Radioactive

• Unstable Nuclei


  Nuclei that will break down to become more stable. Their instability could be caused by having too many neutrons, not enough neutrons, or just too much energy in the nucleus.
• Radioactive decay


  Nucleus decays by releasing energy and/or particles, until it reaches a stable form
• Ionising radiation


  Radioactive emissions that can knock off electrons, creating an ion
• An individual radioactive decay is random - it can't be predicted
• Four Types of Nuclear Radiation

  • Alpha
  • Beta-minus (Beta)
  • Beta-plus
  • Gamma
• Alpha


  A helium nucleus - 2 protons & 2 neutrons, Relative Charge +2, Mass (u) 4
• Beta-minus (Beta)


  Electron, Relative Charge -1, Mass (negligible)
• Beta-plus


  Positron, Relative Charge +1, Mass (negligible)
• Gamma


  Short-wavelength, high-frequency electromagnetic wave, Relative Charge 0, Mass 0
• Different types of radiation have different penetrating powers, and so can be stopped by different types of materials
• Investigating Radiation Types


  1. Record background radiation count rate

  2. Place unknown source near Geiger counter and record count rate

  3. Place sheet of paper between source and Geiger counter, record count rate

  4. Repeat with 3 mm thick sheet of aluminium
• Radiation Types


  • Alpha - Strongly ionising, Slow speed, Absorbed by paper or a few cm of air
  • Beta-minus (Beta) - Weakly ionising, Fast speed, Absorbed by -3 mm of aluminium
  • Beta-plus - Very weakly ionising, Speed of light, Annihilated by electron-positron annihilation
  • Gamma - Weakly ionising, Absorbed by many cm of lead, or several m of concrete
• Alpha particles can easily pull electrons off atoms, causing about 10,000 ionisations per mm in air for each alpha particle
• Alpha particles quickly ionise many atoms and lose all their energy, making alpha-sources suitable for use in smoke alarms
• Alpha particles are dangerous if ingested as they quickly ionise body tissue in a small area, causing lots of damage
• Beta particles ionise about 100 atoms per mm in air, causing much less damage to body tissue than alpha particles
• Beta radiation is commonly used for controlling the thickness of a material
• Gamma Radiation


  • Even more weakly ionising than beta radiation, so will do even less damage to body tissue
  • Can be used in medicine for diagnosis and treatment of cancer
• Radiation use in medicine has benefits and risks, the key is trying to use methods which reduce the risks while still giving the desired results
• Sources of Background Radiation

  • Radioactive radon gas in the air
  • Radioactive isotopes in the ground and buildings
  • Cosmic radiation from space
  • Radioactive carbon-14 and potassium-40 in living things
  • Radiation from medical or industrial sources
• Inverse Square Law


  The intensity of gamma radiation decreases by the square of the distance from the source
• Investigating the Inverse Square Law

  1. Set up Geiger counter and measure background radiation

  2. Place radioactive source at distance d and measure count rate

  3. Double the distance to 2d and measure count rate again

  4. Repeat for 3d, 4d etc.

  5. Correct data for background radiation and plot graph - should show intensity drops to a quarter when distance doubles
• Radioactive decay is completely random, you can't predict which atom's nucleus will decay when
• Decay Constant


  The probability of a given nucleus decaying per second
• Half-Life


  The average time it takes for the number of unstable nuclei to halve
• The longer the half-life of an isotope, the longer it stays radioactive
• Becquerel (Bq)


  Unit of activity, 1 Bq = 1 decay per second
• Decay constant (A)


  Probability of a given nucleus decaying per second, unit is s⁻¹
• Half-life (T½)


  Average time it takes for the number of unstable nuclei to halve
• Uses of radioactive substances

  • Radioactive dating
  • Medical diagnosis
  • Sterilisation
  • Smoke alarms
• Long half-life radioactive substances can be dangerous in waste products
• Standard nuclear notation


  • Proton number (Z)
  • Nucleon number (A)
  • Element symbol
• Nuclei are unstable if they have too many/few neutrons, too many nucleons, or too much energy
• Alpha decay


  1. Proton number decreases by 2

  2. Nucleon number decreases by 4
• Beta-minus decay


  1. Proton number increases by 1

  2. Nucleon number stays the same
• Beta-plus decay


  1. Proton number decreases by 1

  2. Nucleon number stays the same
• Gamma radiation is emitted when a nucleus has excess energy after alpha or beta decay
• Isotopes of carbon


  • ⁶C, ¹²C, ¹³C
• Factors affecting nuclear stability

  • Too many neutrons
  • Too few neutrons
  • Too many nucleons
  • Too much energy
• Spontaneous fission limits the number of nucleons a nucleus can contain