Faustino Gomez | University of Santiago de Compostela (original) (raw)

Papers by Faustino Gomez

Physics in Medicine & Biology, 2020

Microdosimetry has been traditionally performed through gaseous proportional counters, although i... more Microdosimetry has been traditionally performed through gaseous proportional counters, although in recent years different solid state microdosimeters have been proposed and constructed for this task. In this paper we analyze the response of solid state devices of micrometric size with no intrinsic gain developed by CNM-CSIC (Spain). There are two major aspects of the operation of these devices that affect the reconstruction of the probability distributions and momenta of stochastic quantities related to microdosimetry. For micrometric volumes the drift and diffusion of the charge carriers gives rise to a partial charge collection efficiency in the peripheral region of the depleted volume. Such effect produce a perturbation of the reconstructed pulse height (i.e. imparted energy) distributions with respect to the actual microdosimetric distributions. The relevance of this deviation depends on the size, geometry and operation conditions of the device. On the other hand, the electronic noise from the single event readout setup poses a limit on the minimum detectable lineal energy when the microdosimeter size is reduced. This article addresses these issues to provide a framework on the physical constraints for the design and operation of solid state microdosimeters.

Journal of Microelectromechanical Systems, 2015

Physics in Medicine and Biology

IEEE Transactions on Nuclear Science

IEEE Transactions on Nuclear Science, 2016

An architectural performance comparison of bandgap voltage reference variants, designed in a 0.18... more An architectural performance comparison of bandgap voltage reference variants, designed in a 0.18 µm CMOS process, is performed with respect to single event transients. These are commonly induced in microelectronics in the space radiation environment. Heavy ion tests (Silicon, Krypton, Xenon) are used to explore the analog single-event transients and have revealed pulse quenching mechanisms in analogue circuits. The different topologies are compared, in terms of cross-section, pulse duration and pulse amplitude. The measured results, and the explanations behind the findings, reveal important guidelines for designing analog integrated circuits, which are intended for space applications. The paper includes an analysis on how pulse quenching occurs within the indispensable current mirror, which is used in every analog circuit.

Microelectronics Reliability, 2015

The purpose of this paper is to show that the charge induced by radiation in a dielectric on whic... more The purpose of this paper is to show that the charge induced by radiation in a dielectric on which a sigmadelta control of dielectric charge is implemented, can be seen as a disturbance in a sliding mode controller. Preliminary experimental results are presented in which a MEMS device is irradiated with X-rays, while the dielectric charge control is continuously being monitored. The charge induced by radiation generates a change in the control bitstream, which is associated to the presence of an external disturbance on the governing control equations.

IFMBE Proceedings, 2013

Neutrons have a high biological effectiveness. Therefore, patient exposition to them represents a... more Neutrons have a high biological effectiveness. Therefore, patient exposition to them represents a relevant issue and consequences to this exposure, regarding secondary cancer induction, must be clarified. The aim of this work was to make a complete study on the production of photoneutrons in radiotherapy facilities. Detailed Monte Carlo simulation, using MCNPX code, was performed to characterize the neutron generation in a Siemens PRIMUS linac operating at 15 MV and to determine the fluence energy distributions inside the treatment room. This information is mandatory to understand the response, under radiotherapy conditions, of a new neutron digital detector developed by our group.

IFMBE Proceedings, 2013

This work presents the results of the Monte Carlo simulation of the neutron fluence energy spectr... more This work presents the results of the Monte Carlo simulation of the neutron fluence energy spectra at 16 representative points inside three anthropomorphic phantoms mimicking an adult and two children. Simulations were carried out using the MCNPX-2.6.0 code, and the calculation of the neutron spectra for each beam incidence was made in a single simulation file, including the treatment room, each phantom and the linac. Results show that pediatric patients are exposed to higher neutron fluence than adults. Consequences in terms of an increased secondary cancer risk should be analysed.

Radiotherapy and Oncology, 2011

Journal of Instrumentation, 2012

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.

Physics in medicine and biology, Jan 21, 2009

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.

Radiation Measurements, 2010

One of the challenges of measuring neutron fluences around medical linacs is the fact that the sc... more One of the challenges of measuring neutron fluences around medical linacs is the fact that the scattered photon fluence is important and higher than the surrounding neutron leakage fluence. Additionally most electron accelerators are pulsed, with repetition rates of the order of hundreds of Hertz, while the pulse duration is in the microsecond range. For this reason, neutron fluence around RT linacs is usually measured through passive methods, with the inconvenience of their time consuming analysis. A new neutron detector, based on the relation between Single Event Upsets (SEU) in digital SRAM memories and the existing thermal neutron fluence, has been developed. This work reports the calibration results of prototypes of this detector, obtained from exposures to the Physikalisch-Technische Bundesanstalt in Braunschweig (PTB) moderated 252 Cf source, to PTB quasi-monoenergetic neutron beams of 0.565 MeV, 1.2 MeV, 5 MeV, 8 MeV and 14.8 MeV, and to the GKSS thermal neutron beam.

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...

Radiotherapy and Oncology

Physics in Medicine & Biology, 2020

Microdosimetry has been traditionally performed through gaseous proportional counters, although i... more Microdosimetry has been traditionally performed through gaseous proportional counters, although in recent years different solid state microdosimeters have been proposed and constructed for this task. In this paper we analyze the response of solid state devices of micrometric size with no intrinsic gain developed by CNM-CSIC (Spain). There are two major aspects of the operation of these devices that affect the reconstruction of the probability distributions and momenta of stochastic quantities related to microdosimetry. For micrometric volumes the drift and diffusion of the charge carriers gives rise to a partial charge collection efficiency in the peripheral region of the depleted volume. Such effect produce a perturbation of the reconstructed pulse height (i.e. imparted energy) distributions with respect to the actual microdosimetric distributions. The relevance of this deviation depends on the size, geometry and operation conditions of the device. On the other hand, the electronic noise from the single event readout setup poses a limit on the minimum detectable lineal energy when the microdosimeter size is reduced. This article addresses these issues to provide a framework on the physical constraints for the design and operation of solid state microdosimeters.

Journal of Microelectromechanical Systems, 2015

Physics in Medicine and Biology

IEEE Transactions on Nuclear Science

IEEE Transactions on Nuclear Science, 2016

An architectural performance comparison of bandgap voltage reference variants, designed in a 0.18... more An architectural performance comparison of bandgap voltage reference variants, designed in a 0.18 µm CMOS process, is performed with respect to single event transients. These are commonly induced in microelectronics in the space radiation environment. Heavy ion tests (Silicon, Krypton, Xenon) are used to explore the analog single-event transients and have revealed pulse quenching mechanisms in analogue circuits. The different topologies are compared, in terms of cross-section, pulse duration and pulse amplitude. The measured results, and the explanations behind the findings, reveal important guidelines for designing analog integrated circuits, which are intended for space applications. The paper includes an analysis on how pulse quenching occurs within the indispensable current mirror, which is used in every analog circuit.

Microelectronics Reliability, 2015

The purpose of this paper is to show that the charge induced by radiation in a dielectric on whic... more The purpose of this paper is to show that the charge induced by radiation in a dielectric on which a sigmadelta control of dielectric charge is implemented, can be seen as a disturbance in a sliding mode controller. Preliminary experimental results are presented in which a MEMS device is irradiated with X-rays, while the dielectric charge control is continuously being monitored. The charge induced by radiation generates a change in the control bitstream, which is associated to the presence of an external disturbance on the governing control equations.

IFMBE Proceedings, 2013

Neutrons have a high biological effectiveness. Therefore, patient exposition to them represents a... more Neutrons have a high biological effectiveness. Therefore, patient exposition to them represents a relevant issue and consequences to this exposure, regarding secondary cancer induction, must be clarified. The aim of this work was to make a complete study on the production of photoneutrons in radiotherapy facilities. Detailed Monte Carlo simulation, using MCNPX code, was performed to characterize the neutron generation in a Siemens PRIMUS linac operating at 15 MV and to determine the fluence energy distributions inside the treatment room. This information is mandatory to understand the response, under radiotherapy conditions, of a new neutron digital detector developed by our group.

IFMBE Proceedings, 2013

This work presents the results of the Monte Carlo simulation of the neutron fluence energy spectr... more This work presents the results of the Monte Carlo simulation of the neutron fluence energy spectra at 16 representative points inside three anthropomorphic phantoms mimicking an adult and two children. Simulations were carried out using the MCNPX-2.6.0 code, and the calculation of the neutron spectra for each beam incidence was made in a single simulation file, including the treatment room, each phantom and the linac. Results show that pediatric patients are exposed to higher neutron fluence than adults. Consequences in terms of an increased secondary cancer risk should be analysed.

Radiotherapy and Oncology, 2011

Journal of Instrumentation, 2012

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.

Physics in medicine and biology, Jan 21, 2009

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.

Radiation Measurements, 2010

One of the challenges of measuring neutron fluences around medical linacs is the fact that the sc... more One of the challenges of measuring neutron fluences around medical linacs is the fact that the scattered photon fluence is important and higher than the surrounding neutron leakage fluence. Additionally most electron accelerators are pulsed, with repetition rates of the order of hundreds of Hertz, while the pulse duration is in the microsecond range. For this reason, neutron fluence around RT linacs is usually measured through passive methods, with the inconvenience of their time consuming analysis. A new neutron detector, based on the relation between Single Event Upsets (SEU) in digital SRAM memories and the existing thermal neutron fluence, has been developed. This work reports the calibration results of prototypes of this detector, obtained from exposures to the Physikalisch-Technische Bundesanstalt in Braunschweig (PTB) moderated 252 Cf source, to PTB quasi-monoenergetic neutron beams of 0.565 MeV, 1.2 MeV, 5 MeV, 8 MeV and 14.8 MeV, and to the GKSS thermal neutron beam.

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...

Radiotherapy and Oncology