Reality → Energy → Nuclear energy → Radioactivity
A mysterious radiation of uranium salts blackening photographic plates was first discovered by Becquerel in 1896, followed soon by further discoveries: the radiation occurs in three different types, it is ionizing, and it is connected with a transmutation of elements and their isotopes [1] . The three different types of radiation were named alpha, beta, and gamma. Alpha radiation consists of helium nuclei (i.e., 2 protons and 2 neutrons), beta radiation of electrons, and gamma radiation of high-energetic photons [2] . Alpha particles can be stopped by a sheet of paper, beta particles by an aluminum plate, but gamma rays penetrate through most materials and interact with the electrons of atoms and molecules of living tissue [3] . Initially, radioactivity was only known from the decay of naturally occurring elements (e.g., uranium, thorium, radium), their isotopes and compounds. Today, more than thousand artificial isotopes, mostly highly radioactive trans-uranium elements, have been created in particle colliders and nuclear reactors [4] .
These discoveries, not restricted to uranium and its compounds, were made by Rutherford, Marie Curie, and others even before Rutherford’s famous experiment that proved the existence of atomic nuclei.
The particles were identified first for beta radiation (by Becquerel in 1900), then for alpha radiation (by Rutherford in 1907), and last for gamma radiation (after the acceptance of the photon as the quantum-mechanical particle of electromagnetic radiation).
The kinetic energy of the relatively slow-moving but massive alpha particle can be larger than that of the high-energetic (moving at speed of light, but massless) photon. The ionizing effect of the alpha radiation on contact can therefore exceed the effect of gamma radiation, although alpha radiation lacks penetration power.