Although the high voltage is needed for the center of cylindrical metal (negative pole) by what has arranged metal stick (anode), radiation is measurable one by one. The intensity (speed) of radiation cannot be measured. Quantity (number) is measured. Sensitivity can be comparatively made high. It is fit for detection of alpha rays and a beta ray. Although a gamma ray is also detectable, it is said that the detection probability is about several %.
The Geiger Mueller counter is decompressed by 0.1 atmospheric pressure, and it fills up with inactive gas (Ar, helium) and halogen gas (bromine) as continuation electric discharge prevention.
Voltage from DC 500V to 700V necessity. The voltage region suitable for this measurement is called Plateau voltage.
Work theory of GM tube.
- Radiation (alpha, beta, gamma) carries out incidence.
- It collides with a gas molecule (inactive gas), and gas is ionized (ionization).
- The generated electron is accelerated by high voltage field.
- Further, an acceleration electron collides gas and is excited.
- When the excited gas return to a base state, ultraviolet rays are generated.
- Ultraviolet rays ionize gas and cause an electronic avalanche.
- An electronic avalanche reaches the anode and is detected as a pulse.
- On the other hand, plus ion gas reduces voltage field.
- Thereby, the electronic avalanche ceases.
- Plus ion gas moves to the negative pole(Since it is a molecule, movement takes time).
- Current flows, and an electronic avalanche is promoted.
- An electronic avalanche does not stop. Then, electric discharge prevention gas is put in.
- Electric discharge prevention gas receives a plus electric charge.
- Although electric discharge prevention gas reaches the negative pole, an electronic avalanche is not promoted at this time.
- Electric discharge prevention gas returns to a molecule.