Geiger Counters

What are Geiger Counters? 

Geiger counters are devices to detect and measure ionizing radiation, as emitted by radioactive sources. The heart of a  geiger counter is the Geiger-Mueller-Tube. This is a gas filled tube, to which a voltage of several 100V is applied. Normally, the gas insulates and no current is drawn. When a radiation particle passes the tube, it triggers a gas discharge, i.e. gas becomes conducting. The resulting current impulse can be amplified and made visible or hearable ("clicking").

There are two main kinds of tubes:

• Glass tubes are only suitable for beta and gamma rays, as any alphas are absorbed in the glass. They can be used for quantitatively measuring gamma dose-rate. 

• Window tubes have a window (usually at one end), which is sealed with a very thin foil or mica. Alphas can penetrate this window, and thus be detected, as well as betas and gammas. As the window is very thin, it is extremely prone to puncture. Window tubes are more expensive and less common. 

A typical Geiger counter circuit looks like this:

Any good Geiger counter should be calibrated against a known radiation source .  The calibration insures that your measurements are correct and your Geiger counter is working properly.