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Nuclear Radiation SensorsIonization ChambersAn ionization chamber typically is a metal cylinder, with an internal electrode running down the axis. A high voltage usually several hundred or thousand volts, is applied between the two. The chamber is filled with a gas, which could be as simple as dry air, or could be an exotic gas like krypton or xenon, depending on the application.When radiation enters the chamber, it ionizes the gas (frees electrons from atoms), allowing electrical current to flow. This current is proportional to the radiation dose rate, hence it gives a good measurement of the radiation level. The main advantage of the ionization chamber as a nuclear radiation sensor is that it is simple and inexpensive to build. The disadvantages include the fact that the electrical current produced is extremely weak, and must be amplified with sophisticated electronic circuitry. Also, the output current tends to drift with time, and the system must be frequently re-zeroed.
Proportional CounterA proportional counter is very similar to an ionization chamber. The major difference is that the applied voltage is higher. As a result, whenever a radiation particle enters the counter and ionizes a gas molecule, the higher voltage accelerates the freed electron(s), which cause them to ionize additional molecules. This causes a higher current to be produced. The current is roughly proportional to the energy of the radiation, hence the name of the detector. The output current is in the form of a single pulse. If this pulse is measured, the energy of the incident radiation can be determined.As with ionization chambers, sophisticated electronic circuitry is required to amplify and measure the output pulses.
Geiger CounterA geiger counter is virtually the same as a proportional counter, except that the voltage is higher still. As a result, the current pulses all have roughly the same level. While radiation energy information is lost, the output pulses are so large that they can be easily counted, without the need for expensive amplifiers. Take a look at our page which explains how geiger counters work.Black Cat Systems offers a full line of geiger counter based nuclear radiation sensors.
Scintillation DetectorsA scintillation detector uses a special crystal, often sodium iodide (NaI) which converts the radiation energy into light. This light is detected using a photo multiplier tube (PMT) which converts it to electrical current and amplifies it. The result is very similar to a proportional counter.The advantage is that the crystals often have a very high efficiency, especially for higher energy gamma rays, so scintillation detectors are extremely sensitive. The disadvantage is primarily cost - the crystals and PMTs are all very expensive items.
Solid State Nuclear Radiation SensorsThere are a variety of solid state detectors available today. These are semiconductors, which directly convert the incident radiation into electrical current, much as a proportional counter tube does, except that rather than gas, a material such as silicon is used. Other common materials are germanium, cadmium zinc telluride, etc.A major advantage of such sensors is their extremely high energy resolution. That is, they are very good at determining exactly what the energy of the incident radiation is. A disadvantage is cost, the detectors themselves are quite expensive, as are the associated electronics required.
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Thinking of buying a surplus CDV-700 or 715 detector? Be sure to read our report first.