Radioactivity

A2 AQA unit 5 syllabus Radioactivity revision points

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Rutherford study

  • Beam of positively charged alpha particles directed at gold film. A detector is placed on the radius of a circle outlining the experiment.
    • This detector is moved manually around the circle to allow it to detect either:
      • particles that go striaght through
      • particles that are deflected at an angle
      • particles that reflect back off the gold foil.
  • Results:
    • most go straight through or are deflected at an angle less than 90 degrees
    • very few bounce back
  • Explanation:
    • most of atom's mass is cincentrated in the centre, the nucleus.
    • nucleus is positiveky charged because the positively charged alpha particles are deflected.
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Alpha partical properties

  • Easily absorbed
  • positive charged
  • Causes strong ionisation (100000 ions per mm at standard pressure)
  • Easily deflected
  • Leaves stright tracks in cloud chamber
  • Few cm range in air
  • stucture consists of 2 protons and 2 neutrons
  • stopped by paper of thin metal foil
  • energy per particle is constant for a given source.
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Beta properties

  • more penetrating than alpha
  • negative charge
  • weaker ionisation ability than alpha (100 ions per mm in air at stand pressure)
  • Deflected in opp direction than alpha particles
  • Less easily dieflected than alpha particles
  • +ve beta emission = electron
  • -ve beta emission = positron
  • Leaves wispy tracks in cloud chamber
  • range up to 1m in air
  • structure: fast moving electron
  • stopped by 5mm aluminium
  • energy per particle varies up to a max for a given source
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Gamma properties

  • Most penetrating
  • uncharged
  • very weak ionising effect
  • magnetic field have no effect
  • No tracks in cloud chamber
  • high energy particle
  • unlimited range in air (inverse square law)
  • consists of photons
  • stopped by several cm of lead
  • constant energy per particle for a given source.
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Ionisation experiment

  • Experiment setup:
    • a source emits into an ionisation chamber, surrounded by an insulated electrode wall.
    • Opposite the source is a central electrode which is wired up to a picoammeter.
  • The chamber
    • air at atm pressure
    • ions created in chamber are attracted to the opp charged electrode where they are discharged
    • current produced is proportional to the no of ions per second created in chamber.
  • Results:
    • Alpha
      • strong ionisation
      • source has to be within a few cm of detector
    • beta
      • weaker ionising effect
      • produces fewer ions per mm along its path than alpha particle
    • gamma
      • weaker ionising effect
      • photons carry no charge so less effect than alpha and beta cells.
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