A-Level Physics: Photons and Particle Interactions

The particles topic at the start of A level Physics is full of short, precise questions: how much energy a photon carries in MeV, whether a photon can create a particle–antiparticle pair, what happens when matter meets antimatter, which exchange particle an interaction needs, and whether a proposed reaction breaks a conservation law. This material trains exactly those skills and complements the particle physics material in this category, which covers the classification of particles and quark composition.

The quiz (12 questions) opens with photon energy: converting the energy of a gamma-ray photon from joules to keV, and finding the longest wavelength that can still produce an electron–positron pair. Pair production questions cover the conditions needed and the minimum energy for a proton–antiproton pair, and annihilation questions ask why an electron and positron at rest produce two photons and what energy each carries. Interaction questions ask which exchange particle mediates beta-minus decay, which processes involve W bosons (beta-plus decay, electron capture, neutrino–neutron interactions) and which boson is exchanged in an electron–proton collision. The last group is about conservation laws: why strange particles decay by the weak interaction, which law rules out a proposed proton–proton reaction, and a full check of muon decay using electron and muon lepton numbers separately.

The flashcards (12 cards) give the photon energy equation, the rest energies of the electron and proton, the pair production condition, the W bosons in beta decay, the quantities conserved in strong and weak interactions, why annihilation gives two photons, electron capture, strange particle production and the role of the W boson.

The written work (8 questions) asks you to calculate the energy and wavelength of annihilation photons, find the threshold for proton–antiproton pair production and explain why a nucleus must be nearby, describe the interaction diagrams for beta-minus and beta-plus decay at quark level, compare electron capture with an electron–proton collision, analyse the production and decay of strange particles, decide with reasons whether three proposed interactions are possible, describe a neutrino–neutron interaction and why such events are rare, and convert between joules, electronvolts and frequency.

The content is based on the particles and radiation section common to A level Physics specifications in England (for example AQA 3.2.1, particles, antiparticles and photons, particle interactions, and the conservation laws). Interaction diagrams are described in words, and constants are given in each question.

• Calculate photon energies and wavelengths using joules, eV and MeV
• Find threshold energies and maximum wavelengths for pair production and the energy of annihilation photons
• Identify the exchange particle and charges in beta decay, electron capture, electron–proton collisions and neutrino interactions
• Explain why strange particles are produced in pairs and decay by the weak interaction
• Test proposed interactions against conservation of charge, baryon number, lepton number and strangeness

Practice material written by Zestly, based on the DfE GCE AS and A level subject content for physics (DFE-00356-2014): particles, antiparticles and photons, and particle interactions.