Inverse Square Law for Gamma Radiation (AQA A-Level Physics): Revision Notes

Method

1. Measure Background Radiation:

• Place the Geiger counter far from the gamma source. Start the stopwatch and measure the total count over 20 minutes. This is the background count rate.

2. Set Initial Distance:

• Position the gamma source 0.600 m from the Geiger counter. Using the metre ruler, carefully measure the distance $X$ between the source and the Geiger counter.
• Start the stopwatch, then use the Geiger counter to record the count $N$ over 5 minutes.

3. Vary the Distance:

• Move the source closer to the Geiger counter in 0.100 m increments, reducing $X$ each time down to a minimum distance of 0.100 m.
• For each new distance, measure the count $N$ over 5 minutes.

4. Repeat for Reliability:

• Repeat the experiment twice more, then calculate the mean count rate $C$ for each distance.

Safety

• Radiation Precautions:
• Exposure to gamma radiation can be harmful. Always handle the source with tongs and do not allow it to touch your skin.
• Keep a safe distance from the source whenever possible, and keep the source in a lead-lined container when not in use to minimise exposure.

Key Concepts

• Inverse-Square Law: The intensity of gamma radiation from a point source decreases in proportion to the square of the distance from the source. This law is applicable for any point source that spreads uniformly in all directions.
• Background Radiation Correction: Subtracting background radiation from each measurement provides an accurate reading of the gamma source's contribution alone.
• Practical Verification: Plotting $\frac{1}{C'}$ vs. $X$ and obtaining a straight line demonstrates that$C \propto \frac{1}{X^2}$, supporting the inverse-square law for radiation.