Radioactive decay

Radioactive decay
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Name	Radioactive Decay
Caption	Diagram of alpha decay, a common decay process.

Radioactive decay. It is the spontaneous process by which an unstable atomic nucleus loses energy by emitting radiation. This fundamental nuclear transformation was first observed by Henri Becquerel in 1896 while studying uranium salts, a discovery that ushered in the field of nuclear physics. The phenomenon is governed by quantum mechanics and is characterized by a statistical, exponential law, with the rate of decay for a given isotope expressed as its half-life.

Overview of radioactive decay

The process is an intrinsic property of certain nuclides, driven by the quest for greater nuclear stability. It results in the transmutation of an element into a different isotope or a completely different element, as famously demonstrated by Ernest Rutherford and Frederick Soddy. The energy released during decay, known as decay energy, originates from the conversion of mass, as described by Albert Einstein's mass-energy equivalence principle. This energy is carried away by the emitted particles or gamma rays, and its study is central to fields like geochronology and nuclear medicine.

Types of radioactive decay

The primary modes are classified by the type of radiation emitted. Alpha decay involves the ejection of a helium-4 nucleus, commonly observed in heavy elements like radium and plutonium. Beta decay encompasses processes where a neutron converts to a proton (β⁻ decay, emitting an electron and an antineutrino) or a proton converts to a neutron (β⁺ decay or electron capture), as seen in isotopes of carbon-14 and potassium-40. Gamma decay involves the emission of high-energy photons from an excited nucleus, often following other decay types, a process studied extensively at institutions like CERN. Other types include spontaneous fission, prevalent in californium, and cluster decay.

Decay processes and mechanisms

These transformations are mediated by the fundamental interactions. Alpha decay is a quantum tunneling process through the nuclear potential barrier, a concept explained by George Gamow. Beta decay is governed by the weak interaction, involving the transformation of quark flavors via the exchange of W and Z bosons. Gamma decay results from the electromagnetic interaction as the nucleus relaxes from an excited state to a lower energy level. The theoretical framework for these processes was significantly advanced by the work of Maria Goeppert-Mayer on nuclear shell structure and Enrico Fermi's theory of beta decay.

Mathematics of radioactive decay

The decay of a population of radioactive atoms is described by an exponential decay law: *N(t) = N₀ e^{-λt}*, where λ is the decay constant. The half-life (t₁/₂), the time for half the atoms to decay, is related by t₁/₂ = ln(2)/λ. This statistical law was first formulated by Soddy and Rutherford. The activity, measured in becquerels or curies, is the rate of decays per unit time. These mathematical relationships are essential for techniques like radiocarbon dating, developed by Willard Libby, and for safety calculations at facilities like the Chernobyl Nuclear Power Plant.

Occurrence and applications

Naturally occurring decay is responsible for the Earth's internal heat from isotopes like uranium-238, thorium-232, and potassium-40. It is harnessed in nuclear power reactors, such as those at the Fukushima Daiichi Nuclear Power Plant, and in nuclear weapons like those developed during the Manhattan Project. In medicine, isotopes like technetium-99m are used in single-photon emission computed tomography (SPECT) imaging, and cobalt-60 sources are used in radiation therapy for cancer treatment. The Voyager program spacecraft use radioisotope thermoelectric generators powered by plutonium-238 decay.

Decay chains and series

Many heavy radioactive nuclides decay through a sequence of intermediate states called a decay chain or series until a stable isotope is reached. The four naturally occurring series originate from uranium-238 (the uranium series, ending in lead-206), uranium-235 (the actinium series, ending in lead-207), thorium-232 (the thorium series, ending in lead-208), and the extinct neptunium-237 series. These chains involve multiple alpha and beta decays and are crucial for understanding nucleosynthesis in supernovae and for radiometric dating methods used on rocks from the Grand Canyon or Moon rocks retrieved by the Apollo program. Category:Nuclear physics Category:Radioactivity