Liquid ionization chamber initial recombination dependence on LET for electrons and photons

Abstract

The possibility of indirect measurements of linear energy transfer (LET) with a liquid ionization chamber (LIC) has been investigated by studying initial recombination losses at different applied voltages. A linear fit is made to the voltage-signal curve and the intersection point of the fit and the voltage-axis is shown to correlate with LET. The LIC applied voltages were 100-700 V, which corresponds to electric field strengths between 0.3 and 2.0 MV m(-1). Several different photon and electron beams have been studied, and by using MCNPX™ the respective LET spectra have been determined. The beam qualities in this study were found to have a fluence averaged LET between 0.17 and 1.67 keV µm(-1) and a corresponding dose averaged LET between 0.97 and 4.62 keV µm(-1). For the experimental data in this study the linear fit method yields consistent results with respect to Monte Carlo simulated LET values. A calibration curve for LET determination is provided for the LIC used in the present work.

Modeling ion recombination in liquid ionization chambers — Improvement and analysis of the two‐dose‐rate method

“…Schmidt has reported electron mobilities of 7 and 99 ± 5 cm 2 V −1 s −1 for isooctane and tetramethylsilane, respectively, indicating that application of the Onsager theory to tetramethylsilane will be much more sensitive to the ionization density, i.e., radiation beam quality, compared to isooctane . Furthermore, it has been shown by Johansson et al that the 120 kVp X‐rays employed in the present investigation are several times more densely ionizing than medium energy photons, such as annihilation photons (511 keV) . It is clear that the Onsager theory cannot be applied to both isooctane and tetramethylsilane, particularly for densely ionizing radiation such as 120 kVp X‐rays, to help interpret the deviations found in our results.…”

“…23 Furthermore, it has been shown by Johansson et al that the 120 kVp X-rays employed in the present investigation are several times more densely ionizing than medium energy photons, such as annihilation photons (511 keV). 26 It is clear that the Onsager theory cannot be applied to both isooctane and tetramethylsilane, particularly for densely ionizing radiation such as 120 kVp X-rays, to help interpret the deviations found in our results.…”

“…As a consequence, it is reasonable to assume that tetramethylsilane can violate the conditional requirement of the two‐dose‐rate method, , more easily than isooctane given that a higher charge density leads to more charge screening and distortion of the collecting electric field. As previously discussed, initial recombination in liquids also has a strong dependence on the radiation beam quality . Users of the two‐dose‐rate method should thus verify the valid range of general collection efficiencies for their applications, even for isooctane, when not in the near saturation region.…”

“…As previously discussed, initial recombination in liquids also has a strong dependence on the radiation beam quality. 26 Users of the two-dose-rate method should thus verify the valid range of general collection efficiencies for their applications, even for isooctane, when not in the near saturation region.…”

Investigating ion recombination effects in a liquid-filled ionization chamber array used for IMRT QA measurements

“…5,6 However, there is a tradeoff-the liquid reduces ion mobility, leading to ion collection times that are approximately 100 times larger than air-filled chambers, which increases ion recombination effects. [7][8][9][10][11][12][13] Ion recombination effects have been studied in LICs and found to be proportional to the frequency and dose of the pulses delivered to the detector, among other parameters that are typically constant during a measurement. The dose and frequency of the pulses at each detector will vary during an IMRT delivery due to the changing multileaf collimator (MLC) aperture and changing machine dose rate (MU/min), respectively.…”