Radioactivity

Ionising Nuclear & Background Radiation

• **Ionisation** is when an atom gains or loses electrons, becoming charged; nuclear radiation can ionise atoms by removing electrons.
• **Background radiation** is the radiation that exists around us all the time from natural and man-made sources.
• Natural sources include **radon gas** (from rocks), **rocks and buildings**, **food and drink**, and **cosmic rays** from space.
• Man-made sources include **medical X-rays**, **nuclear waste**, **nuclear fallout**, and **nuclear accidents**.
• Radiation is detected using a **Geiger–Müller tube**; the **count rate** (counts per second) measures the number of decays detected.
• The count rate decreases as the distance from the source increases because radiation spreads out.

Types of Radioactive Decay

• **α (α) particles** are helium nuclei (2 protons, 2 neutrons), charge +2, high ionising ability, stopped by paper.
• **β (β) particles** are fast-moving electrons, charge -1, moderate ionising ability, stopped by a few mm of aluminium.
• **Gamma (γ) rays** are electromagnetic waves, no charge, low ionising ability, reduced by thick lead.
• Penetrating power increases from α to β to γ; ionising ability decreases from α to β to γ.
• α has a range of a few cm in air, β a few tens of cm, gamma is infinite (but intensity decreases).

Changes During Radioactive Decay

• In **α-decay**, the nucleus loses 2 protons and 2 neutrons; mass number decreases by 4, atomic number decreases by 2.
• In **β-decay**, a neutron changes into a proton and an electron; mass number stays the same, atomic number increases by 1.
• In **γ-decay**, a gamma ray is emitted; mass number and atomic number do not change, but the nucleus becomes less energetic.
• Decay equations must balance mass and atomic numbers on both sides.
• Example α-decay: $^{212}_{84}Po \rightarrow ^{208}_{82}Pb + ^{4}_{2}\alpha$
• Example β-decay: $^{24}_{11}Na \rightarrow ^{24}_{12}Mg + ^{0}_{-1}\beta$

Half-Life

• **Half-life** is the time taken for half the nuclei in a sample to decay (or for activity to halve).
• Half-life is constant for a given isotope and can range from fractions of a second to billions of years.
• After one half-life, 50% remains; after two, 25%; after three, 12.5%; and so on.
• Half-life can be determined from an **activity–time graph** by reading the time for activity to fall to half its initial value.
• Example: If a 10 mg sample decays to 2.5 mg, that is ¼ remaining $= 2$ half-lives; if half-life is 1.17 min, time $= 2.34$ min.

Uses of Radioactivity

• **α radiation** is used in **smoke detectors**: α ionises air, smoke absorbs α, triggering the alarm.
• **β radiation** is used to **measure thickness** of materials (e.g., paper): more absorption indicates thicker material.
• **Gamma radiation** is used to **sterilise medical equipment** and **irradiate food** to kill bacteria.
• **Gamma rays** are used in **radiotherapy** to treat cancer: beams are directed at tumours to kill cancer cells.
• **Radioactive tracers** (e.g., in PET scans) help diagnose cancer and track substances in the body.

Dangers of Radioactivity

• Ionising radiation can damage **DNA**, causing **mutations** or **cancer**; high doses can cause **cell death** and **skin burns**.
• Radiation dose is measured in **sieverts (Sv)**; acute exposure can lower white blood cell count, increasing infection risk.
• Safety precautions: **reduce exposure time**, **increase distance** (use tongs), and use **shielding** (lead, concrete, water).
• Radioactive sources should be stored safely in lead-lined containers, and workers wear **lead aprons** and **dosimeters**.

Structure of an alpha particle (helium nucleus): 2 protons and 2 neutrons.

Atomic structure of carbon-12; radioactive isotopes have unstable nuclei that decay.

Particle arrangement in solids, liquids, and gases; radioactive decay is a nuclear process, not affected by state.