Free Radioactivity & half-life questions for IGCSE Physics (0625), part of Nuclear physics. Each question has the answer and the marking points an examiner would look for.
Generate a full Radioactivity & Half-life worksheet →All Nuclear physics questionsA detector near a radioactive source records 660 counts per minute. The background count rate is 20 counts per minute. The half-life of the source is 6 hours.
What count rate does the detector record 18 hours later?
Answer: C
A sample of a radioactive isotope is left for a time equal to 4 half-lives.
What fraction of the original nuclei have not yet decayed?
Answer: C
What is meant by the half-life of a radioactive isotope?
Answer: D
Background radiation is always present.
Name two natural sources of background radiation.
Explain how a corrected count rate is found.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | radon gas; rocks; cosmic rays; food Any 2 of: radon gas; rocks and buildings; cosmic rays; food and drink B2 | 2 |
| (b) | subtract background measure the background count rate and subtract it from the measured count rate B1 | 1 |
A radioactive sample has a half-life of 6 hours. Its initial activity is 2400 Bq.
Define half-life.
Calculate the activity after 18 hours.
Explain why the measured count rate never falls to exactly zero.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | time for half the nuclei to decay the time taken for half the nuclei in a sample to decay (or for the activity to halve) B1 | 1 |
| (b) | 300 Bq 3 half-lives C1 300 Bq A1 | 2 |
| (c) | background radiation background radiation is always present B1 | 1 |
A radioactive sample is placed near a detector. The background count rate is 24 counts/min. At time t = 0 the measured count rate is 1224 counts/min. The half-life of the source is 2 hours.
State two sources of background radiation.
The source emits beta particles. Describe the change that happens in a nucleus when a beta particle is emitted, and state the change in its proton number and its nucleon number.
Explain why the detector readings vary even when the count rate is constant on average.
Suggest why a source with a half-life of a few hours is chosen as a medical tracer, rather than one with a half-life of several years.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | rocks / radon; cosmic rays; medical; food; nuclear waste Any 2 of: radon gas / rocks and buildings; cosmic rays; food and drink; medical sources; nuclear power / waste B2 | 2 |
| (b) | neutron becomes proton plus electron; proton number +1; nucleon number unchanged a neutron changes into a proton and an electron (emitted) B1 proton number increases by 1 B1 nucleon number unchanged B1 | 3 |
| (c) | decay is random radioactive decay is random (spontaneous) B1 | 1 |
| (d) | activity high enough to detect during test; decays quickly so patient receives little dose afterwards long enough to be detected during the procedure B1 decays quickly so the patient is not exposed to radiation for long B1 | 2 |
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