The history of the Geiger Counter goes all the way back to 1908 and the scientist Hans Geiger, who first developed his device for detecting ionizing radiation. Subsequently, some years later in 1928, based on suggestions from his colleague Walther Muller, Geiger refined his design to create the Geiger-Muller tube, upon which most contemporary Geiger counters are based.
The classic design consists of a metal tube, the Geiger Mueller or GM tube, usually with a glass or mica window at one end, through which particles enter the device. Through the center of the tube runs a metal wire with a strong positive charge. This sealed tube is filled at low pressure with an inert gas such as argon.
When an ionizing particle enters the device, it collides with gas atoms and strikes electrons from their orbit around the nucleus of the gas atoms. In addition, an ionizing particle striking the outside of the tube can also knock electrons off the metal casing. In either case, these free electrons then are attracted to the positively charged central wire, gaining energy by the attraction. As they approach the wire they knock electrons off other atoms, creating an avalanche or cascade effect which results in a pulse of current large enough to be detected. A "quenching gas" is usually added to the filler gas so that these avalanches will dissipate over time and not continue indefinitely, which would cause inaccuracy or electronic failure.
The Geiger counter can detect the pulses produced by ionizing particles because the metal casing of the Geiger-Muller tube acts as a cathode, with the central wire acting as an anode. The anode transfers the pulses of current through a resistor, where they are converted to pulses of voltage. The voltage pulses are then recorded by a counting device.
Geiger counters are capable of detecting alpha, beta, gamma, and x-radiation, although they cannot determine the type, energy of the detected particles. The design of the specific counter determines how well it can detect the various types of radiation. For example, gamma and x-radiation can penetrate a metal casing without difficulty, but a glass or mica window is necessary to allow the low-penetration particles that comprise alpha and beta radiation to reach the inside of the detector. Other factors, including the gas used to fill the tube, also affect the efficiency of detection.
The most straightforward reading from a Geiger counter is simply the count of particles detected, or counts per minute (cpm). Conversion to other measures can be misleading, since various designs will detect more or less of any given type of radiation. Nevertheless, a well-constructed and well-calibrated Geiger counter can offer several standard units of measurement, with the understanding that the readings apply only to the types of radiation that particular model is able to accurately detect. The Sievert and the rem are the most common measures of radiation dosage, with 1 Sievert (Sv) equal to 100 rems (R). A rem can be further divided into 1,000 millirems (mR or mrem). The U.S. Nuclear Regulatory Commission (NRC) states that a person in an occupation not involving radioactive materials is exposed to 100 mrem per year of normal background radiation, and should avoid more than 100 additional mrem per year.
The Geiger counter, is sometimes also referred to as a Survey Meter, is different from the dosimeter in that a dosimeter is designed to measure the amount of radiation, of all types, absorbed in a certain amount of time. In simplest terms, a Geiger counter is used to detect radiation in an area or on an object, while a dosimeter is used to monitor radiation exposure to a person over an extended period of time. For example, laboratory technicians who work with radioactive materials use film-badge dosimeters, which are worn for weeks or months, then processed to show the amount of radiation absorbed during that time. If the dosimeters indicate that a laboratory's personnel are receiving unexpected levels of radiation, a Geiger counter would be used to pinpoint the specific source of the unintended radiation