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ionization chamber
1. A device that measures the intensity of ionizing radiation.
2. A device used to detect and measure ionizing radiation, consisting of a gas-filled tube with electrodes at each end between which a voltage is maintained.

Radiation that ionizes gas molecules in the tube causes a current between the electrodes, the strength of which is a function of the radiation's intensity.

3. A gas-filled enclosure fitted with electrodes between which electric current flows upon ionization of the gas by incident radiation, the electrodes being maintained at a potential difference just sufficient to collect ions thus produced without causing further ionization.
4. The device for the detection and measurement of ionizing radiation.

It consists basically of a sealed chamber containing a gas and two electrodes between which a voltage is maintained by an external circuit.

When ionizing radiation; such as, a photon, enters the chamber (through a foil-covered window), it ionizes one or more gas molecules.

The ions are attracted to the oppositely charged electrodes; their presence causes a momentary drop in the voltage, which is recorded by the external circuit.

The observed voltage drop helps identify the radiation because it depends on the degree of ionization, which in turn depends on the charge, mass, and speed of the photon.

Geiger-Müller counter

A Geiger-Müller counter results from the application of a still-higher voltage across the electrodes of a proportional counter.

Individual particles of various kinds and energies entering a Geiger-Müller counter produce essentially the same large output pulse, making the instrument an excellent counter of individual particles.

The mixture of gases within a Geiger counter quenches the avalanche of ions produced by a single particle of radiation so that the device can recover to detect another particle.

An additional significant increase in voltage causes a continuous current to flow through the gas between the electrodes, rendering the device useless for detecting radiation.

—Compiled from "ionization chamber", Encyclopædia Britannica; 2010;
Encyclopædia Britannica Online; May 22, 2010.