Yttrium silicate powders doped with Eu3+ and codoped with Eu3+ and Yb3+ were prepared by combustion synthesis. The x-ray powder diffraction data showed the presence of Y2SiO5 and Y2Si2O7 crystalline phases. Singly doped (1 wt %) sample illuminated with ultraviolet light (λ=256 nm) showed the characteristic red luminescence corresponding to D50→F7J transitions of Eu3+. The Judd–Ofelt intensity parameters were calculated from experimental data and the radiative and nonradiative relaxation rates were estimated. The results showed that the nonradiative relaxation rate is smaller in yttrium silicate compared to yttrium oxide powder, a reference material, prepared under similar conditions. Codoped samples were exposed to near-infrared laser excitation (λ=975 nm) and the red luminescence of Eu3+ was also observed. In this case, the luminescence is achieved due to a cooperative upconversion (CUC) process involving energy transfer (ET) from pairs of ytterbium ions to europium ions. The ET rate was estimated by fitting a rate equation model with the dynamics of CUC red emission.
Collagen extraction depends on the state of bone preservation, and the acidity of Brazilian soils often prevents the use of this material for radiocarbon dating. When available, however, bone samples constitute very important chronological records for both archaeological sites and natural depositional sites of specific animals. The extraction of collagen was performed using two filters, the first aiming to remove insoluble contaminants, and the second, a vivaspin ultrafilter 30KD to retain large molecular weight materials. The collagen was liofilized and converted to CO2 by combustion in sealed quartz tubes with CuO and Ag. The graphite was produced by zinc reduction in independently sealed Pyrex™ tubes. In order to verify the accuracy of this protocol, we analyzed a modern bone and four previously dated fragments, including those from the Sixth International Radiocarbon Intercomparison (SIRI), and a fragment of human bone from the Amourins site, a Brazilian shellmound. The results for the known age material are in agreement with the expected and the studied sector of Amourins shellmound was dated 4100–3900 years cal BP from a chronological model performed with charcoal dating found in different stratigraphic layers. Samples were dated at the radiocarbon laboratory of Universidade Federal Fluminense (LAC-UFF) in Brazil.
Discutimos o problema da descarga de um capacitor associado a outro, idêntico e inicialmente descarregado. É sabido que, com ou sem a presença de resistência elétrica no circuito, o estado final de equilbrio, assim como a energia dissipada no processo, são sempre os mesmos. O mecanismo de dissipação no caso em que não há resistência elétrica é a irradiação do sistema e no caso em que a resistência elétrica está presente ocorrem ambos, irradiação e dissipação por efeito Joule, devido à interação dos elétrons de condução com a rede cristalina, de forma a sempre satisfazer o teorema de Poynting. A diferença entre ambos os processos está no intervalo de tempo em que o sistema atinge o equilbrio e, consequentemente, no espectro da radiação eletromagnética emitida por este sistema.
In paleoenvironmental research, several proxies are used to reconstruct climate and vegetation. The establishment of a chronological framework allows for the association of different proxies and correlation of events happening in different geographic areas. Cultural deposits, such as the shellmounds found along the coast of Brazil, play an important role in paleoenvironmental interpretations. Here, we have employed anthracological analysis in charcoal fragments from the Amourins shellmound, located at the margins of the Guanabara Bay, Rio de Janeiro. This allowed for the taxonomic identification and selection of short-lived trees and specific parts of plants for accurate radiocarbon dating. We recorded genera and families typical of the Atlantic Forest, restinga forest, open restinga and mangrove. The 14C ages of charred nuts from different occupational layers range from 3807 ± 35 to 3503 ± 70 BP and a sequential chronological model was built, relating the predominance of mangrove vegetation to the period between 4130–3960 cal BP.
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