Inside a Geiger-Muller tube

 

Here is a view into one of our Geiger-Muller tubes. Note the cylindrical central anode. The window is made of a mineral called mica which has a low mass per unit area and thus allows alpha to pass relatively uninterrupted. There is low pressure gas inside - perhaps 1/10th of atmospheric pressure. When one radioactive detection occurs, it starts with the ionisation of one gas atom to make an ion-electron pair. The electrons are pulled towards the central anode, being accelerated as they go. They gain so much energy that they are able to ionise more atoms to create more electrons and so on. This electron avalanche effect produces enough to be detected.