Multiscale approach to spin transport in magnetic multilayers

This article discusses two dual approaches to spin transport in magnetic multilayers: a direct, purely quantum, approach based on a Tight-Biding model (TB) and a semiclassical approach (Continuous Random Matrix Theory, CRMT). The combination of both approaches provides a systematic way to perform multi-scales simulations of systems that contain relevant physics at scales larger (spin accumulation, spin diffusion...) and smaller (specular reflexions, tunneling...) than the elastic mean free paths of the layers. We show explicitly that CRMT and TB give consistent results in their common domain of applicability

Focal Spot Size Estimation for a 4 MeV Small-size Betatron Using Digital X-ray Detector

One of the main advantages of small-size betatrons is small focal spot size. However there is no conventional procedure of focal spot size measurement for betatrons. Techniques described in standards are suitable only for low- and mid-energy tubes. The task become more complex if the focal spot size should be estimated quickly using modern digital detectors. In this paper we present preliminary results of focal spot size measurement of a 4 MeV small-size betatron with two different procedures using digital x-ray detector

Safety margin sensitivity analysis for model selection in nuclear power plant probabilistic safety assessment

The safety assessment of Nuclear Power Plants makes use of Thermal-Hydraulic codes for the quantification of the safety margins with respect to upper/lower safety thresholds, when postulated accidental scenarios occur. To explicitly treat uncertainties in the safety margins estimates within the Risk-Informed Safety Margin Characterization (RISMC) framework, we resort to the concept of Dynamic Probabilistic Safety Margin (DPSM). We propose to add to the framework a sensitivity analysis that calculates how much the Thermal-Hydraulic (TH) code inputs affect the DPSM, in support to the selection of the most proper probabilistic safety assessment method to be used for the problem at hand, between static or dynamic methods (e.g., Event Trees (ETs) or Dynamic ETs (DETs), respectively). Two case studies are considered: firstly a Station Black Out followed by a Seal Loss Of Coolant Accident (LOCA) for a 3-loops Pressurized Water Reactor (PWR), whose dynamics is simulated by a MAAP5 model and, secondly, the accidental scenarios that can occur in a U-Tube Steam Generator, whose dynamics is simulated by a SIMULINK model. The results show that the sensitivity analysis performed on the DPSM points out that an ET-based analysis is sufficient in one case, whereas a DET-based analysis is needed for the other case

Spin torque and waviness in magnetic multilayers: a bridge between Valet-Fert theory and quantum approaches

We develop a simple theoretical framework for transport in magnetic multilayers, based on Landauer-Buttiker scattering formalism and Random Matrix Theory. A simple transformation allows one to go from the scattering point of view to theories expressed in terms of local currents and electrochemical potential. In particular, our theory can be mapped onto the well established classical Valet Fert theory for collinear systems. For non collinear systems, in the absence of spin-flip scattering, our theory can be mapped onto the generalized circuit theory. We apply our theory to the angular dependance of spin accumulation and spin torque in non-collinear spin valves

New microfocus bremsstrahlung source based on betatron B-18 for high-resolution radiography and tomography

New microfocus source of hard bremsstrahlung (photon energy > 1 MeV), based on the betatron B-18 with a narrow Ta target inside, for high-resolution radiography and tomography is presented. The first studies of the source demonstrate its possibilities for practical applications to detect the microdefects in products made from heavy materials and to control gaps in joints of parts of composite structures of engineering facilities. The radiography method was used to investigate a compound object consisting of four vertically arranged steel bars between which surfaces were exposed gaps of 10 [mu]m in width. The radiographic image of the object, obtained with a magnification of 2.4, illustrates the good sensitivity of detecting the gaps between adjacent bars, due to the small width of the linear focus of the bremsstrahlung source

Mesoscopic versus macroscopic division of current fluctuations

We investigate the current shot noise at a three terminal node in which one of the branches contains a noise generating source and the correlations are measured between the currents flowing through the other two branches. Interestingly, if the node is macroscopic, the current correlations are positive, whereas for a quantum coherent mesoscopic node antibunching of electrons leads to negative correlations. We present specific predictions which permit the experimental investigation of the crossover from the quantum mechanical noise division to the macroscopic noise division

Photon-assisted electron-hole shot noise in multiterminal conductors

Motivated by a recent experiment by Reydellet et al. [Phys. Rev. Lett. 90, 176803 (2003)] we discuss an interpretation of photon-assisted shot noise in mesoscopic multiprobe conductors in terms of electron-hole pair excitations. ac voltages are applied to the contacts of the sample. Of interest are correlations resulting from the fact that electrons and holes are generated in pairs. We show that with two out-of-phase ac potentials of equal magnitude and frequency, applied to different contacts, it is possible to trace out the Hanbury Brown Twiss exchange interference correlations in a four probe conductor. We calculate the distribution of Hanbury Brown Twiss phases for a four-probe single channel chaotic dot

A pragmatic approach for measuring maintainability of dpra models

Dynamic Probabilistic Risk Assessment (DPRA) is a powerful concept that is used to evaluate design and safety of complex industrial systems. A DPRA model uses a conceptual system representation as a formal basis for simulation and analysis. In this paper we consider an adaptive maintenance of DPRA models that consist in modifying and extending a simplified model to a real-size DPRA model. We propose an approach for quantitative maintainability assessment of DPRA models created with an industrial modeling tool called PyCATSHOO. We review and adopt some metrics from conceptual modeling, software engineering and OO design for assessing maintainability of PyCATSHOO models. On the example of well-known "Heated Room" test case, we illustrate how the selected metrics can serve as early indicators of model modifiability and complexity. These indicators would allow experts to make better decisions early in the DPRA model development life cycle