Examinando por Autor "Velasquez, A. A."
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Ítem Design and construction of an electromechanical velocity modulator for Mössbauer spectroscopy(SPRINGER, 2011-11-01) Velasquez, A. A.; Carmona, A.; Velasquez, D.; Angel, L.; Universidad EAFIT. Departamento de Ciencias Básicas; Electromagnetismo Aplicado (Gema)In this paper we report the design, construction and characterization of an electromechanical velocity modulator for application in Mössbauer spectroscopy. The modulator was constructed with copper coils, Neodymium magnets, steel cores and polymeric membranes. The magnetic field in the driving and velocity sensing stages was analyzed by the finite element method, which showed a linear relation between the magnetic field in the region of motion of both coils and the position of the coils within the steel cores. The results obtained by computational simulation allowed us to optimize geometries and dimensions of the elements of the system. The modulator presented its first resonance frequency at 16. 7 Hz, this value was in good agreement with that predicted by a second order model, which showed a resonant frequency of 16. 8 Hz. The linearity of the velocity signal of the modulator was analyzed through an optical method, based on a Michelson-Morley interferometer, in which the modulator moved one of the mirrors. Results showed a satisfactory linearity of the velocity signal obtained in the sensing coil, whose correlation with a straight line was around 0. 99987 for a triangular reference waveform. © 2011 Springer Science+Business Media B.V.Ítem Implementation of a preamplifier-amplifier system for radiation detectors used in Mössbauer spectroscopy(SPRINGER, 2014-01-01) Velasquez, A. A.; Arroyave, M.; Universidad EAFIT. Departamento de Ciencias Básicas; Electromagnetismo Aplicado (Gema)We report the assembly and testing of a preamplification and amplification system for pulses produced by gaseous radiation detectors commonly used in Mössbauer spectroscopy. The system is composed by a pair of commercial integrated circuits A203 and A206, which operate as charge sensitive preamplifier-shaping amplifier and linear amplifier-low level discriminator, respectively. The integrated circuits were interconnected in the unipolar output mode and placed inside a metallic shielding, which prevents noise amplification for a suitable signal-noise ratio. The system was tested by irradiating a proportional counter LND-45431 with characteristic X rays of 6.3 keV and gamma rays of 14.4 keV emitted by a Mössbauer radioactive source of 57Co (Rh). Unipolar pulses with Gaussian profile were obtained at the output of the linear amplifier, whose amplitudes were close to 0.4 V for 6.3 keV X rays and 1.4 V for 14.4 keV gamma rays. Pulse height spectra showed that the system allows a satisfactory identification of the X-rays and gamma rays emitted by the 57Co source, giving the possibility to make a good selection of the 14.4 keV peak for having a suitable signal-noise ratio in the Mössbauer spectra. Absorption percentages of 14 % were found by taking the Mössbauer spectra of a natural iron absorber. The assembly and tests of the system are presented through this paper. © 2013 Springer Science+Business Media Dordrecht.Ítem Synthesis and characterization of Cu2+ substituted magnetite(SPRINGER, 2011-11-01) Morales, A. L.; Velasquez, A. A.; Urquijo, J. P.; Baggio, E.; Universidad EAFIT. Departamento de Ciencias Básicas; Electromagnetismo Aplicado (Gema)Samples of magnetite, both pure and doped with divalent copper, Fe3-xCuxO4, with x = 0, 0.05, 0.10 and 0.20 atm.%, were synthesized hydrothermally. The samples were characterized by Atomic Absorption Spectroscopy, Mössbauer Spectroscopy, X-ray diffraction, Scanning Electron Microscopy and SQUID magnetometry. The analyses made by the above techniques showed that as the Cu2+ concentration increases, a simultaneous reduction in the magnetic and structural parameters takes place, namely: magnetic hyperfine interactions at octahedral sites, particle size and lattice constant. Degradation in the particles morphology as well as a distribution of their size were also observed. Our study points two important effects of Cu2+ in magnetite, the first one is its incorporation within the structure, replacing Fe2+ ions and decreasing both the magnetic hyperfine interactions at octahedral sites and the bulk magnetization, the second one is the contraction of the crystalline lattice of magnetite, because incorporation of Cu2+ within the structure, generation of vacancies or both simultaneous effects. © 2011 Springer Science+Business Media B.V.