Free-air ionization chambers

Abstract

This paper reviews the current status of free-air ionization chamber standards for the determination of air kerma in low- and medium-energy x-rays. It describes the underlying definitions and concepts that permit the air kerma to be determined from a measurement of the charge produced in a free-air chamber. The most commonly used design, the parallel-plate free-air chamber, is discussed in detail, in particular with a view to optimization of the design parameters to minimize the overall uncertainty of the air-kerma determination. Various correction factors and their uncertainties are discussed, with an emphasis on the use of the Monte Carlo technique in the evaluation and optimization of certain corrections. Alternative free-air chamber designs are outlined. The results of international comparisons of free-air chamber standards are presented.

Absolute x-ray dosimetry on a synchrotron medical beam line with a graphite calorimeter

“…The electron loss correction factor k e accounts for energetic electrons failing to deposit all their energy in the collecting volume, resulting in reduced collected charge. 8,15 Increasing the plate separation reduces the magnitude of the electron loss, but at the expense of decreasing the field strength and therefore increasing the ion recombination losses. The design of the free-air chamber is therefore a compromise.…”

“…12,14 The recombination can be lowered by reducing the plate separation on a free-air chamber, but this is limited by the rapid increase of electron loss with decreased plate separation. 8,15,16 The electron loss becomes particularly problematic at energies above 100 keV for the free-air chamber used in this work. 12 As this higher energy range is desirable for suitable penetration in radiotherapy applications, electron loss is unavoidable.…”

Spatial distribution of air kerma rate and impact of accelerating voltage on the quality of an ultra soft X-ray beam generated by a cold cathode tube in air

“…Free-air ionization chambers (IC) are considered to be primary standard instruments for measurement of X-ray Air-Kerma and exposure determination (Burns and Buermann, 2009). In this work an original millimeter-sized parallel plate free air IC is used for measuring the air kerma of the USX beam with energy 1.5 keV.…”

Technical Note: An investigation of polarity effects for wide‐angle free‐air chambers

“…First, its beam-defining aperture has a diameter of 8 cm, while that of a conventional FAC is typically 1 cm or less. 7 The large aperture increases the sensitivity of the device because photons are accepted over a wide angular range.…”