Radiation detectors are instruments designed to detect and measure subatomic particles, such as those emitted by radioactive materials, produced by particle accelerators, or observed in cosmic rays. There are three main types of radiation detectors: gas ionization based detectors, scintillation detectors, and semiconductor detectors. Gas ionization based detectors include the ionization chamber, the proportional counter, and the Geiger-Müller meter. These detectors are used to detect alpha, beta, and gamma radiation.
The Geiger counter is one of the oldest and simplest particle detectors. It was developed in the early 20th century by Hans Geiger and Wilhelm Muller shortly after the discovery of radioactivity. A schematic diagram of a Geiger counter is shown here: a wire electrode runs along the centerline of a cylinder having conductive walls. The tube is normally filled with a monatomic gas, such as argon, at a pressure of about 0.1 atmospheres.
A high voltage, slightly lower than that required to produce a discharge in the gas, is applied between the walls and the central electrode. When a rapidly moving charged particle enters the tube, it ionizes some of the gas molecules in the tube, triggering a discharge. The result of each ionizing event is an electrical pulse that can be amplified to activate headphones or a speaker, making the counter useful in searching for radioactive minerals or in studies to check for radioactive contamination. The counter provides very little information about the particles that activate it because the signal comes from the same size no matter how it is activated. However, one can learn quite a bit about the radiation source by inserting various amounts of shielding between the source and the counter to see how the radiation is attenuated. Because of the risks of exposure to ionizing radiation, most radiation detectors are concerned with detecting and measuring this specific form of radiation.
Film detectors are good at determining radiation levels and are commonly used for radiation safety. These ubiquitous sources of radiation are called background radiation, and all radiation detectors have to deal with it, which they often do by shielding it. Another term used to characterize radiation is to identify it as ionizing or non-ionizing radiation. A topographic meter is a portable radiation detector which typically measures the amount of radiation present and provides this information on a numerical display in units of counts per minute, counts per second, or microroentgen (µR) or microrem (µrem) per hour. The second important type of detector used in radiation detection instruments are scintillation detectors. These devices use photomultiplier tubes to detect Cerenkov radiation which is emitted when charged particles pass through certain materials at speeds faster than light can travel through them.
A subset of these detectors are spectroscopic personal radiation detectors (SPRDs) which measure the energy spectrum of emitted radiation to identify its specific radionuclide. Radiation dosimetry is another common use for these devices. Radiation badges are used by medical personnel, workers in the nuclear industry, and many other workers exposed for work reasons around the world. One type of lift meter called a teletector features a telescopic rod that can be used to measure high radiation dose levels from approximately 4 meters away, providing an additional level of separation between the operator and the radiation source.
