Papers by richard sanchez
Applied Surface Science, 2007
We present a magnetic study of vanadium oxide nanotubes (VO x -NTs) with polyaniline (PAni). Tran... more We present a magnetic study of vanadium oxide nanotubes (VO x -NTs) with polyaniline (PAni). Transmission electron microscopy (TEM) shows the tubular shape and the multi-wall structure of the nanotubes. The static magnetic susceptibility measured at different magnetic fields shows a Curie behavior, while the magnetization versus magnetic field presents a non-linear dependence at low temperatures. Both experiments can be explained by the presence of paramagnetic ions with S = 1/2. Using Electron Paramagnetic Resonance technique, we identified as V 4+ (3d 1 ) the paramagnetic ions present in the nanostructures. All the experimental results can be explained by a fraction between 14% and 18% of V 4+ with respect to the total V atoms in the system. #
We report magnetization experiments in two magnetically isolated ferromagnetic nanotubes of perov... more We report magnetization experiments in two magnetically isolated ferromagnetic nanotubes of perovskite La$_{0.67}$Ca$_{0.33}$MnO$_3$. The results show that the magnetic anisotropy is determined by the sample shape although the coercive field is reduced by incoherent magnetization reversal modes. The temperature dependence of the magnetization reveals that the magnetic behavior is dominated by grain surface properties. These measurements were acquired using a Silicon micro-mechanical oscillator working in its resonant mode. The sensitivity was enough to measure the magnetic properties of these two samples with a mass lower than 14 picograms and to obtain for the first time the magnetization loop for one isolated nanotube.
Journal of Quantitative Spectroscopy & Radiative Transfer, 1991
We present a family of diffusion approximations to the equation of radiative transfer, parameteri... more We present a family of diffusion approximations to the equation of radiative transfer, parameterized functionally by a function χ(ω, R). Here ω is the effective (treating emission as scattering) single scatter albedo, and R is roughly speaking the magnitude of the dimensionless spatial ...
Transport Theory and Statistical Physics, 1990
An iterative algorithm is proposed for simultaneously estimating the optical thickness of a homog... more An iterative algorithm is proposed for simultaneously estimating the optical thickness of a homogeneous, plane-parallel medium and the albedo of an obscured underlying surface. Only remote measurements outside the medium are required of the ingoing and outgoing radiance. The method incorporates analytically-computed first derivatives of the unknowns that are obtained from the Fn method of transport theory. The ill-posed nature
Transport Theory and Statistical Physics, 1997
The two-region Milne problem results of Ganapol and Pomraning are used in a proposed solution to ... more The two-region Milne problem results of Ganapol and Pomraning are used in a proposed solution to an inverse transport problem in which there are two adjacent, optically-thick, isotropic-scattering layers. Equations are obtained for approximately estimating the mean number of secondaries of each region using only the normally-directed angular fluxes and the scalar fluxes and currents at the external boundaries. The
Transport Theory and Statistical Physics, 1983
Equations are developed with which to evaluate the coefficients used in the one-speed Boltzmann-F... more Equations are developed with which to evaluate the coefficients used in the one-speed Boltzmann-Fokker-Planck transport equation for strongly anisotropic scattering of photons or neutrons. The inverse equations, which are derived for a homogeneous slab target, require only measurements outside the medium. One set of equations is useful when the external illumination is nonuniformly incident over the surface, and the other
Applied Optics, 1994
We solve an inverse problem of ocean optics for estimating spatially dependent absorption and sca... more We solve an inverse problem of ocean optics for estimating spatially dependent absorption and scattering coefficients and for determining sources such as fluorescence, bioluminescence, or Raman scattering. The solution requires in situ measurement of the downward and upward plane irradiances and scalar irradiances and a priori estimation of the angular shape of the volume scattering function. Both an explicit algorithm and an implicit one are developed from new two-stream radiative-transfer equations that utilize an asymptotic radiance approximation to close the set of equations. A comparison of numerical tests for the two algorithms is given.
Applied Optics, 1992
An algorithm is developed for estimating the spatial location and magnitude of a bioluminescent r... more An algorithm is developed for estimating the spatial location and magnitude of a bioluminescent radiation source from measurements of the in situ irradiance and scalar irradiance at two depths. The algorithm is based on the principle of photon conservation. The most direct application of the algorithm requires that the absorption coefficient be known, but the algorithm is useful even if that coefficient is unknown. Numerical tests and an error analysis have been done to test the algorithm numerically. In addition we show that if the estimated source magnitude is nearly constant, that value can be used to estimate the vertical attenuation coefficient of the radiation field.
Applied Optics, 1998
A method was developed for determining the spatial distribution of a source from downward and upw... more A method was developed for determining the spatial distribution of a source from downward and upward irradiance measurements at a single wavelength in seawater of known optical properties. The algorithm uses measurements at two depths located an arbitrary distance apart and solves two nonlinear equations for two parameters that fit a globally exponential or linear source shape. Complex spatially dependent source shapes can be estimated from an irradiance profile by piecing together estimates from neighboring measurement pairs. Numerical tests illustrate the sensitivity of the algorithm to depth, measurement spacing, chlorophyll concentration, sensor noise, and uncertainty in the a priori assumed inherent optical properties. The algorithm works well with widely spaced measurements, moderate sensor noise, and uncertainties in the optical properties regardless of whether the assumed and true profiles are the same shape.
British Journal of Anaesthesia, 1967
Radiographs of the position of the epidural catheter were taken in each of 90 patients in whom th... more Radiographs of the position of the epidural catheter were taken in each of 90 patients in whom this form of analgesia was employed for surgery. Radiopaque dye was injected through the catheter. In 33 patients the average length of catheter threaded into the epidural space was 20 cm; the desired cephalad direction was followed in 16. When the average length of catheter inserted was 12.5 cm the tip followed the cephalad direction in 27 of 57 patients. Straight, curled up, and winding patterns were observed, and in 6 of 90 patients the catheter passed out of the epidural space.
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Papers by richard sanchez