1,721,604 research outputs found
On dissipated energy density in compression for concrete
Full text linkAn experimental investigation on drilled cylindrical concrete specimens in compression over a large scale range (1:19) has been carried out to evaluate the variation of some mechanical parameters by varying specimen size. The peculiarity of the present investigation consists in exploring very small specimen dimensions. The experimental results show scale effects on dissipated energy density rather than on uniaxial compressive strength. A theoretical explanation for such a phenomenon, based on fractal hypothesis, is presented and a comparison between experimental and theoretical values is discussed. © 2006 Elsevier Ltd. All rights reserved
The Ibis-Picsit detector onboard Integral
No full textPICsIT is the high-energy detector layer of the IBIS Imager, composed of 4096 CsI(Tl) scintillator detectors 8.4 x 8.4 x 300 mm in size with PhotoDiode readout. The detector operates in the 175 keV-20.4 MeV range and its data generation modes make it possible to collect information from single events and multiple coincident events. PICsIT is surrounded by the active BGO VETO and is located about 3 metres below the coded mask. The entire PICsIT plane is physically divided into 8 modules and logically divided into smaller units. The overall performance of the plane is directly related to the behaviour of each individual pixel, including its electronics, the system interconnection logic, and interaction with the other sub-systems. Pixels and electronic parameters were monitored constantly during instrument assembly. The following report describes PICsIT design and contains a summary of on-ground test results
PICsIT detector for γ-ray astronomy: Pixels qualification campaign
No full textPosition sensitive detectors coupled to coded masks are used for imaging in high energy astronomy. PICsIT is the gamma-ray position sensitive detector of the IBIS telescope to be flown onboard the INTEGRAL satellite in 2001. PICsIT detector functionalities and scientific performances as a whole are described in details elsewhere in this Conference (Labanti et al. (Nucl. Instr. and Meth. A, this conference)). In this work, we present the aim, methodology, and results of the tests and calibrations performed on the first qualification lot of 136 PICsIT pixels carried out at SCIONIX laboratories in The Netherlands, in April 1999. Before being accepted for PICsIT, each detection unit has in fact to be fully characterized in terms of energy resolution and lower energy threshold. The principal aim of PICsIT pixel qualification campaign described in this work has been to measure the key parameters of each CsI(Tl)+photodiode detection unit: CsI(Tl) crystals light output, gain variation with temperature, energy resolution degradation due to scintillation light production and collection, inhomogeneity in the crystal, and the variation of these quantities after a thermo-vacuum stress treatment
Strain localization in a continuum as an instability event
No full textThe paper presents a novel physical model, based on perturbation theory, to describe localization pattern formation in a solid material as a result of system instabilities. Such kind of approach has been inspired by the theory of population dynamics. In particular, the sinergetic phenomenon of strain localization into a stressed continuum, and its subsequent evolution to cohesive cracking, is obtained through the competition of an external source of energy (e.g., strain energy) and of the internal behavior of the material. The hypothesis of mobile energy entities within material bulk is put forward. These entities, which under low strain conditions are evenly distributed throughout the body, can be considered as strain quanta. The quantization of mechanical quantities is not new in continuum and fracture mechanics, [see, e.g., Novozhilov (1969, Prik Mat Mek 33:212-222)]. With increasing strain, a certain critical point is reached when the homogeneous situation becomes unstable and the strain quanta begin to aggregate into bands, leading to periodic strain localization patterns. The model, which is only theoretical at this stage, can be applied to the particular case of dry snow avalanches. In these cases, snow avalanche triggering is due toinstability (onset of sliding onto a weak plane) and is controlled by external loading (e.g., weight of the slope, load by skiers) and by internal factors (e.g., temperature changes, snow phase transformations etc.). © Springer Science+Business Media B.V. 2006
Fermi surface study of LaRu2Si2 and of heavy-fermion CeRu2Si2 above the Kondo temperature
No full textThe Two-dimensional (2D) angular correlation of the positron annihilation radiation of the heavy-fermion system (formula presented) was measured above the Kondo temperature (formula presented) and compared to that of the reference isostructural non-(formula presented)-electron system (formula presented) The (formula presented)-space densities of the two compounds, obtained via the Lock-Crisp-West folding of the 3D-reconstructed electron-positron momentum densities, were very similar. These results are in reasonable agreement with the band structure calculated for (formula presented) using the local-density approximation (LDA). Conversely, in the case of (formula presented) a clear discrepancy between the LDA calculation and the experiment appears unless the Fermi level (formula presented) is raised by (formula presented) After the (formula presented) adjustment the calculated Fermi surfaces are rather similar and in agreement with both experiments. The implications of this similarity on the physics of the heavy fermions are discussed. © 2002 The American Physical Society
Positron- electron autocorrelation function study of e-center in phosphorus-doped silicon
No full textTwo dimensional Fourier transformed angular correlation of annihilation radiation (2D-FT-ACAR) spectra have been taken for 1019 cm -3 Phosphorus-doped Si in the as grown state and after being subjected to 1.8MeV e- fluences of 2x1018cm -2. In the spectra of the irradiated samples, the zero-crossing points are observed to displace outwards from the Bravais lattice positions. It is suggested that this results from positrons annihilating with electrons in localized orbitals at the defect site. An attempt is made to extract just the component of the defect's positron-electron autocorrelation function that relates to the localized defect orbitals. It is argued that such an extracted real-space function may provide a suitable means for obtaining a mapping of localized defect orbitals
Positron-electron autocorrelation function study of E-center in silicon
No full textA study was conducted on the positron-electron autocorrelation function of defect center in silicon. The positron annihilation lifetime spectroscopy (PALS) spectra taken on the as-grown and defected samples of silicon were studied. It was found that the positron binding energy to the defect could be estimated from the ratio of the positron-electron autocorrelates for bulk crystal and defect trapped positron states
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