ionizing radiation enters the Geiger tube through a mylar window at the front.
The radiation (in the form of alpha, beta and gamma) ionizes a low pressure gas inside the tube creating positive and negatively charged ions. the negativly charged ions are attracted towards a central electrode which is held at a high positive potential (typically about 500V dc). The tiny current that flows into the central electrode also passes through a high resistance an consequently a voltage pulse is created across it. A capacitor is then used (100pf polystyrene) to block the 500V dc but allow the pulse through to a detector which then amplifies and shapes it before being counted.
During the time the gas in the tube is being ionized by the incoming radiation it is 'dead' to any other incoming radiation. This time is known as the 'dead time' for the tube and it puts an upper limit on it's counting rate.
The window at the front is there to prevent the low pressure gas escaping but allowing the radiation in.
there's a small break in the circuit inside a GM tube. when particles enter the tube, i.e alpha or beta particles, they ionize the air particles in the small break and thus complete the circuit enabling it to conduct electricity. the circuit is connected to a counter which then counts the amount of radiation and a loudspeaker so that it makes a click whenever radiation is detected
The geiger counter detects Alphas, Beta and Gamma Particles given off by different materials.
The more detected the higher level of radiation.
That's the simple description... I think we went in to more depth at the Naval Nuclear Power School but That was 20+ years ago...
Alpha, beta and gamma radiation from radioactive sources enter the GM tube and excite the gas in the GM chamber and cause a cascade effect which sends a current through the detector, which is amplified and sent to a loudspeaker to give the clicking sound.
Google it up.