Radioactive Decay

This simulation shows 400 unstable nuclei, green when intact and grey after decay, with a flash at each decay. A graph plots the live count against a gold theory curve. Students can change the half-life between 1 and 10 seconds, show or hide the curve and rerun the experiment. Readouts give nuclei left, half-lives elapsed, expected number and the half-life measured from the data. Teachers can use it to show that decay is random for single nuclei but predictable for large numbers.

• Run the experiment with Half-life 4 s and note how many Nuclei left remain after exactly one half-life.
• Press Re-run experiment several times and see that the random count never matches the curve exactly.
• Change Half-life to 10 s and compare how slowly the green nuclei disappear.
• Switch off Show theory curve and ask students to draw their own curve from the live points.
• Compare Measured half-life with the value on the slider after each run.

1. 1Define half-life as the time for half the nuclei to decay.
2. 2Explain why decay is random for one nucleus yet predictable for many.
3. 3Predict the fraction remaining after one, two and three half-lives.
4. 4Interpret an exponential decay graph.

Questions teachers ask