Ion backflow studies with a triple-GEM stack with increasing hole pitch

Gas Electron Multipliers have undergone a very consistent development since their invention in 1997. Their production procedures have been tuned in such a way that nowadays it is possible to produce foils with areas of the order of the square meter that can operate at a reasonable gain, uniform over large areas and with a good stability in what concerns electrical discharges. For the third run of LHC, they will be included in the CMS and ALICE experiments after significant upgrades of the detectors, confirming that these structures are suitable for very large experiments. In the special case of Time Projection Chambers, the ion backflow and the energy resolution are sensitive issues that must be addressed and the GEM has shown to be able to deal with both of them. In this work, a stack of three GEMs with different pitches has been studied as a possible future approach for ion-backflow suppression to be used in TPCs and other detection concepts. With this approach, an ion backflow of 1 % with an energy resolution of 12 % at 5.9 keV has been achieved with the detector operating in an Ar/CO2 (90/10) mixture at a gain of ~ 2000.Comment: 15 pages, 11 figure

Analysis of laminated beams using the natural neighbour radial point interpolation method

Neste trabalho aplica‐se o método sem malha natural neighbour radial point interpolation method (NNRPIM) à análise unidimensional de vigas laminadas, considerando a teoria de Timoshenko. O NNRPIM combina o conceito matemático dos vizinhos naturais com a interpolação radial pontual. Os diagramas de Voronoï permitem impor a conectividade nodal e construir a malha de fundo para efeitos de integração, por intermédio das células de influência. É apresentada a construção das funções de interpolação NNRPIM, sendo, para estas, usada a função de base radial multiquadrática. As funções de interpolação geradas possuem continuidade infinita e a propriedade de delta Kronecker, o que facilita a imposição das condições de fronteira, uma vez que estas podem ser impostas com o método da imposição direta, tal como no método dos elementos finitos (FEM). De modo a obter o campo de deslocamentos e de deformações, a teoria de deformação de Timoshenko para vigas sujeitas a esforços transversos é considerada. Vários exemplos numéricos de vigas isotrópicas e vigas laminadas são apresentados de modo a demonstrar a convergência e a exatidão da aplicação proposta. Os resultados obtidos são comparados com soluções analíticas disponíveis na literatura.In this work, a meshless method, “natural neighbour radial point interpolation method” (NNRPIM), is applied to the one‐dimensional analysis of laminated beams, considering the theory of Timoshenko. The NNRPIM combines the mathematical concept of natural neighbours with the radial point interpolation. Voronoï diagrams allows to impose the nodal connectivity and the construction of a background mesh for integration purposes, via influence cells. The construction of the NNRPIM interpolation functions is shown, and, for this, it is used the multiquadratic radial basis function. The generated interpolation functions possess infinite continuity and the delta Kronecker property, which facilitates the enforcement of boundary conditions, since these can be directly imposed, as in the finite element method (FEM). In order to obtain the displacements and the deformation fields, it is considered the Timoshenko theory for beams under transverse efforts. Several numerical examples of isotropic beams and laminated beams are presented in order to demonstrate the convergence and accuracy of the proposed application. The results obtained are compared with analytical solutions available in the literature.Peer Reviewe

Dengue disease, basic reproduction number and control

Dengue is one of the major international public health concerns. Although progress is underway, developing a vaccine against the disease is challenging. Thus, the main approach to fight the disease is vector control. A model for the transmission of Dengue disease is presented. It consists of eight mutually exclusive compartments representing the human and vector dynamics. It also includes a control parameter (insecticide) in order to fight the mosquito. The model presents three possible equilibria: two disease-free equilibria (DFE) and another endemic equilibrium. It has been proved that a DFE is locally asymptotically stable, whenever a certain epidemiological threshold, known as the basic reproduction number, is less than one. We show that if we apply a minimum level of insecticide, it is possible to maintain the basic reproduction number below unity. A case study, using data of the outbreak that occurred in 2009 in Cape Verde, is presented.Comment: This is a preprint of a paper whose final and definitive form has appeared in International Journal of Computer Mathematics (2011), DOI: 10.1080/00207160.2011.55454

Segmentação e simulação de contornos em imagens através de processos físicos

O objectivo principal deste trabalho é, a partir de duas imagens de um mesmo objecto em dois instantes distintos, simular as formas intermédias que o objecto assume quando o seu comportamento é guiado por princípios físicos. Para tal, é preciso começar por segmentar o objecto nas duas imagens em questão extraindo o seu contorno, após definição manual de um contorno inicial em cada uma delas. Seguidamente, cada um dos contornos definidos evoluirá automaticamente ao longo de um processo iterativo até alcançar a fronteira do objecto desejado. Para isso, é construído um modelo deformável para cada um dos contornos usando o método dos elementos finitos. Posteriormente, a evolução temporal do modelo físico até ao contorno final desejado é obtida resolvendo a equação de equilíbrio dinâmico que contrabalança as forças externas e internas virtualmente aplicadas no objecto modelizado.Para simular a deformação entre os dois contornos obtidos na segmentação, é utilizada análise modal complementada com técnicas de optimização para estabelecer a correspondência entre os dados pontuais dos mesmos. Após esta fase de emparelhamento, o campo dos deslocamentos entre os dois contornos é simulado através da equação de equilíbrio dinâmico

Determination of objects contours using physical principles

Segmentation, the identification of an object represented in a static image or along image sequences,is one of the most common and complex tasks in the domain of Computational Vision. Usually,whenever we intend to extract higher level information from images, we need to start by segmentingthem.The main goal of this work is to segment an object represented in an image by extracting its contourafter defining an initial contour for it; this coarse contour will evolve along an iterative process untilit reaches the frontier of the desired object, figure 1. For that purpose, a deformable model is used,whose behaviour is driven by physical principles

First in-beam studies of a Resistive-Plate WELL gaseous multiplier

We present the results of the first in-beam studies of a medium size (10$\times$10 cm$^2$) Resistive-Plate WELL (RPWELL): a single-sided THGEM coupled to a pad anode through a resistive layer of high bulk resistivity ($\sim$10$^9 \Omega$cm). The 6.2~mm thick (excluding readout electronics) single-stage detector was studied with 150~GeV muons and pions. Signals were recorded from 1$\times$1 cm$^2$ square copper pads with APV25-SRS readout electronics. The single-element detector was operated in Ne\(5% $\mathrm{CH_{4}}$) at a gas gain of a few times 10$^4$, reaching 99$\%$ detection efficiency at average pad multiplicity of $\sim$1.2. Operation at particle fluxes up to $\sim$10$^4$ Hz/cm$^2$ resulted in $\sim$23$\%$ gain drop leading to $\sim$5$\%$ efficiency loss. The striking feature was the discharge-free operation, also in intense pion beams. These results pave the way towards robust, efficient large-scale detectors for applications requiring economic solutions at moderate spatial and energy resolutions.Comment: Accepted by JINS

Micromegas operation in high pressure xenon: charge and scintillation readout

The operational characteristics of a Micromegas operating in pure xenon at the pressure range of 1 to 10 bar are investigated. The maximum charge gain achieved in each pressure is approximately constant, around 4x10^2, for xenon pressures up to 5 bar and decreasing slowly above this pressure down to values somewhat above 10^2 at 10 bar. The MM presents the highest gains for xenon pressures above 4 bar, when compared to other micropattern gaseous multipliers. The lowest energy resolution obtained for X-rays of 22.1 keV exhibits a steady increase with pressure, from 12% at 1bar to about 32% at 10 bar. The effective scintillation yield, defined as the number of photons exiting through the MM mesh holes per primary electron produced in the conversion region was calculated. This yield is about 2x10^2 photons per primary electron at 1 bar, increasing to about 6x10^2 at 5 bar and, then, decreasing again to 2x10^2 at 10 bar. The readout of this scintillation by a suitable photosensor will result in higher gains but with increased statistical fluctuations.Comment: 22 pages, 11 figure