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What is 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
of the above definitions, it may be useful in the exams.

An Explanation of Half-life.

A radioactive material will have some nuclei that are stable
and some that are unstable. The stable nuclei don't change,
that is what stable means. In the picture below,
the unstable nuclei (shown as brown balls) will change
into stable nuclei (shown as purple balls) and emit radioactivity.

Radioactive Decay

Half-life is a measure of the time taken for
the unstable nuclei to change into stable nuclei.


Different substances do this at different rates.

Some do it very quickly and half of the unstable nuclei decay
in less than one second.
For example, lithium-8 has a half-life of only 0·85 seconds.

Some do it very slowly and half of the unstable nuclei take
billions of years to decay.
For example, uranium-238 has a half-life of 4·51 billion years.

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

Two Half-Lives

Continued on the next page.

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