Unit 8: Nuclear physics

• Activity: The rate of decay of the radioactive nuclei in a given isotope. It is proportional to the total number of nuclei in the sample and is measured in Becquerels.
• Half-Life: The average time it takes for the number of radioactive nuclei in a sample to halve.
• Inverse Square-Law: A law that governs the intensity of gamma radiation. It means that the intensity of radiation at any point is inversely proportional to the square of the distance from its source.
• Irradiation: The exposure of an object to radiation. The exposed object does not become radioactive.
• Mass Defect: The difference in mass between a nucleus and the sum of the masses of its constituent nucleons.
• Alpha Decay: The emission of an alpha particle (2 protons and 2 neutrons) from an unstable nucleus (usually one with too much mass) to make it more stable. Alpha radiation is strongly ionising and is stopped by a few centimetres of air or a sheet of paper.
• Moderator: A material in nuclear reactors that absorbs energy from fast moving neutrons, to slow them down to speeds that can be absorbed by fissile neutrons to induce fission.
• Beta Decay: The emission of a beta particle when a proton turns into a neutron (or vice versa) in an unstable nucleus. Beta minus radiation is weakly ionising. Beta plus radiation is immediately annihilated by electrons.
• Radioactive Dating: The use of radioactive isotopes with known half-lives to date objects. The isotope that is usually used is Carbon-14.
• Atomic Mass Unit: A unit used to express atomic masses. One AMU is equal to the one twelfth of the mass of a carbon atom.
• Background Radiation: Radiation that is found in small quantities all around us. It originates from natural sources such as rocks and cosmic rays as well as man-made sources such as nuclear accidents and medical sources.
• Radioactive Waste: The waste produced from the products of fission reactions. Since the waste is unstable and radioactive, it must be stored and handled carefully.
• Binding Energy: The amount of energy required to split a nucleus into all its separate constituent nucleons. It is equivalent to the mass defect.
• Contamination: The introduction of radioactive material to another object. The object is consequently radioactive.
• Chain Reaction: The process of the neutrons released by a fission reaction inducing further fissile nuclei to undergo fission.
• Control Rods: Rods found in nuclear reactors to absorb neutrons and control the rate of reaction. They can be raised or lowered depending on the rate required.
• Critical Mass: The smallest mass of fissile material required in a fission reactor for a chain reaction to be sustained.
• Coolant: A substance that passes through nuclear reactors and is responsible for removing heat from the core. This heat is then used to generate energy.
• Fission: The splitting a nucleus, to form two smaller daughter nuclei, neutrons and energy.
• Fusion: The joining of two smaller nuclei to form a larger nucleus and to release energy.
• Gamma Decay: The emission of gamma rays from an unstable nucleus that has too much energy. Gamma radiation is only very weakly ionising but requires several centimetres of lead to be stopped.