Cambridge IGCSE Physics 5 Nuclear Physics Exam Questions

About Cambridge IGCSE Physics 5 Nuclear Physics

Nuclear physics in Cambridge IGCSE Physics 0625 is split into the nuclear model of the atom and radioactivity. You need to describe the structure of the atom, use nuclide notation, explain isotopes and outline nuclear fission and fusion. Radioactivity covers background radiation, detection with a counter, the properties of alpha, beta and gamma emissions, decay equations, half-life and safe handling of sources. Extended candidates also explain the alpha-particle scattering experiment, deflection of emissions in fields, and corrected count rates in half-life calculations. Examiners frequently ask for balanced decay equations, half-life calculations from graphs or tables, and choices of radiation for a given use.

What you need to know

• 5.1 The atom An atom has a small, dense, positively charged nucleus of protons and neutrons, surrounded by electrons. Atoms form positive ions by losing electrons and negative ions by gaining them. Extended candidates describe alpha-particle scattering as evidence for the nuclear model.
• 5.1 The nucleus Proton number Z is the number of protons; nucleon number A is protons plus neutrons, written as A over Z before the symbol. Isotopes have the same proton number but different nucleon numbers. Fission is the splitting of a large nucleus; fusion is the joining of small nuclei; both release energy.
• 5.2 Detection and background radiation Background radiation comes from radon gas, rocks and buildings, food and drink, and cosmic rays. Count rate is measured with a detector such as a Geiger-Müller tube in counts/s or counts/minute. Corrected count rate = measured count rate - background count rate.
• 5.2 Types of emission Alpha particles are helium nuclei: very ionising, stopped by paper. Beta particles are high-speed electrons: moderately ionising, stopped by a few mm of aluminium. Gamma rays are electromagnetic radiation: weakly ionising, reduced by thick lead or concrete.
• 5.2 Radioactive decay Decay is random and spontaneous. Alpha decay reduces A by 4 and Z by 2; beta decay (a neutron changes to a proton and an electron) increases Z by 1 with A unchanged; gamma emission changes neither. Decay happens because the nucleus is unstable.
• 5.2 Half-life Half-life is the time taken for half the nuclei of a sample of an isotope to decay, or for the count rate to halve. Read it from a decay curve or calculate it from data. Uses depend on half-life and penetration, e.g. alpha in smoke alarms, beta for thickness monitoring, gamma for sterilising.
• 5.2 Safety Ionising radiation can damage cells and cause cancer. Reduce exposure by limiting time, increasing distance, using shielding and handling sources with tongs; store sources in lead-lined containers.

Common mistakes in 5 Nuclear Physics

• Saying the count rate falls to zero after two half-lives. It halves each half-life, so after two half-lives a quarter remains.
• Forgetting to subtract background radiation before finding half-life from count rates.
• Saying gamma rays are particles or have charge. Gamma is electromagnetic radiation with no mass and no charge.
• Balancing decay equations incorrectly. Nucleon numbers and proton numbers must add up to the same totals on each side; a beta particle is written with A = 0 and Z = -1.
• Saying radioactive decay can be sped up by heating. Decay is random and not affected by temperature or chemical conditions.

Try three 5 Nuclear Physics exam-style questions