Radioactivity refers to the spontaneous process by which an unstable atomic nucleus loses energy by emitting radiation. This transformation, known as radioactive decay, results in the nucleus changing into a more stable configuration, often forming a different element or an isotope of the same element. Discovered by Henri Becquerel in 1896, radioactivity is a cornerstone of nuclear physics and underpins the operation of nuclear reactors, medical diagnostics, and various industrial applications.
The primary modes of radioactive decay involve the emission of specific particles or electromagnetic energy:
Alpha decay occurs in heavy nuclei where an alpha particle (a helium nucleus, consisting of two protons and two neutrons) is emitted. This reduces the atomic number by two and the mass number by four. Alpha particles are relatively large and heavy, meaning they have a short range in matter and can be stopped by a sheet of paper or the outer layer of skin.
Beta decay involves the transformation of a neutron into a proton, or vice versa, within the nucleus, leading to the emission of a beta particle (an electron or a positron).
Beta-minus () Decay: A neutron converts into a proton, emitting an electron () and an antineutrino (). The atomic number increases by one, while the mass number remains unchanged.
Beta-plus () Decay (Positron Emission): A proton converts into a neutron, emitting a positron () and a neutrino (). The atomic number decreases by one, and the mass number remains unchanged.
Beta particles are lighter and have a longer range than alpha particles, requiring thicker materials like aluminium to block them.
Gamma decay involves the emission of high-energy photons (gamma rays) from an excited atomic nucleus. This process typically follows other decay modes, as the nucleus settles from an excited energy state to a lower, more stable state. Gamma rays are a form of electromagnetic radiation, similar to X-rays but typically of higher energy. They have no mass or charge and are highly penetrating, requiring dense materials like lead or concrete for effective shielding.
The half-life of a radioactive isotope is the time required for half of the initial quantity of radioactive nuclei in a sample to decay. It is a fundamental characteristic of each radioisotope and varies widely, from fractions of a second to billions of years. Half-life is crucial for characterising the longevity of radioactive materials, their potential hazards, and for applications such as carbon-dating or medical isotope usage.
The decay of radioactive nuclei follows an exponential law, described by:
where is the number of radioactive nuclei remaining at time , is the initial number of nuclei, and is the decay constant. The half-life is related to the decay constant by:
Activity is the rate at which radioactive nuclei decay in a sample. It is measured in Becquerel (Bq), where 1 Bq equals one disintegration per second, or in Curie (Ci), where . Activity is directly proportional to the number of radioactive atoms present:
Understanding radioactivity is paramount for nuclear safety. The ionising radiation emitted during decay can cause biological damage, necessitating stringent radiation-protection-principles and robust radioactive-waste-management strategies. Conversely, controlled utilisation of radioactivity is indispensable for nuclear power generation, medical diagnostics and therapies, industrial gauging, and scientific research. The careful management and comprehension of this natural phenomenon are critical for harnessing its benefits while mitigating its risks.