ABSTRACT
Radioactivity is a natural phenomenon that allows to peek into earth's past and the history of humankind through radioactive dating. Its effects when absorbed by human beings can also be measured. Matter consists of atoms that can be described in a simple way as a central part named the nucleus, containing neutrons and protons, surrounded by an electron cloud. Atomic elements have the same number of protons but the number of neutrons may change. Isotopes can be stable or unstable. Stable nuclei, the majority on earth, last "forever", that is, they always keep the same number of neutrons and protons. The decay of a type of nucleus into another one usually happens due to the emission of particles α or β and radiation γ. In some radionuclides there are also emissions of neutrons and protons and even other types of reactions such as spontaneous fission, common in some heavy nuclei.
