Radioactivity, Uses & DangersEdexcel IGCSE Physics: Revision notes

Background radiation is the ionising radiation present in the environment at all times, from both natural and artificial sources.

Sources from Earth include: rocks and soil containing radioactive isotopes (e.g. granite), radon gas, and radioactive material in building materials and food.

Sources from space include: cosmic rays from the Sun and other stars.

Artificial sources also contribute, including medical procedures (e.g. X-rays) and, to a much smaller extent, nuclear industry and fallout.

The activity of a radioactive source is the number of unstable nuclei decaying per second, measured in becquerels (Bq). Activity always decreases over time, because there are fewer undecayed nuclei left to decay.

The half-life of a radioactive isotope is the time taken for the activity (or the number of undecayed nuclei) to fall to half its original value. Half-life is different for different isotopes — it can range from fractions of a second to billions of years.

Half-life calculations can be done numerically (halving repeatedly) or graphically, by reading the time taken for the activity to drop to half its value on a decay graph.

Radioactivity has many practical uses, exploiting the properties of specific radiation types:

• Medicine: radioactive tracers (often gamma or short half-life sources) to diagnose conditions by tracking their path through the body; radiotherapy to treat cancer by destroying tumour cells; sterilising medical equipment using gamma rays.
• Industry: checking the thickness of materials (e.g. paper, metal sheets) using beta radiation, since the amount passing through varies with thickness; sterilising food using gamma rays to kill bacteria and extend shelf life; smoke detectors using a weak alpha source.

These two terms are often confused but describe different hazards:

• Contamination occurs when radioactive material (particles) gets onto or inside an object or person (e.g. on skin, in food, inhaled). The source of radiation is now on/in the contaminated object, so exposure continues until it is removed or decays.
• Irradiation occurs when an object or person is exposed to radiation from a source that remains external and separate. Exposure stops as soon as the object/person moves away from the source, or the source is removed.

Ionising radiation is hazardous because it can:

• Cause mutations in the DNA of living organisms.
• Damage cells and tissue, which can lead to radiation sickness or, at high doses, cell death.
• Lead to problems from the disposal of radioactive waste, since some waste remains hazardous for very long periods; risks are reduced by storing waste securely (e.g. sealed in concrete/glass, deep underground) and monitoring it long-term.