The older a particular radioactive material, the __________ radiation it emits. In fact it takes the same amount of time for the activity (decays per second) to fall by __________ whatever the start value. This length of time is known as the half life.
This is because there is a 50:50 chance that a particular nuclei will decay in each half life period. This means that the more radioactive nuclei there are, each with a __________ chance of decaying in each half life, the __________ the activity will be.
Therefore the half life is also the time it takes for the number of parent atoms in a sample to __________.
This idea can be used to date materials. Uranium isotopes, which have a very long half-life, decay via a series of relatively short-lived radioisotopes to produce stable isotopes of lead. The relative proportions of uranium and lead isotopes in a sample of igneous rock can, therefore, be used to date the rock. The proportions of the radioisotope potassium-40 and its stable decay product argon can also be used to date igneous rocks from which the gaseous argon has been unable to escape.
It is more accurate to use a computer to measure and plot the activity. This is espcially true when dealing with isotopes with a very long half life. Even then the results of radioactive dating contain significant uncertainties and we must be careful not to be too precise with our calculations.