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 or quantum 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").

This was a very basic explanation. For a more detailed and advanced treatment, see this article which was copied from Caltech.

There are two main kinds of tubes:

A typical Geiger counter circuit looks like this:

Variant b) has the advantage of proper grounding, so that the tube is electrically shielded by its cylindrical cathode.

Geiger counters can be bought complete or as kits from many electronic and surplus stores, but can also be self-designed. The following plans will give you some clues.

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Jochen Kronjaeger