Recrutamento da algarobeira (Prosopis juliflora (SW) D.C.) em condições de campo, Petrolina-PE.

A Algarobeira vem apresentando ampla distribuição no Semiárido do Nordeste do Brasil, em decorrência da boa adaptação das condições ambientais e pela dispersão causada pelo pastejo dos animais, que são criados de forma livre, apresentado maiores densidades em locais mais úmidos.1 CD-ROM

Densidade e composição florística de vegetação de caatinga em áreas ocupadas por algarobeira (Prosopis juliflora (SW) D.C.) em Petrolina-PE, Dormentes-PE, Juazeiro-BA e Jaguarari-BA.

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An instrument for low-level measurements of the leakage current from high-voltage biased detectors

Resistive Plates Chambers (RPC) are detectors biased at High-Voltage (HV) in excess of 4 kV. When fired by a particle, they develop a large signal current that can be read across a small resistance, 100 Omega or so. A characterization has been made of their ageing as a function of the behaviour of their leakage current with time. An array of 10 detectors has been developed for this purpose. We present the instrument designed and built to perform a continuous and automatic monitoring of the leakage current from each detector of the array, while the system is taking data. For the particular biasing set-up adopted, the current has been measured in series to the terminal connected to the HV of every channel. Since the small value of the currents, order of tens of nA, a special circuit solution and special precautions have been adopte

Response of microchannel plates to single particles and to electromagnetic showers

We report on the response of microchannel plates (MCPs) to single relativistic particles and to electromagnetic showers. Particle detection by means of secondary emission of electrons at the MCP surface has long been proposed and is used extensively in ion time-of-flight mass spectrometers. What has not been investigated in depth is their use to detect the ionizing component of showers. The time resolution of MCPs exceeds anything that has been previously used in calorimeters and, if exploited effectively, could aid in the event reconstruction at high luminosity colliders. Several prototypes of photodetectors with the amplification stage based on MCPs were exposed to cosmic rays and to 491 MeV electrons at the INFN-LNF Beam-Test Facility. The time resolution and the efficiency of the MCPs are measured as a function of the particle multiplicity, and the results used to model the response to high-energy showers.Comment: Paper submitted to NIM

Response of microchannel plates in ionization mode to single particles and electromagnetic showers

Hundreds of concurrent collisions per bunch crossing are expected at future hadron colliders. Precision timing calorimetry has been advocated as a way to mitigate the pileup effects and, thanks to their excellent time resolution, microchannel plates (MCPs) are good candidate detectors for this goal. We report on the response of MCPs, used as secondary emission detectors, to single relativistic particles and to electromagnetic showers. Several prototypes, with different geometries and characteristics, were exposed to particle beams at the INFN-LNF Beam Test Facility and at CERN. Their time resolution and efficiency are measured for single particles and as a function of the multiplicity of particles. Efficiencies between 50% and 90% to single relativistic particles are reached, and up to 100% in presence of a large number of particles. Time resolutions between 20ps and 30ps are obtained.Comment: 20 pages, 9 figures. Paper submitted to NIM

The introduced tree Prosopis juliflora is a serious threat to native species of the Brazilian Caatinga vegetation.

Despite its economic importance in the rural context, the Prosopis juliflora tree species has already invaded millions of hectares globally (particularly rangelands), threatening native biodiversity and rural sustainability. Here we examine seedling growth (leaf area, stem diameter, plant height) and seedling mortality across five native plant species of the Caatinga vegetation in response to competition with P. juliflora. Two sowing treatments with 10 replications were adopted within a factorial 2 × 5 randomized block design. Treatments consisted of P. juliflora seeds sowed with seeds of Caesalpinia ferrea, Caesalpinia microphylla, Erythrina velutina, Mimosa bimucronata and Mimosa tenuiflora (one single native species per treatment), while seeds of native species sowed without P. juliflora were adopted as controls. Overall, our results suggest that P. juliflora can reduce seedling growth by half and cause increased seedlingmortality amongwoody plant species.Moreover, native species exhibit different levels of susceptibility to competitionwith P. juliflora, particularly in terms of plant growth. Such a superior competitive ability apparently permits P. juliflora to establishmonospecific stands of adult trees, locally displacing native species or limiting their recruitment. The use of less sensitive species, such as C. ferrea and M. tenuiflora, to restore native vegetation before intensive colonization by P. juliflora should be investigated as an effective approach for avoiding its continuous spread across the Caatinga region

Performance of a Tungsten-Cerium Fluoride Sampling Calorimeter in High-Energy Electron Beam Tests

A prototype for a sampling calorimeter made out of cerium fluoride crystals interleaved with tungsten plates, and read out by wavelength-shifting fibres, has been exposed to beams of electrons with energies between 20 and 150 GeV, produced by the CERN Super Proton Synchrotron accelerator complex. The performance of the prototype is presented and compared to that of a Geant4 simulation of the apparatus. Particular emphasis is given to the response uniformity across the channel front face, and to the prototype's energy resolution.Comment: 6 pages, 6 figures, Submitted to NIM

A new concept for streamer quenching in resistive plate chambers

In this paper we propose a new concept for streamer quenching in Resistive Plate Chambers (RPCs). In our approach, the multiplication process is quenched by the appropriate design of a mechanical structure inserted between the two resistive electrodes. We show that stable performance is achieved with binary gas mixtures based on argon and a small fraction of isobutane. Fluorocarbons, deemed responsible for the degradation of the electrode inner surface of RPC detectors, are thus fully eliminated from the gas mixture. This design {also resulted} in a simplified assembly procedure. Preliminary results obtained with a few prototypes of ``Mechanically Quenched RPCs'' and some prospects for future developments are discussed