A series of isotopes into which a radioisotope is gradually transmuted by alpha decay and beta minus decay. The last element of the series is the stable isotope. The unstable isotopes decay to achieve a more stable arrangement of neutrons and protons in the nucleus, but usually do not decay directly into the stable isotope, but into another radioisotope. This process happens in two ways. Alpha decay, in which an alpha particle (a helium nucleus made up of two protons and two neutrons) is released from the nucleus. This reduces the atomic number of the decaying element by two and its mass number by four. In beta-minus decay, an electron and an antineutrino are released, while a neutron in the nucleus changes to a proton. This increases the atomic number by one while the mass number remains unchanged. Because of this fact, the mass number of the atoms in the series is always a multiple of four. We therefore distinguish four mutually independent decay series: the thorium, neptunium, uranium (also known as uranium-radium), and the actinium (also known as uranium-actinium) series. By knowing the decay series, we can, for example, determine the composition of ancient rocks, determine their age, or predict the behaviour of radioactive waste.