Radioactivity, Half-Life and Nuclear Uses
Radioactivity is the spontaneous and random decay of unstable atomic nuclei, releasing ionising radiation as the nucleus moves towards a more stable configuration. GCSE physics covers the nature and properties of alpha, beta and gamma radiation, balancing nuclear equations for alpha and beta decay, half-life, the difference between irradiation and contamination, and the beneficial uses and hazards of nuclear radiation.
Before you start
Make sure you're comfortable with these topics first:
Method
- Learn the identity and properties of each radiation type: alpha (a helium nucleus, 2 protons + 2 neutrons, strongly ionising, stopped by paper or a few cm of air); beta (a fast electron from the nucleus, medium ionising, stopped by a few mm of aluminium); gamma (an electromagnetic wave from the nucleus, weakly ionising, needs thick lead or concrete to absorb significantly).
- For a nuclear equation, check that mass number (top) and atomic number (bottom) both balance across the arrow: alpha decay reduces mass number by 4 and atomic number by 2; beta decay leaves mass number unchanged and increases atomic number by 1 (a neutron becomes a proton).
- For a half-life calculation, work out how many whole half-lives fit into the given time, then halve the starting activity, mass or count rate that many times (or use the given data to work backwards and find the half-life itself).
- Distinguish irradiation (exposed to radiation from a source that stays separate; exposure stops once removed from the source) from contamination (unwanted radioactive particles get onto or into an object or person, which then continues to be exposed even after the original source is gone).
- For a 'choose the best source' question, weigh up the type of radiation needed (for example, gamma or beta to escape the body for a tracer; alpha where radiation should not travel far, as in a smoke detector) against a suitable half-life (long enough to be useful, short enough to limit unnecessary dose).
- For separate Physics questions on fission or fusion, keep the two processes distinct: fission splits a large unstable nucleus after it absorbs a neutron; fusion joins two light nuclei together and powers stars.
Worked example
A sample of a radioactive isotope has an initial activity of 800 Bq. Its half-life is 6 hours. Calculate the activity of the sample after 24 hours.
- Work out the number of half-lives that have passed: 24 hours / 6 hours per half-life = 4 half-lives.
- Halve the activity once for each half-life: after 1 half-life, 800 / 2 = 400 Bq.
- After 2 half-lives: 400 / 2 = 200 Bq. After 3 half-lives: 200 / 2 = 100 Bq.
- After 4 half-lives: 100 / 2 = 50 Bq.
- Final answer: the activity after 24 hours is 50 Bq.
Practice questions
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Q1Define the term 'half-life' of a radioactive isotope.Show answer
Answer: The time taken for the number of nuclei of the isotope in a sample to halve, or for the count rate (activity) from the sample to fall to half its initial value.
Q2State the number of protons and the number of neutrons in an alpha particle.Show answer
Answer: 2 protons and 2 neutrons.
Q3A GM tube detects a count rate of 240 counts per minute from a source. After 15 days the count rate has fallen to 30 counts per minute. Show that this data is consistent with a half-life of 5 days.Show answer
Answer: 15 days / 5 days = 3 half-lives. 240 -> 120 -> 60 -> 30 counts per minute, which matches the given final count rate, so the data is consistent with a 5-day half-life.
Q4Explain the difference between irradiation and contamination.Show answer
Answer: Irradiation is exposure to radiation from a source that remains separate from the object or person, so exposure stops once they move away from the source; contamination is when unwanted radioactive particles get onto or into an object or person, which then continues to be exposed to radiation even after the original source is removed.
Q5Explain why americium-241, an alpha emitter, is suitable for use in a smoke detector.Show answer
Answer: The alpha radiation only has to travel a short distance across the detector chamber to ionise the air and allow a small current to flow; smoke entering the chamber absorbs the (short-range) alpha particles, reducing the current and triggering the alarm. A beta or gamma source would be less suitable, as they would pass through the smoke without being absorbed as effectively.
Q6State one source of natural background radiation and one source of artificial (man-made) background radiation.Show answer
Answer: Natural: radon gas from rocks and soil, or cosmic rays from space (any one). Artificial: medical X-rays, or fallout from nuclear weapons testing, or waste from the nuclear industry (any one).
Q7Radium-226 (atomic number 88) decays by alpha emission. State the atomic number of the nucleus formed.Show answer
Answer: 86 (88 - 2, since alpha decay reduces the atomic number by 2).
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
Radium-226 (atomic number 88, mass number 226) decays by alpha emission to form an isotope of radon. (a) State the mass number and atomic number of the radon nucleus produced. (b) Write a balanced nuclear equation for this decay.
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A student uses a protactinium-234 generator in a school experiment to investigate radioactive decay. The initial count rate recorded is 960 counts per minute. The student finds that the count rate has fallen to 240 counts per minute after 144 seconds. Calculate the half-life of protactinium-234 shown by this data.
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A hospital needs to choose a radioactive source to use as a medical tracer, which is injected into a patient and tracked using an external detector as it moves through the body. Explain, using ideas about the type of radiation emitted and half-life, why a source such as technetium-99m, which has a short half-life and emits gamma radiation, is more suitable for this purpose than a source with a long half-life that emits alpha radiation.
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See real GCSE Science past-paper questions, with official mark schemes →
Free printable worksheet
Want more practice on paper? Download the radioactivity, half-life and nuclear uses worksheet pack - 19 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 27 of GCSE Physics Workbook, the whole course as one free printable PDF.
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