Aplicaciones del diagrama de Loedel

Se analiza el choque de Lewis Tolman y la paradoja de los mellizos mediante el diagrama de Loedel. Desde 35 años aproximadamente el Dr. Enrique Loedel ha venido publicando en español en diversas revistas especializadas de Argentina y Uruguay variados aspectos de la Teoría de la Relatividad. Entre estos trabajos se ha destacado por su sencillez un método gráfico para manejar la transformación de Lorentz que permite visualizar los pormenores de los distintos problemas. La recopilación de los problemas resueltos por ese método gráfico, que llamo diagrama de Loedel apareció actualizada hasta 1955 en el libro “Física Relativista” del mismo autor. Faltan allí la solución de los problemas mencionados, objeto de este trabajo. La paradoja de los mellizos ha tomado vigencia a partir de 1959. Sin embargo, la motivación esencial es revalidar el mérito de prioridad científica del Dr. Enrique Loedel. Robert W. Brehme publica una ligera variante del diagrama de Loedel, sin mencionarlo; poco más tarde Francsi W. Sears con el título “Some applications of the Brerme diagram” se refiere al mismo tema sin mencionar a Loedel

Galileo, gigante del mundo occidental

Quiero destacar que me resultó muy grato, como profesor universitario, el hecho de haber tenido, gracias a la gentileza del señor ministro de Educación, Dr. C. Aleonada Aramburú, la oportunidad de participar en las deliberaciones de la 139 Asomblea General de la UNESCO. Fue motivo de mi admiración el ejemplo constante de organización en sus mínimos detalles, así como el alto grado de eficiencia e información por parte de su director general, profesor Maheu, y de todos sus colaboradores técnicos, administrativos y científicos, acreedores por momentos de nuestras más cálidas felicitaciones. Con motivo de cumplirse el 4° centenario del nacimiento de Ga- lileo Galilei rendimos un cálido homenaje a su memoria, y bajo los auspicios de la misma UNESCO se dieron a conocer los aspectos sobresalientes de la vida del sabio bajo el título: “Galileo Galilei una nueva visión del universo”. Al término de la citada asamblea visité numerosas catedrales, fuentes, acueductos y variadas obras de arte, deteniéndome con especial interés en las construidas con anterioridad al Renacimiento. Esos esqueletos de humanismo exponen con claridad que antes de Galileo ya contaba el mundo occidental con obreros habilidosos y pacientes, ingenieros, arquitectos y artistas capaces de tales obras imperecederas. Suplían la falta de ciencia con el fruto de su inspiración y el cúmulo de práctica en el manejo de la Estática y Resistencia de materiales; sólo les faltaba la capacidad de forjar conceptos científicos. Los griegos, después de superar la etapa mítica, formaron la facultad de pensar por intermedio de conceptos, lógicamente, de concebir el mundo como un hecho objetivo. Este modo de pensar se desarrolla durante el Renacimiento y adquiere su faz culminante con Galileo y Newton

Formar científicos en condiciones de adversidad

Esta reseña de mi trayectoria, con cuya solicitud me ha honrado la AAPC, intenta transmitir algunas reflexiones sobre cómo facilitar la investigación científica. La preparación de la misma me ha permitido reconocer algunas ideas que han surgido a partir de mis experiencias como investigador y docente a lo largo de mi quehacer universitario y que vale la pena compartir. No abundaré en detalles demasiado particulares que probablemente no tengan un valor general que sí tienen otros. Excepto algunas menciones ineludibles, estas cuestiones son complementarias de lo que muchos investigadores ya han expuesto en las excelentes reseñas que pueden encontrarse en las páginas de la AAPC sobre las condiciones políticas y sociales en las décadas que nos ha tocado desarrollar la investigación en la Argentina.Instituto de Física La Plat

Formar científicos en condiciones de adversidad

Esta reseña de mi trayectoria, con cuya solicitud me ha honrado la AAPC, intenta transmitir algunas reflexiones sobre cómo facilitar la investigación científica. La preparación de la misma me ha permitido reconocer algunas ideas que han surgido a partir de mis experiencias como investigador y docente a lo largo de mi quehacer universitario y que vale la pena compartir. No abundaré en detalles demasiado particulares que probablemente no tengan un valor general que sí tienen otros. Excepto algunas menciones ineludibles, estas cuestiones son complementarias de lo que muchos investigadores ya han expuesto en las excelentes reseñas que pueden encontrarse en las páginas de la AAPC sobre las condiciones políticas y sociales en las décadas que nos ha tocado desarrollar la investigación en la Argentina.Instituto de Física La Plat

Activities of 232Th, 226Ra, 40K, and 137Cs in surface soil and external dose assessment at two zones of Buenos Aires Province, Argentina

In the frame of establishing the radiological baseline of the Buenos Aires Province, Argentina, this work assesses the total outdoor external dose and the activity values of 40K, 226Ra, 232Th and 137Cs in superficial soils of La Plata and Lima cities. In the latter, the most important nuclear emplacement of the country is located. At La Plata, 49 sites were analyzed, which correspond to the green public spaces of the city while at Lima 6 sites around the nuclear power plants were studied. For La Plata, the external dose contribution and activity distribution maps of 40K, 226Ra, 232Th and 137Cs were obtained using the Kriging geo-statistical method. Activity values of 226Ra and 232Th resulted lower than their worldwide average ones, while the 40K levels were higher than the corresponding one. The activity dataset was analyzed together with the activity values determined previously in unperturbed soils of the region. The Pearson correlation coefficient was re-calculated, obtaining correlations more significant when all activity data were considered. No correlation between activity superficial distributions and the height above sea level or the geomorphological units was found. The activity values obtained in Lima soils lie in the range of the values determined for La Plata soils. Like in La Plata, 137Cs activity around the nuclear facility could be associated only with the nuclear power tests. The average total external annual dose for La Plata inhabitants are in the range of 0.19 to 0.25 mSv/year, similar to those determined for Lima residents. The present work complements previous reports contributing to the establishment of the radiological baseline for soils of Buenos Aires Province, Argentina, the province with the most important nuclear activities of the country.Instituto de Física La Plat

Lattice-dynamics of the intermediate oxide of tin

The lattice-dynamical properties of the intermediate oxide of tin (IO) have been investigated over the temperature range 80–295 K. The logarithm of the recoil-free fraction varied linearly with temperature. However, a Debye model does not give a good account of this dependence. By comparison with the other oxides of the Sn-O system, the recoil-free fraction for tin atoms with 2+ and 4+ oxidation states of the IO were estimated. At room temperature, they are 0.463 and 0.323 for 4+ and 2+ states respectively.Facultad de Ciencias Exacta

Hyperfine and radiological characterization of soils of the province of Buenos Aires, Argentina

The depth profile concentration of both natural and anthropogenic gamma-ray-emitter nuclides were determined in soil samples collected in an area located at 34° 54.452′ S, 58° 8.365′ W, down to 50 cm in depth, using an hyper-pure Ge spectrometer. The soil samples were also characterized by means of Mössbauer spectrometry and X-ray diffraction. The activities of 238U and 232Th natural chains remain constant in depth at 41 Bq/kg and 46 Bq/kg, respectively, while the 40K activity increases from 531 Bq/kg to 618 Bq/kg between 2.5 cm y 25.5 cm of depth. The only anthropogenic detected nuclide is 137Cs, whose activity changes form 1.4 Bq/kg to values lower than the detection limit (LD) for depths below 25 cm, exhibiting a maximum at 10 cm beneath the surface. The Mössbauer spectra show two magnetic sextets associated with α-Fe2O3 and Fe3O4, as well as two Fe+3 Fe+2 doublets, probably originated in octahedral and tetrahedral sites of paramagnetic phases. The Fe3+ paramagnetic signal relative fraction increases up to 82% at the expense of the α-Fe2O3 one when de depth increases. No correlation between Fe3O4 and the 137Cs was identificated.Facultad de Ciencias ExactasInstituto de Física La Plat

Hyperfine and radiological characterization of soils of the province of Buenos Aires, Argentina

The depth profile concentration of both natural and anthropogenic gamma-ray-emitter nuclides were determined in soil samples collected in an area located at 34° 54.452′ S, 58° 8.365′ W, down to 50 cm in depth, using an hyper-pure Ge spectrometer. The soil samples were also characterized by means of Mössbauer spectrometry and X-ray diffraction. The activities of 238U and 232Th natural chains remain constant in depth at 41 Bq/kg and 46 Bq/kg, respectively, while the 40K activity increases from 531 Bq/kg to 618 Bq/kg between 2.5 cm y 25.5 cm of depth. The only anthropogenic detected nuclide is 137Cs, whose activity changes form 1.4 Bq/kg to values lower than the detection limit (LD) for depths below 25 cm, exhibiting a maximum at 10 cm beneath the surface. The Mössbauer spectra show two magnetic sextets associated with α-Fe2O3 and Fe3O4, as well as two Fe+3 Fe+2 doublets, probably originated in octahedral and tetrahedral sites of paramagnetic phases. The Fe3+ paramagnetic signal relative fraction increases up to 82% at the expense of the α-Fe2O3 one when de depth increases. No correlation between Fe3O4 and the 137Cs was identificated.Facultad de Ciencias ExactasInstituto de Física La Plat

Phase transformations induced by implantation of ¹²C⁻ ions into α-Fe and AISI 304 and 316 stainless steels studied by CEMS and SEM

Samples of AISI 304 and 316 stainless steels, initially in austenitic (first set) and martensitic states (second set) and α-Fe (third set), were implanted with 180 keV¹²C⁻ to a dose of 10¹⁷ atoms/cm² at room temperature. Surfaces were examined by SEM (scanning electron microscopy) and the crystalline-phase fractions were estimated through CEMS (conversion electron Mossbauer spectroscopy). Different grades of etching were produced by sputtering during the implantations on the stainless steel samples depending on the previous crystallographic states. CEMS data reveal the γ → α′ transformation in the initially martensitic samples and no noticeable modifications as a result of the implantation on α-Fe and austenitic samples.Facultad de Ciencias Exacta

Excitation functions for the production of 90 Nb and 88 Y by irradiation of zirconium with deuterons

Excitation functions for reactions induced by deuterons on natural zirconium and90Zr targets have been measured radiochemically with the stacked foil technique between ≈9 and 27 MeV. The observed reactions are Zr(d,xn)90 Nb, Zr(d,axn)88Y,90Zr(d, 2n)90Nb and90Zr(d, α)88Y. The excitation functions for the reactions91Zr(d, 3n)90Nb and91Zr(d, αn)88Y have been deduced from the results mentioned above. Calculations of the excitation function for the (d, 2n) reaction have been performed with two different treatments, each one taking into account two competitive mechanisms. The compound-statistical model plus Hittmair's stripping theory accounts quite well for the (d, 2n) cross sections observed. However, the agreement obtained with Peaslee-Otozai's theory is excellent and the set of parameters used more reasonable. It has been assumed that the stripping mechanism can have no contribution to the (d, α) reaction. Accordingly, calculations for this reaction have been done using the compound-statistical model and the entire absorption process followed by the evaporation of an α particle. No good agreement is obtained with either theory.Facultad de Ciencias Exacta