ABSTRACT In this work we present a model of signal temporal development in ionization chambers an... more ABSTRACT In this work we present a model of signal temporal development in ionization chambers and we use it to determine ionic mobilities and relative densities of charge carriers in non-ultrapure liquid isooctane using a liquid-filled ionization chamber dosimeter. The detector has been irradiated with a low dose rate, short pulsed photon beam generated with a medical LINAC. Ionic mobilities have been obtained by studying the temporal development of the readout signal and fitting it to a model for low dose rate beams where recombination is negligible. The best fit has been obtained for 3 ionic species with mobilities k1 = (2.22±0.22) × 10-8, k2 = (3.37±0.43) × 10-8, k3 = (19.69±2.59) × 10-8 m2 V-1 s-1 and relative densities n1 = 0.5 (n1 is not a fitting parameter), n2 = 0.23±0.03 and n3 = 0.27±0.03.
An accurate computation of the collection efficiency due to recombination has a capital importanc... more An accurate computation of the collection efficiency due to recombination has a capital importance in order to perform high-precision dose measurements with ionization chambers. The two-voltage method, developed to compute charge collection efficiency in gas chambers, cannot be directly applied to liquid-filled ionization chambers (LICs) due to the ionized charge strong dependence on the collection electric field, which is caused by initial recombination. It is shown that in order to apply the two-voltage method to parallel-plate LICs it is necessary to introduce explicitly the slope of the ionization charge yield with respect to the polarization voltage applied in the chamber. A three-voltage method is also proposed to avoid the introduction of the slope parameter. That method allows decoupling initial and general recombination. It leads to a nonlinear system of equations which can be solved using numerical methods.
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2015
ABSTRACT Liquid-filled ionisation chambers (LICs) are interesting detectors for the dosimetry of ... more ABSTRACT Liquid-filled ionisation chambers (LICs) are interesting detectors for the dosimetry of radiotherapy beams due to their water-equivalent response and high spatial resolution. Isooctane is the liquid most often used as an active medium, but other hydrocarbons, particularly tetramethylsilane (TMS), can be suitable for dosimetry.
Radiotherapy methods have been substantially improving during the last years. Nevertheless, the p... more Radiotherapy methods have been substantially improving during the last years. Nevertheless, the patients' risk, as consequence of the radiation used in the treatment, keeps being of important concern. In particular, the peripheral dose, usually from photons and neutrons in conventional therapy with linacs, represents an important topic to study. The objective is to provide an adequate radiation protection to the patient, that allow to evaluate the best strategy for treatment and take into account the risk-benefit balance due to the out-of-field dose delivered. Although photon doses have been deeply contemplated, the neutron contamination from high energy photon beams is still a subject of research and discussion. In the present work, we introduce a new digital device that permits real time evaluation of the equivalent dose in organs during any therapeutic session. For this purpose, we have obtained a huge experimental data collection, considering different photon energies, linac...
Introducción Los tratamientos de Radioterapia de Intensidad Modulada (IMRT) o radiocirugía utiliz... more Introducción Los tratamientos de Radioterapia de Intensidad Modulada (IMRT) o radiocirugía utilizan haces de fotones estrechos (campos pequeños en relación al rango lateral de partículas cargadas) y/o conformados con un contorno irregular. En este tipo de campos las cámaras de ionización convencionales pueden presentar problemas, debidos a la falta de equilibrio de partículas cargadas (CPE) y a la presencia de gradientes elevados de dosis. Los procedimientos de control de calidad asociados a este tipo de tratamientos demandan dosímetros de alta resolución espacial. En este trabajo se presenta una cámara de ionización con líquido como volumen activo de 0.5 mm 3 . Esta tecnología permite una relación señal ruido alta en un volumen activo pequeño, y una mejor equivalencia a tejido que el aire [1]. Las pruebas que se han realizado, cuyos resultados se incluyen en este trabajo, muestran un comportamiento comparable al del diodo sin blindaje. Se concluye por tanto que el detector es aprop...
ABSTRACT In this work we present a model of signal temporal development in ionization chambers an... more ABSTRACT In this work we present a model of signal temporal development in ionization chambers and we use it to determine ionic mobilities and relative densities of charge carriers in non-ultrapure liquid isooctane using a liquid-filled ionization chamber dosimeter. The detector has been irradiated with a low dose rate, short pulsed photon beam generated with a medical LINAC. Ionic mobilities have been obtained by studying the temporal development of the readout signal and fitting it to a model for low dose rate beams where recombination is negligible. The best fit has been obtained for 3 ionic species with mobilities k1 = (2.22±0.22) × 10-8, k2 = (3.37±0.43) × 10-8, k3 = (19.69±2.59) × 10-8 m2 V-1 s-1 and relative densities n1 = 0.5 (n1 is not a fitting parameter), n2 = 0.23±0.03 and n3 = 0.27±0.03.
An accurate computation of the collection efficiency due to recombination has a capital importanc... more An accurate computation of the collection efficiency due to recombination has a capital importance in order to perform high-precision dose measurements with ionization chambers. The two-voltage method, developed to compute charge collection efficiency in gas chambers, cannot be directly applied to liquid-filled ionization chambers (LICs) due to the ionized charge strong dependence on the collection electric field, which is caused by initial recombination. It is shown that in order to apply the two-voltage method to parallel-plate LICs it is necessary to introduce explicitly the slope of the ionization charge yield with respect to the polarization voltage applied in the chamber. A three-voltage method is also proposed to avoid the introduction of the slope parameter. That method allows decoupling initial and general recombination. It leads to a nonlinear system of equations which can be solved using numerical methods.
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2015
ABSTRACT Liquid-filled ionisation chambers (LICs) are interesting detectors for the dosimetry of ... more ABSTRACT Liquid-filled ionisation chambers (LICs) are interesting detectors for the dosimetry of radiotherapy beams due to their water-equivalent response and high spatial resolution. Isooctane is the liquid most often used as an active medium, but other hydrocarbons, particularly tetramethylsilane (TMS), can be suitable for dosimetry.
Radiotherapy methods have been substantially improving during the last years. Nevertheless, the p... more Radiotherapy methods have been substantially improving during the last years. Nevertheless, the patients' risk, as consequence of the radiation used in the treatment, keeps being of important concern. In particular, the peripheral dose, usually from photons and neutrons in conventional therapy with linacs, represents an important topic to study. The objective is to provide an adequate radiation protection to the patient, that allow to evaluate the best strategy for treatment and take into account the risk-benefit balance due to the out-of-field dose delivered. Although photon doses have been deeply contemplated, the neutron contamination from high energy photon beams is still a subject of research and discussion. In the present work, we introduce a new digital device that permits real time evaluation of the equivalent dose in organs during any therapeutic session. For this purpose, we have obtained a huge experimental data collection, considering different photon energies, linac...
Introducción Los tratamientos de Radioterapia de Intensidad Modulada (IMRT) o radiocirugía utiliz... more Introducción Los tratamientos de Radioterapia de Intensidad Modulada (IMRT) o radiocirugía utilizan haces de fotones estrechos (campos pequeños en relación al rango lateral de partículas cargadas) y/o conformados con un contorno irregular. En este tipo de campos las cámaras de ionización convencionales pueden presentar problemas, debidos a la falta de equilibrio de partículas cargadas (CPE) y a la presencia de gradientes elevados de dosis. Los procedimientos de control de calidad asociados a este tipo de tratamientos demandan dosímetros de alta resolución espacial. En este trabajo se presenta una cámara de ionización con líquido como volumen activo de 0.5 mm 3 . Esta tecnología permite una relación señal ruido alta en un volumen activo pequeño, y una mejor equivalencia a tejido que el aire [1]. Las pruebas que se han realizado, cuyos resultados se incluyen en este trabajo, muestran un comportamiento comparable al del diodo sin blindaje. Se concluye por tanto que el detector es aprop...
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