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 mm3 (plane-parallel) and 20 mm3 (cylindrical). The liquids used as sensitive media in the chambers are either isooctane (C8H18), tetramethylsilane (Si(CH3)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(-9)C Gy(-1) mm(-3) for 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 microGy min-1 (sigma < 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 degree C. The response is almost independent of the direction of the radiation for the plane-parallel LIC.
Wickman
G
G
Radiation Physics Department, Umeå University, Sweden.
Johansson
B
B
Bahar-Gogani
J
J
Holmström
T
T
Grindborg
J E
JE
D055592:Biophysical Phenomena; D001703:Biophysics; D001918:Brachytherapy; D004582:Electronics, Medical / Q000295:instrumentation; D004867:Equipment Design; D006801:Humans; D017785:Photons; D011874:Radiometry / Q000295:instrumentation* / Q000592:standards / Q000706:statistics & numerical data; D011879:Radiotherapy Dosage; D012680:Sensitivity and Specificity; D013679:Technology, Radiologic; D013696:Temperature; D014867:Water
D014867:Water