Radioactivity

Radiations

• Alpha (α): helium nucleus ⁴₂He; strongly ionising; stopped by paper or a few cm of air.
• Beta (β): fast electron; moderately ionising; stopped by a few mm of aluminium.
• Gamma (γ): electromagnetic wave; weakly ionising; reduced by thick lead or concrete.
• In a magnetic or electric field, α and β are deflected in opposite directions; γ is not deflected.

Nuclear equations

• Alpha decay: mass number falls by 4, atomic number by 2.
• Beta decay: mass number unchanged, atomic number rises by 1.

Example: ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He.

Half-life

The time taken for half the radioactive nuclei in a sample to decay.