2.1 Radioactivity

Radioactivity is a phenomenon related to the spontaneous transmutation (decay) of an unstable atomic nucleus accompanied by emission of radiation. In this definition, a number of terms is used that need to be explained. In the following, a brief summary of the definitions and fundamentals is given. For more details, the reader may refer to a suitable textbook (e.g. Turner, 2007; Magill and Galy, 2005).

The nucleus of an atom consists of protons and neutrons and is surrounded by a cloud of orbiting electrons forming the shell of the atom (Figure 1). The mass of a proton is about 2,000 times heavier than that of an electron. The proton carries a positive and the electron a negative elementary charge. The neutron is electrically neutral and only slightly heavier than the proton (Figure 1). In a neutral atom, the number of electrons is equal to the number of protons, which is called the atomic number Z. While the mass of an atom is nearly equal to the mass of its nucleus (about 2×10–27 to 400×10–27 kg), the size (diameter) of the atom is controlled by the atomic shell (about a fraction of a nanometre), that is about 10,000 times larger than the nucleus.

The term nuclide defines an atomic species with a given number of protons and neutrons in the nucleus. The number of protons and neutrons is called the atomic mass number A=N+Z, where N is the number of neutrons and Z the number of protons (atomic number). The common name of proton and neutron is nucleon, that is A is the number of nucleons. A nuclide is characterised by the symbol , where X stands for the chemical element to which the nuclide belongs. For example the nuclide of carbon with 12 nucleons is described by .

Nuclides belonging to the same chemical element, that is having the same atomic number but different atomic mass numbers, are called isotopes. For example the following nuclides are isotopes of the element carbon: (most abundant carbon isotope), (heavy stable carbon isotope) and (heavy unstable carbon isotope). Since the chemical element is defined by the atomic number, the latter can be omitted in the nuclide symbol. Hence, the above carbon isotopes can be written as 12C, 13C and 14C.

Nucleons are bound in the nucleus by nuclear forces, also called strong forces. These are short-range attractive forces that exist between protons and neutrons and are strong enough to overcome the electrostatic repulsion which exists between the charged protons. Like atomic electrons of the shell, nucleons ‘occupy’ discrete energy levels of the nucleus. The situation in the nucleus is, however, complicated by the fact that two types of nucleons, protons and neutrons, are to be accommodated. The energy levels involved are of the order of megaelectron volt, much higher than the energies of atomic electron levels. Because the charged protons experience an electrostatic repulsion between them, which the uncharged neutrons do not, the proton levels appear at slightly higher energies than the neutron levels. As in the case of atomic electrons, each nuclear level can only hold a certain number of protons or neutrons. Because of the higher energy of the proton levels, stable nuclei tend to have slightly more neutrons than protons. There are possible transitions between neutron and proton energy levels. These transitions lead to the emission of particles and transformation of one kind of nucleons to another. If the nucleus has, for example a single proton in a high energy level with a vacancy for a neutron in a lower level, that proton can transmute into a neutron and emit a particle with positive charge, called a positron, and a gamma quantum that takes the excess energy. This process is called beta plus decay. The opposite process, called beta minus decay, occurs if a nucleus has too many neutrons and thus transmutes a neutron into a proton.

The gamma quantum is a photon that has no rest mass and no charge. Therefore, the gamma transition changes neither Z nor A of the emitting nucleus. Like X-rays, the gamma quantum is equivalent to electromagnetic radiation. Thus, the alternative term is gamma ray (radiation). The energy of the gamma quantum Eγ is given by the energy difference between the corresponding energy states E2 (higher state) and E1 (lower state):

where h is Planck’s constant=6.626068×10–34 m2 kg–1 s–1 and f the frequency of the equivalent electromagnetic radiation. In most cases, the nucleus remains in the higher energy state for less than 10–14 s.

The basic condition for the existence of stable nuclides is the following: the total energy of a stable nucleus must be lower than the total energy of any other combination of the same number of neutrons and protons. If this condition is not fulfilled, the nucleus is unstable and will spontaneously decay into another configuration. The latter process is called radioactive decay and the decaying nuclide is called a radionuclide. There are radionuclides that can decay in two or three different ways, but the majority of radionuclides decay in a single way. In either case, radioactive decay changes Z and N of the nucleus. The product nuclide, also called the daughter nuclide, may also be radioactive and transmute spontaneously, as happens in the natural decay series (Chapter 1). In the following sections, a short overview of the various radioactive decay types will be given.