The amount of radiation emitted from a radioactive material is known as the activity. It is measured in Becquerels. If a source has an activity of 800 Becquerels, that means that 800 radioactive decays are taking place every second. School sources tend to have activities of kilobequerels (kBq) where 1 kbq = 1000 Bq. The old unit of activity was the Curie (Ci). Typically, school sources have activities of a few microCuries (μCi) since 1 μCi = 37 kBq. Don’t be surprised to find the activities of older sources given in μCi.
Note that if your radiation detector reads 100 counts per second, that does not mean that the source that it is pointing at has an activity of 100 Bq. This is because:
- Radiation tends to spread out from its source. A detector only intercepts a small amount.
- Some radiation is absorbed by the air.
- Detectors are not 100% efficient at detecting radiation. Indeed, some are only 1% efficient at detecting gamma radiation. Only 1 in 100 gamma photons entering the detector are actually detected.
