Half Life

The activity of a radioactive sample always DECREASES over time. This is because it becomes LESS likely for a decay to occur as there are FEWER UNDECAYED nuclei after every DECAY.

The TIME it takes the ACTIVITY or the NUMBER of UNDECAYED NUCLEI to decrease to HALF of the ORIGINAL value is known as the HALF LIFE.

The half-life of a radioactive isotope can be DEFINED in TWO ways:

1. The time it takes for the number of UNDECAYED NUCLEI of an isotope in a sample to HALVE.

2. The time it takes for the COUNT RATE (or ACTIVITY) from a sample containing the isotope to fall to HALF its initial level.

You can also express the nuclei as a RATIO of UNDECAYED to DECAYED nuclei:

Half Life in Graphs

The ACTIVITY, COUNT-RATE or NUMBER OF UNDECAYED NUCLEI can all be represented on a graph as a DECREASING curve.

The half life of the sample can be found by finding the time taken for the INITIAL VALUE to halve.

To do this, find the INITIAL VALUE on the y-axis and HALVE it.

Then draw a HORIZONTAL LINE from the half value to the curve, and draw a VERTICAL LINE from the curve to the x-axis.

The time you find on the x-axis is the HALF LIFE.

In the above example , the half life is 3 DAYS.