Particle Detectors

The Geiger counter is one of the oldest and simplest of the many particle detectors. The counter was developed in the early part of the twentieth century by Hans Geiger and Wilhelm Muller, shortly after the discovery of radioactivity. A schematic diagram of a Geiger counter is shown here. A wire electrode runs along the center line of a cylinder having conducting walls. The tube is usually filled with a monatomic gas such as argon at a pressure of about 0.1 atmosphere. A high voltage, slightly less than that required to produce a discharge in the gas, is applied between the walls and the central electrode. A rapidly moving charged particle which gets into the tube will ionize some of the gas molecules in the tube, triggering a discharge. The result of each ionizing event is an electrical pulse which can be amplified to activate ear phones or a loud speaker, making the counter useful in searches for radioactive minerals or in surveys to check for radioactive contamination. The counter provides very little information about the particles which trigger it because the signal from it is the same size no matter how it is triggered. However, one can learn quite a bit about the source of radiation by inserting various amounts of shielding between source and counter to see how the radiation is attenuated.