Atoms and radiation

• In 1804, John Dalton decided that all matter was made up of tiny particles called atoms.
• J.J Thomson discovered the electron in 1897. This lead to the plum pudding model - a ball of positive charge with negative electrons embedded in it
• Ernest Rutherford's gold foil experiment showed that most mass is concentrated at the centre of an atom (the nucleus)
• Niels Bohr proposed that electrons orbit the nucleus in shells. Further evidence suggested that the nucleus contained positively charged particles called protons
• In 1932, James Chadwick discovered the neutron, which was a particle that had a mass but no charge.
• If an electron gains energy by absorbing EM radiation, they more to a higher energy level further from the nucleus.
• If they lose energy and release EM radiation, the atom moves to a lower energy level closer to the nucleus
• Isotopes are atoms of the same element with different numbers of neutrons.
• Alpha Particles

  • Helium nuclei (Two neutrons and two protons)
  • Not very penetrating - stopped by paper
  • Strongly ionising
• Beta particles

  • High speed electron
  • Moderately penetrating - stopped by aluminium
  • Moderately ionising
  • For every beta particle emitted, a neutron in the nucleus turns into a proton
• Gamma rays

  • EM waves with a short wavelength
  • Very highly penetrating - stopped by concrete or lead
  • Very weakly ionising as they pass through rather than collide with atoms
• During alpha decay, its atomic number reduces by 2 and its mass number reduces by 4
• During beta decay, the atomic number increases by 1
• The half-life is the time taken for one half of the radioactive material to decay
• The count rate is the number of radiation counts per second
• A Geiger counter measures the intensity of radiation using a scintillation detector which produces light when it detects radiation
• Sources of background radiation

  • Naturally occurring unstable isotopes
  • Cosmic rays from the sun
  • Radiation due to human activity
• Exposure to radiation is irradiation, this happens when an object is exposed to a source of radiation