Physics Narrative for 14-16
Ionising radiations, unstable nuclei and rates of decay
Ionising radiations are emitted by unstable nuclei called
radioactive nuclei (for historical reasons).
Contamination (atoms containing unstable nuclei moving to a material) and irradiation (ionising radiations travel from the unstable nuclei to a material) are fundamentally different. Contamination may lead to irradiation as the unstable nuclei can still emit ionising radiations in their new locations: irradiation will not lead to contamination.
Photons and charged particles can both be emitted by unstable nuclei, which depends on the nucleus. The energy of these emissions also depends on the emitting nucleus.
Ionising particles deplete their kinetic store of energy as they ionise. Photons may ionise as they are absorbed.
As the number of unstable nuclei decrease by becoming stable, so the activity of that source will decrease.