Core Practical 15: Investigating Gamma Radiation Absorption (Edexcel International A Level Physics)

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Core Practical 15: Investigating Gamma Radiation Absorption

Aim of the Experiment

  • To investigate the absorption of gamma rays by different thicknesses of lead

Variables:

  • Independent variable = Thickness of lead
  • Dependent variable = Count rate
  • Control variables:
    • Radioactive source
    • Distance of GM tube to source
    • Location / background radiation

Equipment List

11-8-cp15-equipment-table_edexcel-al-physics-rn

11-8-cp15-apparatus_edexcel-al-physics-rn

  • Resolution of measuring equipment:
    • Ruler = 1 mm
    • Geiger-Müller tube = 0.01 μS/hr

Method

11-8-cp15-equipment-set-up_edexcel-al-physics-rn

  1. Connect the Geiger-Müller tube to the counter and, without any sources present, measure background radiation over a five-minute period
    • Record this value
    • Calculate the average background rate per minute
  2. Measure the thickness of the lead absorbers using Vernier calipers at three points on each sheet.
    • For each sheet record the average thickness
  3. Place the radioactive source a fixed distance of 10 cm away from the tube
  4. Record the count rate over one minute
  5. Repeat steps 3 and 4 a further two times, recording the count rate each time
  6. Place the thinnest absorber directly in front of the gamma ray source
  7. Repeat steps 3-5
  8. Replace the sheet with another thickness and continue taking three readings per thickness

Analysis of Results

  • If the count over that interval falls to background levels (allow for a little random variation), then the radiation has all been absorbed
  • You will be able to determine the thickness of the lead required to absorb gamma radiation

Evaluating the Experiment

Systematic Errors:

  • Make sure that the source is stored well away from the counter during the experiment
  • Conduct all runs of the experiment in the same location to avoid changes in background radiation levels

Random Errors:

  • The accuracy of such an experiment is improved with using a reliable source of radiation with a long half-life and an activity well above the natural background level

Safety Considerations

  • When not using a source, keep it in a lead-lined container
  • When in use, try and keep a good distance (a metre or so) between yourself and the source
  • When handling the source, do so using tweezers (or tongs) and point the source away from you
  • Wash your hands and remove your outer layer of clothing after handling a radioactive source

Exam Tip

When answering questions about the core practicals you could try to remember the acronym SCREAMS:

  • S: Which variable will you keep the same
  • C: which variable should you change
  • R: what will you do to make your experiment reliable
  • E: what special equipment and equations are required
  • A: how will you analyse your results
  • M: which variable will you measure
  • S: what safety precautions will you take?

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