1,721,125 research outputs found
X-ray spectra reconstruction from analysis of attenuation data: A Back Scattering Thomson source application
No full textIt is known that useful information about the spectral distribution of an X-ray beam can be extracted by measuring and analyzing its transmission curve. This kind of approach to the characterization of the distribution in energy of photons is justified when the direct measurements of the energy of a single photon becomes too expensive or cannot just be performed. Thomson Scattering sources can produce up to 10(8) photons within a pulse 10 ps long. Hence the rate of incidence of photons is too high to apply traditional X-ray spectroscopy methods while methods based on the transmission curve can still be used. In order to propose an application of these techniques in the characterization of a Thomson Scattering source we studied an iterative statistical algorithm (Expectation-Maximization) used as a regularization method on simulated measurement computed starting from a calculated energy distribution with peak energy of 20 keV. Results show that this method can give good approximations in the low energy range (approximately 20 keV) and that it is at least sensible to a small amount of radiation in the higher part of the energy range (approximately 70 keV). The robustness of the method against non-ideal experimental conditions is also considered. (C) 2009 Elsevier B.V. All rights reserved
Application of an expectation maximization method to the reconstruction of X-ray-tube spectra from transmission data
No full textCMOS APS detector characterization for quantitative X-ray imaging
No full textAn X-ray Imaging detector based on CMOS Active Pixel Sensor and structured scintillator is characterized for quantitative X-ray imaging in the energy range 11-30 keV. Linearity, dark noise, spatial resolution and flat-field correction are the characteristics of the detector subject of investigation. The detector response, in terms of mean Analog-to-Digital Unit and noise, is modeled as a function of the energy and intensity of the X-rays. The model is directly tested using monochromatic X-ray beams and it is also indirectly validated by means of polychromatic X-ray-tube spectra. Such a characterization is suitable for quantitative X-ray imaging and the model can be used in simulation studies that take into account the actual performance of the detector
White Beam Differential Phase and Dark Field Imaging at High Resolution
Full text linkX-ray phase-contrast imaging (XPCI) can extend the capabilities of conventional radiography and, by
exploiting phase effects, make visible those details that lack enough absorption contrast [1]. Several
approaches have been proposed for XPCI by using synchrotron radiation, microfocal and extended
labortory sources [2]. We focus here on edge illumination [3] in view of its properties of high resolution,
sensitivity, robustness and achromaticity [4-6]. The latter is of particular interest for the study reported
here, where we used the direct beam from a bending magnet, aiming at making use of a spectral
distribution as broad as possible
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Application of an expectation maximization method to the reconstruction of X-ray-tube spectra from transmission data
Full text linkAn expectation maximization method is applied to the reconstruction of
X-ray tube spectra from transmission measurements in the energy range
7 − 40 keV. A semiconductor single-photon counting detector, ionization
chambers and a scintillator-based detector are used for the experimental
measurement of the transmission. The number of iterations required to reach
an approximate solution is estimated on the basis of the measurement error,
according to the discrepancy principle. The effectiveness of the stopping rule
is studied on simulated data and validated with experiments. The quality
of the reconstruction depends on the information available on the source
itself and the possibility to add this knowledge to the solution process is
investigated. The method can produce good approximations provided that
the amount of noise in the data can be estimated
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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