1. Atoms and ionizing radiation

Structure of the atom

Atoms are made up of a nucleus of nucleons (protons and neutrons) and a ring of electrons. Each atom is characterized by its atomic number Z (number of protons equal to the number of electrons in the non-ionized state) and its mass number A (number of nucleons equal to the sum of the numbers of protons and neutrons in the atomic nucleus). Two atoms are said to be "isotopes" if they have the same number of protons, but a different number of neutrons, e.g. ${}_6^{14}\text{C}$ (6 protons and 8 neutrons) and ${}_6^{12}\text{C}$...