Conservation laws in particle interactionsAQA A-Level Physics: Flashcards

Exam questions on Conservation laws in particle interactions

1. Caesium-137 in stored nuclear waste decays by beta-minus emission. Inside each decaying nucleus a neutron changes into a proton, and an electron is emitted at high speed together with one other, very weakly interacting, particle.
   Show that charge and baryon number are both conserved in the decay n → p + e⁻ + $\bar{\nu}_e$.2 marks
2. A positron emission tomography (PET) scanner uses a tracer containing fluorine-18. Each fluorine-18 nucleus decays by beta-plus emission: a proton in the nucleus changes into a neutron and a positron is emitted together with a neutrino.
   A free proton does not decay in this way, although a proton inside some nuclei can. Using conservation of energy, explain why.2 marks
3. At a particle accelerator a beam of negative pions is fired into a target of liquid hydrogen, so that pions collide with protons. Data: a pion π⁻ has charge −1, baryon number 0 and strangeness 0; a proton has charge +1, baryon number +1 and strangeness 0. The Λ⁰ has charge 0, baryon number +1 and strangeness −1. The K⁰ has charge 0, baryon number 0 and strangeness +1. The K⁻ has charge −1, baryon number 0 and strangeness −1. The Σ⁺ has charge +1, baryon number +1 and strangeness −1. Strangeness is conserved in these collisions.
   Show that the interaction π⁻ + p → K⁰ + Λ⁰ obeys the conservation of charge, baryon number and strangeness.3 marks