As defined by the United States Nuclear Regulatory Commission (NRC), a topographic meter is any portable radiation detection instrument specially adapted to inspect an area or individual to establish the existence and quantity of radioactive material present. Collecting only primary ions provides information on true radiation exposure (energy and intensity). However, meters require sensitive electronics to amplify the signal, which makes them quite expensive and delicate. The extra expense and care required are justified when accurate measurements of radiation exposure over a range of radiation energies need to be made. This may be necessary when measuring Bremsstrahlung radiation produced by an X-ray generator.
An ion chamber inspection meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used. This unique use of the CT chamber requires that the active volume response be uniform along its entire axial length, a restriction that is not required in other full immersion cylindrical chambers. Small ventilated air ionization chambers with a volume of 0.01 to 0.3 cm3 are considered suitable for measuring field parameters up to 2 cm × 2 cm. An ionization chamber and an electrometer require calibration before use and, with a triaxial connection cable, tools are required for calibration of the radiation beam. Some types have very thin mica or aluminum entrance windows for the ionization chamber and can be used to detect β particles, as well as x-rays or γ rays.
This mode of operation of an ionization chamber allows the detection of individual events, but not of their energy (i.The 451B's site inspection capabilities make it suitable for a wide range of end users, and the ionization chamber detector enables rapid response time to radiation from leaks, scattering, beams, and pitting. The basic concept of the ionization chamber is extremely versatile, allowing specialized devices to be designed for specific applications. A protective electrode is typically provided in the chamber to further reduce chamber leakage and ensure improved field uniformity in the active or sensitive volume of the chamber, with advantages in charge collection. The generic design concept is a gas-filled chamber with positive and negative electrodes, placed on opposite sides of the chamber or in a concentric cylinder geometry. Noble gas ionization chambers are simple, resistant to radiation, and are easily constructed in the 4π geometry used for accurate measurements of gamma-ray source activity (Suzuki et al.).
Proportional counters work on successive ionization by collision between ions and gas molecules (charge multiplication); in the proportional region, amplification occurs (approximately 103-104 times) for the primary ions to obtain enough energy in the vicinity of the thin central electrode to cause more ionization in the detector. Due to the high voltage, the initial ionization causes an “avalanche of secondary ionizations”, so that the gas is essentially completely ionized. The name of the device is based on the proportionality of the total ionization to the total energy of the ionizing radiation. They act as solid-state ionization chambers when exposed to radiation and, like scintillation detectors, belong to the class of solid-state detectors. Self-reading pocket dosimeters in the form of a pen, consisting of an ionization chamber that functions as a condenser, fully charged (corresponding to zero dose) before use. Like the proportional counter, the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas.
An ion chamber survey meter is a device used for measuring radiation exposure over a range of energies. It consists of a gas-filled chamber with positive and negative electrodes placed on opposite sides or in a concentric cylinder geometry. It can detect β particles as well as x-rays or γ rays and can be used for gamma radiography or site inspections. It requires calibration before use and has advantages such as reduced leakage and improved field uniformity.
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