The random nature of radioactive decay
The The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom. The plural of nucleus is nuclei. of An atom that has unstable nuclei that can break apart or change, releasing radiation. are unstable. They break down and change into a completely different type of atom. This is called radioactive decay.
For example, carbon-14 decays to nitrogen-14 when it emits beta radiation. As this breakdown occurs, the activity of any radioactive source becomes less. This activity is measured in becquerels.
It is not possible to predict when an individual atom might decay. But it is possible to measure how long it takes for half the nuclei of a piece of radioactive material to decay. This is called the The time it takes for the number of nuclei of a radioactive isotope in a sample to halve. Also defined as the time it takes for the count rate from a sample containing a radioactive isotope to fall to half its starting level. of the An isotope of an atom of an element that releases ionising radiation. Isotopes of an element have more or fewer neutrons than each other. Also called a radioisotope..
Half-life definitions
There are two definitions of half-life - but they mean essentially the same thing:
- the time it takes for the number of nuclei of the isotope in a sample to halve
- the time it takes for the Measure of the amount of radiation reaching a detector in a given time, usually shown as counts per second or counts per minute. from a sample containing the isotope to fall to half its starting level
Different radioactive isotopes have different half-lives. For example, the half-life of carbon-14 is 5,730 years, but the half-life of francium-223 is just 20 minutes.
Graph of count rate against time
It is possible to find out the half-life of a radioactive substance from a graph of the count rate against time. The graph shows the decay curve for a radioactive substance.
The count rate drops from 80 to 40 counts a minute in two days, so the half-life is two days. In the next two days, it drops from 40 to 20 - it halves. In the two days after that, it drops from 20 to 10 - it halves again - and so on.