Polarity effect in plane‐parallel ionization chambers using air or a dielectric liquid as ionization medium

Abstract

A plane-parallel ionization chamber having a sensitive volume of 2 mm3 and using the dielectric liquid tetramethylsilane as the sensitive medium instead of air is described. In the design of the chamber special attention was given to the factors that can cause unwanted currents in the cable, stem, or the chamber dielectric material. The chamber has been tested with respect to the polarity effect in regions of radiation fields where ordinary plane-parallel ionization chambers will often fail. These regions are the build-up region in photon fields, and the region close to the practical range for electrons where nonelectronic equilibrium is significant. Experimental results show that, despite the extremely small ionization volume in the liquid ionization chamber, the polarity effect never exceeds a few tenths of a percent in field positions where well-known commercially available chambers with much less spatial resolution designed for measurements in radiation therapy fields can show polarity effects of 5% to 30%. The origin of spurious currents and how they must be minimized in the design of either a liquid- or gas-filled ionization chamber is discussed.

Ion recombination and polarity effect of ionization chambers in kilovoltage x‐ray exposure measurements

“…The polarity effect depends strongly on the medium between the plates of chamber and is the highest for air ͑р30%͒ and the lowest ͑Ͻ0.3%͒ for dielectric medium. 21 If the charges collected at negative and positive potentials are denoted by Q Ϫ and Q ϩ , respectively, then k pol is defined as the absolute value of the ratio: 5…”

Ionization chamber, electrometer, linear accelerator, field size, and energy dependence of the polarity effect in electron dosimetry

“…The plane-parallel and cylindrical chamber results are similar to those found in previous studies. 3,8 Figure 3͑a͒ shows a comparison of the standard deviation of the curve fits for the two PTW 23343 plane-parallel chambers. This figure suggests a possible chamber dependence in the polarity error at low mean energies for the two planeparallel chambers tested, although the difference is small.…”

“…In addition, it has been suggested that some ionization chambers require an adjustment period of several minutes in order for the readings to stabilize. [2][3][4] The net effect is an approximate doubling of the time required to take measurements, which can be unacceptable for many physicists in busy departments.…”

Polarity effect in commercial ionization chambers used in photon beams with small fields

“…Williams and Agarwal [11] developed energy-dependent correction factors for four commercially available ionization chambers used in electron dosimetry. Wickman and Holmstrom [12] studied the polarity effect in a custom-built plane-parallel ionization chamber and found that the design of the chamber affected the magnitude of the polarity effect. Other authors have characterized the polarity effects of ionization chambers that had unique design features [13,14].…”