P7 - Radioactivity

• Radioactive decay


  • When nuclear radiation is emitted by unstable atomic nuclei so that they become more stable
  • A random process
• Radiation can knock electrons out of atoms in a process called ionisation
• Alpha radiation

  • alpha particles emitted
  • nucleus loses two protons and two neutrons
  • highest ionising power
  • travels a few cm in air
  • stopped by a sheet of paper
• Beta radiation


  • an electron is ejected
  • a neutron changes into a proton and an electron
  • higher ionising power than gamma, lower than alpha
  • travels around a meter in air
  • stopped by a sheet of metal
• Gamma radiation


  • short-wavelength, high-frequency electromagnetic radiation
  • some energy transferred away from the nucleus
  • low ionising power
  • virturally unlimited range in air
  • stopped by thick lead or metres of concrete
• Activity


  • the activity of a radioactive source is the rate of decay of an unstable nucleus
  • measured in Bq (becquerels)
  • 1 Bq = 1 decay per second
  • detectors (e.g. a Geiger-Muller tube) record count rate
  • count rate = number of detected decays per second
• Half-life


  • The time for:
  • half the number of unstable nuclei in a sample to decay
  • the count rate or activity of a source to half
  • Can be found from a graph of its count rate or activity against time
• To find the reduction in activity after a given number of half-lives:


  • calculate the activity after each half-life
  • subtract the final activity from the original activity
• Net decline = reduction in activity / original activity
• Irradiation


  • when an object is exposed to nuclear radiation
  • cause harm through ionisation
  • prevented by shielding, removing, or moving away from the radiation source
• Contamination


  • when atoms of a radioactive material are on or in an object
  • cause harm through ionisation
  • object remains exposed to radiation as long as it is contaminated
  • contamination can be very difficult to remove
• Protection against irradiation and contamination


  • maintain a distance from the radiation source
  • limiting time near the source
  • shielding from the radiation
• Ionising radiation


  • Living cells can be damaged or killed by ionising radiation
  • The risk depends on the half-life of the source and the type of radiation
  • Alpha radiation is dangerous inside the body because it affects all the surrounding tissue - outside the body it only affects the skin and eyes because it cannot penetrate further
  • Beta and gamma radiation are danngerous outside and inside the body because they can penetrate into tissues
• Radiation dose

  • Measured in sievert (Sv)
  • Measures the health risk of exposure to radiation
  • Depends on the type and amount of radiation
• Background radiation


  • The radiation that is around us all the time
  • Comes from:
  • natural sources like rocks and cosmic rays
  • nuclear weapons and nuclear accidents
  • Always present but the levels are higher in some locations and in some jobs
• Nuclear radiation in medicine - exploration of internal organs


  • gamma-emitting tracers are injected or swallowed by a patient
  • gamma cameras then create an image showing where the tracer was
  • the half-life of the tracer must be short enough so most of the nuclei decay shortly after the image is taken to limit the patient's radiation dose
• Control or destruction of unwanted tissue

  • 1 - narrow beams of gamma radiation can be focused on tumour cells to destroy them
  • gamma is used because it can penetrate from outside the body
  • 2 - beta or gamma-emitting implants can be placed inside (or next to) tumours
  • their half-lives must be long enough to be effective, but short enough that it does not continue to irradiate the patient after treatment
• Nuclear fission


  • When a large, unstable nucleus absorbs an extra neutron and splits into two smaller nuclei of roughly equal size
  • During nuclear fission:
  • gamma radiation is emitted and energy is released
  • two or three neutrons are emitted that can go on to cause a chain reaction
  • The chain reaction in a power station reactor is controlled by absorbing neutrons
  • Nuclear explosions are uncontrolled chain reactions
  • On rare occasions an unstable nucleus splits apart without absorbing a neutron, which is called spontaneous fission
• Nuclear fusion

  • When two light nuclei join to make a heavier one
  • Some of the mass is converted to energy and transferred as radiation