Radioactive decay
Science
Demonstration of the radioactive decay
Let's look again at a radioactive source with the help of the scintillation meter. In the test tube is a radioactive source, a compound of Proto actinium in solution. A stopwatch in started and the meter reading noted a 112 counts per second and it appears to be falling. 60 seconds later, the meter reading is down to 63 counts. Again, the general trend of readings is downwards. Another minute on the count is 35 and still falling. The energy emitted by this radioactive source is getting weaker.
The readings can be plotted on a graph like this. The activity of the source reduces by half every 72 seconds. The time taken for the activity to fall from any value to half that value is known as the half life. So Proto actinium has a half life of 72 seconds. Notice that activity will never cease altogether. Some radioactive sources decay very quickly with a half life of only microseconds. Others have a very long half life running into millions of years. On average, different radioactive elements take different times to decay. Looking at an individual atom, it's completely impossible to tell when it's going to pop. It could happen in the next few seconds, it might take a million years, there's absolutely no way of knowing. But when you look at a large number of atoms altogether, it's clear that on average, some radioactive materials decay much faster than others.
The average time it takes half the atoms in a large sample to decay is called the half life. It takes more than 24,000 years for half a sample of plutonium to decay. But the half life of other elements is measured in minutes or seconds, so their radioactivity disappears much more quickly. For this reason, radioactive elements with long half lives tend to be more dangerous than those with short half lives.