The ionization chamber is an input device that is used to measure the absorbed dose in radiation beams from X-ray generators and cobalt sources. It consists of a gas-filled cavity surrounded by a conductive outer wall and having a central collecting electrode. A protective electrode is generally provided in the chamber to further reduce leakage from the chamber and ensure improved field uniformity in the active or sensitive volume of the chamber. The detectors are located directly in front of the image receiver, which is where X-ray exposure is measured just before entering the image receiver.
The leakage current of an ionization chamber is composed of a dose-independent and a dose-dependent part. The dose independent part can be measured in the absence of radiation (including background) and is due to leaks in the cable, connectors (dirt and moisture) and the first amplifier (low noise, low polarization). This leakage current can be subtracted from the current during irradiation. The dose-dependent part is difficult to measure, but can be kept at low levels by reducing radiation to the stem of the ionization chamber and to the cable, connectors and electronic equipment.
In tests using fluoroscopy, the irradiation geometry (field size, focus, distance from skin, and projection) and irradiation times vary individually from patient to patient. If the detector mounted in the tube housing is “transparent” to X-rays, both focal and extrafocal radiation will pass through its sensitive volume. If attenuation in the air can be neglected, X-rays transmitted through the detector will pass through all planes perpendicular to the central axis of the beam downstream of the beam. This means that if the integration of the air kerma over the beam area extends across the entire plane, then the KAP will be invariant with the distance from the X-ray tube, provided that the beam is contained by the KAP meter. A transmission ionization chamber generally consists of layers of PMMA coated with conductive material.
Graphite, a commonly used coating material, is close to the equivalent of air and introduces a low energy dependence for air kerma measurements. However, graphite coating is inconvenient in transmission chambers since it is not transparent to light. Therefore, light-transparent materials are mainly used. These materials contain high atomic number elements such as indium and tin which results in a relatively strong energy dependence compared to graphite-coated chambers. Transmission ionization chambers are used as detectors for KAP (or DAP) meters.
In addition to fluoroscopy, KAP meters are also widely used in general radiography and are increasingly being installed in modern dental radiography equipment. For out-of-beam measurements such as those required in radiation shielding, higher sensitivities are desirable. Proportional meters are more sensitive than ionization chambers and are suitable for measurements in low-intensity radiation fields. 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 primary ions to obtain enough energy in the vicinity of a thin central electrode to cause more ionization in the detector. The amount of charge collected from each interaction is proportional to the amount of energy deposited in meter gas by interaction. Personal direct-read monitors are widely used to provide a direct dose reading at any time and to track doses received in daily activities and special operations (e.g., self-reading pocket pen-shaped dosimeters).
These consist of an ionization chamber that functions as a condenser which is fully charged (corresponding to zero dose) before use. After exposure to radiation for a period of time, ionization produced in chamber discharges condenser; exposure (or air kerma) is proportional to discharge which can be read directly against light through built-in eyepiece. A special emulsion photographic film is included in a light-tight enclosure in a windowed box or holder with appropriate filters. The card holder creates a distinctive pattern on film that indicates type and energy of radiation received. While filter is suitable for photons with energy above 100 keV, use of multiple filter system is necessary for lower energy photons since film is not equivalent to tissue; filter system must be used to flatten energy response especially at lower photon beam qualities to approximate response of tissue equivalent material. Optical density allows evaluation of cumulative doses of different radiation sources using different filters and comparing densitometric results with calibration films exposed to known doses of well-defined radiation different types.
Films are adversely affected by many external agents such as heat, liquids, excessive moisture; latent image undeveloped film fades over time limiting possible periods use 3 months under ideal conditions. OSL systems contain thin layer aluminum oxide (Al2O) sandwiched within heat-sealed filter package. During analysis OSL stimulated with selected frequencies laser light producing luminescence proportional radiation exposure; special filter patterns provide qualitative information about conditions during exposure. OSL dosimeters are highly sensitive (from up 10 μSv with accuracy ± 10 μSv 10 Sv photon beams 5 keV 40 MeV) making them particularly suitable individual monitoring low-radiation environments; dosimeters can retested several times without losing sensitivity can used up 1 year. When ionization chambers not most suitable detectors side profile measurements alternative use 2D detectors such scintillation detectors14,15 Gafchromic films16,17 both types detectors detector responses must verified terms linearity.
