Physics 9702 · syllabus 11

Cambridge A Level Physics 9702 — Particle physics topical past papers

Inside the atom: the nuclear model from α-particle scattering, nuclide notation, α, β and γ radiation, and quarks, hadrons and leptons.

Questions: 390
Years: 2009–2026
With mark scheme: 100%
Questions 2022–2026: 128

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What Particle physics covers

When α-particles are fired at a thin gold foil, nearly all pass straight through, but a very small fraction are deflected through large angles. From this you infer that an atom's positive charge and almost all of its mass sit in a tiny nucleus, with electrons around it. The nucleus contains protons and neutrons; the proton number Z identifies the element, the nucleon number A counts protons and neutrons together, and isotopes share Z but differ in their number of neutrons.

Unstable nuclei emit α-particles (helium nuclei), β⁻ particles (electrons), β⁺ particles (positrons, the antiparticle of the electron) or γ-ray photons. Nucleon number and charge are conserved in every decay. α-particles from a given decay have discrete energies, but β-particles have a continuous spread of energies because an antineutrino or a neutrino carries off a share. Masses at this scale are often given in unified atomic mass units.

Protons and neutrons are not fundamental: each is made of three quarks, while leptons such as the electron and the neutrino are fundamental. There are six flavours of quark with charges of +⅔e or −⅓e; hadrons are either baryons, of three quarks, or mesons, of a quark and an antiquark; and β decay changes one quark flavour into another. This topic is AS content, examined on Papers 1 and 2.

In the syllabus

The headings this topic has in the Cambridge International 2025–2027 syllabus, as the board prints them.

11 Particle physics
11.1 Atoms, nuclei and radiation
11.2 Fundamental particles

Read the syllabus on the board’s website

What you should be able to do

Explain what the results of α-particle scattering show about the size and charge of the nucleus
Write and balance α and β decay equations in nuclide notation
Describe the nature, charge and relative mass of α, β⁻, β⁺ and γ radiation
Explain why β-particle energies form a continuous range while α energies are discrete
Work out the charge of a hadron from its quarks, and give the quark composition of the proton and neutron
Describe the change in quark composition in β⁻ and β⁺ decay

Key ideas

Nuclide notation, e.g. ²³⁸₉₂U: nucleon number A at the top, proton number Z at the bottom, neutrons = A − Z
Quark charges: up, charm and top +⅔e; down, strange and bottom −⅓e; each antiquark has the opposite charge
Proton = uud (charge +e); neutron = udd (charge 0)
β⁻ decay: n → p + e⁻ + ν̄, with d → u; β⁺ decay: p → n + e⁺ + ν, with u → d
1 u = 1.66 × 10⁻²⁷ kg

How Particle physics has been examined

From 2022 to 2026, Particle physics was on Paper 2 · AS Level Structured Questions in 11 of 11 exam series, worth about 13% of that paper’s marks.

How Particle physics has been examined, by paper
Paper	Questions	Share of marks	Series
Paper 1 · Multiple Choice	95	7%	11 of 11
Paper 2 · AS Level Structured Questions	33	13%	11 of 11

Momentum’s own count, over the questions in its bank for these years: a series is one exam session, every zone included, and a question filed under two chapters counts in both. Median marks per question: 1. Some of these questions sit in a bank chapter that also covers a neighbouring topic, so the counts can run a little high.

Filed in Momentum under: Particle & Nuclear Physics.

A worked example

Thorium-232, with proton number 90, decays to a stable nucleus through a chain of six α decays and four β⁻ decays. Find the nucleon number and proton number of the final nucleus, and state the quark change in each β⁻ decay.

Each α decay removes 4 nucleons and 2 protons, so six α decays remove 24 nucleons and 12 protons
Each β⁻ decay turns a neutron into a proton: A is unchanged and Z rises by 1, so four β⁻ decays add 4 to Z
Nucleon number: 232 − 24 = 208
Proton number: 90 − 12 + 4 = 82, which is lead
In each β⁻ decay a down quark becomes an up quark (udd → uud), with an electron and an electron antineutrino emitted

Answer: Nucleon number 208 and proton number 82 (lead-208); in each β⁻ decay a down quark changes to an up quark

Written by Momentum for this page, with its own numbers; it is not taken from a past paper.

Mistakes that cost marks

Drawing the wrong conclusion from each α-scattering observation — the undeflected majority shows the atom is mostly empty space; the rare large deflections show charge and mass are concentrated in a tiny nucleus
Leaving out the neutrino or antineutrino in a β decay, or using the wrong one — β⁻ emits an antineutrino and β⁺ a neutrino, and without it the continuous β energy spread cannot be explained
Changing the nucleon number in β decay — a neutron becomes a proton or the reverse, so A stays the same and Z changes by one
Calling a meson a baryon because it is made of quarks — a baryon has three quarks; a meson has one quark and one antiquark
Giving a positron the mass of a proton — it has the mass of an electron and the opposite charge

Exam technique

In decay equations, check that the top numbers and the bottom numbers each balance before moving on
When describing α-particle scattering, keep each observation next to its conclusion: most pass straight through, so…; a few are deflected through more than 90°, so…
For quark questions, write each quark's charge as a fraction and add them; a hadron's total must be a whole number of e

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