Radioactive elements are both naturally occurring and man-made. They can have carcinogenic and other health effects, yet their radiation is undetectable by human senses, even in lethal doses.
The most commonly detected radiation is alpha, beta and gamma. Alpha--helium nuclei--and beta--electrons--are high-energy particles, and gamma is high-energy electromagnetic radiation. These types are called ionizing radiation, because of their ability to break apart atoms into charged particles called ions.
No detector is sensitive to every type of radiation. Some instruments are designed for particle detection, while others are capable of detecting and measuring particle and wave types. Two basic types of instruments are used to detect radiation: particle counters and dose measuring instruments, or dosimeters.
A common and highly sensitive particle detecting device is the Geiger-Muller counter. The detector is a gas-filled tube with a central wire, or anode, for collecting electrons and an outer cathode for collecting positive ions. The tube contains helium, neon or argon gas, with halogens--chlorine, bromine or fluorine--added. High voltage is applied to the tube. An ionizing particle passing through the tube strikes gas molecules, ionizing them. The ions are accelerated toward the electrodes by high voltage, striking more gas molecules. The result is a cascade of electrons and positive ions, and a pulse of electric current that registers as a "count" on the instrument. In a Geiger-Muller tube, the relationship between particle energy and the number of ions produced is non-proportional, which means it can only count particles--the instrument cannot measure radiation "dose." Geiger-Muller counters are used to screen various environmental samples, to measure body fluid activity and to conduct radiation surveys.
A radiation dosimeter is a small device worn on the body for measuring personal radiation exposure. Among the many types in use, the optically stimulated luminescence (OSL) dosimeter is one of the latest designs as of November 2010. More sensitive than other types, it uses an aluminum oxide strip detector to record beta and gamma radiation. The analysis involves exposure of the strip to specific frequencies of laser light, which cause the aluminum oxide to become luminescent in proportion to the amount of radiation energy deposited in it. A report of exposure results is generated based on the measured luminescence. Another advantage of the OSL dosimeter is results can be read up to a year after exposure. OSL dosimeters are worn on the torso or a finger.