Prog Med Phys.  2014 Mar;25(1):46-52. 10.14316/pmp.2014.25.1.46.

Determining Ion Collection Efficiency in a Liquid Ionization Chamber in Co-60 Beam

• Choi SH
• Kim CH

• KMID: 1910558
• DOI: http://doi.org/10.14316/pmp.2014.25.1.46

Abstract

Liquid ionization chamber is filled with liquid equivalent material unlike air filled ionization chamber. The high density material allow very small-volume chamber to be constructed that still have a sufficiently high sensitivity. However liquid ionization chamber should be considered for both initial recombination and general recombination. We, therefore, studied using the Co-60 beam as the continuous beam and the microLion chamber (PTW) for comparing the ion collection efficiency by Greening theory, two-dose rate method and our experiment method. The measurements were carried out using Theratron 780 as the cobalt machine and water phantom and 0.6 cc Farmer type ionization chamber was used with microLion chamber in same condition for measuring the charge of microLion chamber according to the dose rates. Dose rate was in 0.125~0.746 Gy/min and voltages applied to the microLion chamber were +400, +600 and +800 V. As the result, the collection efficiency by three method was generally less than 1%. In particular, our experimental collection efficiency was in good agreement within 0.3% with Greening theory except the lowest two dose rates. The collection efficiency by two-dose rate method also agreed with Greening theory generally less than 1%, but the difference was about 4% when the difference of two dose rates were lower. The ion recombination correction factors by Greening theory, two-dose rate method and our experiment were 1.0233, 1.0239 and 1.0316, respectively, in SSD 80 cm, depth 5 cm recommended by TRS-398 protocol. Therefore we confirmed that the loss by ion recombination was about 3% in this condition. We think that our experiment method for ion recombination correction will be useful tool for radiation dosimetry in continuous beam.

Keyword

Liquid ionization chamber; Collection efficiency; Ion recombination

MeSH Terms

Cobalt
Radiometry
Recombination, Genetic
Silver Sulfadiazine
Water
Cobalt
Silver Sulfadiazine
Water

Figure

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  Fig. 1. Diagram showing the setup of the 0.6 cc ionization chamber and microLion liquid ionization chamber in water phantom.

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  Fig. 2. The ionization current of air filled ionization chamber (left) and microLion liquid ionization chamber (right) versus applied voltage for Co-60 beam.

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  Fig. 3. General correction efficiency relative to the dose rates (Gy/min) of the microLion chamber in different voltages (+800, +600 and +400) determined by Greening theory.

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  Fig. 4. Comparison of correction efficiency obtained by the Greening theory, two dose rate method and our experiment method.

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