Radioactivity is the emission of atomic particles from a nucleus of an atom. Therefore, protons and neutrons are radiated in every direction. This emission of particles is called radiation, which consists of alpha, beta, and gamma radiation. This phenomenon results from an instability of the atomic nucleus in certain atoms, such as an uranium atom, whereby the nucleus experiences a spontaneous but measurably delayed nuclear transition or transformation with the resulting emission of radiation. Thus, the essence of radioactivity lies in the spontaneous change in the composition of the atomic nucleus, which is in either the ground state or a long-lived excited (metastable) state. These transformations are accompanied by the emission of elementary particles.
All the known types of radioactive transformations are the result of fundamental microcosmic interactions, that is, strong interactions (nuclear forces) or weak interactions. Strong interactions are responsible for those transformations accompanied by the release of such nuclear particles as alpha particles, protons, and fragments of nuclear fission. Weak interactions are seen in the beta decay of nuclei. Electromagnetic interactions are responsible for quantum transitions, which are accompanied by the emission of gamma radiation, between various energy states of the same nucleus. Since these transitions are not related to a change in the composition of the nucleus, they are not, according to current classification, considered radioactive transformations. The concept of radioactivity has also been extended to include the beta decay of neutrons.
Types of radioactive transformations
The known types of radioactivity comprise elementary (one-step) transformations and complex (two-step) transformations. Elementary transformations include alpha decay, all types of beta decay (with the release of an electron or positron or with the capture of an orbital electron), spontaneous nuclear fission, proton radioactivity, two-proton radioactivity, and two-neutron radioactivity. In the case of beta decay, the rather long lifetime of the nucleus derives from the nature of the weak interactions. All other types of elementary radioactive processes are caused by nuclear forces. The slowing of such processes to time intervals ≥ 10–12 sec is caused by the presence of potential barriers (coulombic and centrifugal), which hinder the escape of nuclear particles.
Two-step radioactive transformations include processes involving both delayed spontaneous fission and the emission of delayed particles, that is, protons, neutrons, alpha particles, tritium nuclei, and 3/He. Delayed processes involve beta decay as a preliminary step, which delays the subsequent, instantaneous emission of nuclear particles. Thus, in the case of two-step processes, the criterion for radioactivity in terms of lifetime is satisfied for the first step only, which occurs as a result of weak interactions.
Historical Outline
The discovery of radioactivity dates back to 1896, when Anoine Henry Becquerel found that uranium emits an unknown type of penetrating radiation, which he termed radioactive. Soon afterward, the radioactivity of thorium was discovered, and in 1898, Pierre Curie and his wife, Marie, discovered two new radioactive elements—polonium and radium. The work of these scientists, together with that of E. Rutherford, established the existence and explained the nature of three types of emissions of radioactive elements—alpha rays, beta rays, and gamma rays. In 1903, Rutherford and F. Soddy found that the emission of alpha rays is accompanied by the transmutation of chemical elements, for example, the conversion of radium to radon. In 1913, K. Fajans (Germany) and Soddy independently formulated the displacement laws characterizing the shifting of an isotope in the periodic system of elements under the various radioactive transformations.
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| A schematic picture of radioactivity, with the two types of particles (proton and neutron), and the three types of radiation (alpha, beta, and gamma ray). |