A Geiger counter (or actually Geiger-Müller counter, GM counter in short) is a simple but sensitive device to register the presence of ionizing radiation.
In three short movies I try to explain a few basics about this detector and radiation in general.
In the first movie we listen to the sound coming from a Geiger counter and learn about the nature of nuclear decay. What can we learn from the irregularity of the click sounds? Does a nucleus have a memory? Is there a connection between this noise and philosophy?
The last movie of this series shows a series of measurements. Using different radiation sources the drastic difference in stopping
power for alpha particles compared to beta and gamma radiation is
illustrated. In addition, we check if we can verify the inverse square
law
and end with a little exercise.
Of course there is more information in text books and on many places on the web.
Maybe Wikipedia is, as so often, a good starting point to dig further, especially on the Geiger counter and on radioactive decay.