Required Practicals / OCR A / Practical 10
10 A2 PAG 10

Inverse square law for gamma radiation (PAG 10)

Verify the inverse square law for gamma radiation by measuring corrected count rate at different distances.

Apparatus

  • Sealed gamma source (Cs-137 or Co-60) in shielded container
  • Geiger-Muller tube and counter
  • Metre rule and retort stand
  • Stopwatch

Safety

  • Handle source with tongs only; never bare hands.
  • Store in shielded container when not in use.
  • Complete radiation risk assessment; minimise exposure time.

Method

  1. Measure background count rate B without the source (count for 5 minutes).
  2. Position source at distance x. Count for at least 2 minutes; corrected count rate $I = C - B$.
  3. Repeat for eight to ten distances.
  4. Plot I vs $1/x^2$: straight line through origin confirms inverse square law.
  5. Also check: plot $\ln I$ vs $\ln x$; gradient should be $-2$.

Key Variables

Independent Distance x
Dependent Corrected count rate I
Controlled Same source; Same GM tube; Background measured separately

Analysis and Results

  • $I = k/x^2$. Plot I vs $1/x^2$: straight line through origin.
  • Plot $\ln I$ vs $\ln x$: gradient $= -2$ confirms the inverse square law.
  • Background subtraction is essential to avoid a non-zero intercept.

Common Errors

  • Forgetting to subtract background count rate.
  • Measuring x from the lead shielding rather than from the source.
  • Too short a counting time at large distances.

Exam-style questions on this practical. Click Show mark scheme to reveal the answer after attempting each question.

Q1 3 marks

The corrected count rate at 20 cm is 180 counts per minute. Calculate the expected count rate at 60 cm.

Q2 2 marks

Explain why alpha radiation would not obey the inverse square law at the same distances used in this experiment.