Radioactivity and Particles

This guide covers Topic 7, Radioactivity and particles, in full — sub-topics (a) Units, (b) Radioactivity and (c) Fission and fusion — from the Pearson Edexcel International GCSE in Physics (4PH1), Issue 4 specification.

Before studying this

No prior nuclear physics study is required, though this topic builds naturally on general atomic structure covered in other sciences.

Syllabus coverage

PEARSON EDEXCEL INTERNATIONAL GCSE PHYSICS (4PH1) — Topic 7

(a) Units — using becquerel (Bq), centimetre (cm), hour (h), minute (min) and second (s).

(b) Radioactivity — describing atomic structure (protons, neutrons, electrons) and nuclide notation; knowing alpha, beta and gamma radiation are ionising and emitted randomly from unstable nuclei; describing the nature of alpha, beta and gamma radiation and their penetrating power and ionising ability; investigating penetration power practically; describing effects on atomic/mass number of each of the four main radiation types, including neutron radiation; balancing nuclear equations (including neutron emission); knowing detection methods (photographic film, Geiger-Müller detector); explaining sources of background radiation; knowing activity decreases over time, measured in becquerels; defining half-life; using half-life for activity calculations including graphically; describing uses of radioactivity in industry and medicine; distinguishing contamination from irradiation; describing dangers of ionising radiation including mutation, cell/tissue damage and waste disposal.

(c) Fission and fusion — knowing nuclear fission, fusion and radioactive decay are energy sources; understanding U-235 fission by neutron collision and energy release; knowing fission of U-235 produces daughter nuclei and neutrons; describing chain reactions; describing the role of control rods and moderator; understanding reactor shielding; explaining the difference between fusion and fission; describing fusion as creating larger nuclei with mass loss and energy release; knowing fusion powers stars; explaining why fusion requires high temperature and pressure to overcome electrostatic repulsion.

Atomic structure and radiation types

An atom consists of protons and neutrons in a central nucleus, surrounded by electrons. Nuclides are written using notation such as ¹⁴₆C, showing mass number (top) and atomic number (bottom). Unstable nuclei emit ionising radiation randomly and spontaneously: alpha (α) particles (strongly ionising, low penetration — stopped by paper/skin), beta (β⁻) particles (moderately ionising and penetrating — stopped by a few mm of aluminium), and gamma (γ) rays (weakly ionising, highly penetrating — requires thick lead or concrete to stop). These are the three types compared for nature, penetrating power and ionising ability.

A fourth type, neutron radiation (uncharged neutrons emitted from an unstable nucleus), is not part of that penetrating-power/ionising-ability comparison — it enters the specification only through its effect on the nucleus during decay (below). As background, neutrons are weakly ionising but highly penetrating, and are best absorbed by hydrogen-rich materials such as water or paraffin wax.

Each type affects the nucleus differently: alpha emission decreases the mass number by 4 and the atomic number by 2; beta-minus emission leaves the mass number unchanged and increases the atomic number by 1; gamma emission changes neither the mass number nor the atomic number, since it is a loss of energy only, with no change in nuclear composition; neutron emission decreases the mass number by 1 and leaves the atomic number unchanged. Nuclear equations must balance both mass and charge.

Background radiation comes from natural sources (rocks, cosmic rays, food) and artificial sources (medical procedures, nuclear industry). Radiation is detected using photographic film or a Geiger-Müller detector.

Half-life

The activity of a radioactive source (its rate of decay, in becquerels) decreases over time. Half-life is the time taken for activity (or the number of undecayed nuclei) to fall to half its initial value, and differs between isotopes.

Worked example. A source has an initial activity of 800 Bq and a half-life of 6 hours. Its activity after 18 hours (3 half-lives):

800 → 400 → 200 → 100 Bq

Contamination means radioactive material has been transferred onto or into an object or person (an ongoing exposure risk); irradiation means exposure to radiation from an external source without transfer of material (exposure stops once the source is removed). Both carry risks including cell mutation and tissue damage; radioactive waste disposal carries its own long-term risk that must be managed.

Uses of radioactivity

Choosing the right source for an application means matching penetrating power to the job. Monitoring the thickness of paper or foil uses a beta source: alpha would be completely absorbed regardless of thickness, and gamma would pass through almost unaffected, so neither would respond to a change in thickness — only beta is partly absorbed, so the count rate reaching the detector varies as the material’s thickness changes. A medical tracer injected into the body uses a gamma source, since gamma penetrates tissue well enough to be detected from outside the body; such tracers are also chosen with a short half-life, so their activity falls away quickly and limits the patient’s overall radiation exposure.

Nuclear fission

Nuclear fission splits a heavy nucleus (e.g. uranium-235) into two smaller “daughter” nuclei plus several neutrons, releasing energy as kinetic energy of the fission products. If these released neutrons strike further U-235 nuclei, a chain reaction can result. In a nuclear reactor, control rods absorb excess neutrons to regulate the reaction rate, a moderator slows fast neutrons to speeds more likely to cause further fission, and shielding protects against escaping radiation.

Nuclear fusion

Nuclear fusion combines smaller nuclei into a larger one, with a loss of mass converted into a release of energy — the process powering stars, including the Sun. Fusion requires extremely high temperature and pressure to overcome the electrostatic repulsion between positively charged nuclei, which is why it does not occur under everyday conditions on Earth.

Common mistakes

• Confusing alpha, beta and gamma radiation’s relative penetrating power and ionising ability — alpha is the most ionising but least penetrating; gamma is the opposite.
• Forgetting nuclear equations must balance both mass number and atomic (charge) number, not just one.
• Mixing up contamination (material transferred, ongoing risk) and irradiation (external exposure, stops when source removed).
• Confusing fission (splitting heavy nuclei) with fusion (combining light nuclei) — both release energy, but by opposite processes, and only fusion requires overcoming electrostatic repulsion between similarly charged nuclei.

Quick revision checklist

• Alpha, beta and gamma: penetrating power and ionising ability
• Half-life calculations, including repeated halving
• Contamination vs irradiation
• Chain reactions, control rods, moderator and shielding in fission
• Fusion as the energy source for stars, and why it needs high temperature/pressure

Related resources

• Magnetism and Electromagnetism — the previous topic
• Astrophysics — the final topic
• Pearson Edexcel International GCSE Physics hub

Written against the Pearson Edexcel International GCSE in Physics (4PH1) specification, Issue 4. Always check the current specification for your examination year.