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

Abstract

Plane-parallel ionization chambers are the instrument of choice for use in electron calibration and dosimetry, but these chambers may exhibit large polarity effects. This study concentrates on measuring the dependence of the polarity error at various mean energies using different linear accelerator, field size, ion chamber, and electrometer combinations. The polarity error was shown to increase for all four ionization chambers as the mean energy at depth decreased, but was always less than one percent at d(max). Polarity error dependence was also observed for similar plane-parallel chambers, varying field sizes, and different linear accelerators, but no polarity error dependence was observed for similar cylindrical chambers and different electrometers. Measurements of the polarity error can be used to develop correction factors for future measurements that will help minimize the time spent performing electron dosimetry and calibrations. These correction factors can be used to calculate the correct reading without the need to reverse the chamber bias, thus reducing the number of measurements required.

The response of prototype plane‐parallel ionization chambers in small megavoltage x‐ray fields

“…Monte Carlo simulations have also been used in order to calculate necessary correction factors for small chambers, including those required for small fields. Ion chambers with both cylindrical 16 and plane-parallel 7,17,18 geometry have been studied for the measurement of various dosimetric parameters, such as relative dose factors ͑RDFs͒, tissue maximum ratios, percentage depth dose, and off-axis ratio, for small fields.…”

Polarity effect on surface dose measurement for an attix parallel plate ionisation chamber

“…They are often used for measurement of dose in the build up region of high energy photon beams and in regions of high dose gradient. Some types of parallel plate ionisation chambers have shown a difference in collected charge between one polarity and another when used in the build up region or at depth [1][2][3][4] . Gerbi 5 studied the Attix chamber for polarity effects in the build up region for a 10cm x 10cm field size and found a value for the polarity ratio of 0.968 and 0.981 for 6MV and 24MV x-rays respectively during the measurement of surface dose.…”

Technical Note: Influence of Compton currents on profile measurements in small‐volume ion chambers

“…Ramsey et al 10 performed a multifactor investigation of polarity effects in electron beams for both plane-parallel and cylindrical chambers. They also noted larger polarity effects for lower mean energies, and found a 0.5% difference in polarity corrections across a range of field sizes.…”

“…9 Ramsey et al conducted an evaluation of the suitability of a single correction factor for polarity effects, but did not work with chambers that had sensitive volumes comparable to the microchambers that exist today. 10 Our work focuses on the influence of Compton currents on three different models of microionization chamber when exposed to electron radiation fields.…”