Figure 3.41 shows a spectrum of the electromagnetic waves. On its left-hand side, there is a region of the γ-radiation. Then, there are the X-rays that depending on the wavelengths are divided into hard, soft, and ultrasoft rays. However, a spontaneous radiation from the matter not necessarily should be electromagnetic: There is the so-called nuclear radiation, which is emission of particles from the atomic nuclei. A spontaneous decay can be of two types: The charged particles (α and β particles, and protons) and uncharged particles that are the neutrons. Some particles are complex like the α particles, which are the nuclei of helium atoms consisting of two neutrons, while other particles are generally simpler, like the β particles that are either electrons or positrons. Ionizing radiations are given that name because as they pass through various media that absorb their energy, additional ions, photons, or free radicals are created.