Physics

• Atom

  Made up of electrons, protons and neutrons
• Electrons

  • Negatively charged
• Protons

  • Positively charged
• Neutrons

  • Neutral
• The atom is overall neutral since it always has the same number of protons and electrons
• In most elements, the protons and neutrons are bound together to form a stable nucleus
• In some elements, the arrangement of the protons and neutrons is not stable, leading to the emission of nuclear radiation
• Radioactive decay

  Unstable nuclei emit nuclear radiation in the form of alpha particles, beta particles or gamma rays in an attempt to become more stable
• Types of nuclear radiation

  • Alpha particles
  • Beta particles
  • Gamma rays
• Alpha particle

  Made up of two protons and two neutrons, same as the nucleus of a helium atom
• Alpha particle

  • Large mass compared to beta particles and gamma rays
  • Positive charge of +2
  • Range in air is 3-5 cm
  • Absorbed by a single sheet of paper
• Beta particle

  A fast moving electron
• Beta decay

  Neutron converted into a proton and electron, electron emitted as beta particle
• Beta particle

  • Small mass compared to alpha, larger than gamma
  • Negative charge of -1
  • Range in air up to a few meters
  • Absorbed by 2-3 mm of aluminium
• Gamma ray

  Electromagnetic wave of very high frequency and energy
• Gamma rays are emitted when the nucleus of an atom undergoes changes to become more stable
• Alpha and beta particles are particles, gamma rays are electromagnetic waves
• Animations of alpha, beta and gamma radiation

  • Alpha particle emitted from nucleus
  • Beta particle emitted from nucleus
  • Gamma ray emitted from nucleus
• Neutral atom

  Number of orbiting electrons is the same as the number of protons in the nucleus
• Ionization

  Addition or removal of an electron from a neutral atom to create an ion
• Positive ion

  Atom loses an electron
• Negative ion

  Atom gains an electron
• Alpha, beta and gamma radiation are all types of ionizing radiation
• Ionizing radiation can cause damage to living tissue by depositing energy in the material it passes through
• Alpha particles


  • Have a large mass compared to beta particles and gamma rays
  • Are the most ionizing type of radiation
  • Can do the most damage to body tissue when inside the body
  • Are stopped by a single sheet of paper, so have low penetrating ability
• How alpha particles cause ionization

  Positively charged alpha particle attracts a negatively charged orbiting electron away from an atom
• Beta particles


  • Are more ionizing than gamma rays but less ionizing than alpha particles
  • Can penetrate the skin but cannot penetrate the whole body like gamma rays
• How beta particles cause ionization

  Negatively charged beta particle repels a negatively charged orbiting electron away from an atom
• Gamma rays


  • Are the least ionizing but the most penetrating type of radiation
  • Are considered the most dangerous when outside the body as they can easily penetrate body tissue
• How gamma rays cause ionization

  Gamma ray is absorbed by an atom and gives its energy to one of the orbiting electrons, allowing the electron to leave the atom
• Ionizing and penetrating ability of radiation types

  • Alpha: High ionizing, low penetrating
  • Beta: Medium ionizing, medium penetrating
  • Gamma: Low ionizing, high penetrating