Geiger-Mueller (G.M.) Tube Counting

Geiger-Mueller (G.M.) Tube Counting

The Geiger-Mueller (G.M.) counter is one of the oldest radiation detector types, having been invented by Geiger and Mueller in 1928. This counting is a widely used for X-ray, gamma-ray, beta-particle and alpha-particle detection. The experiments that are designed to accomplish this purpose deal with the operating plateau of the Geiger tube, resolving-time correction, half-life determination, and the basic nuclear counting principles.

These counters are based on ionization. In common with proportional counters, they employ gas multiplication to greatly increase the charge represented by the original ion pairs formed along the radiation track, but in a fundamentally different manner. It consists of two electrodes with a gas at reduced pressure between the electrodes. The outer electrode is usually a cylinder, while the inner electrode is a thin wire positioned in the center of the cylinder. The voltage between these two electrodes is maintained at such a value that virtually any ionizing particle entering the Geiger tube will cause a comprehensive electrical avalanche within the tube. The Geiger tube used in this experiment is called an end-window tube because it has a thin window at one end through which the ionizing radiation enters. In the G-M tube, substantially higher electric fields are created that enhance the intensity of each avalanche. Under proper conditions, a situation is created in which one avalanche can itself trigger a second avalanche at a different position within the tube. Therefore, a self-avalanche multiplication chain is created at the critical value of the electric field.

The Geiger counter can only measure the number of photons or charged particles that were detected. The voltage pulse from the avalanche is typically greater than one volts. These pulses are large enough that they can be counted with a 2650 timer and counter without amplification. Though, pulse inversion is necessary.

A major disadvantage of these counters is their unusually large dead time, which greatly exceeds that of any other commonly used radiation detector. Therefore, these detectors are limited to relatively low counting rates and a dead time correction must be applied in situations in which the counting rate is moderate.

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