OxfordAQA International A Level Physics 3.3 Particles, Radiation and Radioactivity Exam Questions

About OxfordAQA International A Level Physics 3.3 Particles, Radiation and Radioactivity

Particles, radiation and radioactivity is the third section of Unit 1 (PH01) on the OxfordAQA International AS course. It covers the constituents of the atom and the evidence from Rutherford scattering, specific charge and nuclide notation, antiparticles with annihilation and pair production, and the properties of alpha, beta-minus, beta-plus and gamma radiation. The work on half-life is extended in the A2 sections on exponential change and nuclear energy. Examiners commonly ask you to complete decay equations with correct nucleon and proton numbers, calculate specific charge, explain the evidence from alpha scattering and use the inverse-square law for gamma radiation.

What you need to know

• Constituents of the atom Protons (charge +1.60 x 10^-19 C), neutrons (no charge) and electrons (charge -1.60 x 10^-19 C). Nuclide notation gives nucleon number A and proton number Z. Specific charge = charge / mass, in C kg-1.
• Rutherford scattering Most alpha particles passed straight through gold foil, so the atom is mostly empty space; a few were deflected through large angles, so the positive charge and most of the mass are concentrated in a tiny nucleus.
• Antiparticles Each particle has an antiparticle with the same mass and opposite charge, such as the positron. In annihilation a particle and its antiparticle convert into two gamma photons. In pair production a photon of energy at least 2mc^2 creates a particle-antiparticle pair.
• Alpha and beta decay Alpha decay emits a helium nucleus, so A falls by 4 and Z falls by 2. Beta-minus decay turns a neutron into a proton, emitting an electron and an antineutrino. Beta-plus decay turns a proton into a neutron, emitting a positron and a neutrino. The neutrino was proposed to explain the continuous energy spectrum of beta particles.
• Gamma radiation and absorption Gamma emission follows alpha or beta decay when the nucleus is left in an excited state. Alpha is stopped by paper or a few cm of air, beta by a few mm of aluminium, and gamma is reduced by several cm of lead. Gamma intensity follows the inverse-square law, I = k / x^2.
• Half-life and background Half-life is the average time for the number of unstable nuclei (or the activity) to halve. Measure and subtract background count rate before analysing data; sources of background include rocks, cosmic rays and medical use.

Common mistakes in 3.3 Particles, Radiation and Radioactivity

• Writing beta-minus decay without the antineutrino or beta-plus decay without the neutrino. Both must appear for lepton number to be conserved.
• Saying alpha particles bounced off the nucleus. They were repelled by electrostatic force from the positive nucleus.
• Forgetting to correct count rates for background before using the inverse-square law or finding half-life.
• Stating that pair production needs a photon energy of mc^2. It must be at least 2mc^2 because two particles are created.
• Calculating specific charge of a nucleus using the atomic mass including electrons, or using the number of nucleons instead of protons for the charge.

Try three 3.3 Particles, Radiation and Radioactivity exam-style questions