Liquid ionization chambers for absorbed dose measurements in water at low dose rates and intermediate photon energies

Two new liquid ionization chamber (LIC) designs, consisting of cylindrical and plane‐parallel configurations, are presented. They are designed to be suitable for high‐precision measurements of absorbed dose‐to‐water at dose rates and photon energies typical for LDR intermediate photon energy brachytherapy sources. The chambers have a sensitive liquid layer thickness of 1 mm and sensitive volumes of 7 mm 3(plane‐parallel) and 20 mm 3(cylindrical). The liquids used as sensitive media in the chambers are either isooctane ( C 8H 18) , tetramethylsilane (Si( CH 3 ) 4 ) or mixtures of these two liquids in the approximate proportions 2 to 1. A chamber filled with such a liquid mixture and with a polarizing voltage of 300 V, provides a volume sensitivity of about 10 − 9C Gy − 1  mm − 3for absorbed dose measurements in water in an x‐ray radiation field with an effective photon energy of 120 keV. In the interval 30 to 140 keV, the relative change in sensitivity is less than ± 2.5%. The leakage current of the chambers is low and stable, which implies that absorbed dose measurements can be done with good reproducibility at dose‐rates as low as 50 μ Gy min − 1( σ < 3%). The long‐term calibration stability was tested for a set of five chambers over a period of more than 1 year. No systematic change in their sensitivity could be observed. The general recombination at a polarizing voltage of 300 V is less than 2% for dose‐rates up to about 100 mGy min − 1. The temperature dependence at room temperature is 0.5% per ° C. The response is almost independent of the direction of the radiation for the plane‐parallel LIC.