Lesson Plan

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Lesson Plan
Grade: Date: 17/01/2026
Subject: Physics
Lesson Topic: Use count rate measured in counts/s or counts/minute
Learning Objective/s:
  • Describe what count rate is and the meaning of cps and cpm.
  • Calculate count rate from recorded counts and time interval.
  • Convert between cps and cpm and predict changes using the inverse‑square law.
  • Estimate a source’s activity when detector efficiency is known.
  • Identify common sources of error and apply corrections to measurements.
Materials Needed:
  • Geiger‑Müller tube with selectable cps/cpm mode
  • Radioactive sources (e.g., ^60Co, ^90Sr, background sample)
  • Adjustable stand for setting source distances (5 cm, 10 cm, 15 cm)
  • Timer/stopwatch
  • Worksheet or spreadsheet for data recording
  • Calculator
  • Safety signage and lead shielding
 Introduction:

Begin with a quick question: “If you double the distance from a source, what happens to the count rate?” Students recall previous work on activity and detector basics. Explain that today they will learn to measure, convert, and analyse count rates, and they will be able to demonstrate the inverse‑square relationship.
Lesson Structure:
  1. Do‑now (5') – Short quiz on activity, decay, and detector types.
  2. Mini‑lecture (10') – Define count rate, introduce formula, units, and conversion; illustrate inverse‑square law with a slide.
  3. Demonstration (8') – Set up GM tube, measure counts for a known source, calculate cps and cpm live.
  4. Guided practice (12') – In pairs, record counts for three sources at 5 cm, 10 cm, 15 cm; fill data table and plot count rate vs 1/r².
  5. Analysis discussion (8') – Interpret graphs, calculate activity for one source using detector efficiency.
  6. Error check (5') – Identify background, timing, and geometry errors; adjust data accordingly.
  7. Exit ticket (2') – Write a correct conversion statement (cps ↔ cpm) and one method to reduce measurement error.
Conclusion:

Review how count rate is measured, converted, and used to estimate activity, emphasizing the inverse‑square relationship and error‑reduction strategies. Collect exit tickets to gauge understanding, and assign homework: complete a worksheet converting between cps and cpm for additional data sets and reflect on how shielding would affect the results.