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It's a Question of Physics: What is a Geiger counter?

The Geiger Counter

Geiger counters are used to detect radioactive emissions, most commonly beta particles and gamma rays. The counter consists of a tube filled with an inert gas that becomes conductive of electricity when it is impacted by a high-energy particle. When a Geiger counter is exposed to ionizing radiation, the particles penetrate the tube and collide with the gas, releasing more electrons. Positive ions exit the tube and the negatively charged electrons become attracted to a high-voltage middle wire. When the number of electrons that build up around the wire reaches a threshold, it creates an electric current. This causes the temporary closing of a switch and generates an electric pulse that is registered on a meter, either acoustically as a click that increases in intensity as the ionizing radiation increases, or visually as the motion of a needle pointer.

Radioactivity can be measured in order to discover the amount of radiation a material emits or the amount of radiation absorbed by a human or mammal. The unit for measuring radioactive emissions is the becquerel (Bq). The Bq indicates the number of decays per second. The roentgen equivalent in man (rem) is an older standardized unit for measuring absorbed dose. The mrem, 1000th of that unit, is the unit used today in medicine.
Geiger Counter

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