Geiger Müller counter tube

The Geiger Müller counter tube is one of the earliest measurement devices for the field of radioactivity. Walter Müller developed it under the auspices of his thesis advisor Hans Geiger at the Christian-Albrechts-Universität in Kiel in the sping of 1928.
In the early days of the development the counter is an evacuated brass tube with a thin wire clamped into it in axial direction. Thus a cylindric condensator is formed which is then operated at voltages around 1.5 kV. If a γ-quantum or β-particle hit the surface of the tube a secondary electron is emitted in the inside which subsequently will be pulled towards the central wire.

Due to the vacuum inside the tube the mean free path of the electron is so long, that it can be accelerated enough to ionise residual particles which in turn produce secondary ions. By this avalanche effect a well measurable current is produced. A resistance of 1GΩ resets the device instantly after the sparkover.

Modern counter tubes are filled with a mixture of noble gases and alcohol at reduced pressure. Hence the high resistance becomes obsolete and the operating voltage can be reduced to a few hundred volts. Besides modern tubes are equipped with a window which allows also for a-particles to pass. Since they are considered inferior to scintillation detectors or semiconductor detectors in terms of precise measurements they are only put to use in civil and educational environments.