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Radioactivity

Half-Life.

1. Half-life is the time taken for half of the radioactive nuclei to decay.

2. Half-life is the time taken for the count rate
to fall to half of its original reading.

There are a number of ways to define half-life.
Remember one of the above definitions, you will need it for the exam.

A radioactive material will have some nuclei which are stable
and some which are unstable.
The stable nuclei don't change, that's what stable means.
The unstable nuclei (brown balls) will change into stable nuclei (purple balls)
and emit radioactivity.

Half-life is a measure of how quickly unstable nuclei change into stable nuclei.

Different materials do this at different rates.
Some do it very quickly and half the unstable nuclei decay
in less than one second (example Lithium-8, half-life 0·85 seconds).
Some do it very slowly and half the unstable nuclei take billions of years
to decay (example Uranium-238, half-life 4·51 billion years).

Remember the half-life is an amount of time.
In the same amount of time, the picture on the right above
will lose half the remaining unstable nuclei.

Continued on the next page.

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