Radioactive decay, also known as nuclear decay or radioactivity, is the spontaneous process through which an unstable atomic nucleus loses energy by emitting radiation. This transformation results in the formation of a different atomic nucleus, which may itself be stable or further radioactive. Understanding radioactive decay is central to nuclear science, particularly in the context of nuclear reactor safety, waste management, and medical applications.
Unstable nuclei, known as radionuclides, undergo various types of decay, each characterised by the emission of specific particles or electromagnetic radiation. The primary modes of decay include:
Alpha decay occurs predominantly in heavy nuclei. An alpha particle, identical to a helium-4 nucleus (He), consisting of two protons and two neutrons, is emitted. This process reduces the parent nucleus's atomic number () by two and its mass number () by four, transforming it into a different element:
Beta decay involves the transformation of a neutron into a proton or vice-versa within the nucleus. There are three main forms:
Beta-Minus () Decay: A neutron converts into a proton, emitting an electron ( or particle) and an electron antineutrino (). The atomic number increases by one, while the mass number remains unchanged.
Beta-Plus () Decay: A proton converts into a neutron, emitting a positron ( or particle) and an electron neutrino (). The atomic number decreases by one, while the mass number remains unchanged.
Electron Capture (EC): An atomic electron is captured by a proton in the nucleus, converting it into a neutron and emitting an electron neutrino. This has the same net effect on the atomic number as decay.
Gamma decay involves the emission of high-energy photons (gamma rays) from an excited nucleus following an alpha or beta decay event. It does not change the atomic or mass number of the nucleus; it merely releases excess energy as the nucleus transitions from an excited state to a lower energy state or its ground state. The excited state is often denoted with an asterisk ( for metastable):
Radioactive decay is a random and probabilistic process for individual nuclei, but for a large sample, it follows a predictable exponential law. The rate of decay, known as activity (), is proportional to the number of radioactive nuclei () present:
Where is the decay constant, a unique value for each radionuclide. The number of remaining nuclei at time can be calculated using:
Where is the initial number of nuclei. Similarly, the activity at time is:
The half-life is the time required for half of the radioactive nuclei in a sample to decay. It is a fundamental characteristic of a radionuclide and is related to the decay constant by:
Understanding radioactive decay is vital for radiation protection and nuclear waste management. The type of radiation emitted, its energy, and the half-life of radionuclides dictate the biological hazard, shielding requirements, and the long-term management strategies for radioactive materials, including spent nuclear fuel.