ABSTRACT
Nuclear medicine and molecular imaging are mostly based on radioactive elements that, when decaying due to an excess of energy, emit radiation in the form of electromagnetic radiation (photons), or by charge-particles (electron, positrons, or alpha particles). The first part of this chapter describes in general the atom and its components and states some definitions important for further reading. There are several ways that a nucleus can decay (by alpha, beta+, and beta decay, electron capture and internal conversion). Each of these processes together with the conditions required for such a decay, are discussed in the chapter. In several of these decays there are also secondary emissions, such as characteristic x-rays and Auger electrons, and these are described together with which conditions they become important. Decay processes are generally described in the literature by decay schemes, and the chapter therefore includes a section on how these schemes are constructed and how to read them. The nature of the decay rate (the activity) is described since this is a fundamental quantity in the nuclear medicine field together with some examples of more complex decays.
