IB Physics HLTopic 7 — Atomic, Nuclear & Particle PhysicsPaper 1 & 2match the isotope to the job~16 min read
Uses of Radioactivity
Radioactivity isn’t just a hazard to be shielded against — it’s a genuinely useful tool. We can date ancient bones, find leaks in buried pipes, sterilise surgical instruments, treat cancer, and even keep smoke detectors working. The trick to every application is the same: pick the right isotope for the job, matching its type of radiation and its half-life to exactly what the task needs.
📚 What you need to know
Choosing an isotope means matching two things: the penetrating power of its radiation and its half-life
Carbon dating uses carbon-14 (half-life ~5730 years) to date once-living material
Carbon dating works reliably for samples between roughly 500 and 60 000 years old
Uranium-lead dating uses the U-238 → Pb-206 chain (half-life 4.5 billion years) to age rocks
Leak detection uses a gamma emitter with a short half-life
Thickness control uses a beta emitter with a long half-life
Smoke detectors use an alpha emitter (americium-241) with a very long half-life
In medicine: gamma for tracers and sterilising, and for treating deep tumours
The two choices that decide everything
Every use of radioactivity comes down to answering two questions about the isotope:
Which radiation? Alpha is easily stopped (good where you want short range), gamma travels far (good where you need to detect from outside or penetrate material).
What half-life? Short means high activity but fades fast (good for medical tracers); long means steady output for years (good for smoke detectors).
Don’t memorise the applications as random facts — reason them out from these two properties. Ask “does the radiation need to travel far or stay contained?” and “does the source need to last years or fade in hours?” Once you frame it that way, you can work out the right isotope for a use you’ve never even seen before. That’s exactly what examiners test.
Every application is just these two questions answered together: the radiation type and the half-life that the task demands.
Carbon dating
Living things constantly take in carbon, including a steady trace of radioactive carbon-14. When an organism dies, it stops absorbing carbon, so its carbon-14 slowly decays away with a half-life of about 5730 years. By comparing how much carbon-14 is left with the amount in living tissue, we can work out how long ago the organism died.
Carbon-14 forms high in the atmosphere when cosmic rays free neutrons that strike nitrogen nuclei:
Formation of carbon-1410n + 147N → 146C + 11p
Why the 500–60 000 year window?
Carbon dating is reliable only for a certain age range:
Younger than ~500 years: so little has decayed that the change is too small to measure accurately — the carbon-14 ratio is still too high to pin down.
Older than ~60 000 years: so much has decayed that the tiny remaining activity gets lost in the background radiation — the ratio is too small to measure.
Extra uncertainty creeps in because the amount of carbon-14 made in the atmosphere varies slightly with time and location.
Think of carbon-14 as a clock that only reads clearly in the “middle” of its life. Too early and the hands have barely moved; too late and they’ve nearly stopped. That readable middle stretch — roughly 500 to 60 000 years — is where the decay is fast enough to measure but not yet swamped by background.
Uranium-lead dating
For anything far older than living matter — like rocks, or the Earth itself — carbon-14 is useless. Instead we use uranium-238, which decays through a long chain ending in stable lead-206, with an enormous half-life of 4.5 billion years. As time passes, the ratio of lead-206 to uranium-238 in a rock steadily rises, and that ratio reveals the rock’s age. This method is how we know the Earth is billions, not millions, of years old.
Industrial uses
Detecting leaks in pipes
A gamma emitter is added to the fluid in a buried pipe. A detector moved along the ground picks up a spike in count rate exactly where fluid is leaking out. Gamma is essential here because it’s the only radiation penetrating enough to pass through metres of soil. The half-life must be short enough not to linger in the supply, but long enough to stay detectable during the survey. Sodium-24 (half-life ~15 hours) is a common choice.
Controlling thickness
To keep a rolled sheet (foil, paper, plastic, steel) at a constant thickness, a beta source sits on one side and a detector on the other. If the sheet thins, more beta gets through, so the rollers adjust. Beta is used because alpha would be blocked entirely and gamma would pass straight through unchanged — only beta responds to small thickness changes. A long half-life keeps the output steady.
Smoke detectors
A tiny alpha source (americium-241) ionises the air, letting a small current flow. When smoke enters, it absorbs the alpha particles, the current drops, and the alarm sounds. Alpha is ideal because it only travels a few centimetres, so it poses no hazard beyond the device. Americium-241’s 460-year half-life means the detector never needs its source replaced.
Medical uses
Radiotherapy: concentrated gamma beams destroy deep tumours; the beam is rotated around the patient so healthy tissue gets a lower dose than the tumour.
Tracers: a short-half-life gamma emitter (like technetium-99m) is introduced into the body and tracked from outside — gamma escapes the body to reach the scanner, and the short half-life limits the patient’s exposure.
Sterilising equipment:gamma kills microbes on sealed instruments without opening the packaging, because it penetrates all sides. It doesn’t make the equipment radioactive — ionising radiation affects only outer electrons, not the nuclei.
WE 1
Four radionuclides are available: A (americium-241, alpha), B (strontium-90, beta-minus), C (cobalt-60, beta-minus + gamma), D (fluorine-18, beta-plus). Choose the best one for (a) sterilising equipment sealed in plastic bags, and (b) monitoring the thickness of thin metal sheet.
(a) Sterilising sealed bags
Alpha and low-energy beta would be absorbed by the bag, so we need penetrating radiation.
C — cobalt-60 (gamma penetrates the packaging)(b) Thickness of thin metal
Alpha is stopped by paper (too weak); gamma passes through all thicknesses (no response).
B — strontium-90 (beta responds to small changes)Reason from penetrating power every time: sterilising needs to reach inside the bag (gamma), thickness gauging needs a radiation that changes when the sheet changes (beta).
WE 2
Explain why a radioactive tracer used in medical imaging should have a short half-life and emit gamma radiation.
Step 1 — why gamma?
Gamma is the most penetrating, so it can escape the body and reach an external detector.
Step 2 — why short half-life?
A short half-life means the activity falls quickly, so the patient isn’t exposed for long — only a small sample is needed.
Gamma to detect from outside; short half-life to limit exposureTwo properties, two reasons. Gamma is about getting the signal out; short half-life is about patient safety. This “type + half-life” framing works for any medical-use question.
⚛ Choosing an isotope
Does the radiation need to travel far or stay contained? Far → gamma; contained → alpha; respond-to-thickness → beta.
How long must the source last? Years → long half-life; hours → short half-life.
Dating once-living material? Carbon-14.
Dating rocks? Uranium-238 → lead-206.
Medical tracer? Short-half-life gamma emitter.
💡 Top tips
Every choice = radiation type + half-life. Reason from both.
Gamma for penetration/detection; beta for thickness; alpha for contained/short-range.
Carbon-14 dates living material (~500–60 000 yr); uranium-lead dates rocks.
Medical tracers: short half-life to limit dose, gamma to escape the body.
Sterilising doesn’t make equipment radioactive — it affects electrons, not nuclei.
⚠ Common mistakes
Using beta or alpha for leak detection — only gamma penetrates the ground
Using gamma for thickness control — it passes through unchanged
Choosing a long half-life for a medical tracer — it should be short
Thinking carbon dating works on rocks — it’s for once-living matter
Claiming gamma sterilisation makes objects radioactive — it doesn’t
Quick recap: Every use of radioactivity comes from matching the radiation type and half-life to the job. Carbon-14 dates once-living material (500–60 000 yr); uranium-lead dates rocks. Gamma suits leak detection, sterilising and tracers (penetrating); beta suits thickness gauging; alpha suits smoke detectors (contained). Medical tracers need short half-lives to limit dose.
Radioactive decay releases energy — that’s what powers a space probe or destroys a tumour. But where does that energy actually come from? The surprising answer is that a nucleus weighs slightly less than its separate parts, and that missing mass becomes energy. Next page: Mass Defect & Binding Energy.
Application questions catching you out?
Book a free meeting and we’ll drill the radiation-type and half-life reasoning behind dating, tracers, thickness gauges and more.
