Half-life
Cambridge IGCSE Physics (0625) · Unit 5: Nuclear physics · 6 flashcards
Half-life is topic 5.2.4 in the Cambridge IGCSE Physics (0625) syllabus , positioned in Unit 5 — Nuclear physics , alongside The atom, The nucleus and Detection of radioactivity. In one line: Definition: The half-life of a radioactive isotope is the time taken for half of the nuclei of that isotope in any sample to decay.
This topic is examined in Paper 1 (multiple-choice) and Papers 3/4 (theory), plus Paper 5 or Paper 6 (practical / alternative to practical).
The deck below contains 6 flashcards — 1 definition — covering the precise wording mark schemes reward. Use the definition card to lock down command-word answers (define, state), then move on to the concept and application cards to handle explain, describe and compare questions.
The term 'half-life' as it relates to radioactive isotopes
Definition: The half-life of a radioactive isotope is the time taken for half of the nuclei of that isotope in any sample to decay.
What the Cambridge 0625 syllabus says
Official 2026-2028 specThese are the exact learning objectives Cambridge sets for this topic. Match the command word (Describe, Explain, State, etc.) in your answer to score full marks.
- Define Define the half-life of a particular isotope as the time taken for half the nuclei of that isotope in any sample to decay; recall and use this definition in simple calculations, which might involve information in tables or decay curves (calculations will not include background radiation)
- Calculate Calculate half-life from data or decay curves from which background radiation has not been subtracted Supplement
- Explain Explain how the type of radiation emitted and the half-life of an isotope determine which isotope is used for applications including: (a) household fire (smoke) alarms (b) irradiating food to kill bacteria (c) sterilisation of equipment using gamma rays (d) measuring and controlling thicknesses of materials with the choice of radiations used linked to penetration and absorption (e) diagnosis and treatment of cancer using gamma rays Supplement
A radioactive isotope has a half-life of 2.0 hours. If a sample initially contains 8000 atoms of the isotope, calculate how many atoms of the isotope will remain after 6.0 hours.
Formula: Number of half-lives = Total time / Half-life; Remaining atoms = Initial atoms / 2^(number of half-lives)
Working: Number of half-lives = 6.0 hours / 2.0 hours = 3
Remaining atoms = 8000 / 2³ = 8000 / 8 = 1000
Answer: 1000 atoms
Explanation: After each half-life, the number of radioactive atoms halves. After 3 half-lives, the initial number of atoms is halved three times.
Define the term 'half-life' as it relates to radioactive isotopes.
Definition: The half-life of a radioactive isotope is the time taken for half of the nuclei of that isotope in any sample to decay.
A radioactive isotope initially has an activity of 800 counts per minute. After 6 hours, the activity is measured as 200 counts per minute. Calculate the half-life of the isotope.
The activity halves twice in 6 hours (800 -> 400 -> 200). Therefore, two half-lives is 6 hours.
Half-life = Total time / Number of half-lives = 6 hours / 2 = 3 hours
Answer: 3 hours. This calculation uses the decrease in activity to determine how many half lives have passed, and then relates that to the total time elapsed.
Explain why determining half-life from data or decay curves is important in managing radioactive waste.
Knowing the half-life of a radioactive isotope in waste is essential because it allows scientists to predict how long the waste will remain hazardous. Isotopes with longer half-lives remain dangerous for a longer period, thus requiring longer-term storage solutions, such as deep geological repositories, to prevent environmental contamination and harm to living organisms. Short half life isotopes decay away quickly, becoming inert.
A radioactive source used in a smoke detector has a half-life of 432 years. If the initial activity of the source is 8.0 Bq, calculate its activity after 864 years.
Activity after two half-lives = Initial Activity / (2^(number of half-lives))
Number of half-lives = Total time / Half-life = 864 years / 432 years = 2
Activity = 8.0 Bq / (2^2) = 8.0 Bq / 4 = 2.0 Bq
Answer: 2.0 Bq. Smoke detectors require long half-lives so they don't need frequent replacement of the radioactive source.
Explain why gamma radiation is suitable for sterilizing medical equipment but alpha radiation is not.
Gamma radiation has a high penetration power, allowing it to pass through packaging and effectively kill bacteria and viruses throughout the equipment. Alpha radiation has very low penetration power; it is easily stopped by even a thin layer of material like paper. Therefore, it cannot reach the bacteria inside wrapped equipment effectively.
Key Questions: Half-life
Define the term 'half-life' as it relates to radioactive isotopes.
Definition: The half-life of a radioactive isotope is the time taken for half of the nuclei of that isotope in any sample to decay.
More topics in Unit 5 — Nuclear physics
Half-life sits alongside these Physics decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Half-life deck
Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.
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