Radionuclides are nuclei that consist of unstable combinations of protons and neutrons, and disintegrate to other nuclei under the emission of radiation (radioactivity). The unit of radioactivity is Becquerel (Bq), indicating 1 decaying nucleus per second. If the decay product is not a stable nucleus, the product will further decay until it reaches a stable end member. Such a chain of decay products is called a decay series. The way the activities of the decay products evolve with time is described by the so-called Bateman equations (see e.g. Evans, 1969). When members of the decay series are in secular-equilibrium (when the formation and decay rates of the nuclei are the same), the activities of all nuclei are equal.
The specific activity or activity concentration, C, is defined as the activity per unit mass of a sample (in Bqkg-1). The relation between activity and mass concentrations, in units like ppm, follows from the natural abundance of the nuclide, its half-life time and the relation between mass and the number of nuclei by Avogadro’s number. The table in Radiation unit conversions presents the conversion between specific activity concentration and mass concentration of the different radionuclides.
Radio nuclei decay mainly by the emission of 𝛼 or β particles or by 𝛾-radiation, each with different properties and energies. The decay-modes presented in next section are the modes that are most relevant for the study of natural radioactivity.
Fission: Very heavy nuclei, as 238U, can disintegrate spontaneously in two parts (spontaneous fission). In this process several neutrons are emitted and the radioactive fragments decay further by emitting 𝛾 and β radiation. For naturally occurring radionuclides, this process is rare.
𝛼-decay: Alpha particles are 4He nuclei, composed of two protons and two neutrons, and are generally emitted by heavy radio nuclei. Alpha particles have a very high rate of energy loss in matter. Consequently, a-particles have a penetration in depth in air of only a few cm, in more dense materials this penetration depth is even lower.
𝛽-decay: Beta particles are fast electrons (e-) or positrons (e+) that result from transfer of a neutron into a proton (β-) or from a proton into a neutron (β+). Also β particles lose their energy rather easily in matter, and are absorbed within a few cm of water.
𝛾-decay: If a nucleus is not stable after one of previously mentioned decay mechanisms but contains a surplus of energy, this energy is lost by the emission of photons (𝛾-radiation), without changing the Z-number (the number of protons in a nucleus) and A-number (the total number of protons and neutrons in a nucleus). These photons have an energy that is distinct for the emitting nucleus and have penetration depths larger than that of a and β particles.