Here is Ivan the geiger counter showing the basic parts.
The Geiger-Müller tube (on the left of the picture) is attached to the counter on the right which has various features such as a display of the number of counts per second, display range and fast/slow meter response. The red cap covers the delicate mica window on the end of the tube.
Below is a basic diagram of a Geiger-Müller tube. This is the most important part of a Geiger counter.
In most Geiger counters the tube is either part of the counter itself or attached to it by a length of cable.
How It Works
A high voltage is applied to the supply wires of the tube.
When ionizing radiation enters the tube it causes atoms in the gas to loose electrons. The free electrons are then attracted to the positive wire electrode in the center of the tube. Because a high voltage is present on the electrode the electrons are pulled towards it and accelerated. As they are pulled towards the electrode, they collide with other atoms which releases more electrons from the gas atoms that are struck.
This causes an avalanche effect which generates an easily detectable pulse of current that the counter which is connected to the tube can turn into a click of sound and an increase in the reading on the meter.
The end of the tube is made of mica as this material allows weak alpha particles to penetrate and be counted. Some Geiger counters do not have this feature and are only sensitive to Beta, Gamma and depending on the model, X-Radiation.
Because the Geiger counter only registers general ionization, it cannot tell the difference between the various types of ionizing radiation. Counters which use other methods of detection and that are able to tell the difference are available.