Particle detectors are instruments designed for the detection and measurement of sub-atomic particles such as those emitted by radioactive materials, produced by particle accelerators or observed in cosmic rays. They include electrons, protons, neutrons, alpha particles, gamma rays and numerous mesons and baryons. Most detectors utilize in some way the ionization produced when these particles interact with matter.

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...