If you could see an individual americium atom you would have no way of telling when it was going to decay. It is a random process (blame quantum physics). What we do know, for a particular radioactive substance ,is the half life. This is the time taken for the activity to half. It is completely independent of the activity.
Here is an example.
Cobalt-60 (which was used by many schools as a source of gamma radiation) has a half life of 5 years. When schools bought these sources, their activities were about 180 kBq.
5 years later, the activity would be 180 ÷ 2 = 90 kBq
Another 5 years later, (10 years after it was bought), the activity would be 90 ÷ 2 = 45 kBq.
| Number of half lives | 0 (new) | 1 | 2 |
| Time (years) | 0 | 5 | 10 |
| Activity (kBq) | 180 | 90 | 45 |
If you keep going, you’ll see why a 30 year-old Co-60 source is pretty useless for everything other than to demonstrate the effects of a source having a relatively short half life.
