Title: InkPhysics / Particles
On every note of this deck, unless a note says otherwise:
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Front: DEFINITION
Define: Hadrons.
Back: Particles that feel the strong interaction: baryons, made of three quarks, and mesons, made of a quark–antiquark pair.

Front: CHECK YOURSELF
Calculate the specific charge of an iron-56 nucleus (Z = 26). Take e = 1.60 × 10^−19 C and the nucleon mass as 1.67 × 10^−27 kg.
Show a hint
Charge counts protons only; mass counts every nucleon.
Back: Charge: Q = 26 × 1.60 × 10^−19 = 4.16 × 10^−18 C.
Mass: m = 56 × 1.67 × 10^−27 = 9.35 × 10^−26 kg.
Specific charge: Q/m = 4.4 × 10^7 C kg^−1. Smaller than a proton's alone, because thirty neutrons added mass without adding any charge.

Front: DEFINITION
Define: Isotopes.
Back: Atoms with the same number of protons but different numbers of neutrons: the same element with a different nucleon number.

Front: CHECK YOURSELF
Write the decay equation for radium-226 (Z = 88) undergoing alpha decay to radon (Rn), and check both balances.
Show a hint
The alpha particle takes four nucleons, two of them protons.
Back: ^226Ra → ^222Rn + ^4He.
Nucleon check: 226 = 222 + 4. Proton check: 88 = 86 + 2, so radon has Z = 86.
Both books balance, and the element changed because Z changed: that is what decay does.

Front: CHECK YOURSELF
A kaon decays into pions. Identify the interaction responsible and give two pieces of evidence for your answer.
Show a hint
What happens to strangeness, and how quickly do these decays happen?
Back: The decay is a weak interaction.
Evidence one: strangeness changes. The kaon carries S = ±1 and pions carry S = 0, and only the weak interaction permits ΔS = ±1; the strong interaction conserves strangeness exactly.
Evidence two: the decay is slow. Strong processes are effectively instantaneous, and the kaon's comparatively long life is the signature of a particle waiting for the weak interaction.

Front: DEFINITION
Define: The weak interaction.
Back: The only interaction that can change quark flavour, as when a down quark becomes an up quark and a neutron becomes a proton in beta decay; its exchange particles are the W^+ and W^− bosons.

Front: DEFINITION
Define: Leptons.
Back: Fundamental particles that do not feel the strong interaction: the electron, the muon, their neutrinos, and their antiparticles.

Front: EQUATION — NOT ON THE DATA SHEET
Specific charge:
specific charge = ?
Back: specific charge = Qm

Front: CHECK YOURSELF
A slow electron and a slow positron annihilate. Find the minimum frequency of each photon produced. (Electron rest energy 0.511 MeV; h = 6.63 × 10^−34 J s.)
Show a hint
One rest energy per photon; convert MeV to joules first.
Back: Each photon carries at least one rest energy: 0.511 MeV = 0.511 × 10^6 × 1.60 × 10^−19 = 8.18 × 10^−14 J.
Minimum frequency: f = E/h = 8.18 × 10^−14 / 6.63 × 10^−34 = 1.2 × 10^20 Hz.
That is gamma radiation, and detecting the two back-to-back gamma photons is precisely how a PET scanner locates the annihilation.

Front: CHECK YOURSELF
In electron capture, a proton in a nucleus absorbs an atomic electron. Write the particle equation, name the exchange particle, and explain how you know the weak interaction is responsible.
Show a hint
What does the proton become, and what must leave to balance the leptons?
Back: p + e^− → n + ν_e: the proton becomes a neutron and an electron neutrino leaves. Charge balances: +1 − 1 = 0.
The exchange particle is the W^+, passed from the proton to the electron.
A proton changed into a neutron, and changing a particle's type is something only the weak interaction can do; neither the electromagnetic nor the strong interaction ever alters identity.

Front: CHECK YOURSELF
Using quark charges, show that the proton's charge is +1 and the neutron's is 0, then write the neutron's decay at quark level.
Show a hint
Add thirds, then change exactly one flavour.
Back: Proton, uud: ⅔ + ⅔ − ⅓ = +1. Neutron, udd: ⅔ − ⅓ − ⅓ = 0. Both are whole charges, as hadrons must be.
Decay: one d becomes a u, so udd → uud + e^− + an electron antineutrino, the W^− mediating.
Charge audit at the vertex, in thirds: −1 = +2 − 3. The books balance, and the neutron's decay is one quark's change of flavour.

Front: DEFINITION
Define: Pair production.
Back: A single high-energy photon converting into a particle–antiparticle pair, usually near a nucleus, which recoils so momentum is conserved.

Front: DEFINITION
Define: Annihilation.
Back: A particle and its antiparticle meeting and converting entirely into two photons, conserving energy and momentum.

Front: CHECK YOURSELF
A student proposes that a free proton decays: p → n + e^+ + ν_e. Audit the conservation laws, then explain what actually forbids the decay. (Rest energies: p 938.3 MeV, n 939.6 MeV, e 0.511 MeV.)
Show a hint
Every ledger passes. So check the one thing left.
Back: Charge: +1 → 0 + 1 + 0 ✓. Baryon number: +1 → +1 ✓. Electron lepton number: 0 → 0 − 1 + 1 = 0 ✓. Strangeness: 0 → 0 ✓. Every quantum number balances.
Energy does not. The products carry at least 939.6 + 0.511 = 940.1 MeV of rest energy, more than the proton's 938.3 MeV, and a free proton has nothing extra to pay with.
Energy conservation forbids the decay, and the proton stays the only stable baryon. Inside a nucleus the binding-energy accounts can differ, which is why β^+ decay happens there.

Front: DEFINITION
Define: Specific charge.
Back: The charge of a particle divided by its mass: Q/m, in C kg^−1.