Exponential Decay of Activity
Quantum and Nuclear

Sweet simulations (radioactivity)

Teaching Guidance for 14-16 IOP RESOURCES

In this activity students shake sweets to model the radioactive decay of a large number of unstable nuclei. You can use it to introduce decay curves.

Preparation & safety

Sweets or chocolates provide a colourful analogy for radioactivate decay, but there should be no eating or drinking in labs. Consumption will also skew results. If you think the temptation to eat sweets might be too great for your students you may want to consider alternatives such as coins or small dice. Whatever you choose source a large number.

Equipment

Each group of students will need:

  • Enough sweets or chocolates with a logo on one side so that each student can have four (eg 100 sweets for a class of 25 students)
  • Four extra sweets for yourself
  • Seven or eight measuring cylinders each with a capacity large enough to hold half the total number of sweets

Procedure

  1. Line up measuring cylinders in a row and ask for a volunteer to be your assistant for counting and collecting sweets.
  2. Write up the total number of sweets on the board and distribute four sweets to each student. Keep four for yourself.
  3. Ask the class to hold sweets in cupped hands like you are and when you shout ‘shake’ to shake them so that the sweets move around inside their hands. You should do the same with yours.
  4. After 5 seconds shout ‘stop’. They should open hands palm up, remove any sweets that are logo-side up. You should do the same.
  5. Ask your assistant to collect all discarded sweets and put them into first measuring cylinder.
  6. Repeat steps 3 to 5 to fill the other cylinders. Time your shouts of ‘shake’ to be at regular intervals for a better model of radioactive decay.

Discussion prompts

  • How many sweets do you think are in the first cylinder? What about the second?
  • For each shake, what are the chances for an individual sweet landing face up?
  • Is it possible to predict when a particular sweet will land face-up?

Teaching notes

Students could ‘place bets’ by writing down predictions for sweet numbers on mini-white boards or post-it notes. Refer back to them at the end of the activity. Discuss results before removing sweets from the cylinders to do any final counts.

The chance of being face-up after a shake for a sweet is 1 in 2, or they could say there is a 50% probability. Emphasise that each shake is an independent event. What happens in one does not depend on what happened in the last or affect a future one. The probability of the single sweet landing face-up is 50% whether it is your first or last shake.

Unstable nuclei in radioactive sources behave in a similar way. The probability of a decay is fixed, but it is not possible to predict when a particular nuclide will decay. Provide an example of a decay curve for a radioactive source to show that it sweeps downwards just like the sweet simulation.

Learning outcome

Students describe a sweet/coin model for unstable nuclei and sketch a decay curve.

This experiment was safety-checked in March 2020.

Exponential Decay of Activity
can be analysed using the quantity Activity
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