Lesson Plan

• Calculate raw and background count rates from given counts and times.
• Determine the corrected count rate by subtracting background and propagate its uncertainty using Poisson statistics.
• Explain why background subtraction is essential for accurate activity measurements.
• Interpret a data table and report results with appropriate uncertainty.
• Apply practical tips to ensure reliable radiation measurements in the laboratory.

• Geiger–Müller counter with counting interface
• Computer/projector for displaying equations and data tables
• Worksheets with sample data and calculation space
• Calculator or spreadsheet software
• Safety goggles and lab coat
• Background radiation source (or simulated data set)

Radiation measurements can be misleading if we ignore the ever‑present background radiation. Students already know how to calculate a simple count rate and understand basic Poisson uncertainty. Today they will learn to subtract the background contribution and report a corrected rate with its uncertainty, demonstrating mastery by completing an accurate calculation.

1. Do‑Now (5'): Quick mental review of the count‑rate formula and Poisson uncertainty.
2. Mini‑lecture (10'): Explain background radiation, introduce raw, background, and corrected rate equations; work through the example on the board.
3. Guided practice (15'): Pairs use the worksheet data table to calculate rates, corrected rate, and its uncertainty; teacher circulates for support.
4. Whole‑class check (5'): Collect answers, discuss common errors, emphasise using the same counting time.
5. Practical tip demonstration (5'): Show how to record background before and after the sample and discuss stability checks.
6. Exit ticket (5'): Each student writes the corrected rate with uncertainty for a new set of counts.

We have reviewed how to subtract background radiation and propagate uncertainties to obtain a reliable activity measurement. For homework, complete the attached worksheet with a different set of raw and background counts and write a brief paragraph explaining why consistent counting times are important.