue such as tumours.
To explore an internal organ, a patient swallows or is injected with a radioactive tracer. Tracers are normally gamma emitters with a short half-life: gamma rays pass out of the body to detectors outside it, and the short half-life means the activity soon falls away. A PET scan is one kind of tracer scan. Its detail goes beyond this course, but it shows the same idea: it uses a tracer that emits positrons (beta-plus particles). When a positron meets an electron in body tissue, both annihilate to produce two gamma rays travelling in opposite directions. Detectors arranged around the patient pick up these paired gamma rays and use them to build a three-dimensional map of where the tracer is concentrating. PET scans are particularly valuable for detecting cancer spread and assessing brain activity, because the tracer is typically a glucose analogue that accumulates in metabolically active tissue.
Radiotherapy exploits the cell-killing power of ionising radiation. A beam of gamma radiation is directed at a tumour from several angles, all focused on the same point. This ensures the tumour receives a high dose while surrounding healthy tissue receives a lower, more manageable dose from each individual beam.
PET scans use a positron-emitting tracer. Positrons annihilate with electrons to produce paired gamma rays, which external detectors pick up. Radiotherapy uses a focused gamma beam to destroy tumour cells, crossing the body from multiple angles.
Background Radiation
Background radiation is the low-level radiation everyone is exposed to constantly. Sources include radon gas seeping from granite rocks, cosmic rays from space, small amounts of radioactive isotopes in food and water, and radiation from medical procedures. The level varies with location — areas with granite geology, for example, have higher radon concentrations. This background level is the baseline against which risks from additional radiation sources are judged.
Nuclear fission occurs when a slow-moving neutron is absorbed by a large, unstable nucleus — typically uranium-235 or plutonium-239. The nucleus splits into two smaller “daughter” nuclei of roughly equal size, releasing energy as kinetic energy of those particles and as gamma radiation.
The split also releases two or three additional neutrons. These can go on to trigger further fission events in nearby nuclei — a chain reaction. In a nuclear reactor the chain reaction is controlled so that on average exactly one neutron from each fission causes the next fission, maintaining a steady rate of energy release. Control rods made of neutron-absorbing material regulate this.
Nuclear fusion is the process that powers stars including our Sun. Two hydrogen nuclei are forced close enough together that the strong nuclear force overcomes their electrostatic repulsion, and they fuse into a helium nucleus. In doing so, some mass is converted into energy — a very large amount of energy relative to the mass involved.
Fusion does not produce long-lived radioactive waste in the same way fission does, and its fuel (hydrogen isotopes) is far more abundant than uranium. For these reasons, controlled fusion is an active area of international research, though achieving the extreme temperatures needed to sustain fusion reactions on Earth remains a major engineering challenge.
Fission: a large nucleus (e.g. uranium-235) splits after absorbing a neutron, releasing energy and more neutrons that can sustain a chain reaction. Fusion: two small nuclei (hydrogen isotopes) join together, releasing large amounts of energy. Both release energy from the nuclear store.