Characterization of tetramethylsilane for liquid-filled ionization dosimeters: Ion mobilities, free-ion yield and general recombination

Abstract

Liquid-filled ionization chambers (LICs) are interesting detectors for the dosimetry of radiotherapy beams due to their water-equivalent response and high spatial resolution. Isooctane is the liquid most often used as an active medium, but other hydrocarbons, particularly tetramethylsilane (TMS), can be suitable for dosimetry.In this work we present a characterization of TMS (Merck, NMR calibration grade, purity>99.7%) for its use in LICs. The characterization consisted of measuring ion mobilities, using low dose 6MV photon pulses from a medical linac, and free-ion yield, using a continuous cobalt-60 beam (the reference beam quality used in radiotherapy dosimetry). Those values were then used to model general recombination in a TMS-filled LIC.Measured ion mobilities, (2.6±0.3)×10−8 and (3.6±0.4)×10−8m2V−1s−1, are similar to mobilities in isooctane, and two- to three-fold lower than some values reported for TMS. Such discrepancy can probably be attributed to the presence of different impurities. On the other hand, free-ion yield values obtained are approximately two-fold higher than for isooctane, in agreement with published data. Such high free-ion yield values result in a higher signal-to-noise ratio and may allow even better spatial resolution to be obtained with TMS-filled LICs. However, it comes at the cost of higher recombination effects that can compromise the operation of the chamber. Such high recombination and the low boiling point of TMS (≃28°C) make isooctane-filled LICs preferable to TMS-filled LICS for radiotherapy applications.