P15.4 – Half-life

Use the term half-life in simple calculations, including the use of information in tables or decay curves.

The half-life of a radioactive substance is the time it takes for parent nuclei in a sample to halve. In other words, half-life of a radioactive substance is the time it takes for the count rate (on a Geiger-Muller counter) from the original substance to fall to half its initial level.

If we start with 1000 unstable nuclei and 10% disintegrate every hour, we would expect 100 nuclei to decay in the first hour leaving 900. Another 10% (900*10%=90) will decay in the next hour leaving 810 and so on. If we were to draw a table to record this decay, it would look like this:

From the table above we can calculate the half-life of this particular radioactive substance to be somewhere between 6 to 7 hours (half of 1000 is 500 and it falls to 500 nuclei in the seventh hour).

In one half-life the number of nuclei decreases by half. In the second half-life it decreases by a further half and so on.