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Application of the two-dose-rate method for general recombination correction for liquid ionization chambers in continuous beams.

Physics in medicine and biology (2010-12-17)
Jonas Andersson, Heikki Tölli
ABSTRACT

A method to correct for the general recombination losses for liquid ionization chambers in continuous beams has been developed. The proposed method has been derived from Greening's theory for continuous beams and is based on measuring the signal from a liquid ionization chamber and an air filled monitor ionization chamber at two different dose rates. The method has been tested with two plane parallel liquid ionization chambers in a continuous radiation x-ray beam with a tube voltage of 120 kV and with dose rates between 2 and 13 Gy min(-1). The liquids used as sensitive media in the chambers were isooctane (C(8)H(18)) and tetramethylsilane (Si(CH(3))(4)). The general recombination effect was studied using chamber polarizing voltages of 100, 300, 500, 700 and 900 V for both liquids. The relative standard deviation of the results for the collection efficiency with respect to general recombination was found to be a maximum of 0.7% for isooctane and 2.4% for tetramethylsilane. The results are in excellent agreement with Greening's theory for collection efficiencies over 90%. The measured and corrected signals from the liquid ionization chambers used in this work are in very good agreement with the air filled monitor chamber with respect to signal to dose linearity.

MATERIALS
Product Number
Brand
Product Description

Sigma-Aldrich
2,2,4-Trimethylpentane, anhydrous, 99.8%
Supelco
2,2,4-Trimethylpentane, analytical standard
Sigma-Aldrich
2,2,4-Trimethylpentane, suitable for HPLC, ≥99%
Sigma-Aldrich
2,2,4-Trimethylpentane, HPLC Plus, for HPLC, GC, and residue analysis, ≥99.5%
Sigma-Aldrich
2,2,4-Trimethylpentane, ReagentPlus®, ≥99%
Sigma-Aldrich
2,2,4-Trimethylpentane, puriss. p.a., ≥99.5% (GC)
Sigma-Aldrich
2,2,4-Trimethylpentane, puriss. p.a., ACS reagent, ≥99.5% (GC)
Sigma-Aldrich
2,2,4-Trimethylpentane, ACS reagent, ≥99.0%
Sigma-Aldrich
Tetramethylsilane, ≥99.0% (GC)
Sigma-Aldrich
Tetramethylsilane, electronic grade, ≥99.99% trace metals basis
Supelco
Tetramethylsilane, analytical standard, for NMR spectroscopy, ACS reagent
Supelco
Density Standard 692 kg/m3, H&D Fitzgerald Ltd. Quality
Sigma-Aldrich
Tetramethylsilane, ACS reagent, NMR grade, ≥99.9%