Radionuclides, also known as radioactive isotopes or radioisotopes, are atoms characterised by an unstable nucleus that spontaneously transforms, or 'decays', into a more stable configuration. During this decay process, the radionuclide emits energy in the form of ionising radiation, such as alpha particles, beta particles, gamma rays, or neutrons. This phenomenon is a cornerstone of nuclear physics and has profound implications across various scientific and industrial disciplines, including nuclear energy generation, medical diagnostics, and environmental monitoring.
The fundamental characteristic of a radionuclide is its nuclear instability. This instability typically arises from an imbalance in the number of protons and neutrons within the atomic nucleus. To achieve a more stable state, the nucleus undergoes one of several types of radioactive decay:
Alpha Decay (): The emission of an alpha particle, which consists of two protons and two neutrons (a helium nucleus). This reduces the atomic number by two and the mass number by four.
Beta Decay (): Involves the transformation of a neutron into a proton ( decay), or a proton into a neutron ( decay or positron emission). Electron capture is another form of beta decay. These processes change the atomic number but not significantly the mass number.
Gamma Decay (): Often follows alpha or beta decay when the nucleus is left in an excited energy state. The nucleus releases excess energy as high-energy photons (gamma rays) without changing its composition.
Each radionuclide decays at a specific, characteristic rate, quantified by its half-life (). The half-life is the time required for half of the radioactive atoms in a sample to decay. The decay process follows first-order kinetics, expressed by the equation:
where is the number of radioactive nuclei remaining after time , is the initial number of nuclei, and is the decay constant, related to the half-life by
.
Radionuclides originate from both natural and artificial sources:
Primordial Radionuclides: These have existed since the Earth's formation and possess very long half-lives, comparable to the age of the Earth. Examples include Uranium-238 (), Thorium-232 (), and Potassium-40 (). Their decay series produce many other shorter-lived radionuclides, such as Radon-222 ().
Cosmogenic Radionuclides: Continuously produced in the Earth's atmosphere due to interactions of cosmic rays with atmospheric gases. Key examples include Carbon-14 () and Tritium ().
These are man-made and typically produced through nuclear reactions in reactors, accelerators, or during nuclear weapons testing. Examples include:
Fission Products: Generated during nuclear fission reactions in reactors, such as Caesium-137 (), Strontium-90 (), and Iodine-131 ().
Activation Products: Formed when stable materials become radioactive after absorbing neutrons, for instance, Cobalt-60 () used in radiotherapy.
Medical and Industrial Isotopes: Specifically produced for diagnostic imaging (e.g., Technetium-99m, ) or industrial applications.
Radionuclides have diverse applications, ranging from electricity generation in nuclear power plants and medical diagnostics (e.g., PET scans) to sterilisation of medical equipment and archaeological dating. However, their ionising radiation poses health risks, necessitating stringent radiation-safety measures. Proper handling, shielding, and disposal programmes are crucial to minimise exposure and protect human health and the environment from their potential hazards.