IGCSE Physics / Applications of radioactivity
Uses of radioactivity in industry
Edexcel International GCSE Physics (4PH1), chapter 24: Applications of radioactivity, specification point 7.14. Free lesson.
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Radioactive sources are used to control thickness, find leaks, detect smoke, irradiate food and date old remains. The radiation is chosen for how easily it passes through materials and how strongly it ionises them.
How beta keeps paper thickness steady
A paper mill checks a moving sheet continuously. A beta source and a detector sit on opposite sides of the paper. The detector measures how many beta particles pass through, so a change in thickness changes the count rate. If the paper becomes thicker, it absorbs more beta particles and the count rate falls. The rollers move closer together to make the paper thinner. If the count rises, they move apart. This feedback keeps the paper thickness steady without stopping the machine.
Why beta is chosen for thickness gauging
The detector needs a count that changes when the material's thickness changes. Beta is partly absorbed, so it gives a useful change in count. Alpha and gamma do not give a clear enough change for paper. Alpha is completely absorbed by paper, whatever its thickness, so the count stays at zero. Gamma passes through almost unaffected, so the count hardly changes as the paper gets thicker. Beta is the right choice for paper and thin aluminium foil. For thick metal sheets, beta cannot get through, so gamma is used instead.
How radiation traces buried leaks
A small gamma tracer is added to the liquid in a buried pipe. Gamma can pass through the soil, allowing a detector above ground to find where the liquid escapes. Along a sound pipe, the detector reads a steady, low count. Above a leak, radioactive liquid enters the soil, so the count jumps. The tracer has a short half-life, so the ground soon stops being radioactive.
How alpha works in a smoke alarm
A smoke alarm uses an alpha source, not gamma. Alpha strongly ionises the air between two plates. The ions carry a small electric current while the air is clear. When smoke enters, it absorbs the alpha particles. The current falls and the alarm sounds. Alpha cannot escape through the plastic case, making the source safe to use indoors. The source has a long half-life, so the alarm can work for years.
How radioactivity is used with food and old remains
Gamma radiation kills bacteria in packed food, helping it last longer. The food does not become radioactive. Carbon-14 dating uses the radioactive carbon taken in by living things. After an organism dies, its carbon-14 decays and is no longer replaced. A lower carbon-14 activity means the sample is older.