Atomic and Nuclear Physics

Atomic structure

• Atom: mostly empty space. Tiny, dense, positively charged nucleus at the centre, surrounded by negatively charged electrons in shells.
• Nucleus: made of protons (positive, relative charge +1, relative mass 1) and neutrons (neutral, relative charge 0, relative mass 1).
• Proton number (atomic number, Z): number of protons. Defines the element.
• Mass number (nucleon number, A): total protons + neutrons. Symbol: A.
• Neutron number: N = A − Z.
• Isotopes: atoms of the same element with the same proton number but different neutron numbers (and therefore different mass numbers). Some isotopes are stable; others are radioactive (unstable nuclei).

Notation: ᴬ_Z X

Example: ²³⁸_₉₂ U has 92 protons, 238 − 92 = 146 neutrons.

The three types of radiation

CCEA questions often ask "which type of radiation is most dangerous inside the body?" — alpha, because it deposits all its energy over a very short range in tissue (highly ionising).

Nuclear equations (decay equations)

Alpha decay: nucleus loses ⁴₂He. A decreases by 4, Z decreases by 2. ²³⁸_₉₂ U → ²³⁴_₉₀ Th + ⁴_₂ He

Beta decay: a neutron converts to a proton and an electron. A unchanged, Z increases by 1. ¹⁴_₆ C → ¹⁴_₇ N + ⁰_₋₁ e

Gamma emission: often follows alpha or beta decay; no change in A or Z. Represents the nucleus releasing excess energy as a photon.

Conservation rules: sum of mass numbers on each side must be equal; sum of proton numbers on each side must be equal.

Half-life

Half-life (t½): time taken for the count rate (or number of undecayed nuclei) to halve.

After n half-lives: N = N₀ × (½)ⁿ or Activity = A₀ × (½)ⁿ.

Half-life is constant for a given isotope — it does not depend on temperature, pressure, or chemical state. CCEA questions often give a decay curve or table and ask you to read off the half-life.

Example: If t½ = 8 days and initial count = 800 Bq:

• After 8 days: 400 Bq
• After 16 days: 200 Bq
• After 24 days: 100 Bq

Nuclear fission

A large unstable nucleus (e.g. U-235) absorbs a slow neutron and splits into two smaller nuclei + 2–3 neutrons + energy (mainly kinetic energy of products and gamma radiation).

Chain reaction: the released neutrons cause further fissions. If uncontrolled → nuclear explosion; if controlled (in reactor) → sustained power generation. A moderator (water/graphite) slows neutrons; control rods (boron) absorb neutrons to regulate the rate.

Nuclear fusion

Two light nuclei (e.g. hydrogen isotopes deuterium ²H + tritium ³H) combine to form a heavier nucleus + energy. Requires very high temperature and pressure (millions of °C) to overcome the electrostatic repulsion of the nuclei.

Fusion releases more energy per nucleon than fission. The Sun's energy comes from fusion (hydrogen → helium). Practical fusion power remains a research challenge (ITER project).

Background radiation

Radiation present in the environment from natural sources (radon gas, rocks, food, cosmic rays) and artificial sources (medical, nuclear industry). Always subtract background count from measured count rate before analysis.

Uses of radioactivity

• Medical imaging/treatment: gamma cameras, PET scans, radiotherapy.
• Smoke detectors: Am-241 (alpha emitter) ionises air to carry current; smoke absorbs alpha → alarm triggers.
• Sterilisation: gamma rays kill bacteria in food and medical equipment.
• Thickness gauges: beta radiation absorbed by materials of different thicknesses.
• Carbon dating (¹⁴C): living organisms maintain fixed ¹⁴C; at death, ¹⁴C decays; ratio ¹⁴C/¹²C gives age.