Radioactivity isn’t just something to shield against — it dates ancient bones, images the inside of the body, guards against house fires, and measures the age of the Earth. The trick behind every use is picking an isotope whose penetrating power and half-life match the job. Get those two right and the application almost designs itself.
Almost every “which isotope should you use?” question is answered by weighing up two things.
Penetrating power decides whether the radiation can reach where it’s needed — and whether it would pass straight through or be safely stopped. You need gamma to get out of a body or through metres of ground, but you want alpha’s tiny range where safety matters.
Half-life decides how long the source stays useful and how long it stays hazardous. A tracer inside a patient wants a short half-life so it fades fast; a smoke detector wants a long one so it never needs replacing.
Living things constantly take in carbon while alive, keeping a fixed proportion of radioactive carbon-14 alongside ordinary carbon-12. The moment an organism dies, it stops absorbing carbon, and its carbon-14 begins to decay with a half-life of about 5730 years. By comparing how much carbon-14 is left against the amount in living tissue, you can work out how long ago it died.
Carbon dating is useless for rocks — they’re far too old and were never alive. Instead, uranium-238 decays through a long chain that ends in stable lead-206, with a half-life of 4.5 billion years. Fresh rock starts with only uranium; over time the ratio of lead to uranium climbs. Measuring that ratio dates the rock, and this is how we know the Earth is billions of years old.
Radioactive tracers are swallowed or injected, then tracked by a detector outside the body to watch how an organ is working. They need a short half-life (so the patient isn’t exposed for long) and are usually gamma emitters (so the radiation escapes the body to be detected). Technetium-99m is the workhorse here: gamma, with a ~6-hour half-life.
Radiotherapy aims gamma radiation at a tumour to kill cancer cells, rotating the beam so healthy tissue gets a lower dose. Less-penetrating beta can treat cancers right at the skin. Sterilisation of medical equipment uses penetrating gamma, which passes through sealed packaging to kill microbes without opening it — and doesn’t make the equipment radioactive, because only the nucleus, not the outer electrons, would need to be changed for that.
Smoke detectors hold a tiny alpha source (americium-241). The alpha ionises the air so a small current flows; smoke absorbs the alpha, the current drops, and the alarm sounds. Alpha is perfect — its short range means it can’t harm people in the room, and its long half-life (~460 years) means it never needs replacing.
Thickness control of foil or paper uses a beta source. Beta passes through thin material but the amount getting through changes if the thickness changes, so a detector can adjust the rollers. Alpha would be blocked entirely and gamma would pass straight through unchanged — only beta is sensitive to small thickness changes.
Leak detection in buried pipes uses a gamma emitter added to the fluid. Gamma is the only radiation penetrating enough to be picked up through metres of soil, and the count rate spikes above the leak. A short-to-medium half-life (like sodium-24’s ~15 hours) keeps the tracer from lingering in the supply.
A piece of ancient wood is found to contain one-eighth of the carbon-14 present in living wood. The half-life of carbon-14 is 5730 years. Estimate the age of the sample.
A factory needs a radioactive source to continuously monitor the thickness of aluminium foil as it is rolled. State which type of radiation should be used, and explain why alpha and gamma would both be unsuitable.
Book a free meeting and let’s work through the tricky bits together.
Book your free meeting
IB Demystified is a trusted online learning platform led by certified IB examiners and educators.
© 2026 IB Demystified LTD ALL RIGHTS RESERVED. Powered by AfiaDigital