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

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

• Resolution of measuring equipment:

  • Ruler = 1 mm

  • Geiger-Müller tube = 0.01 μS/hr

Method

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