Charge collection efficiency in ionization chambers exposed to electron beams with high dose per pulse

Abstract

The correction for charge recombination was determined for different plane-parallel ionization chambers exposed to clinical electron beams with low and high dose per pulse, respectively. The electron energy was nearly the same (about 7 and 9 MeV) for any of the beams used. Boag's two-voltage analysis (TVA) was used to determine the correction for ion losses, k(s), relevant to each chamber considered. The presence of free electrons in the air of the chamber cavity was accounted for in determining k(s) by TVA. The determination of k(s) was made on the basis of the models for ion recombination proposed in past years by Boag, Hochhäuser and Balk to account for the presence of free electrons. The absorbed dose measurements in both low-dose-per-pulse (less than 0.3 mGy per pulse) and high-dose-per-pulse (20-120 mGy per pulse range) electron beams were compared with ferrous sulphate chemical dosimetry, a method independent of the dose per pulse. The results of the comparison support the conclusion that one of the models is more adequate to correct for ion recombination, even in high-dose-per-pulse conditions, provided that the fraction of free electrons is properly assessed. In this respect the drift velocity and the time constant for attachment of electrons in the air of the chamber cavity are rather critical parameters because of their dependence on chamber dimensions and operational conditions. Finally, a determination of the factor k(s) was also made by zero extrapolation of the 1/Q versus 1/V saturation curves, leading to the conclusion that this method does not provide consistent results in high-dose-per-pulse beams.

Ion recombination correction factors () for Varian TrueBeam high‐dose‐rate therapy beams

“…In clinical practice, this condition is usually satisfied. However, the two‐voltage method has been shown to be notably inaccurate for both low electric field strength (6) and very large doses per pulse 14 , 15 . Piermattei et al, (14) for example, showed substantial error in the recombination correction estimated by the two‐voltage technique for a high dose‐per‐pulse intraoperative electron therapy accelerator.…”

Dosimetry for electron Intra-Operative RadioTherapy: Comparison of output factors obtained through alanine/EPR pellets, ionization chamber and Monte Carlo-GEANT4 simulations for IORT mobile dedicate accelerator

“…In order to determine the absorbed dose, the chamber signal is corrected by using many factors as indicated in IAEA 398 protocol [41]. In particular, the ion recombination correction factor k s was calculated following the method proposed in Laitano et al [6] for each investigated fields in order to reduce the inaccuracy in the measurements due to the high dose per pulse with respect to the conventional electron beam [8,42,6]. The values used in this work are reported in Table 1.…”

“…The main difference is related to the high dose per pulse of IORT dedicated LINACs that does not allow the direct use of ionization chamber as recommended in the protocol IAEA 398 for the absolute dose determination because of ion recombination effects [6]. Indeed, because of the high density of electric charge produced in the ionization chamber's volume per radiation pulse, the correction factor k s for ion recombination is overestimated if the correction methods recommended by the current international dosimetry protocols are used [7][8][9].…”

Current saturation in free-air ionization chambers with chopped synchrotron radiation

“…From the calculated values of ! and measured values of in air, fitting expressions have been deduced (Hochhauser et al, 1994;Laitano et al, 2006). Fig.…”