PROPERTIES OF LIQUID IONIZATION CHAMBERS AT LDR BRACHYTHERAPY DOSE-RATES

Properties such as sensitivity, general recombination and reproducibility of liquid-filled parallel-plate ionization chambers for dosimetry in low-dose-rate brachytherapy radiation fields have been evaluated. Two different dielectric liquids, isooctane (C(8)H(1)8) and tetramethylsilane (Si(CH3)(4)), have been used as sensitive media in chambers having a coin-shaped sensitive volume of 3 mm in diameter and 1 mm thickness. An electric field strength of 300 kV m(-1) was found to be optimal with respect to sensitivity, leakage current and general recombination. At absorbed dose rates from 0.1-100 mGy min(-1) the ionization charge measurements at an irradiation time of 1 min showed a reproducibility better than 1%, and a general recombination not exceeding 0.5%. The calibration-absorbed dose to water against ionization charge at a Co-60 reference source-did not show any significant change over an observation time of one year for any of the chambers.