Flashcards · 9702 · AS Topic 11

Particle physics (AS Level) flashcards

12 cards on AS Topic 11, subtopics 11.1 to 11.2. Pick a subtopic or study them all.

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What did the alpha-particle scattering experiment show?: Most alpha particles pass straight through a thin gold foil, but a very few are deflected through large angles. So the mass and positive charge of an atom are concentrated in a tiny, dense nucleus.
Nucleon number and proton number: Nucleon number A: protons + neutrons. Proton number Z: protons only. A nuclide is written with A as the superscript and Z as the subscript before the symbol.
Composition, mass and charge of α, β⁻, β⁺ and γ radiation: α: 2 protons + 2 neutrons (helium nucleus), 4 u, +2e. β⁻: electron, about 0 u, −e. β⁺: positron, about 0 u, +e. γ: electromagnetic radiation, no mass, no charge.
Balance the alpha decay of uranium-238 (Z = 92) to thorium: ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂α. Nucleon number: 238 = 234 + 4. Proton number: 92 = 90 + 2.
What extra particle is emitted in β⁻ decay, and in β⁺ decay?: β⁻: an electron antineutrino. β⁺: an electron neutrino. (The positron emitted in β⁺ decay is the antiparticle of the electron: same mass, opposite charge.)
Why do α particles have discrete energies but β particles a continuous range?: In β decay the released energy is shared in a variable proportion between the β particle and the (anti)neutrino. In α decay the fixed energy release is shared between just two bodies.

The six quark flavours, and the charges of up, down and strange: Flavours: up, down, strange, charm, top, bottom. Up: +⅔e. Down: −⅓e. Strange: −⅓e. Their antiquarks have the opposite charges.
Quark composition of the proton and neutron, with a charge check: Proton uud: +⅔ + ⅔ − ⅓ = +1. Neutron udd: +⅔ − ⅓ − ⅓ = 0. So protons and neutrons are not fundamental.
Baryons: Hadrons made of three quarks, for example the proton and the neutron.
Mesons: Hadrons made of one quark and one antiquark, for example the pion and the kaon.
Leptons: Fundamental particles such as the electron and the neutrino. They are not made of quarks and are not hadrons.
Quark change in β⁻ decay and in β⁺ decay: β⁻: a down quark changes to an up quark (n → p + e⁻ + antineutrino). β⁺: an up quark changes to a down quark (p → n + e⁺ + neutrino).