Different half-lives of radioactive isotopes — AQA GCSE Physics
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Different half-lives of radioactive isotopes explained
Half-life values span an extraordinary range, from tiny fractions of a second to billions of years.
Read the full explanation
Uranium-238 has a half-life of about 4.5 billion years, carbon-14 about 5730 years, technetium-99m about 6 hours and some isotopes decay in microseconds. The consequence matters more than the numbers: a short half-life means each nucleus is very likely to decay soon, so for the same number of nuclei the activity is high and the decay curve is steepest at the start. A long half-life means a low activity that persists for a very long time. In a sample holding equal numbers of polonium-209, half-life 125 years, and polonium-210, half-life 138 days, the polonium-209 has the lower activity precisely because its half-life is longer.
Students should be able to explain why the hazards associated with radioactive material differ according to the half-life involved.
The half-life sets how the hazard is spread over time. A short half-life gives a high activity, so a lot of radiation is emitted in a short period and the dose received in that time is large, but the source decays away quickly and soon becomes safe. A long half-life gives a much lower activity for the same number of nuclei, yet the material stays hazardous for thousands of years, which is why high-level waste from power stations is buried, stored in cooling ponds, sealed in metal containers, encased in concrete or transported in secure vessels. The same reasoning drives medical choices: a tracer uses a short half-life isotope so its activity falls quickly once the scan is done, limiting the patient's dose.
Your focus
- Compare the activities of two isotopes present in equal numbers, using their half-lives.
- Convert half-lives given in different units so that two isotopes can be ranked.
- Match decay curves to isotopes by relating initial steepness to half-life.
Show all 6 objectives
- Explain why a short half-life source and a long half-life source are hazardous in different ways.
- Suggest a suitable precaution for storing long-lived radioactive waste and justify it using half-life.
- Choose between two isotopes for a medical tracer and defend the choice in terms of dose received.
Different half-lives of radioactive isotopes exam tips
Marking Points
- one mark for the range running from fractions of a second to millions or billions of years
- one mark for a short half-life giving a high activity from the same number of nuclei
- one mark for the isotope with the shortest half-life having the steepest curve at the start
- one mark for a comparison in the correct direction, such as polonium-209 having a lower activity than polonium-210 in the same sample
- one mark for a short half-life meaning a high activity and so a large dose over a short time
- one mark for a short half-life source quickly decaying away and becoming safe
- one mark for a long half-life meaning a lower activity but a hazard lasting a very long time
- one mark for a named precaution for long-lived waste, such as burying it, storing it in cooling ponds or metal containers, or covering it with concrete
Examiner Tips
- 💡Convert to a common unit before comparing two half-lives; days against years catches people out.
- 💡State the activity comparison first and then the half-life reason, matching the order the mark scheme expects.
- 💡Remember that comparisons of activity only work directly when the numbers of atoms are equal, which the question will usually tell you.
- 💡Answer with both halves of the trade-off, activity now against duration later, because a one-sided answer caps the marks.
- 💡For waste questions, give a physical precaution such as burial or concrete encasement; do not suggest simply storing it for one half-life, as it remains hazardous.
- 💡Bring in the radiation type when a source is named, since alpha, beta and gamma need different handling.
Common Mistakes
- assuming a long half-life automatically means a more dangerous source
- comparing half-lives given in different units, such as 125 years and 138 days, without converting
- reading a steeper decay curve as a longer half-life
- saying an isotope with more nuclei remaining later must be decaying faster
- stating the comparison of activities without giving the half-life as the reason, when the reason mark depends on it
- writing that a long half-life is more dangerous without saying why the hazard persists
- claiming a short half-life source is harmless because it decays quickly
- ignoring the type of radiation emitted, which also controls the hazard
- saying waste is safe after one half-life, when only half of it has decayed
- labelling waste as high, medium or low level instead of naming an actual precaution