7. Radioactivity

• Atom


  Positively charged nucleus made of protons and neutrons, surrounded by negatively charged electrons orbiting the nucleus
• Nucleus


  • Radius is a lot smaller than the radius of the entire atom, almost all the mass of the atoms lies in the nucleus
• Isotopes


  Forms of an element's atom with the same number of protons but a different number of neutrons
• Nuclide


  Distinct nucleus, where X is the symbol of the element, A is the mass (nucleon) number, and Z is the atomic (proton) number
• Radioactive decay


  Spontaneous transformation of an unstable nucleus into a more stable one by the release of radiation, a random process
• Alpha decay


  A heavy nucleus emits an alpha particle (helium nucleus), the nucleus changes to that of a different element
• Beta decay


  A neutron turns into a proton and emits a beta particle (electron), the nucleus changes to that of a different element
• Gamma emission


  After a previous decay, a nucleus with excess energy emits a gamma particle (electromagnetic radiation)
• Neutron radiation


  In neutron-rich nuclides, one or more neutrons are ejected, the nucleus becomes a new isotope of the original element
• Particles


  • Proton (relative mass 1, relative charge +1), Neutron (relative mass 1, relative charge 0), Electron (relative mass 0.0005, relative charge -1)
• Radiation detection methods


  • Photographic film, Geiger-Muller tube
• Background radiation


  Weak radiation that can be detected from external sources, including cosmic rays, radioactive rocks, food/drink, fallout, medical sources, nuclear power plants
• Activity


  The number of decays which occur per unit time, measured in becquerels (Bq where 1 Bq = 1 decay per second)
• Half-life


  The time taken for half the nuclei to decay, or the time taken for the activity to halve, it is different for different radioactive isotopes and is constant
• Industrial uses of radioactivity

  • Smoke detectors, thickness monitoring
• Medical uses of radioactivity

  • Sterilisation of equipment, diagnosis and treatment
• Contamination


  When a radioactive source has been introduced into or onto an object, the contaminated object will be radioactive for as long as the source is in or on it
• Irradiation


  When an object is exposed to a radioactive source which is outside the object, the irradiated object does not become radioactive
• Effects of radiation exposure

  • Can destroy living cell membranes by ionisation, causing the cells to die, or damage DNA which causes mutations that could lead to cancer
• Radiation safety measures


  • Minimising time of exposure, keeping big distance from source, using shielding, carefully disposing of radioactive waste
• Nuclear fission


  The process of splitting a nucleus, when a uranium-235 nucleus absorbs a thermal neutron, it splits into two daughter nuclei and 2 or 3 neutrons, releasing energy in the process
• Nuclear reactor


  • Control rods absorb neutrons to keep the number of neutrons such that only one fission neutron per event goes on to induce further fission, moderator slows down neutrons, coolant prevents overheating, reactor core is a thick steel vessel in a building with thick reinforced concrete walls that act as radiation shields
• Nuclear fusion


  The process of fusing two nuclei to form a larger nucleus, there is a small loss of mass accompanied by a release of energy, how the sun and other stars release energy
• Nuclear fusion does not happen at low temperatures and pressures because the electrostatic repulsion of the protons is too great