Course: Separate Physics only | Tier: Foundation + Higher | Read the notes, then try the questions on paper.
Notes
Background radiation
Background radiation is around us all the time. Natural sources: rocks (e.g. radon gas) and cosmic rays from space. Man-made sources: fallout from nuclear weapons testing and nuclear accidents. The level can vary with where you live and your job. Radiation dose is measured in sieverts (Sv); 1000 millisieverts (mSv) = 1 Sv.
Half-life and hazard
Isotopes with a long half-life stay radioactive for a very long time (e.g. nuclear waste must be stored safely for thousands of years). Isotopes with a short half-life decay quickly, so they are only hazardous for a short time.
Uses in medicine
- Tracers for exploring internal organs: a gamma emitter with a short half-life is injected or swallowed and detected outside the body. Gamma passes out of the body; the short half-life means it doesn’t stay radioactive for long.
- Radiotherapy to control or destroy unwanted tissue such as tumours, using beams of gamma radiation or implanted sources.
Nuclear fission
Fission is the splitting of a large, unstable nucleus such as uranium-235 or plutonium-239. The nucleus usually has to absorb a neutron first. It splits into two smaller nuclei of roughly equal size, releasing two or three neutrons, gamma rays and energy. The released neutrons can cause more fissions – a chain reaction. In a nuclear reactor the chain reaction is controlled (control rods absorb neutrons); in a nuclear weapon it is uncontrolled.
Nuclear fusion
Fusion is the joining of two light nuclei to form a heavier nucleus. Some of the mass is converted into energy, released as radiation. Fusion is the energy source of stars.
Questions
Q1 (F) Give two natural sources of background radiation.
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Rocks (e.g. radon gas from granite) and cosmic rays from space.
Q2 (F) What unit is radiation dose measured in?
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Sieverts (Sv), often given in millisieverts (mSv).
Q3 (F) What is nuclear fission?
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The splitting of a large, unstable nucleus (e.g. uranium-235) into two smaller nuclei, releasing energy, neutrons and gamma rays.
Q4 (F) What is nuclear fusion, and where does it happen naturally?
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The joining of two light nuclei to form a heavier nucleus, releasing energy. It happens in stars, such as the Sun.
Q5 (F/H) Describe how a chain reaction happens in a nuclear reactor.
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A uranium-235 nucleus absorbs a neutron and splits, releasing two or three more neutrons. These neutrons are absorbed by other uranium nuclei, which also split and release more neutrons, and so on. Control rods absorb some neutrons to keep the reaction steady.
Q6 (F/H) Why is a gamma emitter with a short half-life used as a medical tracer?
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Gamma passes out of the body so it can be detected outside, and it is weakly ionising so causes less damage. A short half-life means the patient is only radioactive for a short time – but long enough for the test to be done.
Q7 (F/H) Why must waste with a long half-life be stored very carefully?
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It stays radioactive for a very long time (possibly thousands of years), so it remains a hazard and must be kept away from people and the environment for all that time.
Q8 (H) Radiotherapy uses radiation to kill cancer cells. Evaluate the use of radiation in medicine, considering benefits and risks.
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Benefits: it can destroy tumours and diagnose problems without surgery. Risks: radiation can damage healthy cells and cause mutations that may lead to cancer. Doses are kept as low as possible and aimed carefully. Judgement: the benefit of treating a serious illness usually outweighs the small extra risk.
