High activity does not necessarily mean high danger

Does the rate of counting of a Geiger–Müller tube indicate how dangerous a source is?

Wrong Track: If the Geiger–Müller tube tube clicks or counts at a high rate, this must mean that there is a very dangerous radioactive source close by.

Right Lines: The rate of clicking or counting of the Geiger–Müller tube measures the activity of the source and is not a direct indicator of how dangerous it is.

You can't tell how dangerous a source is just from the activity

Thinking about the learning

When observing demonstrations with radioactive sources and a Geiger–Müller tube, students are likely to associate a high rate of counting with a high level of danger from a source.

Helen: Wow! Look at the counter. It's flying round! I wouldn't like to get too close to that!

Thinking about the teaching

The rate of counting of the Geiger–Müller tube is not a direct measure of the potential danger of a source. For example, it could be that the high rate of counting is due to a high-activity beta source that, in fact, has a less harmful effect at close range on human tissue than an alpha source of similar activity. Thus if both sources were swallowed or breathed in, the highly ionising alpha source would be much more dangerous. The beta source is releasing electrons at a high rate but these have a smaller damaging effect on human tissue. So, you can't tell simply by the rate of clicking of a Geiger–Müller tube how dangerous a radioactive source is.