Lesson Plan

• Describe the concept of half‑life and its relationship to exponential decay.
• Apply the method of using raw count data (including background) to calculate half‑life by locating the half‑value point.
• Perform linear interpolation between two data points to estimate the half‑life when the exact half‑value is not recorded.
• Interpret a decay curve to read or interpolate the half‑life directly from the graph.
• Identify common errors when background radiation is not accounted for and explain why a constant background does not alter the half‑life determination.

• Geiger‑Müller counter or radiation detector with data‑logging capability
• Computer with spreadsheet software (e.g., Excel) or graphing calculator
• Projector and screen for displaying tables and graphs
• Printed worksheet containing raw data tables and practice questions
• Graph paper or digital plotting tool for drawing decay curves
• Ruler and calculator for interpolation calculations

Begin with a quick demonstration: show a live count of a radioactive source on the screen and ask students what they notice as time passes. Recall that they have already studied exponential decay and the definition of half‑life. Explain that today they will learn how to determine the half‑life directly from raw data, even when background radiation has not been removed, and they will be able to check their answers using a simple interpolation method.

1. Do‑now (5 min): Students answer a short question on the exponential decay formula and define half‑life.
2. Teacher mini‑lecture (10 min): Review the effect of constant background radiation and introduce the step‑by‑step method using tabulated data.
3. Guided practice – Method 1 (15 min): Work through the example table together, calculate N₀/2, locate the interval, and perform linear interpolation.
4. Guided practice – Method 2 (10 min): Plot the same data on a quick graph, draw the half‑value line, and read/interpolate the half‑life from the curve.
5. Independent worksheet (15 min): Students complete two practice questions, one using raw data and one using a decay curve, with teacher circulating for support.
6. Check for understanding (5 min): Quick exit ticket where each student writes the half‑life they obtained and one common pitfall to avoid.

Summarise that half‑life can be extracted from raw counts by locating the half‑value point and, if needed, applying linear interpolation, whether using tables or graphs. Remind students to verify that the background remains constant and to avoid mixing background‑subtracted and raw data. For the exit ticket, they submit their calculated half‑life and a brief explanation of why background does not affect the result. Homework: complete the additional practice set in the textbook and bring any questions to the next class.