5.2

• radioactivity arises from unstable nuclei which may occur naturally or be produced in reactors
• count-rates measured from a radioactive source need to be corrected by subtracting the contribution from the background radiation from the readings
• alpha particles:

  • highest mass
  • highest charge
  • highest kinetic energy
  • least penetrating power
  • greatest ionising power
• beta particles:

  • greater penetrating power than alpha
  • kinetic energy is lesser
  • ionising power is lesser
• gamma emissions:

  • no charge
  • almost no mass
  • little interaction with matter
  • highest penetrating power
• in a diffusion cloud chamber:

  • alpha particles - straight thick tracks
  • beta particles - thin stright tracks and slower ones gave short, twisted, thicker tracks
• radioactive decay - emission of an alpha or beta particle from an unstable nucleus
• stable nuclides:

  • number of neutrons = number of protons for the lightest
  • number of neutrons greater than number of protons for the heaviest
• unstable nuclides:

  • disintegration tends to produce new nuclides nearer the stability line and continues until a stable nuclide is formed
  • nuclide above the stability line decays by beta negative emission
  • nuclide below the stability line decays by beta positive emission
  • nuclei with more than 82 protons usually emit an alpha particle when they decay
• half life - time taken for half of the nuclei to decay, half of the original nuclei to decay
• activity - average number of disintegrations per second of a sample is its activity
• safety precautions:

  • exposure time to radiation should be minimised
  • distance between source and a person should be kept as large as possible
  • people should be protected by the use of shielding which absorbs the radiation