The ionization chamber is the simplest of all gas-filled radiation detectors, and is widely used to detect and measure certain types of ionizing radiation; X-rays, gamma rays and beta particles. Conventionally, the term "ionization chamber" is used exclusively to describe detectors collecting all charges made by direct ionisation in the gas through the application of an electric field. It only uses the discrete cost created by any interaction between incident radiation and gas, and does not involve the mechanism of gas multiplication used by other radiation instruments, such as a Geiger-MÃÆ'¼ller counter or a proportional counter.
The ion space has a good uniform response to radiation through various energies and is the preferred means of measuring high levels of gamma radiation. They are widely used in the nuclear power industry, research laboratories, radiography, radiobiology, and environmental monitoring.
Video Ionization chamber
Principle of operation
The ionization chamber measures the charge of the number of ion pairs made in the gas caused by the incident radiation. It consists of a gas-filled room with two electrodes; known as anode and cathode. The electrode may be in parallel plate Ionization Chambers: PPIC, or a cylindrical arrangement with an internal coaxial anode wire.
The voltage potential is applied between the electrodes to create an electric field in the fill gas. When the gas between the electrodes is ionized by incident ionizing radiation, ion pairs are made and the resulting positive ions and dissociated electrons move into the electrode of the opposite polarity under electric field influence. This produces an ionized current measured by a series of electrometers. The electrometer must be capable of measuring very small output currents located in the femtoamperes region to picoamperes, depending on the design space, the radiation dose and the applied voltage.
Each ion pair creates a deposit or removes a small electrical charge to or from the electrode, so the charge accumulates proportional to the number of ion pairs made, and hence the radiation dose. Continuous loading generates an ionization current, which is a measure of the total ionizing dose that enters the chamber. However, space can not distinguish between types of radiation (beta or gamma) and can not produce a radiation energy spectrum.
The electric field also allows the device to work continuously by cleaning the electrons, which prevents the gas from being saturated, where no more ion can be collected, and by preventing the recombination of ion pairs, which will reduce the ion current. This mode of operation is referred to as the "current" mode, which means that the output signal is a continuous current, rather than a pulse output as in the case of a Geiger-MÃÆ'¼ller or a proportional counter.
Referring to the graph of the accompanying ion pair collection, it can be seen that in the operating area of ​​"ion space" the ion pair assembly is effectively constant over the given voltage range, due to its relatively low electric field strength, ion space does not have "multiplication effect". This is in contrast to the Geiger-MÃÆ'¼ller tube or proportional counterpart in which the secondary electrons, and finally the double avalanche, greatly amplify the original ion-current charge.
Maps Ionization chamber
Room type and construction
The following room types are commonly used.
Free air space
It is an open space to the atmosphere, where gas fills the ambient air. The domestic smoke detector is a good example of this, where the natural flow of air through the room is necessary so that the smoke particles can be detected by changes in ion currents. Another example is an application in which ions are made outdoors but carried by air or forced gas.
Room pressure
Vented chamber
These spaces are usually cylindrical and operate at atmospheric pressure, but to prevent the entry of water vapor, a filter containing desiccants is installed in the ventilation ducts. This is to stop the formation of water vapor in the interior, which otherwise would be introduced by the "pump" effect of atmospheric atmospheric pressure changes. These spaces have cylindrical bodies made of aluminum or plastic a few millimeters thick. The material is chosen to have an atomic number similar to air so that the walls are said to be "air equivalents" above the various radiant energy rays. It has the effect of ensuring the gas in the room acts as if it is part of an infinitely large gas volume, and improves its accuracy by reducing gamma interaction with wall materials. The higher the atomic number of wall materials, the greater the chance of interaction. Wall thickness is a trade-off between maintaining air effects with thicker walls, and increasing sensitivity by using thinner walls. These rooms often have end windows made of thin enough material, such as mylar, so that the beta particles can enter the gas volume. Gamma radiation enters through the end window and side walls. For handheld wall thickness instruments are made as uniform as possible to reduce photon directionality although the beta window response is clearly highly directional. Ventilated spaces are susceptible to small changes in efficiency with air pressure and correction factors can be applied to highly accurate measurement applications.
Low-pressure space closed
This is similar in construction to a ventilated space but is sealed and operating at or around atmospheric pressure. They contain a special fill gas to improve detection efficiency because free electrons can be easily captured in air-filled spaces by electronegative neutral oxygen, to form negative ions. These rooms also have the advantage of not requiring ventilation and dryer. The final window of the beta limits the differential pressure of tolerable atmospheric pressure, and the common material is stainless steel or titanium with a typical thickness of 25 Âμm.
High pressure chamber
Room efficiency can be further enhanced by the use of high pressure gas. Usually atmospheric pressure of 8-10 can be used, and a variety of noble gases are used. Higher pressures result in greater gas density and thus greater likelihood of collisions by gas filling and ion pair creation by incident radiation. Due to the increased wall thickness required to withstand this high pressure, only gamma radiation can be detected. These detectors are used in survey meters and for environmental monitoring.
Space shape
Viewfinder
Most commonly used for the measurement of radiation therapy is the cylindrical space or "thimble". The active volume is placed inside the thimble-shaped cavity with the inner conductive surface (cathode) and the central anode. The bias voltage applied in the cavity collects ions and produces a current that can be measured with an electrometer.
Space parallel plates
The parallel plate space is formed like a small disc, with a circular collecting electrode separated by a small gap, usually 2 mm or less. The upper disc is very thin, allowing near-surface dose measurements much more accurate than is possible with cylindrical space.
Monitor space
The monitor room is usually a parallel plate ion chamber placed in a radiation beam to continuously measure the intensity of the beam. For example, in the head of a linear accelerator used for radiotherapy, a multi-cavity ionization chamber can measure the intensity of the radiation emission in different areas, providing the symmetry of the file and the flatness information.
Research room and calibration
Early versions of the ion space were used by Marie and Pierre Curie in their original work in isolating radioactive material. Since then ion space has become a widely used tool in laboratories for research and calibration purposes. To do this various forms of bespoke booths, some using liquids as ionized media, have evolved and used. Ion Space is used by national laboratories to calibrate key standards, and also to transfer these standards to other calibration facilities.
History Room
Condenser space
The condenser chamber has a secondary cavity inside the rod acting as a capacitor. When this capacitor is fully charged, every ionization in the thimble removes this charge, and the charge changes can be measured. They are only practical for beams with energy of 2 MeV or less, and high stem leakage makes them incompatible with proper dosimetry.
Extrapolation room
Similar in design to parallel plate spaces, the upper plate of the extrapolation chamber can be lower using a micrometer screw. Measurements can be taken with different plate spacing and extrapolated to a zero plate distance, ie a dose without a room.
Instrument type
Hands held
The ion space is widely used in handheld radiation survey meters to measure beta and gamma radiation. They are highly preferred for high dose rate measurements and for gamma radiation they provide good accuracy for energy above 50-100 keV.
There are two basic configurations; an "integral" unit with space and electronics in the same case, and a "two-piece" instrument that has a separate ion chamber probe attached to an electronic module by a flexible cable.
The integral instrument space is generally in front of the case facing down, and for the beta/gamma instrument there is a window at the bottom of the casing. It usually has a shear shield that allows discrimination between gamma and beta radiation. The operator closes the shield to exclude the beta, and thus can calculate the rate of each type of radiation.
Some hand-held instruments generate audible clicks similar to those generated by G-M counters to assist operators, who use audio feedback in radiation surveys and contamination checks. Since the ion space works in the current mode, not the pulse mode, it is synthesized from the radiation rate.
Installed
For industrial process measurements and interlocks with continuous high radiation levels, ion space is the preferred detector. In this app only space is located in the measurement area, and electronics are located at a distance to protect them from radiation and connected by cable. Installed instruments can be used to measure gamma ambient for personnel protection and usually sound an alarm above a predetermined level, although the Geiger-Müller tube instrument is generally preferred where high accuracy is not required.
General precautions used
Humidity is a major problem affecting the accuracy of ion space. The internal volume of the chamber must remain completely dry, and the ventilator type uses a desiccant to assist with this. Due to the very low current generated, the leakage current must be kept to a minimum to maintain its accuracy. The hygroscopic humidity not visible on the dielectric surface of the wires and connectors can be sufficient to cause leakage currents that will overwhelm any radiation induced ion currents. This requires a careful cleaning of the chambers, terminations and cables, and subsequent drying in the oven. The "guard ring" is generally used as a design feature on higher voltage tubes to reduce leakage through or along the tubular insulator's support surfaces, which can require resistance in the order of 10 13 Ã,.
For industrial applications with remote electronics, ion chambers are housed in separate enclosures that provide mechanical protection and contain dryers to remove moisture that may affect termination resistance.
In installations where space is remotely from measuring electronics, readings may be affected by external electromagnetic radiation acting on the cable. To overcome this the local converter module is often used to translate very low ion space currents to pulse trains or data signals associated with incident radiation. It is immune to electromagnetic effects.
Apps
Nuclear industry
Ionization space is widely used in the nuclear industry because they provide outputs that are proportional to the radiation dose. They found widespread use in situations where constant high doses were being measured because they had a larger operating lifetime than standard Geiger-Müllller tubes, which suffered from damaged gases and were generally confined to the lives of about 10 11 count events. In addition, Geiger-MÃÆ'¼ller tubes can not operate above about 10 4 counts per second, due to dead time effects, whereas there is no similar limit on ion space.
Smoke detector
The ionization chamber has found widespread and useful use in smoke detectors. In smoke detectors, ambient air is allowed free entry into the ionization chamber. The space contains a small amount of americium-241, which is an alpha particle emitter that produces a constant ion current. If smoke enters the detector, it interrupts this current because the ion attacks the smoke particles and is neutralized. This drop in triggered an alarm. The detector also has a sealed reference room but is ionized in the same way. Comparison of ion currents in two chambers allows compensation for changes due to air pressure, temperature, or source aging.
Medical radiation measurements
In medical physics and radiotherapy, the ionization chamber is used to ensure that the dose delivered from the therapy unit or radiopharmaceutical is intended. The device used for radiotherapy is called a "dosimeter reference", while those used for radiopharmaceuticals are called radioisotope dose calibrations - an improper name for the radionuclide radioactivity calibrator , used for radioactivity measurements but not absorbed doses. A space will have calibration factors established by national standard laboratories such as ARPANSA in Australia or NPL in the UK, or will have factors determined by comparison to standard transfer space traceable to national standards on user sites.
Application usage guides
In the UK, HSE has issued a user guide to select the right radiation measurement instrument for the particular application in question. It covers all the technology of radiation instruments, and is a useful comparative guide to the use of ion space instruments.
See also
- Absorbed dose
- Dosimetry
- The Sievert Room
- Stops power of radiation particles
Note
References
Source of the article : Wikipedia
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