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

Abstract

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.

Liquid-filled ionization chamber temperature dependence

“…In air ionization chambers, the measured charge is proportional to the gas density, thus decreasing with increasing temperature. On the contrary, it has been experimentally verified in several studies for different LIC designs with isooctane and tetramethylsilane [1,2,4] as sensitive medium, that LIC response increases when temperature is also increased. This response dependence on temperature is mainly explained through the variation of the initial recombination in dielectric liquids [5].…”

“…Liquid-filled ionization chambers (LIC) are currently used in radiotherapy both for dosimetry [1,2] and portal imaging [3]. Usually, the room temperature liquids used in these devices are liquid hydrocarbons.…”

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

“…Research in LICs for radiotherapy has grown in the last two decades due to the good characteristics that they present for dosimetry, namely a near water-equivalent response, and the attainable high spatial resolution due to the high density of the ionization medium (when compared to air), which is especially important in the verification of small and/or high gradient fields such as those present in many advanced radiotherapy techniques. Several prototypes and commercial devices, as well as dosimetry methods, have been developed and characterized [1][2][3][4][5][6][7][8][9][10][11]. Isooctane (2,2,4-trimethylpentane, C 8 H 18 ) is the liquid most commonly used in LICs due to its good physico-chemical properties.…”

On the property of measurements with the PTW microLion chamber in continuous beams

“…There are several chamber and liquid specific characteristics that can have an impact on the 87 usability of LICs, these include particle type and energy dependence, temperature dependence and leakage currents. An investigation of these effects in LICs can be found in Wickman et al 16 Leakage 89 currents set a lower limit for permissible dose rates that may be accurately measured by LICs,…”

“…However, the initial recombination effect may not depend only on the polarizing 104 voltage but also on radiation beam type and energy, as the radiation quality will influence the density 105 of ions created along an incident ionizing particle track. Wickman et al 16 which is the result of a 120 minutes production time at a cyclotron current of 60 µA, as specified by 141 the manufacturer. The activity, which is contained in liquid form (2 ml), was transported by high 142 pressure helium gas to a hot cell in the PET radiation chemistry laboratory at the hospital.…”