How Ion Chamber Survey Meters Work

An ion chamber survey meter is a device used to measure the amount of ionizing radiation present in an environment. It works by creating an electric field between two electrodes, an anode and a cathode. This electric field causes electrons from each ion pair to move towards the anode, while the positively charged gas atom is attracted to the cathode. This results in an electrical signal that is amplified, correlated with exposure and displayed as a value.

Ionization chambers with transparent X-ray plates made of aluminized plastic or thin metal mesh are used for the detection of fluorescent radiation. They act as solid-state ionization chambers when exposed to radiation and, like scintillation detectors, belong to the class of solid-state detectors. A CT camera is often referred to as a pencil chamber because its active volume comprises a thin cylinder 100 mm in length (sometimes longer). Multi-channel xenon ionization chambers pressurized to 20 bar were developed in the 1970s and 1980s and were successfully used in several clinical computed tomography (CT) scanners. 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.

Self-reading pocket dosimeters in the form of a pen, consisting of an ionization chamber that functions as a condenser, are fully charged (corresponding to zero dose) before use. 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. Radiation indicators are considered, whereas ionization chambers are used for more quantitative measurements. 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. A gas ionization chamber measures charge from the number of ion pairs created within a gas caused by incident radiation.

With reference to the attached ion pair collection graph, it can be seen that in the operating region of the ion chamber, the charge of a collected ion pair is effectively constant over an applied voltage range, since due to its relatively low electric field strength, the ion chamber has no multiplication effect. Proportional meters are more sensitive than ionization chambers and are suitable for measurements in low-intensity radiation fields. Ionization chambers are widely used in the nuclear industry, as they provide an output proportional to the radiation dose. They find wide use in situations where a constant high dose rate is measured, as they have a longer service life than standard Geiger-Müller tubes, which suffer from gas breakage and are generally limited to a lifetime of approximately 1011 counting events. When ionization chambers are not the most appropriate detectors for side profile measurements, an alternative is to use 2D detectors such as scintillation detectors and Gafchromic films. 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.

When the atoms or gas molecules between the electrodes are ionized by the incident ionizing radiation, ion pairs are created and the resulting positive ions are created and the dissociated electrons move to the electrodes of the opposite polarity under the influence of the electric field.

Leave Reply

Your email address will not be published. Required fields are marked *