Framed in our paleoenvironmental research project, we have observed a high diversity and frequency of diatoms of the genus Luticola D.G. Mann in sediments of the last 11,000 years of Barrancas locality (S 23° 18’ 08,7”; W 66° 05’ 15,2”; 3666 m asl). We identified ten taxa, two of them are new species -Luticula punae and L. juliae-, and we described 2 different forms of L. mollis. Two of these species are new records for Argentina and one for South America.

En el marco de las investigaciones paleoambientales que estamos desarrollando en la localidad de Barrancas (23° 18’ 08,7” S; 66° 05’ 15,2” O; 3666 m snm) detectamos, en sedimentos de los últimos 11.000 años, una alta diversidad y frecuencia de representantes del genero Luticola D.G. Mann. Se identificaron 10 especies, de las cuales dos son nuevas (Luticola punae sp. nov. y Luticola juliae sp. nov.). Se describen dos morfotipos de Luticola mollis. Dos de estas especies se citan por primera vez para Argentina y una para Sudamérica.

An experimental setup to determine the K-shell photoelectric cross-section of Dy, Ta, Pt and Au atoms was implemented at the Nuclear Analytical Laboratory (LAN) of the InSTEC. Bremsstrahlung photons, produced by <img width=26 height=15 src="http://img/revistas/nuc/n54/e01035413.jpg">-<img width=15 height=13 src="http://img/revistas/nuc/n54/e02035413.jpg"> beta particles hitting a thin Ni converter, were used to irradiate the target under study. A HPGe detector, coupled to standard nuclear instrumentation, collected the incident and transmitted spectra. A sharp decrease in intensity at the K-shell binding energy was observed in the transmitted spectra. The photon beam divergence effects were corrected with a calibration curve calculated with Monte Carlo simulations (MCNPX 2.6). In order to establish accurately the cross section at the K-edge energy, the obtained data was processed by two methods: fitting the total cross section to a sigmoidal function, as well as the cross section branches around the K-edge to the empirical law <img width=59 height=13 src="http://img/revistas/nuc/n54/e03035413.jpg">. The Empirical Law method was introduced in this work to minimize the detector resolution effects. The results were compared with experimental and theoretical values showing the best agreement when the thinner targets were used. For the first time the photoeffect cross section at the K-edge energy for Pt is reported at first time.

Se determina la sección eficaz fotoeléctrica de la capa K de los átomos Dy, Ta, Pt y Au en un arreglo experimental desarrollado en el Laboratorio Analítico Nuclear del InSTEC. Los blancos bajo estudio se irradiaron con fotones de frenado producidos en un radiador de Ni por las partículas beta emitidas por una fuente de <img width=26 height=15 src="http://img/revistas/nuc/n54/e01035413.jpg">-<img width=15 height=13 src="http://img/revistas/nuc/n54/e02035413.jpg">. Los espectros incidentes y de transmisión se colectaron en un detector de germanio hiperpuro, acoplado a su instrumentación nuclear estándar. En los espectros de transmisión se observó un decrecimiento agudo de la intensidad correspondiente a la energía del borde K. Los efectos de interacciones múltiples del haz fotónico en las láminas blanco se corrigieron a través de una curva de calibración calculada mediante simulaciones Monte Carlo (MCNPX 2.6). Con vistas a garantizar la mejor precisión en la determinación de la sección eficaz para la energía del borde K, los datos obtenidos se ajustaron según dos comportamientos funcionales en esta región: una sigmoide y una ley empírica del tipo <img width=59 height=13 src="http://img/revistas/nuc/n54/e03035413.jpg">. Este último método se introdujo en el trabajo y permite minimizar los efectos resolutivos. Los resultados obtenidos se compararon con valores teóricos y experimentales, mostrando mayor concordancia cuando se emplean blancos finos. Se reporta, por primera vez, la sección eficaz de fotoefecto en el borde K del platino (Pt).

O Laboratório Virtual, enquanto estratégia de ensino, tem a finalidade de complementar a ação docente em sala de aula, a fim de trabalhar conceitos abstratos de maneira experimental. Para isso, filmamos sistemas reais com o objetivo de observar todo o movimento do objeto em estudo junto com um instrumento que possibilitou a medida de sua posição. Posteriormente, incorporou-se ao vídeo um código de tempo, de modo que, ao transformar-se o filme em quadros independentes, as imagens extraídas permitiram medir a posição ocupada pelo corpo em instantes sucessivos e conhecidos. Com a tabela de posição por tempo, toda a evolução dinâmica do sistema pode ser obtida. O projeto foi desenvolvido e aplicado em disciplinas de mecânica no curso de Licenciatura do Instituto de Física da Universidade de São Paulo. O Laboratório conta com experimentos sobre movimentos de translação, rotação e de fluidos, dois dos quais foram usados para exemplificar a proposta de construção desse laboratório, suas motivações iniciais, o processo de concepção de um experimento, suas etapas de filmagem, edição e disponibilização em um ambiente virtual, que pode ser acessado em http://www.fep.if.usp.br/~fisfoto. Explicaremos o uso pedagógico do laboratório virtual fornecendo detalhes sobre o modo de acesso aos dados e guias de estudo, a etapa de coleta dos dados pelos estudantes e a proposta de análise. Mostraremos resultados típicos obtidos dos experimentos, tanto do ponto de vista de interpretação dos fenômenos em si quanto da maneira como os estudantes internalizaram os diversos conteúdos envolvidos, que vão dos princípios físicos e interpretação estatística das informações obtidas ao uso de planilhas de cálculo, métodos de redução de dados e redação de sínteses e relatórios.

The Virtual Laboratory is a complementary educational activity which aims to work with abstract concepts from an experimental point of view. Real systems were recorded to show the whole motion of an object along with an instrument for position measurement. A time code was added to the recorded video; hence the object position can be measured at known times in the extracted frames. From the position vs. time table, the dynamical evolution of the system can be obtained. This project was developed and applied to Bachelor of Education degree students of the Instituto de Fisica - Universidade de São Paulo. This Laboratory includes experiments on translational, rotational and fluid motion, from which two were used to describe the initial motivation for the laboratory development, the steps taken to build an experiment, from its conception to recording, editing, and making it available through a virtual interface, which can be accessed by the URL http://www.fep.if.usp.br/~fisfoto. We will explain the pedagogical character of the virtual laboratory, and detail the access path to page content, study guides, and data, as well as the analysis proposed to the students. Typical results achieved with this experiment will be shown, both on the physical phenomenon interpretation and on the way the students internalize different aspects, ranging from physical concepts to statistical interpretation of the information to datasheet calculation, data reduction methods and report preparation techniques.

A diatomological analysis of thirteen high altitude wetlands, between 3500-4683 m a. s. l. of Jujuy Province (Argentina) was performed. Fifty one genera and one hundred and fifty seven infrageneric taxa, excluding Navicula sensu stricto species, were identified; three of them are new records for Argentina and 43 are new for Jujuy Province. A new combination is proposed: Craticula cuspidata var. gracilis. The finding of Cylindrotheca gracilis (Bréb.) Grunow is important, since it has not been previously mentioned for this kind of environments. Amphora veneta Kützing var. veneta was recorded in more than 90 % of the studied waterbodies while Nitzschia hungarica Grunow, Craticula cuspidata (Kützing) D. G. Mann var cuspidata and Luticola cohnii (Hilse) Lange-Bertalot were found in more than 60% of the samples.

Se estudió la flora diatomológica perteneciente a 13 humedales de altura ubicados entre los 3500-4683 m s. n. m. de la Provincia de Jujuy (Argentina). Se identificaron, excluyendo las especies de Navicula sensu stricto, 51 géneros y 157 taxones infragenéricos, de los cuales tres son nuevas citas para Argentina y 43 se registran por primera vez para Jujuy. Se propone una nueva combinación: Craticula cuspidata var. gracilis. Muy interesante resultó el hallazgo de Cylindrotheca gracilis (Bréb.) Grunow, nunca antes mencionado para este tipo de ambientes. Sólo Amphora veneta Kützing estuvo presente en más del 90 % de los cuerpos de agua estudiados mientras que Nitzschia hungarica Grunow, Craticula cuspidata (Kützing) D. G. Mann var. cuspidata y Luticola cohnii (Hilse) Lange-Bertalot fueron halladas en más del 60% de las muestras.

Among the secondary detection effects in gamma-ray spectroscopy with HPGe detectors that reduce the peak area and must be properly taken into account in accurate measurements, the less well known is counting loss by beta and gamma-ray sum coincidence. The fraction of lost counts was estimated assuming that a photon detection event can sum either with the coincident beta-ray or its bremsstrahlung in the detector capsule or surrounding materials. The beta-ray was tracked through all the materials between source and detector. The result was calculated as a sequence of integral expressions evaluated numerically, without resorting to simulation. It is shown that the counting losses due to this effect are negligible for most calibration sources in typical detector arrangements.