Principle of radioisotopic dating
Well-known examples are uranium and thorium, but also included are naturally occurring long-lived radioisotopes, such as potassium-40.
If there are multiple particles produced during a single decay, as in beta decay, their relative angular distribution, or spin directions may not be isotropic.Alpha decay is one type of radioactive decay, in which an atomic nucleus emits an alpha particle, and thereby transforms (or "decays") into an atom with a mass number decreased by 4 and atomic number decreased by 2. A radioactive nucleus with zero spin can have no defined orientation, and hence emits the total momentum of its decay products isotropically (all directions and without bias). However, for a collection of atoms, the collection's expected decay rate is characterized in terms of their measured decay constants or half-lives. The half-lives of radioactive atoms have no known upper limit, spanning a time range of over 55 orders of magnitude, from nearly instantaneous to far longer than the age of the universe.Subsequently, the radioactive displacement law of Fajans and Soddy was formulated to describe the products of alpha and beta decay.The early researchers also discovered that many other chemical elements, besides uranium, have radioactive isotopes.Decay products from a nucleus with spin may be distributed non-isotropically with respect to that spin direction, either because of an external influence such as an electromagnetic field, or because the nucleus was produced in a dynamic process that constrained the direction of its spin.Such a parent process could be a previous decay, or a nuclear reaction.), and the process produces at least one daughter nuclide.The energy of an excited nucleus may be emitted as a gamma ray in a process called gamma decay, or that energy may be lost when the nucleus interacts with an orbital electron causing its ejection from the atom, in a process called internal conversion.Another type of radioactive decay results in products that vary, appearing as two or more "fragments" of the original nucleus with a range of possible masses.The first decay processes to be discovered were alpha decay, beta decay, and gamma decay.Alpha decay occurs when the nucleus ejects an alpha particle (helium nucleus).