A Transport Equation Theory of Electron Scattering

The use of the Boltzmann transport equation to describe electron scattering in electron microscopy and electron probe microanalysis is discussed. A method of solution is given in which the transport equation is divided into angle and energy intervals. This gives rise to a number of coupled first order differential equations. Separation into forward and backward travelling components of the electron flux distribution enables the correct boundary conditions to be imposed. Solutions are derived which take the form of matrix operators analytic in both depth and target thickness. These matrices allow derivation of other physical quantities such as X-ray or Auger electron production. Calculations using this method are fast and accurate. Results are presented showing angular distributions of backscattered electrons and the variation of the backscattered fraction with angle of incidence and atomic number. The variations of backscattered, transmitted and absorbed fractions with target thickness are presented. The theory has also been applied to the calculation of the energy distributions of backscattered electrons, energy dissipation and X-ray production as functions of depth and the Auger backscattering factor. It appears that electron scattering in thick targets is not amenable to treatment using simple models. This is because most of the features of interest are determined by a combination of medium angle scattering (\u3c 20°) and large angle scattering (20-90°). Nevertheless certain approximations within the present framework, which describe multiple scattering correctly, can give some useful insights

Dose, exposure time, and resolution in Serial X-ray Crystallography

The resolution of X-ray diffraction microscopy is limited by the maximum dose that can be delivered prior to sample damage. In the proposed Serial Crystallography method, the damage problem is addressed by distributing the total dose over many identical hydrated macromolecules running continuously in a single-file train across a continuous X-ray beam, and resolution is then limited only by the available molecular and X-ray fluxes and molecular alignment. Orientation of the diffracting molecules is achieved by laser alignment. We evaluate the incident X-ray fluence (energy/area) required to obtain a given resolution from (1) an analytical model, giving the count rate at the maximum scattering angle for a model protein, (2) explicit simulation of diffraction patterns for a GroEL-GroES protein complex, and (3) the frequency cut off of the transfer function following iterative solution of the phase problem, and reconstruction of an electron density map in the projection approximation. These calculations include counting shot noise and multiple starts of the phasing algorithm. The results indicate counting time and the number of proteins needed within the beam at any instant for a given resolution and X-ray flux. We confirm an inverse fourth power dependence of exposure time on resolution, with important implications for all coherent X-ray imaging. We find that multiple single-file protein beams will be needed for sub-nanometer resolution on current third generation synchrotrons, but not on fourth generation designs, where reconstruction of secondary protein structure at a resolution of 0.7 nm should be possible with short exposures.Comment: 19 pages, 7 figures, 1 tabl

Determination of antibiotic residues and withdrawal time in milk from the town of Cartago, Valle del Cauca

P?ginas 25-31Recurso Electr?nicoEl control de residuos de antimicrobianos, en productos de origen animal, es de gran importancia, ya que estos pueden ocasionar problemas de salud al consumidor, as? como interferencias en los procesos tecnol?gicos y p?rdidas econ?micas significativas para la industria; por ello, resulta conveniente establecer sistemas adecuados de control de residuos de los antimicrobianos utilizados en el ganado vacuno lechero y as? entregar un producto de excelente calidad para el consumidor. En este estudio, se emplearon seis antimicrobianos (ceftiofur, espiramicina, tilosina, oxitetraciclina, amoxicilina, cloxacilina) que se administraron v?a intramuscular y una asociaci?n (cloxacilina-ampicilina) administrado v?a intramamaria en 115 vacas cruzadas (Holstein-ceb?). Se tomaron muestras de leche a 1, 3 y cada 12 horas postratamiento y se analizaron con la prueba microbiol?gica Delvotest SP-NT. Los resultados indicaron que el 89,3% de los animales no presentaron residuos de ceftiofur, mientras que el 65,2% tuvieron residuos de oxitetraciclina, en rangos de 72 y 156 horas. Residuos de tilosina se obtuvieron entre 108 y 168 horas, con un pico m?ximo a las 108 horas (58,8%), el cual coincide con la detecci?n de residuos de espiramicina (69,2%). Se detectaron residuos de amoxicilina a partir de las 48 y hasta las 84 horas, con una frecuencia del 65% a las 84 horas. Finalmente, los tratamientos intramamarios con la asociaci?n cloxacilina-ampicilina indicaron que la detecci?n de residuos disminuye despu?s de las 60 horas (42,9%).ABSTRACT. Control of antimicrobial residues in animal products is very important, since they may cause consumer?s health problems and interfere with technological processes causing significant economic losses to the food industry. Thus, it is necessary to establish suitable control systems for antimicrobials used in dairy cattle and in this way to deliver a high quality product to the consumers. In this study, we used 6 antimicrobials (ceftiofur, spiramycin, tylosin, oxytetracycline, amoxicillin and cloxacillin-ampicillin association) that were administered via intramuscular route and one antimicrobial (cloxacillin-ampicillin association) administered via intramammary in 115 crossbred cows (Holstein-Zebu), to evaluate antimicrobial residues in milk. Samples of milk were taken, at 1, 3, and every 12 hours post-treatment and analyzed by the microbiological test Delvotest SP-NT). The results indicated that 89,3% of the animals did not present residues of Ceftiofur, whereas 65,2% had residues of Oxytetracycline between 72 and 156 hours post-treatment. Residues of Tylosin were found between 108 to 168 hours, with a peak at 108 hours (58.8%), which coincides with the detection of residues of Spiramycin (69.2%). Residues of amoxicillin were detected at 48 hr and still remained at 84 hours, with a frequency of 65% by this time. Finally, intramammary treatment with Ampicillin- Cloxacillin indicated that the detection of residues decreased after 60 hours (42.9%)

El papel de los recursos del territorio local en el contexto de una propuesta alternativa para la ense?anza de las ciencias naturales en los estudiantes de grado primero de primaria de la instituci?n Liceo Isabel Sarmiento

66 p. Recurso Electr?nicoLas ciencias naturales han sido durante la historia una signatura tediosa y mon?tona en la que los estudiantes son privados de conocer la naturaleza y sumergidos en el mundo de los textos y las l?neas, due?os de la verdad absoluta sobre los fen?menos naturales, por otra parte la pedagog?a ha buscado estrategias para promover el aprendizaje y la aprehensi?n de las ciencias naturales convirtiendo estas en una asignatura interesante para los estudiantes en la que puedan experimentar e interactuar con la naturaleza descubriendo por si mismos los que los libros describen. Un docente inquieto e interesado en el aprendizaje de sus estudiantes busca herramientas para estimular el aprendizaje de su asignatura, por esto es importante utilizar los recursos que el ambiente local aporta para facilitar su labor, esta investigaci?n contribuye al que hacer docente otorgando una caja de herramientas con distintas actividades y elementos pedag?gicos como la construcci?n de ambientes de aprendizaje para realizar las clases de ciencias naturales aprovechando cada uno de los recursos. Facilitando la aprensi?n de los temas y estimulando el inter?s de los estudiantes por los fen?menos del mundo que los rodea. Palabras claves: recursos, ciencias, aprendizaje, asignaturas, ambientes, territorio local, actividades, herramientas.?The natural sciences have been during the history a tedious and monotonous signature in which the students are deprived of knowing the nature and immersed in the world of the texts and the lines, owners of the absolute truth on the natural phenomena, on the other hand the pedagogy to sought strategies to promote learning and apprehension of the natural sciences making these an interesting subject for students in which they can experience and interact with nature discovering for themselves what the books describe. A restless teacher interested in learning their students is looking for tools to stimulate the learning of their subject, so it is important to use the resources that the local environment contributes to facilitate their work, this research contributes to the teacher by providing a toolbox with different activities and pedagogical elements as the construction of learning environments to realize the classes of natural sciences taking advantage of each one of the resources. Facilitating the apprehension of the subjects and stimulating the interest of the students by the phenomena of the world that surrounds them. Keywords: resources, science, learning, subjects, environments, local territory, activities, tools

Constraining the neutrino magnetic moment with anti-neutrinos from the Sun

We discuss the impact of different solar neutrino data on the spin-flavor-precession (SFP) mechanism of neutrino conversion. We find that, although detailed solar rates and spectra allow the SFP solution as a sub-leading effect, the recent KamLAND constraint on the solar antineutrino flux places stronger constraints to this mechanism. Moreover, we show that for the case of random magnetic fields inside the Sun, one obtains a more stringent constraint on the neutrino magnetic moment down to the level of \mu_\nu \lsim few \times 10^{-12}\mu_B, similar to bounds obtained from star cooling.Comment: 4 pages, 3 figures. Final version to appear in Phys. Rev. Let