Particle physics

11.1 Atoms, nuclei and radiation

Alpha-particle scattering

Alpha particles were fired at a very thin gold foil in a vacuum.

Nuclei have diameters of about $10^{-15}$ m, while atoms are about $10^{-10}$ m across.

The nuclear atom and nuclides

The nucleus contains protons and neutrons (together called nucleons), and electrons orbit it. A nuclide is written $^{A}_{Z}\text{X}$:

• $A$ is the nucleon number, which is protons plus neutrons.
• $Z$ is the proton number.

Isotopes are forms of the same element, with the same number of protons but different numbers of neutrons. For example, $^{235}_{92}\text{U}$ and $^{238}_{92}\text{U}$.

The unified atomic mass unit is $1\ \text{u} = 1.66 \times 10^{-27}$ kg, about the mass of one nucleon.

Radiation

Antiparticles and neutrinos

An antiparticle has the same mass as its particle but the opposite charge. The positron is the antiparticle of the electron.

• $\beta^-$ decay also emits an electron antineutrino, $\bar{\nu}$.
• $\beta^+$ decay also emits an electron neutrino, $\nu$.

Alpha particles from a given decay have discrete energies. Beta particles have a continuous range of energies, because the energy released is shared between the beta particle and the (anti)neutrino in varying proportions.

Decay equations

Nucleon number and charge are conserved in nuclear processes.

$^{238}_{92}\text{U} \rightarrow\, ^{234}_{90}\text{Th} + ^{4}_{2}\alpha$

$^{14}_{6}\text{C} \rightarrow\, ^{14}_{7}\text{N} + ^{0}_{-1}\beta + \bar{\nu}$

$^{11}_{6}\text{C} \rightarrow\, ^{11}_{5}\text{B} + ^{0}_{+1}\beta + \nu$

11.2 Fundamental particles

Quarks

A quark is a fundamental particle. There are six flavours. Each has an antiquark with the opposite charge.

Hadrons

Protons and neutrons are not fundamental. They are made of quarks.

• A hadron is any particle made of quarks.
• A baryon has three quarks. Proton = uud (charge $+\tfrac{2}{3} + \tfrac{2}{3} - \tfrac{1}{3} = +1$). Neutron = udd (charge $+\tfrac{2}{3} - \tfrac{1}{3} - \tfrac{1}{3} = 0$).
• A meson has one quark and one antiquark.

Quark changes in beta decay

$\beta^-: \quad \text{d} \rightarrow \text{u} + e^- + \bar{\nu} \qquad (\text{a neutron becomes a proton})$

$\beta^+: \quad \text{u} \rightarrow \text{d} + e^+ + \nu \qquad (\text{a proton becomes a neutron})$

Leptons

Electrons and neutrinos are fundamental particles called leptons. They are not made of quarks, and they do not count as hadrons.