19,042 research outputs found
Generic guide concepts for the European Spallation Source
The construction of the European Spallation Source (ESS) faces many
challenges from the neutron beam transport point of view: The spallation source
is specified as being driven by a 5 MW beam of protons, each with 2 GeV energy,
and yet the requirements in instrument background suppression relative to
measured signal vary between 10 and 10. The energetic particles,
particularly above 20 MeV, which are expected to be produced in abundance in
the target, have to be filtered in order to make the beamlines safe,
operational and provide good quality measurements with low background.
We present generic neutron guides of short and medium length instruments
which are optimized for good performance at minimal cost. Direct line of sight
to the source is avoided twice, with either the first point out of line of
sight or both being inside the bunker (20\,m) to minimize shielding costs.
These guide geometries are regarded as a baseline to define standards for
instruments to be constructed at ESS. They are used to find commonalities and
develop principles and solutions for common problems. Lastly, we report the
impact of employing the over-illumination concept to mitigate losses from
random misalignment passively, and that over-illumination should be used
sparingly in key locations to be effective. For more widespread alignment
issues, a more direct, active approach is likely to be needed
Intermittency in Hall-magnetohydrodynamics with a strong guide field
We present a detailed study of intermittency in the velocity and magnetic
field fluctuations of compressible Hall-magnetohydrodynamic turbulence with an
external guide field. To solve the equations numerically, a reduced model valid
when a strong guide field is present is used. Different values for the ion skin
depth are considered in the simulations. The resulting data is analyzed
computing field increments in several directions perpendicular to the guide
field, and building structure functions and probability density functions. In
the magnetohydrodynamic limit we recover the usual results with the magnetic
field being more intermittent than the velocity field. In the presence of the
Hall effect, field fluctuations at scales smaller than the ion skin depth show
a substantial decrease in the level of intermittency, with close to monofractal
scaling.Comment: 10 pages, 8 figure
Locating the Source of Diffusion in Large-Scale Networks
How can we localize the source of diffusion in a complex network? Due to the
tremendous size of many real networks--such as the Internet or the human social
graph--it is usually infeasible to observe the state of all nodes in a network.
We show that it is fundamentally possible to estimate the location of the
source from measurements collected by sparsely-placed observers. We present a
strategy that is optimal for arbitrary trees, achieving maximum probability of
correct localization. We describe efficient implementations with complexity
O(N^{\alpha}), where \alpha=1 for arbitrary trees, and \alpha=3 for arbitrary
graphs. In the context of several case studies, we determine how localization
accuracy is affected by various system parameters, including the structure of
the network, the density of observers, and the number of observed cascades.Comment: To appear in Physical Review Letters. Includes pre-print of main
paper, and supplementary materia
Block Gauss and anti-Gauss quadrature with application to networks
Approximations of matrix-valued functions of the form WT f(A)W, where A ∈Rm×m is symmetric, W ∈ Rm×k, with m large and k ≪ m, has orthonormal columns, and f is a function, can be computed by applying a few steps of the symmetric block Lanczos method to A with initial block-vector W ∈ Rm×k. Golub and Meurant have shown that the approximants obtained in this manner may be considered block Gauss quadrature rules associated with a matrix-valued measure. This paper generalizes anti-Gauss quadrature rules, introduced by Laurie for real-valued measures, to matrix-valued measures, and shows that under suitable conditions pairs of block Gauss and block anti-Gauss rules provide upper and lower bounds for the entries of the desired matrix-valued function. Extensions to matrix-valued functions of the form WT f(A)V , where A ∈ Rm×m may be nonsymmetric, and the matrices V, W ∈ Rm×k satisfy VT W = Ik also are discussed. Approximations of the latter functions are computed by applying a few steps of the nonsymmetric block Lanczos method to A with initial block-vectors V and W. We describe applications to the evaluation of functions of a symmetric or nonsymmetric adjacency matrix for a network. Numerical examples illustrate that a combination of block Gauss and anti-Gauss quadrature rules typically provides upper and lower bounds for such problems. We introduce some new quantities that describe properties of nodes in directed or undirected networks, and demonstrate how these and other quantities can be
computed inexpensively with the quadrature rules of the present paper
ERP Characterization of Sustained Attention Effects in Visual Lexical Categorization
As our understanding of the basic processes underlying reading is growing, the key role played by attention in this process becomes evident. Two research topics are of particular interest in this domain: (1) it is still undetermined whether sustained attention affects lexical decision tasks; (2) the influence of attention on early visual processing (i.e., before orthographic or lexico-semantic processing stages) remains largely under-specified. Here we investigated early perceptual modulations by sustained attention using an ERP paradigm adapted from Thierry et al. [1]. Participants had to decide whether visual stimuli presented in pairs pertained to a pre-specified category (lexical categorization focus on word or pseudoword pairs). Depending on the lexical category of the first item of a pair, participants either needed to fully process the second item (hold condition) or could release their attention and make a decision without full processing of the second item (release condition). The P1 peak was unaffected by sustained attention. The N1 was delayed and reduced after the second item of a pair when participants released their attention. Release of sustained attention also reduced a P3 wave elicited by the first item of a pair and abolished the P3 wave elicited by the second. Our results are consistent with differential effects of sustained attention on early processing stages and working memory. Sustained attention modulated early processing stages during a lexical decision task without inhibiting the process of stimulus integration. On the contrary, working memory involvement/updating was highly dependent upon the allocation of sustained attention. Moreover, the influence of sustained attention on both early and late cognitive processes was independent of lexical categorization focus
Active galactic nuclei synapses: X-ray versus optical classifications using artificial neural networks
(Abridged) Many classes of active galactic nuclei (AGN) have been defined
entirely throughout optical wavelengths while the X-ray spectra have been very
useful to investigate their inner regions. However, optical and X-ray results
show many discrepancies that have not been fully understood yet. The aim of
this paper is to study the "synapses" between the X-ray and optical
classifications.
For the first time, the new EFLUXER task allowed us to analyse broad band
X-ray spectra of emission line nuclei (ELN) without any prior spectral fitting
using artificial neural networks (ANNs). Our sample comprises 162 XMM-Newton/pn
spectra of 90 local ELN in the Palomar sample. It includes starbursts (SB),
transition objects (T2), LINERs (L1.8 and L2), and Seyferts (S1, S1.8, and S2).
The ANNs are 90% efficient at classifying the trained classes S1, S1.8, and
SB. The S1 and S1.8 classes show a wide range of S1- and S1.8-like components.
We suggest that this is related to a large degree of obscuration at X-rays. The
S1, S1.8, S2, L1.8, L2/T2/SB-AGN (SB with indications of AGN), and SB classes
have similar average X-ray spectra within each class, but these average spectra
can be distinguished from class to class. The S2 (L1.8) class is linked to the
S1.8 (S1) class with larger SB-like component than the S1.8 (S1) class. The L2,
T2, and SB-AGN classes conform a class in the X-rays similar to the S2 class
albeit with larger fractions of SB-like component. This SB-like component is
the contribution of the star-formation in the host galaxy, which is large when
the AGN is weak. An AGN-like component seems to be present in the vast majority
of the ELN, attending to the non-negligible fraction of S1-like or S1.8-like
component. This trained ANN could be used to infer optical properties from
X-ray spectra in surveys like eRosita.Comment: 15 pages, 7 figures, accepted for publication in A&A. Appendix B only
in the full version of the paper here:
https://dl.dropboxusercontent.com/u/3484086/AGNSynapsis_OGM_online.pd
Concept for a Time-of-Flight Small Angle Neutron Scattering Instrument at the European Spallation Source
A new Small Angle Neutron Scattering instrument is proposed for the European
Spallation Source. The pulsed source requires a time-of-flight analysis of the
gathered neutrons at the detector. The optimal instrument length is found to be
rather large, which allows for a polarizer and a versatile collimation. The
polarizer allows for studying magnetic samples and incoherent background
subtraction. The wide collimation will host VSANS and SESANS options that
increase the resolution of the instrument towards um and tens of um,
respectively. Two 1m2 area detectors will cover a large solid angle
simultaneously. The expected gains for this new instrument will lie in the
range between 20 and 36, depending on the assessment criteria, when compared to
up-to-date reactor based instruments. This will open new perspectives for fast
kinetics, weakly scattering samples, and multi-dimensional contrast variation
studies.Comment: 18 pages, 10 figure
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