Radioactive decay and half-life

The idea

Unstable nuclei give out radiation randomly. Alpha particles (two protons and two neutrons) are highly ionising, travel a few centimetres in air and are stopped by paper. Beta particles (fast electrons from the nucleus) travel about a metre in air and are stopped by a few millimetres of aluminium. Gamma rays (electromagnetic waves) are weakly ionising and need thick lead or concrete. Activity is measured in becquerels (Bq). The half-life is the time for the number of unstable nuclei, or the activity, to halve. Contamination is radioactive material on or in an object; irradiation is exposure to radiation from outside.

1. Name the radiation from its penetration or ionising power.
2. For alpha decay: mass number − 4, atomic number − 2.
3. For beta decay: mass number unchanged, atomic number + 1.

Watch out Irradiation does not make an object radioactive; contamination does.