# Activity changes over time

Physics Narrative for 14-16

## Exponential changes – fixed chance leads to reducing activity

There is a very precise and important result that follows from the inability to alter the chance that an ionising radiation is emitted from a nucleus. Exponential decay over time is a consequence of the constant fractional decay of the nuclei. This is often modelled by throwing multi-sided dice. The lower the chance of decay in any one period, the greater the number of sides the dice should have. This change in chance for a decay, and the number of nuclei you start with, both rescale the graph but do not alter its shape. The shape of the curve is controlled by very exact mathematics – the rate of change at any time (the activity of the source) depends on the number present at that time. The change in the number of nuclei depends only on the number of nuclei present and on the kind of nuclei. Each kind of nucleus has a fixed and immutable chance of decaying in each interval – there is absolutely nothing we can do to change it. It was this that led Einstein to the famous statement: God does not play dice

. Here it appears that he was wrong.

There are significant consequences. If we make something radioactive then there is no way to reduce the activity. If there is a nuclear accident, the only clean-up remedy is to seal off the unstable nuclei and wait.