Constitutive laws for the neutron density current

In this technical note, a fractional wave equation for the average neutron motion in nuclear reactor is considered. This representation covers the full spectrum of the average neutron transport behavior, i.e., Fickian and non-Fickian effects. The fractional diffusion model retains the main dynamic characteristics of the neutron motion in which the relaxation time associated with a rapid variation in the neutron flux contains a fractional exponent that can be manipulated to obtain the best representation of the neutron transport phenomena. The detrended fluctuation analysis (DFA) method is presented in this paper to estimate the fractional exponent.     

LOCALIZING THE IRIS THROUGH MEMETIC ALGORITHMS

An iris recognition system requires efficient image processing techniques in order to duly represent and interpret the iris structural characteristics of an individual. The first processing stage should be the identification of the iris region in an eye image. This work introduces the application of evolutionary algorithms to localize the iris region in an eye image. A method based on memetic algorithms was proposed and used to find the circles that represent the external iris border and the pupil border. This method is applied after detecting the edges of the image through gradient algorithms. The efficiency of the memetic algorithm in solving the problem was compared to the application of the Wildes' method, which uses the Circular Hough Transform (CHT), a well-known algorithm employed to find circles in an edged image. To test the algorithms, images from the UBIRIS database (Proença and Alexandre 2005 Proença , H. and L. A. Alexandre . 2005 . UBIRIS: A noisy iris image database . ICIAP 2005, 13th Int. Conf. on Image Analysis and Processing , Cagliari, Ital. Lect. Notes Comput. Sci. 3617 : 970 – 977 . [Google Scholar]) were used.     

Supercontinuum generation in non-silica fibers

The development of super continuum sources is driven by the requirements of a wide range of emerging applications. This paper points out how non-silica fibers are of benefit not only because their broad mid-IR transparency enables continuum generation in the 2–5 μm region but also since the high intrinsic nonlinearity of the glasses reduces the power-threshold for devices at wavelengths below 2 μm. For these glasses, the material zero-dispersion wavelength is typically shifted to long wavelengths compared to silica so dispersion tailoring is key to creating sources based on practical, near-IR, solid state pump lasers. We show how modeling work has produced fiber designs that provide flattened dispersion profiles with high nonlinearity coefficients and zero-dispersion wavelengths in the near-IR. Building on this flexibility, modeling of the pulse dynamics then demonstrates how coherent mid-IR supercontinuum sources could be developed. We also show the importance of the second zero-dispersion wavelength using bismuth fibers as an example. Nonlinear mode-coupling is shown to be a factor in larger core fibers for high-power applications. Demonstrations of supercontinuum in microstructured tellurite fibers, all-solid lead–silicate (SF57) fibers and in bismuth fibers and tapers are then reported to show what has been achieved experimentally using a range of materials and fiber geometries.     

Special purpose processor for parameter identification of CNC second order servo systems on a low-cost FPGA platform

The parameter identification process has an important role in servo systems tuning on computer numerical control (CNC) machine tools. The parameter identification is mostly done off-line due to the high computational load carried by the identification algorithms. In this work, we present a novel hardware architecture for an application specific processor implementing the recursive least-squares algorithm for the on-line identification. The developed processor is simulated and tested with the necessary elements to perform the CNC machine servo system control. The system was implemented in a low-cost FPGA, getting a maximum sample frequency of 40 kSps.     

Optical fibre nanotaper sensors

This paper reviews the work carried out in the recent years on sensors which exploit optical fibre nanotapers (OFN). After a brief introduction on the morphology, fabrication and properties of OFN, this paper will review the various OFN sensors. This type of sensors are extremely compact and relatively robust, are usually unperturbed by electromagnetic interference and can be interrogated remotely. In this review sensors will be grouped in three categories according to their morphology: linear sensors, resonant sensors and tip sensors. While linear and resonant sensors exploit the fraction of power propagating in the evanescent field (outside the nanotaper physical boundary), tip sensors exploit light confined within the nanotaper to excite/detect variations within a very limited area.     

Lognormal switching times for titanium dioxide bipolar memristors: origin and resolution

We measured the switching time statistics for a TiO(2) memristor and found that they followed a lognormal distribution, which is a potentially serious problem for computer memory and data storage applications. We examined the underlying physical phenomena that determine the switching statistics and proposed a simple analytical model for the distribution based on the drift/diffusion equation and previously measured nonlinear drift behavior. We designed a closed-loop switching protocol that dramatically narrows the time distribution, which can significantly improve memory circuit performance and reliability.     

The point-characteristic function, wavefronts, and caustic of a spherical wave refracted by an arbitrary smooth surface

The aim of this paper is to obtain expressions for the k-function, the wavefront train, and the caustic associated with the light rays refracted by an arbitrary smooth surface after being emitted by a point light source located at an arbitrary position in a three-dimensional homogeneous optical medium. The general results are applied to a parabolic refracting surface. For this case, we find that when the point light source is off the optical axis, the caustic locally has singularities of the hyperbolic umbilic type, while the refracted wavefront, at the caustic region, locally has singularities of the cusp ridge and swallowtail types.     

Development of a framework for the risk assessment of Na-Tech accidental events

Natural events impacting on chemical and process plants may cause severe accidents, triggering the release of relevant quantities of hazardous substances. The present study focused on the development of the tools needed to build up a general framework allowing the extension of quantitative risk assessment procedure to include the analysis of the industrial accidents caused by natural events. Specific methods and models were developed to allow the quantitative assessment of risk caused by two categories of “Na-Tech” accidents: accidents triggered by earthquakes and accidents triggered by floods. The approach allows the identification of the different damage modes expected for process equipment and of the accidental scenarios that may be triggered. The damage models developed allow the calculation of the damage probability of equipment items due to the natural events. A specific methodology was issued to take into account the consequences of the possible contemporary failure of several process units due to the impact of the natural event. The procedure allows the calculation of the overall individual and societal risk indexes including the multiple-failure scenarios caused by the impact of natural events. The overall methodology was applied to the analysis of specific case studies.     

Porous electrodes supported on ion-exchange membranes as electrochemical detectors for supercritical fluid chromatography

A conveniently assembled electrochemical cell, exploiting a porous electrode supported on a moist perfluorinated ion-exchange polymer, is proposed for profitable electrochemical detection in supercritical fluid chromatography. It consists of a porous Pt working electrode, contacted by the mobile phase from the chromatographic column, which is chemically deposited onto one side of a Nafion membrane. The rear uncoated side of this membrane, acting as a solid polymer electrolyte, is contacted by an electrolyte solution (1 M NaCl) contained in an internal compartment equipped with a Pt counter electrode and a Ag/AgCl, Cl(-) 1 M reference electrode. Ferrocene, eluted with supercritical carbon dioxide through a Spherisorb column installed in a supercritical fluid chromatographic system, was used as electroactive prototype analyte to test the performance of this detector, which turned out to be quite better than that provided by a conventional on-line UV absorbance detector. The recorded peaks were characterized by both a good reproducibility (4.5%) and a linear dependence of their height and area, which extended over a wide concentration range ( approximately 3 orders of magnitude). Moreover, they were not interfered by possible solvent front, unlike peaks recorded by the UV detector. The detection limit, estimated for a signal-to-noise ratio of 3 (4.2 x 10(-11) mol), was lower by approximately 1 order of magnitude than that found for the UV detector. Finally, the long-term stability of this detector was satisfactory in that only a approximately 6% decrease in the current responses was observed after a rather long period (2 months) of continuous use.     

Validation of the 3D code SARUMAN in a stability and quench experiment with the superconducting magnet SAFFO

Experiments on stability and quench propagation were performed on a NbTi hollow conductor test module allowing heat conduction only between adjacent layers, while the turns in a layer are thermally insulated (2-D winding). The measurements performed over a wide range of operating conditions were used as the experimental data-base for the comparison with the results of the simulations done using the quench analysis code SARUMAN. In the paper the experimental results are compared with the predictions of a 2-D version of SARUMAN, which show that the simulation is able to reproduce the general features of the quench propagation experiments.