Desarrollo de técnicas neutrónicas aplicadas al estudio de la dinámica de la materia condensada

Este proyecto, continuación del 06/C185, contempla desarrollos en diversos aspectos relacionados con el uso de técnicas neutrónicas utilizadas en el estudio de sistemas de interés tanto para la ciencia básica como aplicada. La actividad experimental prevista se realizará principalmente en el Centro Atómico Bariloche (acelerador lineal LINAC) que es utilizado para la producción de neutrones en forma pulsada. Este proyecto está dedicado principalmente al estudio de diversas problemáticas relacionadas con el procesamiento de los datos experimentales obtenidos con las técnicas de dispersión inelástica profunda de neutrones (DINS), y sección eficaz total (SET). La técnica DINS tiene como principal objetivo obtener las distribuciones de impulsos de los átomos que componen la muestra, y ha mostrado ser particularmente útil a tal fin. En este sentido, hemos formulado un nuevo método de análisis de datos, corrigiendo serias inconsistencias encontradas en los métodos tradicionales las cuales arrojan dudas sobre resultados claves consignados en la literatura, como ser temperaturas efectivas de sistemas que contienen átomos livianos, y supuestas anomalías en la sección eficaz del hidrógeno. Recientemente esta técnica ha sido implementada en el Laboratorio de Neutrones del CAB, lo que permite llevar adelante un programa experimental independiente con el objeto de estudiar la dinámica de diferentes sistemas, validar la técnica para la obtención de valores absolutos de secciones eficaces neutrónicas epitérmicas, y extender su campo de acción a su empleo en espectrometría de masa no destructiva, entre otras aplicaciones. Por su parte la técnica SET, que constituye un valioso complemento de otras técnicas neutrónicas más detalladas, también se encuentra operativa en nuestro Laboratorio y ha probado ser de gran utilidad a fin de estudiar la estructura y la dinámica de diversos sistemas. En publicaciones previas hemos mostrado que mediante esta técnica simple se puede obtener con apreciable sensibilidad el espectro de frecuencias de sistemas dispersores incoherentes, y que la capacidad de esta técnica para estudiar propiedades estructurales y dinámicas aún no se encuentra debidamente explotada. Con esta técnica se realizarán determinaciones estructurales de alcoholes deuterados, mientras que de las correspondientes especies normales se extraerán parámetros dinámicos. Se realizarán mediciones de SET variando la temperatura de la muestra, obteniéndose así información experimental confiable de gran utilidad a los efectos de modelar la interacción de neutrones térmicos con tales sistemas. Otro aspecto importante de la actividad que realizaremos está relacionada con colaboraciones con grupos extranjeros para la aplicación de los conocimientos generados en el desarrollo de las técnicas de análisis propuestas. Estudiaremos la dinámica del helio líquido, a través de actividades experimentales que desarrollaremos en el Institut Laue Langevin de Grenoble (Francia) y colaboraremos en el desarrollo de software de procesamiento de datos para la línea de difracción de neutrones de aquél Instituto.The scope of this project covers several aspects related with the use of neutronic techniques employed in the study of systems of interest in basic and applied research. The experimental activity will be performed at Centro Atómico Bariloche. A pulsed electron LINAC is available, that allows to obtain 25 MeV electrons, which are employed to produce neutrons. The main purpose of this project is the study of several aspects related with the experimental data processing in the Deep Inelastic Neutron Scattering (DINS), and Neutron Total Cross Sections (NTCS) techniques. The main objective of DINS is to obtain the momentum distributions of the atoms of the sample under study. We have formulated a new data treatment procedure, that corrects serious flaws found in the usual modus operandi, that poses doubts on some published key results, notably anomalies in the Hydrogen cross sections in light water/heavy water mixtures. Recently this technique has been implemented at Centro Atómico Bariloche, and we are able to perform an independent experimental program to study the dynamics of different systems, to obtain absolute values of neutron total cross sections, and extend its range of applicability to non-destructive mass spectrometry. On the other hand NTCS has proven to be a valuable complement of more sophisticated neutron techniques, and is also operative at our Laboratory. Recently we showed its ability to apply it to the study of the frequency spectra of different systems, which presently is not fully exploited

Experimental corrections in neutron diffraction of ambient water using H/D isotopic substitution

We report on the procedure to treat neutron diffraction data, in order to obtain the differential cross sections, a step that is essential in the analysis of structural information for liquids and amorphous materials. The treatment is applied to new measurements that have been made on the D4C diffractometer for eight different isotopic compositions of H2O/D2O mixtures at ambient temperature. The procedure is based on Monte Carlo simulations to perform multiple scattering, attenuation and detector efficiency corrections, applied iteratively. The normalization process allows the direct comparison of measured data from different samples, and requires the knowledge of the total cross-section at the incident neutron energy. They high contrast between the cross sections (coherent and incoherent) for heavy and light water allows to test the procedure under very different conditions of multiple scattering effects.Fil: Dawidowski, Javier. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Area de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Area de Investigación y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Bariloche); ArgentinaFil: Cuello, Gabriel Julio. Institut Laue Langevin; Francia. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Formalism for obtaining nuclear momentum distributions by the Deep Inelastic Neutron Scattering technique

We present a new formalism to obtain momentum distributions in condensed matter from Neutron Compton Profiles measured by the Deep Inelastic Neutron Scattering technique. The formalism describes exactly the Neutron Compton Profiles as an integral in the momentum variable $y$. As a result we obtain a Volterra equation of the first kind that relates the experimentally measured magnitude with the momentum distributions of the nuclei in the sample. The integration kernel is related with the incident neutron spectrum, the total cross section of the filter analyzer and the detectors efficiency function. A comparison of the present formalism with the customarily employed approximation based on a convolution of the momentum distribution with a resolution function is presented. We describe the inaccuracies that the use of this approximation produces, and propose a new data treatment procedure based on the present formalism.Comment: 11 pages, 8 figure

Multiple scattering and attenuation corrections in Deep Inelastic Neutron Scattering experiments

Multiple scattering and attenuation corrections in Deep Inelastic Neutron Scattering experiments are analyzed. The theoretical basis is stated, and a Monte Carlo procedure to perform the calculation is presented. The results are compared with experimental data. The importance of the accuracy in the description of the experimental parameters is tested, and the implications of the present results on the data analysis procedures is examined.Comment: 19 pages, 8 figure

Diffraction Properties and Application of 3D Polymer Woodpile Photonic Crystal Structure

We present a new technique for modification of diffraction and optical properties of photonic devices by surface application of polymer Three-Dimensional (3D) woodpile Photonic Crystal (PhC) structure. Woodpile structure based on IP-Dip polymer was designed and fabricated by Direct Laser Writing (DLW) lithography method based on nonlinear Two-Photon Absorption (TPA). At first, we investigated diffraction properties of woodpile structure with a period of 2 μm. The structure was placed on a glass substrate, and diffraction patterns were measured using laser sources with different wavelengths. After diffraction properties investigation, the fabricated structures were used in optoelectronic devices by their surface application. Our polymer 3D PhC woodpile structures were used for radiation properties modification of light emitting devices - optical fiber and Light Emitting Diode (LED) and for angular photoresponse modification of InGaAsN-based photodiode. The modification of the far-field radiation patterns of optical fiber and LED and spatial modulation of light coupling into photodiode chip with applied structures were measured by goniophotometer. Quality of fabricated structures was analyzed by a Scanning Electron Microscope (SEM)

The Physics of Liquid Para-Hydrogen

Macroscopic systems of hydrogen molecules exhibit a rich thermodynamic phase behavior. Due to the simplicity of the molecular constituents a detailed exploration of the thermal properties of these boson systems at low temperatures is of fundamental interest. Here,we report theoretical and experimental results on various spatial correlation functions and corresponding distributions in momentum space of liquid para-hydrogen close to the triple point. They characterize the structure of the correlated liquid and provide information on quantum effects present in this Bose fluid. Numerical calculations employ Correlated Density-Matrix(CDM)theory and Path-Integral Monte-Carlo(PIMC)simulations. A comparison of these theoretical results demonstrates the accuracy of CDM theory. This algorithm therefore permits a fast and efficient quantitative analysis of the normal phase of liquid para-hydrogen.We compare and discuss the theoretical results with available experimental data.Comment: 14 pages, 7 figure

Emisiones de gases de efecto invernadero : atmósfera cargada

Fil: Dawidowski, Laura. Universidad de Buenos Aires. Facultad de Ingeniería; Argentina.Fil: Gómez, Darío. Universidad de Buenos Aires. Facultad de Ingeniería; Argentina.Durante los últimos cien años la humanidad ha incrementado la\nconcentración en la atmósfera de gases y partículas que por sus\ncaracterísticas físico-químicas tienen el potencial de modificar el\nclima. Los llamados gases de efecto invernadero (dióxido de carbono,\nmetano, oxido nitroso y otros gases que contienen flúor, tales como\nlos hidroflurocarbonos, clorofluorocarbonos y el hexafluoruro de\nazufre) se caracterizan por su capacidad de absorber la radiación\ninfrarroja emitida por la Tierra. Las partículas en suspensión, también\nllamadas aerosoles atmosféricos, juegan un rol doble dependiendo de\nsu composición y tamaño, ya sea reflejando la radiación que proviene\ndel sol e induciendo un enfriamiento, o absorbiendo la radiación\nemitida por la Tierra potenciando así el efecto de los mencionados\ngases

Superfluid 4He dynamics beyond quasiparticle excitations

The dynamics of superfluid 4He at and above the Landau quasiparticle regime is investigated by high precision inelastic neutron scattering measurements of the dynamic structure factor. A highly structured response is observed above the familiar phonon-maxon-roton spectrum, characterized by sharp thresholds for phonon-phonon, maxon-roton and roton-roton coupling processes. The experimental dynamic structure factor is compared to the calculation of the same physical quantity by a Dynamic Many-body theory including three-phonon processes self-consistently. The theory is found to provide a quantitative description of the dynamics of the correlated bosons for energies up to about three times that of the Landau quasiparticles.Comment: 5 pages, 3 figure

Efficient procedure for the evaluation of multiple scattering and multiphonon corrections in inelastic neutron-scattering experiments

10 págs.; 7 figs.; 1 tab.We present a full set of procedures to evaluate the experimental corrections needed to derive physically meaningful quantities from the measured neutron intensities in inelastic neutron-scattering experiments. Multiple-scattering corrections are evaluated by means of a Monte Carlo code, in which a combination of experimental data and the Synthetic Model is used to account for neutron-molecule interactions. Multiphonon corrections are treated with an iterative scheme. To illustrate the procedure the densities of vibrational states of deuterated water and ice near room temperature are evaluated from data measured in a chopper spectrometer. ©1998 American Physical SocietyPeer Reviewe