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EXPERIMENT 8: Build a Geiger Counter (A Class Project)

THINGS YOUR CLASS WILL NEED: Geiger counter components (see parts list and circuit diagram). Gamma ray source.*

Chances are you’ve heard of a Geiger counter before. This versatile instrument is one of the most useful pieces of equipment ever developed for detecting the presence of radioactive emissions.

A Geiger counter is a relatively simple device. Its heart is the Geiger tube, a chamber filled with a mixture of special gases and equipped with a pair of internal electrodes that must be charged at high voltage.

Normally, the gas inside a charged Geiger tube does not conduct electricity. However, if a radioactive emission strikes the tube, the gas is momentarily ionized, and it becomes a conductor for a split second. Thus, a pulse of electric current flows through the tube, from electrode to electrode. The electrical circuit of the Geiger counter is designed so that a click is produced by the headphones each time a pulse of current flows through the tube. Each click you hear means that a radioactive emission has struck the Geiger tube.

*Your school may have a gamma ray source or a chunk of radioactive ore. If not, try to find an old luminous-dial watch or clock. These time pieces, unlike newer models, have a small amount of radioactive material, radium, mixed in with the dial phosphors. If all else fails, a low level gamma ray source intended for educational use is available by mail order (see inside front cover).