排序方式: 共有69条查询结果,搜索用时 15 毫秒
1.
D. Cavouras I. Kandarakis D. Nikolopoulos I. Kalatzis G. Kagadis N. Kalivas A. Episkopakis D. Linardatos M. Roussou E. Nirgianaki D. Margetis I. Valais I. Sianoudis K. Kourkoutas N. Dimitropoulos A. Louizi C. Nomicos G. Panayiotakis 《Applied physics. B, Lasers and optics》2005,80(7):923-933
In this study Y3Al5O12: Ce powder scintillator was evaluated for use in X-ray imaging detectors. This phosphor, also known as YAG: Ce scintillator or P-46 phosphor, is a non-hygroscopic, emitting green light with very short decay time. These properties are very attractive for X-ray imaging. Y3Al5O12: Ce powder was used to prepare various test screens (33–166 mg/cm2). Absolute luminescence efficiency measurements were performed for various X-ray tube voltages (50–130 kVp). In addition parameters related to image quality such as the modulation transfer function and the detective quantum efficiency were examined. A theoretical model, describing radiation and light transfer, was employed to fit experimental data and to estimate values of optical parameters. Absolute efficiency was found to decrease with X-ray tube voltage. Highest efficiency was obtained for the 107 mg/cm2 screen. Light attenuation coefficients were close to those of green emitting rare earth scintillators. At low spatial frequencies the detective quantum efficiency was high for the 107–166 mg/cm2 screens. The light emission efficiency and imaging performance of Y3Al5O12: Ce was not better than currently employed scintillators. However due to its very fast response and high spectral compatibility to optical sensors it may be considered for use in digital imaging detectors. 相似文献
2.
M. Araceli Snchez-Snchez Cristina Conde Beatriz Gmez-Aylln David Ortega-DelCampo Aristeidis Tsitiridis Daniel Palacios-Alonso Enrique Cabello 《Entropy (Basel, Switzerland)》2020,22(11)
Automated border control systems are the first critical infrastructure point when crossing a border country. Crossing border lines for unauthorized passengers is a high security risk to any country. This paper presents a multispectral analysis of presentation attack detection for facial biometrics using the learned features from a convolutional neural network. Three sensors are considered to design and develop a new database that is composed of visible (VIS), near-infrared (NIR), and thermal images. Most studies are based on laboratory or ideal conditions-controlled environments. However, in a real scenario, a subject’s situation is completely modified due to diverse physiological conditions, such as stress, temperature changes, sweating, and increased blood pressure. For this reason, the added value of this study is that this database was acquired in situ. The attacks considered were printed, masked, and displayed images. In addition, five classifiers were used to detect the presentation attack. Note that thermal sensors provide better performance than other solutions. The results present better outputs when all sensors are used together, regardless of whether classifier or feature-level fusion is considered. Finally, classifiers such as KNN or SVM show high performance and low computational level. 相似文献
3.
N. I. Kavallaris A. A. Lacey C. V. Nikolopoulos D. E. Tzanetis 《Mathematical Methods in the Applied Sciences》2007,30(13):1507-1526
We estimate the blow‐up time for the reaction diffusion equation ut=Δu+ λf(u), for the radial symmetric case, where f is a positive, increasing and convex function growing fast enough at infinity. Here λ>λ*, where λ* is the ‘extremal’ (critical) value for λ, such that there exists an ‘extremal’ weak but not a classical steady‐state solution at λ=λ* with ∥w(?, λ)∥∞→∞ as 0<λ→λ*?. Estimates of the blow‐up time are obtained by using comparison methods. Also an asymptotic analysis is applied when f(s)=es, for λ?λ*?1, regarding the form of the solution during blow‐up and an asymptotic estimate of blow‐up time is obtained. Finally, some numerical results are also presented. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
4.
Rafael Mayorga-Gonzlez Miguel Rivera-Torrente Nikolaos Nikolopoulos Koen W. Bossers Roozbeh Valadian Joaquín Yus Beatriz Seoane Bert M. Weckhuysen Florian Meirer 《Chemical science》2021,12(24):8458
Metal–Organic Frameworks (MOFs) have the potential to change the landscape of molecular separations in chemical processes owing to their ability of selectively binding molecules. Their molecular sorting properties generally rely on the micro- and meso-pore structure, as well as on the presence of coordinatively unsaturated sites that interact with the different chemical species present in the feed. In this work, we show a first-of-its-kind tomographic imaging of the crystal morphology of a metal–organic framework by means of transmission X-ray microscopy (TXM), including a detailed data reconstruction and processing approach. Corroboration with Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM) images shows the potential of this strategy for further (non-destructively) assessing the inner architecture of MOF crystals. By doing this, we have unraveled the presence of large voids in the internal structure of a MIL-47(V) crystal, which are typically thought of as rather homogeneous lattices. This challenges the established opinion that hydrothermal syntheses yield relatively defect-free material and sheds further light on the internal morphology of crystals.TXM-tomography unraveled large macropore defects within a MIL-47(V) MOF crystal. These pores do not seem to be well connected and they show a preferential orientation. 相似文献
5.
Georgios Patermarakis Konstantinos Moussoutzanis Nikolaos Nikolopoulos 《Journal of Solid State Electrochemistry》1999,3(4):193-204
A new method has been developed capable of describing the incorporation of electrolyte anions along the pore wall surface
and across both the barrier layer and the pore wall oxide after the establishment of the steady state of growth of porous
anodic Al2O3 where other methods cannot be applied to obtain reliable results. The knowledge of the nature/composition of anodic oxides
as regards the incorporation of species like electrolyte anions is of specific importance for both the understanding of the
electrochemical mechanism of oxide production and growth and the scientific and technological applications of porous anodic
Al2O3 films. The method consists of the selection and use of a suitable catalytic probe reaction on porous anodic oxides at thicknesses
varying from a value near zero up to the maximum limiting thickness and the treatment of the experimental reaction rate results
by a properly developed mathematical formalism. This method was employed in anodic Al2O3 films prepared in H2SO4 anodizing electrolyte at a constant bath temperature and different current densities using as a probe reaction the decomposition
of HCOOH on these oxides, which is almost exclusively a dehydration reaction, at relatively high reaction temperatures, 350 °C
and 390 °C, where the effect of other species except SO4
2− incorporated in the oxide on the reaction rate is eliminated. It has been shown that the fraction of the intercrystallite
surfaces occupied by SO4
2− follows a parabola-like distribution. It has a significant value at the pore base surface, depending on the current density,
then it passes through a maximum along the pore wall surface and across both the barrier layer and the pore walls near the
pore bases at positions depending on the current density and then becomes almost zero at the mouths of the pores of the oxide
with the maximum limiting thickness and at both the Al2O3/Al interface and cell boundaries. The maximum value of the surface coverage is almost independent of the current density
and is always near 1, showing an almost complete saturation of intercrystalline surfaces at these positions. The above distribution
of surface coverage predicts a qualitatively similar distribution of the SO4
2− bulk concentration across both the barrier layer and pore wall oxide around the pore bases. The method may be improved and
developed further either for a more detailed investigation of the above films or to investigate films prepared in other pore-forming
electrolytes.
Received: 30 July 1998 / Accepted: 30 September 1998 相似文献
6.
Weakly quasi-threshold graphs form a proper subclass of the well-known class of cographs by restricting the join operation. In this paper we characterize weakly quasi-threshold graphs by a finite set of forbidden subgraphs: the class of weakly quasi-threshold graphs coincides with the class of {P 4, co-(2P 3)}-free graphs. Moreover we give the first linear-time algorithm to decide whether a given graph belongs to the class of weakly quasi-threshold graphs, improving the previously known running time. Based on the simplicity of our recognition algorithm, we can provide certificates of membership (a structure that characterizes weakly quasi-threshold graphs) or non-membership (forbidden induced subgraphs) in additional ${{\mathcal O}(n)}$ time. Furthermore we give a linear-time algorithm for finding the largest induced weakly quasi-threshold subgraph in a cograph. 相似文献
7.
N.A.?Papadogiannis L.A.A.?Nikolopoulos D.?Charalambidis G.D.?Tsakiris Email author P.?Tzallas K.?Witte 《Applied physics. B, Lasers and optics》2003,76(7):721-727
We present experimental results in which a second-order effect, namely two-photon ionization of atomic He induced by a superposition of harmonics, is observed. The harmonics are generated in a Xe gas-jet using a 790-nm 10-Hz femtosecond Ti:sapphire laser and are subsequently focused into a He gas-jet with a Kirkpatrick–Baez arrangement. The superposition is formed by using a thin In filter and it comprises the 7th to 13th harmonics. Solving the time-dependent Schrödinger equation for He in a polychromatic laser field, the He + ion yield is calculated as a function of the total XUV intensity. Using the calculated yield and taking into account the focusing and transmission properties of the arrangement, the number of He + ions produced per laser pulse is estimated and is found to be in reasonable agreement with its measured value. The total number of ions produced non-resonantly follows a nearly quadratic dependence on the harmonic intensity, thus establishing the feasibility of a second-order auto-correlation measurement of the superposition of harmonics, i.e., of a direct temporal characterization of attosecond pulse trains. 相似文献
8.
9.
Nikolopoulos D Korgiopoulou C Mavropoulos K Liakopoulos G Karabourniotis G 《Talanta》2008,76(5):1265-1268
Dimethylsulfoxide (DMSO) is a widely used solvent for the extraction of chlorophylls (Chls) from leaves of higher plants. The method is preferred because the time-consuming steps of grinding and centrifuging are not required and the extracts are stable for a long time period. However, the extraction efficiency of this solvent is not comparable among plant species, whereas the particular leaf anatomical characteristics responsible for this unevenness remain unknown. In order to examine the influence of leaf anatomy on the extraction efficiency of DMSO (i.e. the concentration of Chls extracted with DMSO as % of the concentration of Chls extracted with 80% acetone), leaves of 19 plant species with different anatomical characteristics were incubated for 40min in DMSO at 65 degrees C. Under these conditions, heterobaric leaves, which are characterized by the occurrence of bundle sheath extensions in the mesophyll, showed lower extraction efficiency of DMSO compared to homobaric leaves and conifer needles. Microscopical observations of DMSO incubated leaf tissues showed that bundle sheath extensions behave as anatomical barriers which prevent the diffusion of DMSO within heterobaric leaves, even after prolonged incubation with the solvent. The effect was stronger in heterobaric leaves possessing thick bundle sheath extensions. The extraction efficiency of DMSO in these leaves was improved by vacuum infiltration of the samples in the presence of warm (65 degrees C) solvent. 相似文献
10.
We investigate whether, and to what extent, the physical phenomenon of long-lifetime resonant electromagnetic states with localized slowly-evanescent field patterns can be used to transfer energy efficiently over non-negligible distances, even in the presence of extraneous environmental objects. Via detailed theoretical and numerical analyses of typical real-world model-situations and realistic material parameters, we establish that such a non-radiative scheme can lead to “strong coupling” between two medium-range distant such states and thus could indeed be practical for efficient medium-range wireless energy transfer. 相似文献