Required Practical: Inverse Square-Law for Gamma Radiation (AQA A Level Physics)

Aim of the Experiment

• The aim of this experiment is to verify the inverse square law for gamma radiation of a known gamma-emitting radioactive source

Variables

• Independent variable = the distance between the source and detector, x (m)
• Dependent variable = the count rate / activity of the source, C
• Control variables
  • The time interval of each measurement
  • The same thickness of aluminium foil
  • The same gamma source
  • The same GM tube

Equipment List

• Resolution of equipment:
  • Metre ruler = 1 mm
  • Stopwatch = 0.01 s

Method

Set up for inverse-square law investigation

1. Measure the background radiation using a Geiger Muller tube without the gamma source in the room, take several readings and find an average
2. Next, put the gamma source at a set starting distance (e.g. 5 cm) from the GM tube and measure the number of counts in 60 seconds
3. Record 3 measurements for each distance and take an average
4. Repeat this for several distances going up in 5 cm intervals

• A suitable table of results might look like this:

Analysing the Results

• According to the inverse square law, the intensity, I, of the gamma radiation from a point source depends on the distance, x, from the source

• Intensity is proportional to the corrected count rate, C, so:

• Rearranging this equation gives:

• Comparing this to the equation of a straight line, y = mx
  • y =
  • x =  (m)
  • Gradient = constant, k

1. Subtract the background radiation from each count rate reading to give the corrected count rate, C
2. Plot a graph of against distance 
3. If it is a straight-line graph, this shows they are directly proportional, and the inverse square relationship is confirmed

Graph for Inverse Square Law Experiment

A straight-line graph verifies the inverse square relationship. If the line does not go through the origin, this indicates the presence of a systematic error in the measurement of distance

Evaluating the Experiment

Systematic errors:

• The main source of systematic error in this experiment is in the measurement of distance
  • It is unlikely that the source is at the end of the tube, and it is unlikely that the detector is at the end of the GM tube
  • This means the measured distance is likely to be smaller than the actual distance
  • By plotting a graph of  against , this discrepancy can be easily read off the graph where the line meets the negative x-axis

The exact positions of the gamma source and the detector in their sealed tubes are not known, so this gives rise to a systematic error in the measurement of distance

• The Geiger counter may suffer from an issue called “dead time”
  • This is when multiple counts happen simultaneously within ~100 μs and the counter only registers one
  • This is a more common problem in older detectors, so using a more modern Geiger counter should reduce this problem

• The source may not be a pure gamma emitter
  • To prevent any alpha or beta radiation from being measured, the Geiger-Muller tube should be shielded with a sheet of 2–3 mm aluminium
  • If alpha or beta emissions make it to the detector, this is likely to affect the shape of the graph, making it curve slightly

Random errors:

• Radioactive decay is random, so repeat readings are vital in this experiment
• Measure the count over the longest time span possible
  • A larger count helps reduce the statistical percentage uncertainty inherent in smaller readings
  • This is because the percentage error is proportional to the inverse-square root of the count

Safety Considerations

• For the gamma source:
  • Reduce the exposure time by keeping it in a lead-lined box when not in use
  • Handle with long tongs
  • Do not point the source at anyone and keep a large distance (as activity reduces by an inverse square law)

• Safety clothing such as a lab coat, gloves and goggles must be worn