A Geiger counter -- or a Geiger-Mueller tube or a GM tube -- detects radiation. A GM tube has a thin wire for a central electrode down the center of a conducting tube. This arrangement is connected to a high-voltage power supply of a few hundred volts.
This geometry produces a radial electric field:
This electric field varies as 1/r which means the electric field gets much stronger as one gets closer to the central wire.
The GM tube contains some inert gas -- like Nitrogen.
When a high-speed electron passes through this GM tube, it will tear some some electrons from the gas atoms in the GM tube. These electrons have very small mass so they are rapidly accelerated toward the central wire by the electric field.
As these stripped-off electrons accelerate toward the central wire, they rip off additional electrons.
These additional electrons are rapidly accelerated toward the central wire. And they, in turn, rip additional electrons from the gas,
An "avalanche" of electrons reaches the central wire. The initial high-energy beta particle (or electron) may have torn electrons from only a very small number of gas atoms. That would be impossible to detect. But this "avalanche" of electrons will be enough to detect.
(c) Doug Davis, 2002; all rights reserved
Iodine Experiment Nuclear Power Return to Ch 30, Nuclear Physics