Every use of radioactivity depends on matching the properties of a source to the task. Every risk comes from the way ionising radiation affects living cells. A good discussion answer joins the two with reasons.
This lesson is part of SPM Physics nuclear physics. It draws on distinguishing types of radiation and interpreting half-life graphs.
What is the matching method?
Use three questions in order.
- Radiation type: does it pass through the material enough, but not completely?
- Half-life: will the source last long enough for the task, without staying active longer than needed?
- Safety: what happens if the radiation escapes?
Worked examples
Paper thickness gauge. Alpha is stopped by thin paper, so none reaches the detector and it is no use.
Gamma passes through almost unchanged, so changes in thickness hardly change the count.
Beta passes through partly, so the count changes with thickness, and beta is suitable. A long half-life keeps the source useful for a long time.
Medical tracer. The radiation must leave the body to be detected, so gamma is suitable. A short half-life means the source decays quickly and exposure ends soon after the test.
Smoke detector. Alpha radiation ionises the air between two plates and a small current flows. Smoke absorbs the alpha radiation, the current falls and the alarm sounds. Alpha is stopped by the casing, so little escapes.
How do you discuss the risks?
Ionising radiation can damage molecules in cells. The risk rises with the amount of radiation received and the time of exposure. Safety ideas are discussed as reasoning: a greater distance reduces exposure, shielding absorbs radiation, and a shorter time reduces the amount received.
These ideas explain why real sources are kept in shielded containers and used under trained supervision. They are not steps for handling materials.
The mistake that costs marks
The common slip is to give a use without a reason, such as “gamma is used for tracers”. Another is to say that radiation makes everything radioactive.
| Answer | Verdict |
|---|---|
| “Beta is used in thickness gauges.” | Weak: no reason |
| “Beta passes partly through thin sheets, so the count changes with thickness.” | Strong: links the property to the use |
Being exposed to radiation is different from being contaminated by a radioactive source. Keep the two ideas apart in your answers.
Check yourself
Explain why a source with a half-life of a few seconds would not be suitable for a thickness gauge running in a factory.
Answer
With a half-life of a few seconds, the source would decay almost completely within a minute, so the count would keep falling. This would be mistaken for a change in thickness.
A thickness gauge needs a source that stays active for a long time so that the count changes only when the thickness changes.
What to study next
Test all five nuclear skills with the nuclear physics practice set. Revisit balancing simple nuclear equations if any equation steps felt unsure.
The units and significant-figure checker supports answer presentation. If you want a teacher to coach your discussion answers, see online one-to-one Physics tuition.