A device to safely perform experiments on radioactivity

This paper describes a cost effective and safe device to perform realistic experiments about the physics of radioactivity in classrooms. It can be used to study both $\alpha -$ and $\beta -$radioactivity as well as $\gamma -$emitters and shows an extremely realistic behaviour. The device, in the form of a Geiger-M"uller tube, was tested during a public lecture and it deceived many people, even among physicists and teachers.