What are devices use to meausure radiation exposure?

A Geiger counter There are several different types of detectors used to quantify radiation exposure. The Geiger counter (GM tube) is one type of meter based on the action of ionizing radiation with the gas in a tube or chamber that is placed in an electric field. Gas is naturally an insulator, meaning most electrical currents do not readily move through a gas. But, if the gas is ionized (electrons are released from the gas atoms) by radiation, then an electrical current can be generated by the motion of the electrons. If the gas is placed in an electric field by applying a positive and negative polarity to the container holding the gas (just like the positive and negative polarity you see on a battery), then ionized, the electrons freed in the gas will travel towards the positive pole, creating a current. The amount of current created will be proportional to the amount of radiation interacting with the gas. So, other gas filled instruments are the ionization chamber and the proportional counter. The ionization chamber is probably a more accurate detector of exposure in a high radiation field than the GM tube. Solid detectors also exist, such as scintillators (give off light in proportion to the amount of radiation interacting with the detector). And, radiation will darken x-ray film. The darkness of the film is also proportional to the amount of radiation interacting with the film/detector. The radiation exposure monitors that radiation workers wear on their bodies are either scintillators or film type detectors.