Radioactive Decay
Radioactive decay is the process in which an atom emits another particle or particles. A radioactive element exhibits fewer decays per unit of time at an exponential rate. The half-life is how long it takes the element to decay to half its original concentration. This process obeys the known conservation laws, including conservation of mass number at low energy.
Radioactive elements can be found naturally (as in radium or uranium) or artificially created in a laboratory. Pierre and Marie Curie and Antoine-Henri Becquerel were among the first to study naturally occurring radioactivity, in substances such as uranium, radium, thorium, and polonium. They jointly won the Nobel Prize in 1903 for studying spontaneous radioactivity. In 1934, Pierre Joliot and Irene Joliot-Curie were the first to induce radioactivity by bombarding a sheet of aluminum to produce a radioactive isotope of phosphorus.
There are several kinds of radioactive decay. They are classified by the type of particle emitted. The major decay types are alpha, beta, and gamma decay. Nuclear fission is sometimes also included in types of decay, but it is usually a large nucleus splitting into two roughly equally sized nuclei, rather than one atom emitting a much smaller particle.
Alpha decay occurs when a larger nucleus emits a Helium-4 nucleus, called an alpha-particle. This occurs when the nucleus would be more stable with fewer protons and neutrons. The energy released when an alpha particle is emitted is equal to the binding energy of that alpha particle-that is, the amount of energy it took for the particle to stay with the rest of the nucleus.
Beta decay occurs when there is an imbalance of protons and neutrons in the nucleus. A neutron can decay to a proton and an electron. In this case, the proton will stay in the nucleus, and the electron (or beta-minus particle) will emerge. Alternately, a proton can decay to a neutron and a positron (or beta-plus particle). Here, the positron will be ejected. In many cases, electron capture (pulling an electron into the nucleus, where it joins with a proton to become an electron) is classified as beta decay as well. All of these processes are mediated by the weak nuclear force.
Gamma decay puts a nucleus in a lower, more favorable energy state. The gamma-particle is actually a photon with about the same amount of energy as is lost by its parent atom. Any atom that can stay together in excited states can exhibit gamma emission.
Radioactive elements may emit many particles before reaching a stable energy. This series of decays can be used to identify an unknown radioactive element, since different nuclei emit at different energies. Radioactive decay can also be used to date a sample of a material by the proportion of radioactive nuclei left and a knowledge of those substances' half-lives.
Radioactive decay has uses in many modern technologies, including medical technologies, nuclear power, and nuclear weapons. Since the beginning of the twentieth century, great progress has been made in discovering both the physical mechanisms and the applications of radioactive decay.
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