A scintillation counter detects and measures emissions of radioactive elements. Radioactivity is the release of particles or energy from elements with too many neutrons. The counter combines a chemical that creates light when hit by emissions and a detector to count the light pulses. There are three types of radioactive decay: alpha, beta, and gamma radiation. All three cause cellular damage to plants and animals. The counter uses solid scintillation chemicals, including inorganic, organic, and plastic phosphors. The counting software is critical in determining the amount of radiation. Photomultipliers are used to detect very small flashes of light. Liquid phase units are useful in laboratory analyses and decontaminating radioactive spills.
A scintillation counter is a device used to detect and measure emissions of radioactive elements. Radioactivity is a release of particles or energy from some elements that contain too many neutrons and can be dangerous to humans, animals and plants. The scintillation counter combines a chemical that creates light when hit by radioactive emissions and a detector to detect and count the light pulses.
Many elements have isotopes, molecules that contain different numbers of neutrons with the same number of protons and electrons. Most isotopes are stable and nothing will happen to change their chemical composition over time. A number of radioactive isotopes, however, will not hold the neutrons in place and will begin to decay radioactively.
There are three main types of radioactive decay and each has different characteristics. Alpha radiation is a particle that combines protons and neutrons and has a relatively low energy, which allows it to be stopped by water or thin metal plates. Beta radiation is high energy electrons released from the element and can penetrate body tissue and protective shielding layers. Gamma radiation is not a particle, but rather an electromagnetic wave, similar to light, which has a very high energy and can only be shielded by layers of dense lead foil.
All three types cause cellular damage to plants and animals because they cause molecules to change when hit by radiation. When a radioactive particle or gamma radiation hits a molecule, it will release electrons into surrounding tissue or the air. If radiation strikes a chemical that emits a flash of light when hit and the light can be detected, a scintillation counter has been created.
There are three types of solid scintillation chemicals, called phosphors, used in meters and they include inorganic, organic and plastic. Inorganic chemicals that can release light, called photons, when hit by radiation include metal iodides and zinc sulfide. Organic phosphors can include naphthalene, anthracene, and other benzene-related compounds. Plastics themselves are not typically phosphors, but chemicals can be combined with a plastic to form a photon generator.
Inorganic chemicals are the best detectors for gamma radiation, organics are best for beta particles, and phosphors embedded in plastics work well for neutron detection. Radioactive isotopes can decay using a variety of methods, so detectors can contain more than one type of sensing element. The counting software used in detectors is critical in determining the amount of radiation, as higher counts indicate the presence of more radioactive elements or the counter is close to radioactivity.
Once the photons of light are created, the other important part is the detector, which sees the photons and counts them. Many meters use a photomultiplier, which is an array of electrodes mounted in a vacuum tube. When a photon of light enters the tube, it is normally too weak to be detected by the electronic circuitry in the scintillation counter. The photon strikes the first electrode, to which an electric voltage is applied.
When struck by light, the electrode releases more electrons, which travel to the second electrode. Each time this happens, more electrons are released and the signal gets stronger. After several passes, which occur very rapidly with electrons traveling at the speed of light, the signal is strong enough to be detected by the counter, which registers the presence of a photon of light and counts it. A photomultiplier is extremely sensitive and can accurately detect very small flashes of light from decaying.
Another type of scintillation counter is a liquid phase unit. These counters can be useful in laboratory analyses, because a sample is placed directly into a liquid composed of a phosphor and a solvent. Any radioactive emissions are detected immediately by the phosphors surrounding the sample, which are then counted.
This technology can be useful when decontaminating a radioactive spill, because wipe tests can be used to check for radioactivity. Small tissue samples are rubbed onto the surfaces and then placed in a liquid scintillation counter. This process can be repeated as needed until the meter shows that the radioactivity is at low levels, called background radiation.
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