In this deck, we'll explore the concept of half-life, a crucial idea for understanding how radioactive substances decay over time. You'll learn why radioactive decay is a random process and how to calculate the decline in radioactivity. We will also distinguish between the hazards of irradiation and contamination, and look at the sources of background radiation that are all around us.

Imagine you're making popcorn. You can't predict exactly which kernel will be the next one to pop. All you know is that after a few minutes, most of them will have popped. Radioactive decay is similar. For a single unstable nucleus, its decay is a completely random event. We can't predict when it will happen. However, for a large number of nuclei, we can predict how long it will take for half of them to decay. This statistical predictability, despite individual randomness, is key to understanding radioactivity.

The predictability of radioactive decay for large numbers of atoms is described by the half-life. The half-life of a radioactive isotope is the time it takes for the number of undecayed nuclei in a sample to be reduced by half. Because the radioactivity, or 'activity', of a sample is proportional to the number of undecayed nuclei, the half-life can also be defined as the time required for the activity to fall to half its initial value. This is measured in units of time, such as seconds, years, or even millions of years.