RESUMO
First measurements with a prototype ionization chamber are described to be applied in online monitoring of modulated fields in radiation therapy. The liquids isooctane, isononane (TMP) and tetramethylsilane (TMS) are used in a high purity grade in order to realize high current signals for electronic read-out in parallel at frequencies exceeding 10 Hz. Signals of more than a factor 4 with respect to isooctane, analysis grade, are obtained. With an electrode structure of 400 pads, a uniformity in efficiency within 1.2% has been measured. The penumbra of a multileaf collimator could be resolved. Theoretical examination verifies that the free electrons in the liquids cause higher signals when the measured currents are compared with expectation for ion transport only.
Assuntos
Análise de Falha de Equipamento/métodos , Octanos/efeitos da radiação , Pentanos/efeitos da radiação , Proteção Radiológica/instrumentação , Radiometria/instrumentação , Radioterapia Conformacional/instrumentação , Silanos/efeitos da radiação , Transdutores , Simulação por Computador , Desenho de Equipamento , Estudos de Viabilidade , Modelos Teóricos , Sistemas On-Line/instrumentação , Projetos Piloto , Proteção Radiológica/métodos , Radiometria/métodos , Dosagem Radioterapêutica , Radioterapia Conformacional/métodos , Reprodutibilidade dos Testes , Sensibilidade e Especificidade , Soluções/efeitos da radiação , Compostos de TrimetilsililRESUMO
The general collection efficiency in the dielectric liquids iso-octane (CaH18; 2-2-4 trimethylpentane) and tetramethylsilane (Si(CH3)4), used as sensitive media in a thimble liquid ionization chamber (LIC) with a liquid layer thickness of 1 mm, has been studied. Measurements were made for continuous radiation at varying dose rates using 140 keV photons from the decay of 99mTc for chamber polarizing voltages of 50, 100 and 500 V. The maximum dose rate in each measurement session was about 150 mGy min(-1). The experimental results were compared with theoretical general collection efficiencies calculated by the equation for the general collection efficiency in gases. The results show that the general collection efficiency in a thimble LIC for continuous radiation can be calculated with the equation for the general collection efficiency in gas ionization chambers, using the same chamber geometry correction factors and analogous characteristic ion recombination parameters for the dielectric liquids.
