Science 7–10 · Year 9
Radioactive decay with 100 dice: chance, half-life and the decay curve
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The idea
Each unstable nucleus decays at random with a fixed chance per unit time, so a large sample loses a constant fraction in each equal interval and the count falls along a curve with a constant half-life.
What you need
- 100 six-sided dice in a lidded tub (or 100 coins)
- results table and graph paper
How to do it
- Shake the tub and tip the dice out. Remove every die showing a six: these have "decayed". Count and record how many remain.
- Return the survivors to the tub and repeat; each throw is one time interval. Continue until fewer than 5 dice remain.
- Plot dice remaining against throw number and read off how many throws it takes to fall from 100 to 50 and from 50 to 25.
- Pool the whole class's data (1000 dice or more) and plot again; compare the smoothness of the two curves.
What you should see
Each throw removes about one sixth of the dice. The expected count after 4 throws is 48 and after 8 throws 23; the half-life is 3.8 throws, and the second halving takes the same number of throws as the first. A single group's curve is jagged; the pooled class curve is smooth. The learner knows it worked when both halvings take about 4 throws, the 100-dice curve lies close to 100 x (5/6)^n, and the pooled curve lies close to the same expected curve drawn for its own starting number, N0 x (5/6)^n: 1000 pooled dice are expected to leave 482 after 4 throws, not 48.
What changes
- What you change
- number of throws (time)
- What you measure
- number of undecayed dice
- What you keep the same
- decay rule (a six decays)
- all dice thrown each time
- starting number
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- After two half-lives nothing is left.
- Each nucleus knows when it is due to decay.
- Half-life is half the time a sample lasts.
Safety card
Hazards
- none
Controls
- none needed
Note
No chemicals or heat. No risk assessment beyond ordinary classroom practice.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Science 7–10 Syllabus (2023)SC5-RXN-01SC5-WS-05
- Australian Curriculum v9AC9S9U06AC9S9I04
Sources
The pages the author read to write this activity.