What Can Ionization Chamber-Type Survey Meters Do?

The ionization chamber is a widely used gas-filled radiation detector, capable of detecting and measuring X-rays, gamma rays, and beta particles. The 451B's site surveying capabilities make it suitable for a range of end users, and the ionization chamber detector provides a rapid response time to radiation from leaks, scattering beams, and pitting. Ion chambers have a relatively low voltage between the anode and cathode, resulting in the collection of only the charges produced in the initial ionization event. Examples of specialized basic ionization chambers include radiation metering meters such as the cutie-foot, some pocket dosimeters, and radionuclide dose calibrators. The basic concept of the ionization chamber is highly versatile, allowing for specialized devices to be designed for specific applications.

Rather than measuring individual ionizing events, the ionization chamber is used to measure the total current resulting from multiple events during a certain integration time in a given radiation detection environment. 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. Some types have very thin mica or aluminum entry windows for the ionization chamber and can be used to detect β particles, as well as x-rays or γ rays. The main difference between a proportional counter and the basic ionization chamber is that a higher voltage is applied between the electrodes in the first. As they can show individual ionizing events, GM meters are generally more sensitive to low levels of radiation than ion chamber instruments.

The name of the device is based on the proportionality of total ionization to the total energy of ionizing radiation. Topographic meters with an ionization chamber provide reasonably accurate estimates of exposure rates (± 10%) over most of the nuclear medicine energy range. The interaction of ionizing radiation with the gas in the chamber creates positive and negative ions, which move toward the electrodes and produce an electric current. Due to the high voltage, the initial ionization causes an “avalanche of secondary ionizations” so that the gas is essentially completely ionized. This mode of operation of an ionization chamber allows for detection of individual events but not their energy (that is, several devices routinely used in nuclear medicine clinics operate on this principle).

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