#nuclear physics

• Actinides — Plural form for the neuter noun 'actinid'; a homogeneous structural group in the periodic table consisting of 15 metallic and radioactive chemical elements, with atomic numbers ranging from 89 (actinium) to 103 (lawrencium). Located in the 'f' block of transition elements in the 7th period, they exhibit similar chemical properties due to the progressive filling of the 5f electron subshell, being recognized for their high density, substantial nuclear instability, and capacity to undergo fission.
• Actinon — A historical and scientific term used to designate the radioactive isotope radon-219 (An), a structural intermediate product within the natural decay chain of actinium or uranium-235. It is a heavy, unstable noble gas with an extremely short half-life of approximately 3.96 seconds, emitting high-energy alpha particles before transforming into polonium-215, monitored in nuclear research laboratories for the study of radioactive emanations.
• Activity — A set of physical, intellectual, or physiological acts, manifestations, or processes carried out consciously and systematically by a human being, a collectivity, or a system, with a specific purpose; the dynamic functioning of an organ or mechanism. In physics, it represents the number of nuclear disintegrations produced in a radioactive substance per unit of time.
• Americium — A synthetic, highly radioactive chemical element of the actinide series, with atomic number 95 and symbol Am. This silver-colored structural transuranium metal, originally obtained by bombarding plutonium with neutrons, is utilized in precision industrial applications, such as household smoke detectors and alpha radiation sources for laboratory analyses.
• Antineutrino — The antiparticle corresponding to the neutrino, characterized by an extremely small (nearly zero) mass, a neutral electric charge, and half-integer spin, possessing an opposite helicity (right-handed) compared to that of the neutrino. This subatomic entity is predominantly emitted during beta-minus decay processes, interacting with matter exclusively through the weak nuclear force and gravity, which makes it extremely difficult to detect.
• Antineutron — The antiparticle corresponding to the neutron, possessing exactly the same mass and spin, but characterized by an opposite baryon number and a magnetic moment of opposite sign relative to the neutron. Although globally electrically neutral, its internal structure is composed of antiquarks (two down antiquarks and one up antiquark), determining its specific annihilation behavior upon contact with ordinary baryonic matter.
• Antiparticle — An elementary particle that has exactly the same mass, lifetime, and spin as a corresponding particle of ordinary matter, but whose electric charges, quantum numbers, and magnetic moments have opposite mathematical signs. Upon direct contact between a particle and an antiparticle, the structural phenomenon of mutual annihilation occurs, converting their entire mass into energy in the form of high-frequency photons.
• Heavy water — A form of water in whose structure the light hydrogen atoms (protium) are completely or highly replaced by its heavy isotope named deuterium (D₂O). This structural chemical compound is characterized by a higher density than ordinary water and is widely used in the nuclear industry as a neutron moderator and cooling agent in CANDU-type power reactors.
• Astatine — A highly unstable radioactive chemical element of the halogen group, with atomic number 85 and symbol At; it is the rarest naturally occurring element in the Earth's crust, produced only transiently through the decay of uranium and thorium, and due to its extremely short half-life (a maximum of 8.1 hours for its most stable isotope), its physical properties are studied almost exclusively through artificial synthesis techniques in particle accelerators.
• Autoradiograph — An image, photographic film, or structural digital record obtained through the autoradiography technique, which highlights the spatial distribution of radioactive substances or isotope-labeled molecules (tracers) previously introduced into a biological tissue, cell, or metallurgical material. The term also denotes the apparatus or technical detection assembly utilized in the laboratory to capture and visualize alpha, beta, or gamma radiation emissions originating directly from the analyzed sample.