Scielo RSS <![CDATA[Revista mexicana de física]]> vol. 60 num. 6 lang. es <![CDATA[SciELO Logo]]> <![CDATA[<b>Quality control of the breast ca treatments on HDR brachytherapy with TLD-100</b>]]> An anthropomorphic Phantom, a female trunk, was built with a natural bone structure and experimental material coated, glycerin and water-based material called JJT to build soft tissue equivalent to the muscle of human tissue, and a polymer (styrofoam) to build the lung as critical organ to simulate the treatment of breast cancer, with high dose rate brachytherapy (HDR) and sources of Ir-192. The treatments were planned and calculated for the critical organ: Lung, and injury of 2 cm in diameter in breast with MicroSelectron HDR system and the software Plato Brachytherapy V 14.1 of the Nucletron (Netherlands) which uses the standard protocol of radiotherapy for brachytherapy treatments. The dose experimentally measured with dosimeters TLD-100 LIF: Mg; Ti, which were previously calibrated, were placed in the same positions and bodies mentioned above, with less than 5% uncertainty. The reading dosimeters was carried out in a Harshaw TLD 4500.The results obtained for calculated treatments, using the standard simulator, and the experimental with TLD-100, show a high concordance, as they are on average a ± 1.1% making process becomes in a quality control of this type of treatments. <![CDATA[<b>Complete solutions of the Hamilton-Jacobi equation and the envelope method</b>]]> It is shown that the parameters contained in any two complete solutions of the Hamilton-Jacobi equation, corresponding to a given Hamiltonian, are related by means of a time-independent canonical transformation and that, in some cases, a generating function of this transformation is given by the envelope of a family of surfaces defined by the difference of the two complete solutions. Conversely, in those cases, one of the complete solutions is given by the envelope of a family of surfaces defined by the sum of the other complete solution and the generating function of the canonical transformation. Some applications of these results to geometrical optics are also given.<hr/>Se muestra que los parámetros contenidos en cualesquier dos soluciones completas de la ecuación de Hamilton-Jacobi, correspondiente a una hamiltoniana dada, están relacionados por medio de una transformación canónica independiente del tiempo y que, en algunos casos, una función generatriz de esta transformación está dada por la envolvente de una familia de superficies definida por la diferencia de las dos soluciones completas. Recíprocamente, en esos casos, una de las soluciones completas está dada por la envolvente de una familia de superficies definida por la suma de la otra solución completa y la función generatriz de la transformación canónica. Se dan también algunas aplicaciones de estos resultados en la óptica geométrica. <![CDATA[<b>Monte Carlo studies of critical phenomena in mixed spin-3/2 and spin-5/2 Ising model on square lattice</b>]]> We used a Monte Carlo simulation to analize the magnetic behavior of Ising model of mixed spins SiA= ±3/2, ±1/2 and σjB = ±5/2, ±3/2, ±1/2, on a square lattice. Were studied the possible critical phenomena that may emerge in the region around the multiphase point (D/|J1|= -3, J2/|J1| = 1) and the dependence of the phase diagrams with the intensities of the anisotropy field of single ion (D/|J1|) and the ferromagnetic coupling of exchange spin SiA (J2/|J1|). The system displays first order phase transitions in a certain range of the parameters of the Hamiltonian, which depend on D/|J1| and |J2\|J1|. In the plane (D/|J1|,kB T/|J1|), the decrease of |D/|J1||, implies that the critical temperature, Tc, increases and the first order transition temperature, Tt, decreases. In the plane (J2/|J1|, kB T/|J1|), Tc increases with the increasing of J2/|J1|, while that Tt decreases. <![CDATA[<b>Polarimetric parameters associated to commercial optical fibers</b>]]> The most important polarimetric parameters are determined for six different types of commercially available optical fibers, at 1550 nm of transmission wavelength. The diattenuation, polarizance, retardance, polarization dependent loss (PDL), among other conventional polarimetric parameters, are determined from the Mueller matrix associated to 1m length of each fiber studied here. An improvement to the data analysis method, reported recently by our group, is presented. Results obtained show the fibers can be used not only as static elements, but also as versatile optical devices, depending on the incident polarization state employed.<hr/>Se determinan los parámetros polarimétricos más importantes para seis distintos tipos de fibras ópticas accesibles comercialmente, a una longitud de onda de transmisión de 1550 nm. La diatenuación, polarizancia, retardancia, pérdidas dependientes de la polarización (PDL), entre otros parámetros polarimétricos convencionales, se obtienen mediante las matrices de Mueller asociadas a 1 m de longitud de cada fibra estudiada aquí. Una mejora al método de análisis de datos, reportado recientemente por nuestro grupo, es presentada. Los resultados obtenidos muestran que las fibras no solo pueden utilizarse como elementos ópticos estáticos, sino como dispositivos versátiles, dependiendo del estado de polarización incidente. <![CDATA[<b>DFT study of interaction between a hydrogen molecule and AgY<i>-zeolite</i></b>]]> The cationic exchange of H by Ag in a HY-zeolite is carried out to study the adsorption of a hydrogen molecule in an AgY-zeolite, which is achieved by means of a BOMD simulation. The chosen zeolite model is representative of adsorbent materials when a cation Ag forms part of surface fragments, and they are considered in this interaction. This study provides a criterion to establish the electronic property of cation promoting a relevant characteristic that must have an adsorbent material. The energy values on this interaction are analyzed to determine an evaluation of the hydrogen adsorption using a AgY-zeolite ring and fragments of 3 tetrahedral sites. The AgY-zeolite ring represents a cavity that exhibits diffusivity after an adsorption process. This study was developed in the density functional theory level with DZVP function bases.<hr/>La simulación inicia con un intercambio catiónico de átomos de hidrogeno por plata, considerando a un modelo representativo de la HY-zeolita, se observa en los resultados una adsorción de una molécula de hidrogeno en la AgY-zeolita. El análisis de este modelo indica que la zeolita es un material representativo de adsorbedor de hidrogeno cuando se realiza un intercambio de catión (H por Ag). El estudio con base en el cálculo de la energía de interacción proporciona un criterio mediante las propiedades electrónicas acerca de la importancia del catión para que ocurra el fenómeno y se obtenga un material adsorbedor. El modelo usado es un anillo de AgY-zeolita además de un fragmento de 3 sitios tetrahedricos. El modelo del anillo de AgY-zeolita representado por una cavidad exhibe difusividad después del proceso de adsorción. Los cálculos de interacción dinámica utilizaron funcionales de intercambio y correlación en DFT con funciones base DZVP. <![CDATA[<b>Diagrama de fase sitio-enlace en dímeros</b>]]> En este trabajo se presenta un cálculo analítico aproximado para caracterizar la percolación en un sistema donde coexisten dímeros de sitios y enlaces en una red cuadrada. Un dímero es la representación más simple de individuos ligados a primeros vecinos. Se calculan funciones de percolación representadas por polinomios, obtenidas mediante enumeración numérica en una celda tamaño N = Lx x Ly para dímeros de sitios y de enlaces. En el límite termodinámico se encuentra que el umbral de percolación para dímeros de sitios (enlaces) es 0.562 (0.486). Para determinar el diagrama de fase sitio - enlace en dímeros, se utilizan operaciones OR y AND en las funciones de percolación que representan las celdas finitas. Tanto los valores del umbral de percolación, como el diagrama de fase sitio - enlace están en correspondencia con sus similares obtenidos mediante simulaciones numéricas.<hr/>This paper presents an approximate analytical calculation to characterize the percolation in a system where coexist bonds and sites dimers. A dimer is the simplest representation of individuals who are bound to nearest neighbors. Percolation functions represented by polynomials, obtained by numerical listing of a cell of size N = Lx x Ly for sites and bonds dimers are calculated. In the thermodynamic limit, the percolation threshold for site dimer (bond dimer) is 0.562 (0.486) on a square lattice, values that are in line with their counterparts obtained by numerical simulations. To determine the site-bond phase diagram of dimers on a square lattice, the OR and AND operations associated to the functions of a finite percolation cell are used and performed well with similar results in numerical simulations shown in the literature. <![CDATA[<b>Micro displacement measured by the grating interferometer with rings pattern</b>]]> Micro displacements are measured with holographic gratings by using a grating interferometer of one order that detects the phase changes in the diffracted orders caused by movement of the grooves in the diffraction gratings. The period can be on the order of fractions of a micron, with high reproducibility and an error of a half period. The basic operating principle involves the superposition of order +1, with order 0. The interferometer system produces standing waves; it works by measuring the intensity variations at the center of a ring-shaped interference pattern, which indicate the phase shift introduced by displacement of the grating grooves. When these rings move to the center of the pattern or to the border, the direction of the grating displacement can be detected; the interferometer system has no moving parts, except for the diffraction grating, and is very stable and robust. This system has the ability to measure micro displacements even with damaged gratings, as long as the gratings diffract evenly. <![CDATA[<b>Desarrollo de una cama de pruebas experimental para sistemas IR-UWBoF-IM/DD basándose en el uso de la plataforma de simulación VPIphotonics</b>]]> En este trabajo se describe el diseño de un sistema denominado radio impulsivo de ancho de banda ultra amplio sobre fibra óptica (IR-UWBoF) implementado en la plataforma de simulación VPItransmissionMaker de la compañía VPIphotonics. El sistema propuesto utiliza la técnica de modulación de intensidad de un laser DFB de onda continua (CW) con detección directa (IM/DD) para transmitir señales IR-UWB con una tasa de transferencia de 1 Gbps a través de un enlace de fibra óptica monomodo estándar (SSMF). La validez del esquema implementado en simulación ha sido confirmada mediante un análisis comparativo entre la densidad espectral de potencia (PSD) de señales IR-UWB obtenidas en el simulador con respecto a la PSD del mismo tipo de señales entregadas por un sistema experimental IR-UWBoF IM/DD. El ancho de banda máximo del sistema experimental es de 10 GHz y está determinado por la respuesta en frecuencia del modulador electro-óptico y los componentes eléctricos utilizados. El resultado del análisis comparativo muestra que el sistema propuesto en simulación puede ser usado como una cama de pruebas experimental en donde se pueden analizar y evaluar transmisiones de señales IR-UWB a través de enlaces SSMF de una forma rápida y precisa.<hr/>In this work we describe the design of an Impulse-Radio Ultra-Wide band over Fiber (IR-UWBoF) system implemented on the VPItransmissionMaker™ simulation software. This IR-UWBoF system uses the simple and low cost intensity external modulation with direct detection (IM/DD) technique to transmit IR-UWB signals at 1 Gbps of data rate through a standard single mode optical fiber (SSMF) link. We validate our proposal by means of a comparative analysis between the Power Spectral Density (PSD) of IR-UWB signals generated by simulation and the PSDs of signals obtained by an experimental IR-UWBoF IM/DD system. The maximum bandwidth of the proposed system is 10 GHz, which it is determined by the frequency response of the electro-optic modulator and the electric devices used. The analysis of results show that the simulated IR-UWBoF system can be used as an experimental testbed to analyze and to evaluate IR-UWB transmissions through optical fiber links in a fast and accurate manner. <![CDATA[<b>Interdigitated capacitance sensors in the mm scale with sub-femtoFarad resolution suitable for monitoring processes in liquid films</b>]]> We propose and analyze a high resolution capacitive sensor appropriate for monitoring physical or chemical processes in liquid films. The proposed sensor is based on a planar interdigitated capacitor with planar electrodes of dimensions in the milimeter scale. The electric field between electrodes extend above the plane of the electrodes up to a few milimeters and thus permits placing a dielectric container with a liquid film on top and still be sensitive to changes in the liquid film. First, we present numerical calculations of the capacitance of an interdigitated sensor as a function of the thickness and dielectric constant of a film placed on top of it using a finite element method. Then, we describe a suitable electronic system to sense with very high-resolution capacitance variations of the sensor by measuring the phase lag and amplitude change of a sinusoidal current signal passing through it. This is accomplished by subtracting a stable sinusoidal current of the same frequency going through a reference device. Initially the system is balanced by adjusting the reference current to cancel out the net output current. In this way, we compensate parasitic capacitances due to electronics, wiring and system hardware as well. When the capacitance of the sensor element varies the system gets unbalanced and a net current appears. The resulting current is measured with a locking-amplifier. To illustrate the sensitivity and resolution achieved by the sensing system, we present results of monitoring the capacitance of the sensor during the evaporation of liquid solvent films and discuss the time-evolution of the registered signals. The floor noise throughout the measurements was in the order of 50 atto-Farads while the signal varied in the range of several femto-Farads.