Edexcel IAL Physics: Nuclear and Particle Physics — Revision Notes

Condensed for the final weeks. For the full explanation, use the Nuclear and Particle Physics study guide.

Scope note: this page covers Unit 4 sub-topic 4.5, Nuclear and Particle Physics. Mass deficit, binding energy, fission and fusion belong to Unit 5 sub-topic 5.4 and are in the Nuclear Decay revision notes. The exchange particles in the “Fundamental forces” table, strangeness and synchrotrons are background beyond the specification.

Atomic structure and accelerators

Nucleon number (mass number, A) is the total protons + neutrons; proton number (atomic number, Z) is protons only. Thermionic emission releases electrons from a heated material, which are then accelerated by electric and magnetic fields to feed accelerators like the linac and cyclotron.

Why high energies are needed: resolving very small distances inside a nucleon requires very short associated wavelengths (via de Broglie), which correspond to high momentum and hence high particle energy.

Relativistic time dilation can significantly increase a short-lived particle’s observed lifetime as its speed approaches c — this is why some particles that should decay almost instantly are observed travelling measurable distances in accelerator experiments.

The standard model

Quarks (up, down, strange, charm, top, bottom) and leptons (electron, muon, tau and their neutrinos) are fundamental. Protons and neutrons are not.

proton  = uud      charge  2(+2/3) + (-1/3) = +1
neutron = udd      charge  (+2/3) + 2(-1/3) = 0

Hadrons feel the strong force: baryons (three quarks) and mesons (quark + antiquark). Leptons do not feel the strong force.

Fundamental forces

Force	Exchange particle	Acts on
Strong	Gluon	Quarks and hadrons
Electromagnetic	Photon	Charged particles
Weak	W⁺, W⁻, Z⁰	All quarks and leptons (i.e. all fermions)
Gravitational	Graviton (hypothetical)	All mass

Only the weak force changes quark flavour, which is why it governs beta decay.

Antiparticles. Every particle has a corresponding antiparticle whose properties — such as charge — are the exact opposite, deducible from (or used to deduce) the particle’s own properties. The symmetry of the standard model predicted the top quark’s existence before it was experimentally observed, a strong piece of evidence for the model itself.

Conservation rules

Charge, baryon number, lepton number (per family), energy and momentum must all balance. Strangeness is conserved in strong and electromagnetic interactions but may change by ±1 in weak ones.

When rejecting a proposed interaction, you must name which rule is violated — saying “it’s impossible” without the reason scores nothing.

Mass–energy equivalence: creation and annihilation

Delta E = c^2 Delta m
1 eV = 1.60 x 10^-19 J      1 MeV = 1.60 x 10^-13 J      1 GeV = 1.60 x 10^-10 J
mass in MeV/c^2 = rest energy in MeV     (e.g. electron 0.511 MeV/c^2, proton 938 MeV/c^2)

Annihilation — a particle and its antiparticle meet and their mass is converted to energy, usually as two photons travelling in opposite directions (so momentum is conserved). For an electron and positron at rest, each photon carries 0.511 MeV.

Pair creation — a photon (or collision energy) of at least the combined rest energy of a particle and its antiparticle can create the pair; an electron–positron pair needs at least 1.02 MeV.

In accelerator collisions the kinetic energy of the colliding particles can become the mass of new particles, which is one reason very high energies are needed.

Particle accelerators

Linear accelerator — alternating p.d. across successive tubes, timed so the particle is always accelerated as it crosses each gap. Tubes get longer because the particle moves faster.

Cyclotron — two dees in a magnetic field, with an alternating p.d. across the gap. The magnetic field keeps the particle circling; radius increases as it speeds up, since r = mv/BQ.

The cyclotron’s limitation is relativistic: as the speed approaches c, the particle’s mass increases, so the time per semicircle is no longer constant and it falls out of step with the alternating field. That is why synchrotrons vary the field or frequency.

Detection: in a magnetic field, the radius of curvature gives momentum (r = p/BQ) and the direction of curvature gives the sign of the charge. A particle and its antiparticle curve in opposite directions.

Exam traps

• Calling protons or neutrons fundamental.
• Saying leptons feel the strong force.
• Not naming the violated conservation rule.
• Forgetting that annihilation produces two photons, not one, so that momentum is conserved.
• Converting GeV to joules without the factor of 10⁹.
• Forgetting the relativistic limit of the cyclotron.

Self-test

1. Which particles are fundamental?
2. Which force changes quark flavour, and what does that explain?
3. An electron and a positron annihilate at rest. What is produced, and what is the energy of each product in MeV?
4. Express 2.0 GeV in joules, and a mass of 938 MeV/c² in kilograms.
5. What limits a cyclotron at high speeds?
6. Distinguish nucleon number from proton number.
7. Why does relativistic time dilation matter for detecting short-lived particles?
8. What did the symmetry of the standard model successfully predict, before it was confirmed experimentally?

Answers: 1. Quarks and leptons; protons and neutrons are composite. 2. The weak force; it explains beta decay, where a down quark becomes an up quark or vice versa. 3. Two photons travelling in opposite directions (so momentum is conserved), each with energy 0.511 MeV, the rest energy of one electron. 4. 2.0 × 10⁹ × 1.60 × 10⁻¹⁹ = 3.2 × 10⁻¹⁰ J; 938 × 1.60 × 10⁻¹³ = 1.50 × 10⁻¹⁰ J, and dividing by c² = (3.00 × 10⁸)² gives 1.67 × 10⁻²⁷ kg. 5. As the particle approaches the speed of light its mass increases, so the time to complete each semicircle is no longer constant and it loses synchronisation with the alternating supply. 6. Nucleon number (mass number) is the total number of protons and neutrons in a nucleus; proton number (atomic number) is the number of protons only. 7. It increases a fast-moving particle’s observed lifetime, so particles that would otherwise decay almost instantly can travel measurable distances and be detected in accelerator experiments. 8. The existence of the top quark, before it was experimentally confirmed.