The performance of liquid ionization chambers, which may prove to be useful tools in the field of radiationdosimetry, is based on several chamber and liquid specific characteristics. The present work investigates the performance of the PTW microLion liquid ionization chamber with respect to recombination losses and perturbations from ambient electric fields at various dose rates in continuous beams.Methods:
In the investigation, experiments were performed using two microLion chambers, containing isooctane (C8H18) and tetramethylsilane [Si(CH3)4] as the sensitive media, and a NACP-02 monitor chamber. An initial activity of approximately 250 GBq 18F was employed as the radiationsource in the experiments. The initial dose rate in each measurement series was estimated to 1.0 Gy min−1 by Monte Carlo simulations and the measurements were carried out during the decay of the radioactive source. In the investigation of general recombination losses, employing the two-dose-rate method for continuous beams, the liquid ionization chambers were operated at polarizing voltages 25, 50, 100, 150, 200, and 300 V. Furthermore, measurements were also performed at 500 V polarizing voltage in the investigation of the sensitivity of the microLion chamber to ambient electric fields.Results:
The measurement results from the liquid ionization chambers, corrected for general recombination losses according to the two-dose-rate method for continuous beams, had a good agreement with the signal to dose linearity from the NACP-02 monitor chamber for general collection efficiencies above 70%. The results also displayed an agreement with the theoretical collection efficiencies according to the Greening theory, except for the liquid ionization chamber containing isooctane operated at 25 V. At lower dose rates, perturbations from ambient electric fields were found in the microLion chamber measurement results. Due to the perturbations, measurement results below an estimated dose rate of 0.2 Gy min−1 were excluded from the present investigation of the general collection efficiency. The perturbations were found to be more pronounced when the chamber polarizing voltage was increased.Conclusions:
By using the two-dose-rate method for continuous beams, comparable corrected ionizationcurrents from experiments in low and medium energy photon beams can be achieved. However, the valid range of general collection efficiencies has been found to vary in a comparison between experiments performed in continuous beams of 120 kVp x ray, and the present investigation of 511 keV annihilation photons. At very high dose rates in continuous beams, there are presently no methods that can be used to correct for general recombination losses and at low dose rates the microLion chamber may be perturbed by ambient electric fields. Increasing the chamber polarizing voltage, which diminishes the general recombination effect, was found to increase the microLion chamber sensitivity to ambient electric fields. Prudence is thus advised when employing the microLion chamber in radiationdosimetry, as ambient electric fields of the strength observed in the present work may be found in many common situations. Due to uncertainties in the theoretical basis for recombination losses in liquids, further studies on the underlying theories for the initial and general recombination effect are needed if liquid ionization chambers are to become a viable option in high precision radiationdosimetry.
The authors would like to thank the Department of Nuclear Medicine at the University Hospital of Norrland (Umeå, Sweden) for the usage of the cyclotron (GE Medical, PETtrace 6) and the PET radiation chemistry laboratory. The authors are also grateful to the following co-workers: Per Egelrud and David Gunnarsson for their help in the operation of the cyclotron, Mattias Ögren and Margaretha Ögren for lending their insights about the working environment in the PET radiation chemistry laboratory, and Jonna Wilén and Kjell Hansson Mild for valuable assistance in measurements of electromagnetic fields.
II.A. Ionization chambers
II.B. Electrometers and high voltage supplies
II.C. Irradiation source and geometry
III.A. Theory and experiments
III.B. Monte Carlo simulation
- Electric fields
- Electric measurements
- Ionization chambers
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