A Geiger counter named after Hans Geiger, a German scientist in the early 20th century who worked on radiation detection, is an instrument that can detect radiation. A particular meter, known as a teletector, is specifically designed to detect gamma and x-ray radiation. Named for its telescopic capability, this device can be extended to about 4 meters (13 feet) to measure very high dose rates without subjecting the user to unnecessary exposure. In addition, these devices typically have the ability to measure dose rates ranging from 0 to 1000 rad per hour.
Radiation detection is achieved through the use of a variety of instruments. The most common type of radiation detector is a Geiger-Mueller (GM) tube, also called a Geiger counter. The number of ion pairs collected increases with increasing voltage. Because the probability of reaching the collecting electrodes significantly exceeds that of encountering an oppositely charged ion, it is possible to achieve a voltage such that all ion pairs produced in the volume by the primary ionizing event are swept from the volume and collected.
At this point, further increases in the applied voltage do not result in an increase in the number of ion pairs collected. The voltage at which this occurs is called the saturation voltage and the region (i.e., gas-filled detectors operating in this region are called ionization chambers). The voltage required to produce saturation in the ionization region depends on the type of gas used, the gas pressure and the dimensions of the chamber. To detect these events and obtain information about radiation, some means must be used to detect light.
Note that gas-filled radiation detectors do not detect all particles, as some of the radiation may pass through and not produce enough ionization in the chamber to be detected.