The use of liquids in ionization chambers for high precision radiotherapy dosimetry

Two dielectric liquids, isooctane and tetramethylsilane (TMS) have been tested as sensitive media in parallel-plate ionization chambers. The ionization chambers use graphite electrodes and have a high spatial resolution due to a coin-shaped ionization volume of 2 mm3 (diameter 3 mm, height 0.3 mm). The detectors show a calibration stability within ± 1% over several years. After a single very large absorbed dose of 10 kGy a sensitivity loss of ∼ 0.1% was found in TMS while the corresponding loss was about 2% in isooctane. In neither liquid did general recombination exceed 2% in pulsed radiation fields and dose rates up to 7 Gy min-1 (PRF = 200 s-1, and dose per pulse = 0.6 mGy). Residual current, reproducibility and sensitivity are more favourable in TMS than in isooctane. Absorbed dose to water calibration factors at various radiotherapy qualities were determined by the use of FeSo4 dosimetry as a reference. A weak, hardly resolvable difference in the calibration factor was found in TMS chambers, and in isooctane chambers this was within 3%. The boiling point of TMS is low, restricting its upper operational temperature to ∼ 26oC. (author)