RP12 - Inverse Square Law for Gamma Radiation

• Safety precautions when working with a radioactive source

  • Limit the time of exposure
  • Display warning signs so people are aware a radioactive source is in use
  • Keep sources an arm's length away from your body at all times, and only ever handle the source using long-handled tongs
• How to handle a radioactive source safely

  1. Use long-handled tongs

  2. Remain at least an arm's length away from it at all times
• How to store a radioactive source safely

  1. Lock it away in a sealed lead-lined container

  2. Display a hazard symbol on the container and at the storage location
• Inverse square law of radiation

  The intensity of radiation is inversely proportional to the square of the distance from the source. As the distance doubles, the intensity quarters.
• Geiger Counter or Geiger-Muller Tube


  Device used to measure a radioactive count rate
• Why is Cobalt-60 a suitable sourcefor this experiment?

  • Safe for use in schools
  • Has a half-life of around 5 years, meaning it can be reused for a number of years
  • The activity is low enough to be safe, but high enough for measurements to be taken easily
• Measuring over a longer period of time

  Reduces measurement uncertainty
• Preliminary recording before bringing radioactive source into lab

  Take background radiation count
• Accounting for background radiation in experimental data

  Subtract background count from counts for each distance to produce corrected counts (C')
• Converting from count to count rate

  Divide count by length of time over which it was taken to acquire the count rate (mean number of counts per unit time)
• Preliminary experiment with source before main experiment

  1. Find maximum distance from source at which you can still get a measurable count

  2. Take recordings closer to source to determine suitable time period over which to take readings
• Graph to confirm inverse square law

  Plot 1/√C' against x - should form a straight line with positive gradient
• Plotting 1/√C' against x removes any systematic error in the distance measurements
• Systematic error in distance measurements

  Location of radioactive source within sealed capsule, and precise location of ionisation in GM tube not known to high precision
• Obtaining systematic distance error from graph of 1/√C' against x

  The straight line will not cross the x-axis at the origin. The difference between x=0m and the point at which the straight line crosses the x-axis will be the systematic error.
• An old gamma source may be unsuitable as its activity may have fallen to a level too low to obtain recordable counts over a large enough range of distances