Antiparticles, photons and annihilationAQA A-Level Physics: Mind map

Exam questions on Antiparticles, photons and annihilation

1. A positron emission tomography (PET) scanner uses a tracer that emits positrons inside a patient. Each positron quickly meets an electron in the tissue and the pair annihilate, producing two gamma-ray photons that are detected by a ring of sensors around the patient. The rest energy of an electron is 0.511 MeV.
   Explain why the annihilation of an electron and a positron at rest produces two photons rather than one, and why the photons travel in opposite directions.2 marks
2. In a bubble chamber experiment, a gamma-ray photon of energy 1.50 MeV travels through liquid hydrogen. Close to a nucleus it disappears and a pair of particles is produced. The rest energy of an electron is 0.511 MeV.
   Calculate the longest wavelength of photon that could produce an electron–positron pair. Give your answer in metres.2 marks
3. Physicists at a research laboratory study the production and annihilation of proton–antiproton pairs. The rest energy of a proton is 938 MeV, and an antiproton has the same rest energy.
   Calculate the minimum frequency of a photon that can produce a proton–antiproton pair.3 marks